Johannes Fink, Houssemeddine Guissouma, Friedrich K. Jondral:
Feature-based Gesture Classification by Means of High Resolution Radar Measurements
[Angenommen] 2017 18th International Radar Symposium (IRS), Prague, Czechia, Mai 28-30, 2017

Abstract:
Continuously improving analog and digital hardware nowadays enables pow- erful waveforms at low costs and thus makes radar an attractive and cheap sensor for new fields of application. One such field is human machine interfaces (HMI). In this work, gesture classification based on high resolution radar measurements as new type of HMI is investigated. For this purpose, wideband radar measurements of human hand and body gestures at a center frequency of 60.5 GHz are recorded. As waveform, a sequence of lin- ear frequency modulated (LFM) chirps with a bandwidth of 7 GHz is employed, allowing simultaneous high resolution measurements of range and radial velocity of multiple tar- gets. Eight different gestures have been studied in this work. From the detector output, different features providing information of the relevant body parts are extracted using a proposed algorithm. These features of both training and test measurements are fed into the following classifiers: 1-nearest neighbor, p-nearest neighbor and polynomial classi- fier. It is shown, that the proposed radar signal processing, filtering and feature extraction methods yield very promising classification rates of over 95 % on the given data. [ back to overview ]

Benjamin Nuss, Johannes Fink, Friedrich K. Jondral:
Cost Efficient Frequency Hopping Radar Waveform for Range and Doppler Estimation
Proceedings of 17th International Radar Symposium (IRS), Krakau, Poland, Mai 10-12, 2016

Abstract:
A simple and cost efficient frequency hopping radar waveform which has been developed to be robust against mutual interference is presented. The waveform is based on frequency hopping on the one hand and Linear Frequency Modulation Shift Keying (LFMSK) on the other hand. The signal model as well as the radar signal processing are described in detail, focusing on the efficient estimation of range and Doppler of multiple targets in parallel. Finally, the probability of interference is estimated and techniques for interference mitigation are presented showing a good suppression of mutual interference of the same type of radar. [ back to overview ]

Friedrich K. Jondral:
Industrie 4.0 – Funk in der Fabrik
Zeitschrift für wirtschaftlichen Fabrikbetrieb (ZWF), Jahrgang 111 (2016), Heft 1-2, S. 59- 62

Abstract:
The digitization of our world irresistibly proceeds towards the internet of things and services. With her industry 4.0 initiative the German Federal Government stimulates growth, employment and innovation. After methods of information technology have been increasingly installed for industrial automation, further developments promise a migration to cyber physical systems in fabrication. In this connection radio communi- cations will increasingly be applied. This contribution is dedicated to the correspond- ing challenges. [ back to overview ]

Noha El Gemayel, Javier Schloemann, R. M. Buehrer, Friedrich K. Jondral:
Improved Indoor Positioning Using the Baum-Welch Algorithm
2015 IEEE Globecom Workshops, San Diego, USA, Dez 6, 2015

Abstract:
In this paper, we examine the exploitation of individual patterns of behavior to enhance indoor positioning of pedestrians. We make use of the fact that, due to habits and needs, a person is likely to be in some locations more often than others. For example, at their work or in their home, a person
is likely to spend more time in some rooms than in others. Therefore, it seems natural to take advantage of established behavior patterns when performing indoor localization in an effort to improve accuracy. Such improvement is particularly beneficial during emergencies, where location inaccuracies may lead to life-threatening delays in response times. In this work, habitual behavior is modeled and learned using a hidden Markov
model. It is shown that, applying the Markov model for location estimation results in more accurate estimates when compared to using a standard particle filter with odometry information. Additionally, transition probabilities as well as position error distributions do not need to be known a priori since they can be learned using the Baum-Welch algorithm. Results show how the Baum-Welch algorithm can even learn the distributions of biased
estimates. On the other hand, it is shown how user feedback can help accelerate the learning process, while guaranteeing good parameter estimation accuracies.
[ back to overview ]

Felix Wunsch, Holger Jäkel, Friedrich K. Jondral:
Performance Evaluation of IEEE 802.15.4 OQPSK and CSS PHY in the Presence of Interference
2015 IEEE 82nd Vehicular Technology Conference, Boston, USA, Sep 6-9, 2015

Abstract:
Industrial environments often exhibit harsh propagation conditions for wireless transmissions. This paper investigates the influence of different interference characteristics on the performance of IEEE 802.15.4 using the OQPSK and Chirp Spread Spectrum (CSS) PHY. Bit error rates are determined for AWGN, Rayleigh fading and self-interference scenarios. GNU Radio, a Software Defined Radio framework, serves as simulation environment. In addition to the implementation of the CSS transmitter and receiver, a low-complexity synchronization algorithm, which only consists of a correlator and a second order Phase Locked Loop (PLL), and its limitations are presented. Furthermore, MAC layer simulations are performed for beacon-enabled (slotted) as well as nonbeacon-enabled (unslotted) mode to make a statement about achievable throughput and latency in the presence of interference. It shows that the CSS PHY clearly surpasses the OQPSK PHY in throughput. Moreover, the simulation results are confirmed by the derivation of the theoretical maximum throughput for the investigated PHY layers.
[ back to overview ]

Sebastian Koslowski, Friedrich K. Jondral:
Short Paper: SIC-MMSE Detection for Filter Bank Multicarrier Systems
2015 IEEE 82nd Vehicular Technology Conference, Boston, USA, Sep 6-9, 2015

Abstract:
Filter bank multicarrier is one of the candidates for next generation mobile radio systems. Using filtered sub-channels reduces side-lopes as well as interference among sub-channels. Symbols overlap in time and are not separated by a cyclic prefix resulting in inter-symbol inference as well as interference from adjacent sub-channels under multi-path propagation. Equalization methods known from OFDM can not be directly applied. However, a linear per-subchannel equalizer with minimum meansquared error has been proposed. This has also been used for iterative decoding and equalization by remodulating decoded symbols to cancel-out interference. In this short paper, we extend this concept and derive a MMSE equalizer using soft interference cancellation. Using soft-information increases performance and enables turbo equalization for filter bank multicarrier systems.
[ back to overview ]

Noha El Gemayel, Holger Jäkel, Friedrich K. Jondral:
A Weighted Least Squares Algorithm for Passive Localization in Multipath Scenarios
2015 IEEE 82nd Vehicular Technology Conference, Boston, USA, Sep 6-9, 2015

Abstract:
This paper presents a weighted least squares algorithm for passive localization using Time Difference of Arrival
in multipath scenarios. The algorithm is based on the direct or one-step approach for position estimation using distributed sensors and requires no knowledge about the number of received multipaths, the transmitted signal or the transmit time. Delayed signal paths arriving due to multipath propagation are considered
and treated as interference to the localization problem. Simulation results show a better and more robust performance of the algorithm, compared to conventional two-step localization algorithms.
[ back to overview ]

Ralph Tanbourgi:
Diversity Combining under Interference Correlation in Wireless Networks
Dissertation, Forschungsberichte aus dem Institut für Nachrichtentechnik des Karlsruher Instituts für Technologie, Band 34, Karlsruhe 2015

Abstract:
A theoretical framework is developed for analyzing the performance of diversity combining under interference correlation. Stochastic models for different types of diversity combining and networks are presented and used for analysis. These models consider relevant system aspects such as network density, path loss, channel fading, number of antennas, and transmitter/receiver processing. Theoretical results are derived, performance comparisons are presented, and design insights are obtained.
[ back to overview ]

Johannes Fink,  Friedrich K. Jondral:
Short Paper: A Numerical Comparison of Chirp Sequence versus OFDM Radar Waveforms
2015 IEEE 82nd Vehicular Technology Conference, Boston, USA, Sep 6-9, 2015

Abstract:
The radar waveforms OFDM and chirp sequence are numerically compared in terms of receiver operating characteristic, accuracy in range and Doppler estimation and necessary signal bandwidth. By means of Monte Carlo simulations, fairly equal performace is shown, which supports a previous analytical result by the authors, which revealed that both signals yield the same baseband signal under narrowband assumption and negligible range-doppler-coupling in the chirp sequence waveform. However, as unambiguous range and Doppler get large, chirp sequence needs prohibitively more bandwidth than OFDM.
[ back to overview ]

Stefan Wunsch,  Johannes Fink,  Friedrich K. Jondral:
Improved Detection by Peak Shape Recognition Using Artificial Neural Networks
2015 IEEE 82nd Vehicular Technology Conference, Boston, USA, Sep 6-9, 2015

Abstract:
Conventional peak detection algorithms are not designed to include information on the expected peak shape. Therefore, commonly used detectors discard this valuable information and do not perform optimally in regard to the given possibilities. Designed and evaluated is a detector based on an artificial neural network, which is employed for pattern recognition in order to exploit the peak shape information. The detector outperforms the best detector using no peak shape information with a detection rate increase of up to 10% at a constant false alarm rate. The proposed detection method is compared with a threshold detector and an ordered statistics constant false alarm rate (OS-CFAR) detector commonly used in radar. The introduced detector provides useful information on the reliability of the peak detection. Furthermore, it is shown that the neural network based detection mechanism is easily employable on hardware because no knowledge about the signal-to-noise ratio of the training data is needed.
[ back to overview ]

Johannes Fink,  Friedrich K. Jondral:
Comparison of OFDM and Chirp Sequence Radar
Proceedings of 16th International Radar Symposium (IRS), Dresden, Germany, June 24-26, 2015

Abstract:
The radar waveforms OFDM and chirp sequence are compared in terms of accuracy, resolution capabilities, receiver operating characteristics, required resources and signal processing. We show, that both waveforms can be parametrized in such a way, that they yield the same baseband signal which can be processed using the same detection and estimation algorithms. This key insight reveals, that in this case, the waveforms perform identical. However, OFDM additionally allows simultaneous communication using the radar signal at the cost of increased signal processing.
[ back to overview ]

Ralph Tanbourgi, Friedrich K. Jondral:
Downlink MIMO Diversity with Maximal-Ratio Combining in Heterogeneous Cellular Networks
IEEE International Conference on Communications (ICC), London, UK, June 8 - 12, 2015

Abstract:
We present a stochastic model for analyzing the performance of multiple-input multiple-output (MIMO) diversity in a downlink heterogeneous cellular network. Multi-antenna receivers are assumed to perform maximal-ratio combining (MRC). We consider interference-blind (IB) MRC and interference-aware (IA) MRC, where the latter takes the interference power at each antenna into account. Using tools from stochastic geometry, we derive the coverage probability for both types of MRC as a function of various tier-specific system parameters, including the number of base station transmit antennas in each tier. The model is then used to compare the performance of IB-MRC and IA-MRC. One important insight is that IA-MRC becomes less favorable than IB-MRC in a transmit-diversity system due to a larger interference correlation across receive antennas.
[ back to overview]

Johannes Fink,  Stefan Wunsch,  Friedrich K. Jondral:
Improved Detector for Continuous Wave Radar Waveforms exploiting Phase Information
2015 IEEE International Radar Conference, Washington D.C., USA, May 11-15, 2015

Abstract:
By transmitting multiple copies of an arbitrary (frequency modulated) continuous wave radar signal, phase information in target echoes can be exploited for detection. The phase information is gained by calculating the sample variance of phase spectra of the multiple echoes. In this manner, phase and amplitude information is available at the detector. The optimal detector decides based on two-dimensional decision regions in the amplitude/phase-variance space. Suboptimal detectors combine two one-dimensional decisions. It is shown, that the proposed method improves detection performance especially at low signal to noise ratios. The optimal detector achieves a gain in detection rate compared to the conventional square law detector of nearly 10% at a signal to noise ratio (SNR) of 3 dB. The performance gain reduces to about 3% at a SNR of 9 dB. One of the suboptimal detectors also outperforms the conventional detector, but achieves lower gains in detection rates than the optimal detector.
[ back to overview ]

Ralph Tanbourgi, Friedrich K. Jondral:
Analysis of Heterogeneous Cellular Networks under Frequency Diversity and Interference Correlation
IEEE Wireless Communications Letters, vol.4, no.1, pp.2 - 5, Feb. 2015

Abstract:
Analyzing heterogeneous cellular networks (HCNs) has become increasingly complex, particularly due to irregular base station locations, massive deployment of small cells, and flexible resource allocation. The latter is usually not captured by existing stochastic models for analytical tractability. In this work, we develop a more realistic stochastic model for a generic K-tier HCN, where users are served in the downlink under frequency diversity. We derive the rate coverage probability for this case, taking into account the interference correlation across different parts of the allocated spectrum. Our results indicate that ignoring this type of correlation may considerably overestimate the offered date rate. Furthermore, the gain of spectrum diversification is significant, particularly at low to moderate target data rates.
[ back to overview]

Michael Schwall, Tamal Bose, Friedrich K. Jondral:
On the Performance of SC-MMSE-FD Equalization for Fixed-Point Implementations
8th International Symposium on Turbo Codes and Iterative Decoding (ISTC), Bremen, Germany, August 18 - 22, 2014

Abstract:
A fixed-point implementation of a minimum mean square error (MMSE) based frequency domain (FD) equalizer with soft interference cancellation (SC) is studied. The equalizer additionally processes \apriori information about the transmitted symbols and is used for turbo equalization. In this paper, we analyze the quantization and the clipping for different fixed-point representations and modulation schemes. The analysis allows to derive efficient representations for all symbols within the equalizer. This procedure is demonstrated for a generic system configuration featuring a 16-QAM. Finally, a fixed-point implementation in an integrated design environment for FPGAs verifies the theoretical studies and shows the device utilizations for different FPGAs that are embedded in current software defined radios. The results show, that on average 10 bits per symbol are required for a near-optimum equalization performance utilizing less than 8% area of state of the art FPGAs.
[ back to overview]

Ralph Tanbourgi, Sarabjot Singh, Jeffrey G. Andrews, Friedrich K. Jondral:
A Tractable Model for Noncoherent Joint-Transmission Base Station Cooperation
IEEE Transactions on Wireless Communications, vol.13, no.9, pp.4959 - 4973, Sept. 2014

Abstract:
Base station (BS) cooperation is set to play a key role in managing interference in dense heterogeneous cellular networks (HCNs). Non-coherent joint transmission (JT) is particularly appealing due to its low complexity, smaller overhead, and ability for load balancing. However, a general analysis of this technique is difficult mostly due to the lack of tractable models. This paper addresses this gap and presents a tractable model for analyzing non-coherent JT in HCNs, while incorporating key system parameters such as user-centric BS clustering and channel-dependent cooperation activation. Assuming all BSs of each tier follow a stationary Poisson point process, the coverage probability for non-coherent JT is derived. Using the developed model, it is shown that for small cooperative clusters of small-cell BSs, non-coherent JT by small cells provides spectral efficiency gains without significantly increasing cell load. Further, when cooperation is aggressively triggered intra-cluster frequency reuse within small cells is favorable over intra-cluster coordinated scheduling.
[ back to overview]

Ankit Kaushik, Ralph Tanbourgi, Friedrich K. Jondral:
Operating Characteristics of Underlay Cognitive Relay Networks
25th IEEE International Symposium on Personal, Indoor and Mobile Communications PIMRC, Sept 2014, Washington D.C., USA

Abstract:
Understanding the performance of cognitive relay networks (CRNs) is of great interest. Recently, stochastic geometry is being used to model and characterize the performance of CRNs. It is a known fact that sensing is an integral part of the CRN, however, in most cases it is not perfect. Moreover, the model inaccuracies caused by simplifications and/or approximations when deriving the analytical expressions for characterizing CRNs may distort their true performance. With no sensing in the system, we determine a lower performance bound (LPB) that can be used to judge the reliability of other systems that include sensing and model approximations. Based on the LPB, the operating characteristics (OC) for the CRN are obtained, which determine the joint performance of the primary and secondary system. Finally, OC are used to investigate the system performance under different scenarios.
[ back to overview]

Ralph Tanbourgi, Harpreet S. Dhillon, Jeffrey G. Andrews, Friedrich K. Jondral:
Dual-Branch MRC Receivers under Spatial Interference Correlation and Nakagami Fading
IEEE Transactions on Communications, vol.62, no.6, pp.1830 - 1844, June 2014

Abstract:
Despite being ubiquitous in practice, the performance of maximal-ratio combining (MRC) in the presence of interference is not well understood. Because the interference received at each antenna originates from the same set of interferers, but partially de-correlates over the fading channel, it possesses a complex correlation structure. This work develops a realistic analytic model that accurately accounts for the interference correlation using stochastic geometry. Modeling interference by a Poisson shot noise process with independent Nakagami fading, we derive the link success probability for dual-branch interference-aware MRC. Using this result, we show that the common assumption that all receive antennas experience equal interference power underestimates the true performance, although this gap rapidly decays with increasing the Nakagami parameter m_i of the interfering links. In contrast, ignoring interference correlation leads to a highly optimistic performance estimate for MRC, especially for large m_i. In the low outage probability regime, our success probability expression can be considerably simplified. Observations following from the analysis include: (i) for small path loss exponents, MRC and minimum mean square error combining exhibit similar performance, and (ii) the gains of MRC over selection combining are smaller in the interference-limited case than in the well-studied noise-limited case.
[ back to overview]

Ralph Tanbourgi, Harpreet S. Dhillon, Jeffrey G. Andrews, Friedrich K. Jondral:
Effect of Spatial Interference Correlation on the Performance of Maximum Ratio Combining
IEEE Transactions on Wireless Communications, vol.13, no.6, pp.3307 - 3316, June 2014

Abstract:
While the performance of maximum ratio combining (MRC) is well understood for a single isolated link, the same is not true in the presence of interference, which is typically correlated across antennas due to the common locations of interferers. For tractability, prior work focuses on the two extreme cases where the interference power across antennas is either assumed to be fully correlated or fully uncorrelated. In this paper, we address this shortcoming and characterize the performance of MRC in the presence of spatially-correlated interference across antennas. Modeling the interference field as a Poisson point process, we derive the exact distribution of the signal-to-interference ratio (SIR) for the case of two receive antennas, and upper and lower bounds for the general case. Using these results, we study the diversity behavior of MRC and characterize the critical density of simultaneous transmissions for a given outage constraint. The exact SIR distribution is also useful in benchmarking simpler correlation models. We show that the full-correlation assumption is considerably pessimistic (up to 30% higher outage probability for typical values) and the no-correlation assumption is significantly optimistic compared to the true performance.
[ back to overview]

Ralph Tanbourgi, Holger Jäkel, Friedrich K. Jondral:
Cooperative Interference Cancellation using Device-to-Device Communications
IEEE Communications Magazine: Special Issue on Smart Device-to-Smart Device Communications, June 2014

Abstract:
The ongoing densification in cellular networks has turned interference into a serious problem in future cellular networks. Since the interference experienced by close-by users in a cellular network is usually correlated, it can be cooperatively suppressed to improve user experience. In this paper, we introduce the idea of cooperative interference cancellation (CIC) between close-by users using device-to-device (D2D) communications for the example of the upcoming 3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) Rel-12 D2D technology. We understand CIC as a new network interference management tool, capable of exploiting interference correlation to improve downlink throughput. We discuss possible deployment scenarios as well as theoretical and practical challenges. To each challenge we provide some possible solutions. Finally, a first feasibility analysis using numerical simulations is presented which demonstrates the potential gains of CIC.
[ back to overview]

Ralph Tanbourgi, Sarabjot Singh, Jeffrey G. Andrews, Friedrich K. Jondral:
Analysis of Non-Coherent Joint-Transmission Cooperation in Heterogeneous Cellular Networks
IEEE International Conference on Communications (ICC), Sidney, Australia, June 10 - 14, 2014

Abstract:
Base station (BS) cooperation is set to play a key role in managing interference in dense heterogeneous cellular networks (HCNs). Non-coherent joint transmission (JT) is particularly appealing due to its low complexity, smaller overhead, and ability for load balancing. However, a general analysis of this technique is difficult mostly due to the lack of tractable models. This paper addresses this gap and presents a tractable model for analyzing non-coherent JT in HCNs, while incorporating key system parameters such as user-centric BS clustering and channel-dependent cooperation activation. Assuming all BSs of each tier follow a stationary Poisson point process, the coverage probability for non-coherent JT is derived. Using the developed model, it is shown that for small cooperative clusters of small-cell BSs, non-coherent JT by small cells provides spectral efficiency gains without significantly increasing cell load. Further, when cooperation is aggressively triggered intra-cluster frequency reuse within small cells is favorable over intra-cluster coordinated scheduling.
[ back to overview]

Ankit Kaushik, M. Rehan Raza, Friedrich K. Jondral:
On the Deployment of Cognitive Relay as Underlay Systems
Proceedings of CrownCom 2014, Oulu (Finland), June 2-4, 2014

Abstract:
The objective of this paper is to extend the idea of Cognitive Relay (CR). CR, as a secondary user, follows an underlay paradigm to endorse secondary usage of the spectrum to the indoor devices. To seek a spatial opportunity, i.e., deciding its transmission over the primary user channels, CR models its deployment scenario and the movements of the primary receivers and indoor devices. Modeling is beneficial for theoretical analysis, however it is also important to ensure the performance of CR in a real scenario. We consider briefly, the challenges involved while deploying a hardware prototype of such a system.
[ back to overview]

Ralph Tanbourgi, Harpreet S. Dhillon, Jeffrey G. Andrews, Friedrich K. Jondral
Dual-Branch MRC Receivers in the Cellular Downlink under Spatial Interference Correlation
(Invited, Best Student Paper)
European Wireless Conference, Barcelona, Spain, May 14 - 16, 2014

Abstract:
Although maximal-ratio combining (MRC) has become a widespread diversity-combining technique, its performance under interference is still not very well understood. Since the interference received at each antenna originates from the same set of interferers, but partially de-correlates over the fading channel, it exhibits a complicated correlation structure across antennas. Using tools from stochastic geometry, this work develops a realistic analysis capturing the interference correlation effects for dual-branch MRC receivers in a downlink cellular system. Modeling the base station locations by a Poisson point process, the probability of a typical dual-branch MRC receiver being covered by its serving base station is derived. For the interference-limited case, this result can be further simplified to an easy-to-use single-integral expression. Using this result, it is shown that ignoring interference correlation overestimates the true performance by 3%–10%, while assuming identical interference levels across antennas underestimates it by <2%. In both cases, however, the true diversity order of dual-branch MRC is preserved. Finally, the performance of MRC and selection combining under spatial interference correlation is compared.
[ back to overview]

Sebastian Koslowski, Martin Braun, Friedrich K. Jondral:
Using Filter Bank Multicarrier Signals for Radar Imaging
IEEE/ION Position, Location and Navigation Symposium (PLANS) 2014, Monterey, CA, USA, May 5-8, 2014

Abstract:
This paper presents an adaption of a joint OFDM communication and radar system to filter bank multicarrier signals. Our results show that any signal which can be represented as a matrix of symbols spread out in the time-frequency domain in a regular fashion can be processed in the same way, making the results obtained from OFDM radar research applicable to FBMC radar. For OFDM, it was shown that the two-dimensional periodogram is a robust and optimal way to obtain a range- Doppler plane; this method can also be applied to FBMC radar. However, the absence of a cyclic prefix in FBMC modulated signals entails a faster deorthogonalisation of the received symbol for targets with a high range. This is mitigated by the higher processing gain in FBMC radar and the exploitation of the intrinsic interference.
[ back to overview]

Noha El Gemayel, Holger Jäkel, Friedrich K. Jondral:
Error Analysis of a Low Cost TDoA Sensor Network
IEEE/ION Position, Location and Navigation Symposium 2014, Monterey, CA, USA, May 5-8, 2014

Abstract:
Geolocation methods are recently gaining a lot of interest due to their new range of applicability. Location based mobile services as well as frequency regulators aiming at efficient spectrum usage are interested in flexible, low cost geolocation systems with high accuracy. One method that meets those requirements is Time Difference of Arrival (TDoA). It has been subject to research for many years now. The main focus of research published about TDoA is presenting new algorithms or calculating estimation bounds in different scenarios. Due to new low cost available hardware solutions, a simple TDoA system can be built and used as a testbed for different algorithms in different real scenarios. This contribution presents an error analysis of a TDoA sensor network using low cost, off-the-shelf software defined radio platforms. The system relies on GPS time stamps provided by the platforms. Five important TDoA error types caused by hardware as well as different channel effects are analyzed. Each error is analyzed in its influence on the position estimate and a possible solution is given. Finally, a general structure of how a smart TDoA system should work is described.
[ back to overview]

Ankit Kaushik, Friedrich K. Jondral:
On the Estimation of Channel State Transitions for Cognitive Radio Systems
2014 Proceedings IEEE Wireless Communications & Networking Conference 2014, Istanbul, Turkey

Abstract:
Coexistence by means of shared access is a cognitive radio application. The secondary user models the slotted primary users channel access as a Markov process. The model parameters, i.e, the state transition probabilities (alpha,beta) help secondary user to determine the channel occupancy, thereby enables secondary user to rank the primary user channels. These parameters are unknown and need to be estimated by secondary users for each channel. To do so, the secondary users have to sense all the primary user channels in every time slot, which is unrealistic for a large and sparsely allocated primary user spectrum. With no other choice left, the secondary user has to sense a channel at random time intervals and estimate the parametric information for all the channels using the observed slots.
[ back to overview]

Michael Mühlhaus, Friedrich Jondral:
A Fully Blind Receiver for Automatic Modulation Classification of MIMO Signals
Proceedings of the 8th Karlsruhe Workshop on Software Radios, Karlsruhe, Germany, March 2014, pp. 148-152

Abstract:
Automatic modulation classification (AMC) finds its applications in spectrum surveillance, in signals intelligence, and nowadays in cognitive radios. Many articles examine the problem of AMC in single input single output (SISO) systems. Modern communications standards often use multiple input multiple outputs (MIMO) transmission that causes a rising interest in the AMC of MIMO signals. Usually in the literature a perfectly synchronized system is assumed. In this contribution, we extend these ideas to a fully blind receiver that does not require any a-priori knowledge of the transmitted signal. We propose methods for blind frequency, symbol rate, timing, channel, and phase estimation, as well as a low complexity AMC algorithm. Furthermore, the entire receiver is verified by the test bed using universal software Radio Platforms (USRP).
[ back to overview]

Michael Schwall, Friedrich K. Jondral:
High-Speed Turbo Equalization for GPP-based Software Defined Radios
The 2013 Military Communications Conference, San Diego, USA, Nov 18-20, 2013

Abstract:
High data rate waveforms for software defined radios (SDR) have to cope with frequency selective fading due to the mobile use in different harsh transmission environments. The received signal needs to be equalized in order to restore the transmitted information. Turbo equalization is a promising approach to deal with the inter-symbol interference occurring at the receiver. The iterative exchange of soft information between the equalizer and the decoder improves the decision reliability and hence reduces the bit error probability compared to conventional receivers. However, the necessary resource-demanding soft-input soft-output algorithms require a high processing performance to ensure real-time capability. In this paper, we will present a highspeed implementation of a turbo equalizer for SDRs with digital signal processing being performed on general purpose processors (GPP). The implementation will utilize linear MMSE filtering and suboptimal algorithms like overlapping sub-trellis MAXLog- MAP decoding, approximations of mathematical operations, parallelization methods such as threading-based pipelining, and processor specific optimizations like single instruction multiple data (SIMD) commands. We will present the processing gains for each optimization level, highlight the performance loss for the suboptimal modifications and analyze the latency introduced by the pipelined processing. So far, transmissions with data rates up to 5.4 Mbit/s can be decoded in real-time with negligible performance loss and tolerable delay.
[ back to overview]

Michael S. Mühlhaus Mengüç Öner, Octavia A. Dobre, Friedrich K. Jondral
A Low Complexity Modulation Classification Algorithm for MIMO Systems
IEEE Communications Letters, Vol. 17, No. 10, October 2013, pp. 1881-1884

Abstract:
A novel algorithm is proposed for automatic modulation classification in multiple-input multiple-output spatial multiplexing systems, which employs fourth-order cumulants of the estimated transmit signal streams as discriminating features and a likelihood ratio test (LRT) for decision making. The asymptotic likelihood function of the estimated feature vector is analytically derived and used with the LRT. Hence, the algorithm can be considered as asymptotically optimal for the employed feature vector when the channel matrix and noise variance are known. Both the case with perfect channel knowledge and the practically more relevant case with blind channel estimation are considered. The results show that the proposed algorithm provides a good classification performance while exhibiting a significantly lower computational complexity when compared with conventional algorithms.
[ back to overview]

Noha El Gemayel, Holger Jäkel, Friedrich K. Jondral
A Hybrid TDOA/RSSD Geolocation System using the Unscented Kalman Filter
2013 IEEE 78th Vehicular Technology Conference, Las Vegas, USA, Sep 2-5, 2013

Abstract:
Geolocation is an important topic nowadays. Among other applications, geolocation can be used by frequency regulators to monitor the time, frequency and space domain. In this case of a passive geolocation system, Time Difference of Arrival (TDOA) is often used. In this paper we analyze a geolocation system based on a combination of TDOA and Received Signal Strength Differences (RSSD). The benefit of combining the two measurement types is shown using the Cramer Rao Lower Bound (CRLB). The suitability of the Unscented Kalman Filter (UKF) as an algorithm for the hybrid geolocation system is investigated. Simulation results show that by using the UKF, the instability of RSSD geolocation, that is often caused by bad geometry, is compensated. The UKF nearly approaches the CRLB and greatly outperforms the previously proposed least squares hybrid solution. Using the UKF, the information about the signal power can be definitely used to enhance the position estimation accuracy.
[ back to overview]

Michael Schwall, David Leuck, Friedrich K. Jondral:
Efficient Fixed-Point Implementation of a SC-MMSE Turbo Equalizer
2013 IEEE 78th Vehicular Technology Conference, Las Vegas, USA, Sep 2-5, 2013

Abstract:
Turbo equalization is a promising approach to deal with the inter-symbol interference when transmitting over frequency selective radio channels. The iterative exchange of soft information between the equalizer and the decoder improves the decision reliability and hence reduces the bit error probability compared to conventional receivers. However, the processing of real-valued soft information with floating point precision causes high memory consumption and complex arithmetic logic units. In this paper, the effects of a fixed-point implementation of linear minimum mean square error (MMSE) based turbo equalizers with soft cancellation (SC) are studied. The applied soft-input soft-output (SISO) algorithms are analyzed concerning the value range and efficient fixed-point representations are derived. Results show, that even for small word lengths of 8 bit, the performance loss is negligible.
[ back to overview]

Ankit Kaushik, Marcus Mueller, Friedrich K. Jondral:
Cognitive Relay: Detecting Spectrum Holes in a Dynamic Scenario
Proceedings of IEEE International Symposium on Wireless Communication Systems ISWCS 2013, Ilmenau

Abstract:
Cognitive radio has been in discussion for more than a decade. Dynamic access to the spectrum is considered its major application to solve the bandwidth demands of the mobile network operator. Literature in the form of paper and books exists. Through Cognitive Relaying, the authors make an attempt to abridge the gap between the theoretical concepts and a practical implementation.
[ back to overview]

Martin Braun, Ralph Tanbourgi, Friedrich K. Jondral:
Co-channel interference limitations of OFDM communication-radar networks
EURASIP Journal on Wireless Communications and Networking, Volume 2013

Abstract:
Orthogonal frequency-division multiplexing (OFDM) communication-radar networks are systems where individual nodes use OFDM signals to both communicate and perform radar simultaneously. As a research subject, such networks are fairly new and lack some research covering fundamental limits. In particular, it is unclear how the reliability of the radar component is affected by a network operation, as several nodes might attempt to access the medium at the same time, thereby increasing interference and reducing the radar capabilities of individual nodes. In this paper, we apply the notion of outage (which was originall y introduced for communication networks) to radar networks and introduce the outage probability as a performance metric. Using stochastic geometry, we are able to give tight bounds on the outage probability and demonstrate how this is useful for testing OFDM radar parametrizations and algorithms. It is possible to show that the outage probability is smaller than 1% for previously suggested OFDM radar parametrizations without having to resort to empirical methods.
[ back to overview]

Ralph Tanbourgi, Holger Jäkel, Friedrich K. Jondral:
Cooperative Relaying in a Poisson Field of Interferers: A Diversity Order Analysis
IEEE International Symposium on Information Theory 2013, Istanbul, Turkey, July 7 - 12, 2013

Abstract:
This work analyzes the gains of cooperative relaying in interference-limited networks, in which outages can be due to interference and fading. A stochastic model based on point process theory is used to capture the spatial randomness present in contemporary wireless networks. Using a modification of the diversity order metric, the reliability gain of selection decode-and-forward is studied for several cases. The main results are as follows: the achievable spatial-contention diversity order (SC-DO) is equal to one irrespective of the type of channel which is due to the ineffectiveness of the relay in the MAC-phase (transmit diversity). In the BC-phase (receive diversity), the SC-DO depends on the amount of fading and spatial interference correlation. In the absence of fading, there is a hard transition between SC-DO of either one or two, depending on the system parameters.
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Johannes Fink, Friedrich K. Jondral, Thomas Bachle, Oliver Prinz
Ultrawideband radar time domain simulation for the analysis of coherent signal processing techniques
2013 14th International Radar Symposium (IRS), vol.2, pp. 1019-1024, June 19-21, 2013

Abstract:
In this paper a method for performing UWB radar time domain simulations is explained and simulation results are demonstrated. The given results are compared to measurements and the simulation errors are discussed. This constitutes a first step towards development of coherent signal processing techniques for UWB radar.
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Johannes Demel, Sebastian Koslowski, Friedrich K. Jondral:
A LTE Receiver Framework Implementation in GNU Radio
2013 Wireless Innovation Forum European Conference on Communications Technologies and Software Defined Radio (SDR-WInnComm-Europe-2013), Munich, Germany Jun. 11-13, 2013

Abstract:
We present an open source LTE receiver framework. Using GNU Radio's block-based signal processing capabilities, various LTE baseband specific functionality has been implemented in dedicated easily reconfigurable blocks. These can be used to decode and analyze arbitrary channels in the LTE downlink signal. As an example we decode the Master Information Block (MIB) transmitted on the Broadcast Channel (BCH). Our work is focused on performance measurements in order to identify critical processing operations on a General Purpose Processor (GPP). By optimizing critical components, e.g. channel estimation, synchronization, we can improve the overall system performance and therefore the system‘s real-time capabilities.
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Michael Schwall, Ruben Thill, Friedrich K. Jondral:
Turbo Equalization: Parallelization Strategies for Multi-Core General Purpose Processors
Proceedings of 23rd Virginia Tech Symposium on Wireless Communications, Blacksburg, May 2013

Abstract:
Turbo equalization is a promising approach to deal with the inter-symbol interference when transmitting over frequency selective radio channels. The iterative exchange of soft information between the equalizer and the decoder improves the decision reliability and hence reduces the bit error probability compared to conventional receivers. However, the necessary resource-demanding soft-input soft-output (SISO) algorithms have to be performed with a high processing power to ensure real-time capability. In this paper, two parallelization strategies for soft interference cancellation and linear minimum mean square error (SC-MMSE) based turbo equalizers are proposed, to efficiently utilize the processing power of multi-core general purpose processors (GPP). The first strategy focuses on the parallelization of the entire turbo process using a pipelined design. Results show that the throughput becomes independent of the number of turbo iterations at the expense of memory consumption and latency. In contrast, the second one increases the granularity of the parallelization by exploiting concurrencies in the applied algorithms. The throughput can also be increased without any additional latency.
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Martin Braun, Nico Otterbach, Jens Elsner, Friedrich K. Jondral:
Rapid Prototyping of a Frequency Hopping Ad Hoc Network System
Proceedings of 23rd Virginia Tech Symposium on Wireless Communications, Blacksburg, May 2013

Abstract:
Wireless networks in-the-loop is a method to develop software radio systems which operate in networks. It eliminates the gap between simulations and live testing by allowing the developer to use the same code for both types of testing. This allows the parallel development of signal processing and protocols in a network context without having to use intermediary tools. We demonstrate this concept using the example of frequency hopping ad hoc networks, which are both challenging on the signal processing side as well as on the network side.
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Noha El Gemayel, Sebastian Koslowski, Friedrich K. Jondral:
A low cost TDOA Localization System: Setup, Challenges and Results
10th Workshop on Positioning, Navigation and Communication 2013 (WPNC'13), Dresden, Germany, March 20-21, 2013

Abstract:
In this paper we present a prototype location estimation system which uses Time Difference of Arrival. The system consists of five low cost sensors that are synchronized using GPS time stamps. The sensors were installed on the rooftops of the university campus. The system is based on software defined radios which enable receiving signals with different carrier frequencies and sampling rates with the aim of applying it as a frequency monitoring and regulating system. The paper presents the expected challenges we face from the system and compares them to the measured results.
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Michael S. Mühlhaus, Mengüc Öner, Octavia A. Dobre, Holger U. Jäkel, Friedrich K. Jondral:
A Novel Algorithm for MIMO Signal Classification Using Higher-Order Cumulants
IEEE Radio and Wireless Symposium, Austin TX, USA, 20-23 Jan. 2013

Abstract:
Automatic modulation classification (AMC) of unknown communications signals is employed in both commercial and military applications, such as cognitive radio, spectrum surveillance, and electronic warfare. Most of the AMC methods proposed in the literature are developed for systems with a single transmit antenna. In this paper, an AMC algorithm for multipleinput multiple-output (MIMO) signals is proposed, which is based on higher-order cumulants. The use of cumulants with different orders, as well as their combinations as feature vectors are investigated. The ideal case of a priori knowledge of the channel state information (CSI) is considered, along with a setting of practical relevance, where the channel matrix is blindly estimated through independent component analysis. The performance of the proposed algorithm with different features is evaluated through simulations and compared with that of the average likelihood ratio test (ALRT).
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Sebastian Koslowski, Jens P. Elsner, Friedrich K. Jondral, Stefan Couturier, Cédric Keip, Olaf Bettinger:
Distributed Localized Interference Avoidance for Dynamic Frequency Hopping ad hoc Networks
2013 Wireless Innovation Forum Conference on Communications Technologies and Software Defined Radio (SDR-WInnComm-2013), Washington D.C., Jan. 8-10, 2013

Abstract:
We present a cognitive spectrum access solution for FH-CDMA ad hoc networks. Building on a cluster-based split phase multi-channel MAC protocol, we propose a mechanism for local interference avoidance through distributed hopset adaptation. Its goal is to identify and substitute channels not suitable for reliable communication. Substitution rules replace channels by locally unused hopsets. This way, interference is mitigated while maintaining orthogonality between nodes’ hopsets. We evaluate the gains of the proposed method with a simplified system model.
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Georg Vallant, Michael Epp, Markus Allen, Mikko Valkama, Friedrich K. ]ondral:
System-Level Mitigation of Undersampling ADC. Nonlinearity for High-IF Radio Receivers
Frequenz, Vol. 66(9-10), pp. 311-319

Abstract:
Over the last years ongoing advances in ADC technology have enabled RF signals to be sampled at IF frequencies. Undersampling is nowadays employed in software-defined radio or radar receivers and offers the possibility to relieve requirements in the analog receiver partition. Unfortunately, when moving to higher IF concepts, this becomes demanding for the ADC itself, because of inherent spurious-free dynamic range (SFDR) roll-off that increases with input frequency. This fact often limits the receiver's IF placement to Nyquist zone (NZ) 2. In this work the emerging concept of Digital Assistance is pursued to give the receiver access to higher NZs while making no compromise on the SFDR. We will present and discuss post correction results for two 16-bit high-speed converters from two different vendors at 120 and 125 MSPS, respectively. The proposed system-level post correction decomposes nonlinearity into a static and a dynamic part. For both ADCs under investigation the degraded SFDR in higher NZs could be improved by up to 15 dB using purely digital linearization technologies, thus increasing the detectability of small signals in the presence of very strong signals or interferers. Near-identical results for both ADCs confirm the general validity of the system-level correction approach.
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Noha El Gemayel, Simon Meier, Friedrich K. Jondral:
On the Applicability of the Residual Weighting Algorithm for TDOA
4th International Congress on Ultra Modern Telecommunications and Control Systems (ICUMT), St. Petersburg, Russia, October 3-5, 2012

Abstract:
Locating a mobile transmitter passively is a chal- lenging task. Recently, the most preferred method for this matter is the Time Difference of Arrival (TDOA) method. In this paper, different techniques for enhancing the TDOA results are introduced. They generally give an idea about the challenges of TDOA. We analyze different ways of using consecutive measure- ments as well as redundant sensors. We do that focusing on one specific algorithm using TDOA measurements, the residual weighting algorithm (RWA). Its main advantages lie in the low computational complexity and the fact that it doesn’t require additional a priori information. We analyze the performance of the algorithm through simulation of different scenarios and present the most preferable application. We show what kind of additional averaging could be used to enhance the results.
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Hanns-Ulrich Dehner, Daniel Figielek, Holger Jäkel, Martin Braun, Friedrich K. Jondral, Klaus Witrisal:
High Data Rate Coexistence-based Channel Coding for Noncoherent Multiband Impulse Radio UWB
IEEE International Conference on Ultra-Wideband (ICUWB), Syracuse, New York, USA, September 17 - 20, 2012

Abstract:
This paper investigates the performance of high data rate coexistence-based channel coding within a noncoherent Multiband Impulse Radio UWB system. It is shown that in the presence of interference the combination of adaptive image-based thresholding and channel coding can lead to a significant performance gain in comparison to thresholding-only. For thresholding two low complex methods, the Otsu algorithm as well as the nonlinear Teager-Kaiser filtering operation, are considered.
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Johannes Fink, Martin Braun, Friedrich K. Jondral:
Effects of Arbitrarily Spaced Subcarriers on Detection Performance in OFDM Radar
2012 IEEE 76th Vehicular Technology Conference, Québec City, Canada, Sep 3-6, 2012

Abstract:
Orthogonal frequency division multiplexing (OFDM) is a transmission technique which divides a given frequency band into subbands such that the subcarriers are mutually orthogonal. Because of its favourable transmission properties, OFDM is widely used in wireless communications. Recently it was shown that it is possible to extract information about distance and speed of obstacles from OFDM scatterings as in a radar system. The combination of communications and radar in a single OFDM system for automotive applications was named OFDM Radar. In this paper a simple multi user access scheme for OFDM Radar is proposed and its implications on radar detector design are discussed. The proposed scheme is a variant of OFDMA based on the idea that each user chooses independently a subset of the available OFDM carriers for transmission. The chosen subcarriers can be spaced arbitrarily. A suitable detector which is furthermore capable of multiple target detection is derived and its performance in various scenarios is demonstrated using Monte Carlo simulations.
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Michael S. Mühlhaus, Mengüc Öner, Octavia A. Dobre, Holger U. Jäkel, Friedrich K. Jondral:
Automatic Modulation Classification for MIMO Systems Using Fourth-Order Cumulants
2012 IEEE 76th Vehicular Technology Conference, Québec City, Canada, Sep 3-6, 2012

Abstract:
Automatic classification of the modulation type of an unknown communication signal is a challenging task, with applications in both commercial and military contexts, such as spectrum surveillance, cognitive radio, and electronic warfare systems. Most of the automatic modulation classification (AMC) algorithms found in the literature assume that the signal of interest has been transmitted using a single antenna. In this paper, a novel AMC algorithm for multiple input multiple output (MIMO) signals is proposed, which employs fourth-order cumulants as features for classification. First, perfect channel state information (CSI) is assumed. Subsequently, a case of more practical relevance is considered, where the channel matrix is unknown and has to be estimated blindly by employing independent component analysis (ICA). The performance of the proposed classification algorithm is investigated through simulations and compared with an average likelihood ratio test (ALRT) which can be considered as optimum in the Bayesian sense, but has a very high computational complexity.
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Ralph Tanbourgi, Xevi Pujol i Molist, Friedrich K. Jondral:
Collision-Balancing Frequency Hopping in Single-Hop Mobile Ad Hoc Networks
2012 IEEE 76th Vehicular Technology Conference, Québec City, Canada, Sep 3-6, 2012

Abstract:
We consider a single-hop frequency hopping (FH) mobile ad hoc network (MANET). To increase throughput orthogonal FH (O-FH) with hop-synchronous hopping can be used, thereby allowing collision-free transmissions under certain circumstances. However, as the MANET grows, hopping sequences must be re-used. As a result, nodes sharing the same sequence will experience deterministic collisions while others having a unique sequence will still communicate collision-free. This results in an unfair interference situation and makes performance guarantees difficult to meet for all nodes simultaneously. In this paper, we propose a collision-balancing FH (CB-FH) scheme to overcome this drawback. The CB scheme builds on O-FH and adapts the sequence set to balance interference among the nodes. We analytically study the characteristics of the adaptive sequences and analyze the resulting interference situation at the nodes. The performance is compared to uncoordinated FH (U-FH) and O-FH. It is found that CB-FH solves the problem of unfairness present in O-FH. Compared to U-FH, CB-FH yields considerable improvements in a wide range. Practical issues are discussed.
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Martin Braun, Manuel Fuhr, Friedrich K. Jondral:
Spectral Estimation-based OFDM Radar Algorithms for IEEE 802.11a Signals
2012 IEEE 76th Vehicular Technology Conference, Québec City, Canada, Sep 3-6, 2012

Abstract:
Recently, OFDM radar has gained attention as new algorithms for range and Doppler estimation specific to OFDM signals have been developed. The major advantage of OFDM is that it is both well-suited for radar processing as well as being suitable for communication. In previous work, we have proposed parametrizations for OFDM signals which suit the radar problem well. However, the proposed algorithms are independent of the OFDM signal parametrization, and it is therefore interesting to see how well they work with other, more commonly available signals. For this paper, we examine a common type of signal–OFDM signals according to the IEEE 802.11a standard–and determine how they could be used in a radar application.
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Jens P. Elsner, Ralph Tanbourgi, Joerg Schmid, Friedrich K. Jondral:
Hopping Strategies for Adaptive FH-CDMA Ad Hoc Networks under External Interference (Slides)
IEEE International Conference on Communications, Ottawa, Canada, Jun 10 - 15, 2012

Abstract:
We discuss the performance of an adaptive FH-CDMA ad hoc network under the influence of external interference where node positions are modeled by a homogeneous Poisson point process. The optimum channel assignment that balances internal network interference due to spatial reuse and external interference is derived analytically for a path loss and Rayleigh fading model. The performances of the resulting hopping strategies are then compared to various suboptimal hopping strategies such as non-adaptive hopping and min-max allocation with constant QoS. It is found that adaptivity offers a benefit only at low to medium node densities and that a good suboptimal strategy can be based on hard exclusion of bad channels (thresholding) with optional min-max allocation to balance the load of active channels.
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Jens P. Elsner, Hosung Park, Jeonghoon Mo, Friedrich K. Jondral:
Multi-Level Orthogonal Hopping Rendezvous
IEEE Communications Letters, Vol. 16, No. 8, August 2012, pp. 1220 - 1223

Abstract:
We propose a rendezvous scheme based on multi-level orthogonal hopping for multi-channel medium access control protocols. Its benefits compared to random hopping are higher throughput due to higher rendezvous probability. Furthermore, it offers improved fairness over other orthogonal hopping schemes.
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Martin Braun, Marcus Müller, Manuel Fuhr, Friedrich K. Jondral:
A USRP-based Testbed for OFDM-based Radar and Communication Systems
Proceedings of 22nd Virginia Tech Symposium on Wireless Communications, Blacksburg, June 2012

Abstract:
In this paper, we present a measurement testbed for OFDM radar which uses USRPs as a front-end. The resulting system, using two USRPs and a laptop, requires little power and can thus be easily installed in vehicles to perform measure- ments for car-to-car or car-to-infrastructure applications. As an example, we show how signals parametrized according to the IEEE 802.11a/p standards can be enhanced by radar functions without any additional spectrum usage and only a little extra signal processing.
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Ralph Tanbourgi, Holger Jäkel, Leonid Chaichenets, Friedrich K. Jondral:
Interference and Throughput in Aloha-based Ad Hoc Networks with Isotropic Node Distribution
IEEE International Symposium on Information Theory 2012, Cambridge, MA, USA, July 1 - 7, 2012 

Abstract:
We study the interference and outage statistics in a slotted Aloha ad hoc network, where the spatial distribution of nodes is non-stationary and isotropic. In such a network, outage probability and local throughput depend on both the particular location in the network and the shape of the spatial distribution. We derive in closed-form certain distributional properties of the interference that are important for analyzing wireless networks as a function of the location and spatial shape. Our results focus on path loss exponents $2$ and $4$, the former case not being analyzable before due to the stationarity assumption of the spatial node distribution. We propose two metrics for measuring local throughput in non-stationary networks and discuss how our findings can be applied to both analysis and optimization.
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Ralph Tanbourgi, Jens P. Elsner, Holger Jäkel, Friedrich K. Jondral:
Optimizing practical adaptive frequency hopping and medium access control in ad hoc networks (invited)
Eighth Workshop on Spatial Stochastic Models for Wireless Networks (SpaSWiN) at IEEE WiOpt 2012, May 14, 2012, Paderborn, Germany

Abstract:
Adaptive frequency hopping (AFH) as proposed, e.g., in IEEE 802.15.2 aims at increasing system reliability in the presence of quasi-static external interference. Practical approaches require autonomous sensing of the interference environment, with the measurements containing both external interference and network self -interference. In prior work, a simplistic model for AFH-based ad hoc networks was developed to analyze how this issue affects the area spectral efficiency (ASE). It was found that the AFH mechanism severely degrades ASE when self-interference is increased. In this paper, we modify the model to account for the correlation between the nodes' adapted hopping sets. We then address the question of how to design the system parameters to achieve optimal performance and avoid the degradation. We discuss different optimization problems and identify sensing techniques that can cope with increased selfinterference. Among these techniques, carrier detection sensing was found to be robust against self-interference while showing good performance. We further discuss cases where joint optimization of the AFH and CSMA mechanisms is beneficial and cases where there is little to be gained.
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Ralph Tanbourgi, Jens P. Elsner, Holger Jäkel, Friedrich K. Jondral:
Adaptive Frequency Hopping in Ad Hoc Networks with Rayleigh Fading and Imperfect Sensing
IEEE Wireless Communications Letters, Vol. 1, No. 3, June 2012, pp. 185 - 188 

Abstract:
A probabilistic model for adaptive frequency hopping (AFH) based wireless ad hoc networks with Rayleigh fading, where interference is due to self- and to slow-varying external interference, is proposed. Different AFH sensing techniques are studied in terms of area spectral efficiency (ASE) and it is shown that self-interference can have a negative effect on the AFH mechanism. In particular, packet error rate and received signal strength sensing suffer when self-interference is high, while carrier detection based sensing remains robust. The proposed model further offers powerful means for system optimization.
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Manuel Fuhr, Martin Braun, Christian Sturm, Lars Reichardt, Friedrich K. Jondral:
An SDR-based Experimental Setup for OFDM-based Radar
Proceedings of the 7th Karlsruhe Workshop on Software Radio Karlsruhe, Germany, March 2012

Abstract:
We present a software radio-based testbed for live testing of OFDM radar algorithms. Unlike previous testing setups, the hardware used (Universal Software Radio Peripherals) was low-cost and low-power, thus making the measurement setup both flexible and easy to use. The hardware can be easily battery powered. Both stationary and mobile measurements in realistic scenarios were performed.
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Michael Schwall, Stefan Nagel, Friedrich K. Jondral:
Code Parallelization for Multi-Core Software Defined Radio Platforms with OpenMP
Journal of Signal Processing Systems, Vol. 69, No. 1, 2012, pp. 67-74

Abstract:
Since the number of processing cores in a General Purpose Processor (GPP) increases steadily, parallelization of algorithms is a well known topic in computer science. Algorithms have to be adapted to this new processor architecture to fully exploit the available processing power. This development equally affects the Software Defined Radio (SDR) technology because the GPP has become an important processor for SDR platforms.

To make use of the entire processing power of a multi-core GPP and hence to avoid system inefficiency, this work provides an approach to parallelize C/C++ code using OpenMP. This application programming interface provides a rapid way to parallelize code using compiler directives inserted at appropriate positions in the code. The processing load can be shared between all available cores. We use Matlab Simulink as a framework for a model-based design and evaluate the processing gain of embedded handwritten C-code blocks with OpenMP support.

We will show that with OpenMP the core utilization is increased. Compared to a single-core GPP, we will present the increase of the processing speed depending on the number of cores. We will also highlight the limitations of code parallelization.

In our results, we will show that a straightforward implementation of algorithms without multi-core consideration will cause an underutilized system.
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Jens P. Elsner, Ralph Tanbourgi, Friedrich K. Jondral:
Multiple Access Interference Mitigation Through Multi-Level Locally Orthogonal FH-CDMA
The 2011 Military Communications Conference, Baltimore, Maryland, USA, Nov 7 - 10, 2011

Abstract:
Multi-level locally orthogonal frequency hopping code division multiple access (MLLO-FH-CDMA) is introduced as a method to reduce self-interference in large scale FH-CDMA ad hoc networks. It is analyzed in a stochastic geometry scenario and verified with simulations. The performance gains of multi-level hopping are shown by comparing it to standard FH-CDMA channel access.
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Jens Elsner, Christian Mohr, Friedrich K. Jondral:
Capacity Regions of Wireless Multi-Channel Ad Hoc Networks
2011 IEEE 74th Vehicular Technology Conference, San Francisco, USA, Sep 5-8, 2011

Abstract:
We consider the influence of FDMA on the capacity region of wireless ad hoc networks with a small number of nodes. It is found that FDMA offers an additional degree of freedom that is beneficial in interference-limited networks. Furthermore, simulation results indicate that static FDMA channel assignment over a time division schedule can achieve rates comparable to dynamic channel assignment.
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Martin Braun, Thomas Schu, Christian Sturm, Friedrich K. Jondral:
On the Single-Target Accuracy of OFDM Radar Algorithms
22nd Annual IEEE International Symposioum on Personal, Indoor and Mobile Radio Communications (PIMRC), Toronto, September 2011

Abstract:
Two OFDM radar algorithms (the Maximum Likelihood and a MUSIC-based algorithm) are introduced and their fundamental limits concerning accuracy are explained. To simplify the analysis, we research the special case of only one target in the radar's detection range, which facilitates the individual effects on the estimation quality. Through simulations, we can show that OFDM radar works well for this case.
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Georg Vallant, Wolfgang Schlecker, Friedrich K. Jondral:
Realization of a Volterra-Based Postdistortion Algorithm for Radar Receivers
46th Workshop on Microelectronics (MPC-Group Baden-Württemberg), Furtwangen, Juli 2011, ISSN 1868-9221, pp. 75-81

Abstract:
Modern Pulse Radar receivers have an increasing demand for high bandwidth and dynamic range. Digitization of the analog signal at IF allows architectural benefits, but places tight requirements on the ADC performance. How high an IF can be placed is severely restricted by the ADC's SFDR roll-off with increasing input frequency. While the SNR remains quite constant and can be improved by various Radar processing gains, SFDR remains the limiting figure of merit, as spurious contributions will cause potential false targets in the Radar processor. Weaker signals of interest will be masked by distortion induced by strong interferers or clutter.

We propose a digital, system-level linearization algorithm for ADCs, which allows increased receiver SFDR at high input frequencies. The model used for identification and correction is a constrained version of the general Volterra series. We focus on the FPGA implementation, which exploits a parallel processing structure in order to perform all computations at the necessary sample rate. The bit-true match between Matlab and VHDL code is verified in ModelSim. For estimating the hardware effort, a synthesis report for standard FPGAs is given. The application of the IP-Core in tandem with a commerical 16-Bit 120 MSPS ADC yielded 25 dB increased SFDR.
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Georg Vallant, Jochen Dederer, Michael Epp, Wolfgang Schlecker, Friedrich K. Jondral:
A Linearization Strategy for Undersampling Analog-to-Digital Converters
2011 International Workshop on ADC Modelling, Testing and Data Converter Analysis and Design and IEEE 2011 ADC Forum, June 30 - July 1, 2011. Orvieto, Italy

Abstract:
The ability of high performance Radar and Broadband Systems to detect weak targets in presence of strong interferers or clutter is given by their Spurious Free Dynamic Range (SFDR).
Although the Signal-to- Noise-Ratio (SNR) necessary for detection may be improved by well-known system processing gains, the dynamic range is ultimately limited by distortion terms caused by nonlinear behaviour of receiver components. The Software Defined Radio (SDR) paradigm assigns the Analog-to-Digital Converter a key role in receiver design. For systems using IF-Subsampling, linearity requirements place a heavy burden on the ADC, as SFDR signifcantly degrades with increasing input frequency.
As a consequence, the ADC can only be used at input frequencies fairly below its intrinsic full power bandwidth, restricting the systems IF placement.
This contribution discusses the possibility of processing ADC output data in the digital domain to achieve improved linearity. The Volterra series approach of nonlinear systems and its constrained variants are discussed. We will show in detail that for higher input frequencies, dynamic errors cause the harmonic terms to loose their in-phase ability; in higher Nyquist zones a frequency-dependend dynamic phase error has to be considered. Assumptions are backed by an evaluation of coherent data from the LTC2208 (16 Bit, 120 MSPS). A specific correction algorithm incorporating the dynamic phase error will be presented, which yielded 25 dB SFDR improvement in the 7th Nyquist Zone (360-420 MHz). The reproducibility of correction results is considered in some detail.
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Moritz Fischer, Martin Braun, Jens P. Elsner, Friedrich K. Jondral:
An Over-the-Air Reconfiguration API for Experimental Cognitive Radio Setups
2011 Wireless Innovation Forum European Conference on Communications Technologies and Software Defined Radios (SDR'11-WInnComm-Europe), 22-24 June, Brussels, Belgium

Abstract:
We present a software solution for over-the-air reconfiguration of remote software radio terminals using the free GNU Radio toolkit, facilitating the development of Cognitive Radio applications. The modular architecture allows for separating the cognitive engine from the signal processing algorithms, thus allowing fast implementation, verification and easier testing.
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Hanns-Ulrich Dehner, Holger Jäkel, Friedrich K. Jondral:
Narrow- and broadband Interference Robustness for OOK/BPPM based Energy Detection
Proceedings of the IEEE International Conference on Communications (ICC 2011), Kyoto (Japan), June 5-9, 2011

Abstract:
An analysis of narrow- and broadband interference robustness within an On-Off Keying/Binary Pulse Position Modulation based noncoherent multiband impulse radio ultra-wideband communication system is presented. Using the energy detector's processing gain closed-form expressions of the noise and interference related second order moment statistics at the output of an energy detection receiver are derived. This allows separate statements on the relative modulation specific processing gain with respect to various interference parameters.
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Nico Grove, Arnold Picot, Friedrich K. Jondral, Jens Elsner:
Why the Digital Dividend will not close the Digital Divide
Intermedia, 39 (2). pp. 32-37. ISSN 0309-118X, May 2011

Abstract:
Technological advance makes it possible to use broadcasting frequencies more efficiently. All over the world, terrestrial broadcasting infrastructure is being updated to switch from legacy analog to digital television. For the same broadcasting coverage less spectrum is required, and a “Digital Dividend” – the freed spectrum – is to be distributed. Many options on how to make best use of these frequencies are being discussed and have to be weighed against each other. Distribution of spectrum is further complicated by the need to provide adequate protection or alternative solutions for incumbent users affected by the change in spectrum allocation. A political goal which is driving the Digital Dividend discussion particularly in Germany, the USA and Australia is to close the so called “Digital Divide”, which refers to the widening gap between people with access to the Internet and those without. Especially in rural areas, the absence of high data rate (“broadband”) Internet access contributes to a widening of the gap. In this article, we would like to answer the questions “Can the Digital Dividend close the Digital Divide or mitigate it?” and suggest, in consequence, economically feasible options for reassignment of frequencies.
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Ralph Tanbourgi, Jens P. Elsner, Holger Jäkel, Friedrich K. Jondral:
Lower Bounds on the Success Probability for ad hoc Networks with local FDMA scheduling
Seventh Workshop on Spatial Stochastic Models for Wireless Networks at IEEE WiOpt 2011, May 9, 2011, Princeton, NJ, USA

Abstract:
This paper studies the performance of ad hoc networks with local FDMA scheduling using stochastic point processes. In such networks, the Poisson assumption is not justified due to interdependencies between points introduced by scheduling. For this reason, an upper bound on the second reduced moment measure is derived. Using this result, two lower bounds on the success probability are given, based on the second order product density and a non-homogeneous Poisson approximation. The relative performance of local FDMA is compared to random channel access. It is shown that the relative outage probability reduction of local FDMA highly depends on the SIR threshold as well as on the ratio of transmission distance to orthogonalization distance. If these two quantities are small, the improvement is high; the number of channels has only a minor effect on the relative improvement.
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Hanns-Ulrich Dehner, Holger Jäkel, Friedrich K. Jondral:
Interference investigations within noncoherent multiband impulse radio UWB
6th IEEE UWB Forum on Sensing and Communications 2011, Graz, Austria. May 5, 2011

Abstract:
Based on an impellent need for power efficient and low complexity communication systems, noncoherent UWB systems are in the focus of research interest. However, as noncoherent UWB systems are characterized by an unlicensed usage of an extremely wide bandwidth there is a potentially high interference resulting from other licensed/unlicensed radio systems. Hence, a crucial concern is the efficient treatment of interference operating simultaneously in the same frequency region. In this presentation an overview on ongoing research activities regarding the efficient treatment of interference is given. Thereby, a noncoherent multiband impulse radio UWB (MIR UWB) system is considered for high data rate communication over short distances. The presentation is structured as follows: At the beginning, an introduction into the MIR UWB system architecture is given. To increase the broad- and narrowband interference robustness of the MIR UWB system the second part focuses on the interference robustness of the noncoherent receiver used in the system. The third part introduces in adaptive coexistence based interference handling which is based on the usage of low complex image processing. Lastly, possible methods of low complex interference mitigation are described. Thereby, a mitigation approach for broadband interference and one for narrowband interference is presented. The presentation concludes with a summary as well as an outlook.
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Ralph Tanbourgi:
Resolving the Connectivity-Throughput Trade-Off in Random Networks
IEEE EUROCON 2011, Lisbon, Portugal

Abstract:
The discrepancy between the upper bound on throughput scaling in wireless networks and the throughput scaling in random networks is due to the connectivity-throughput trade-off. In a random network with $\lambda$ nodes, throughput scales by a factor of $\sqrt{\log{\lambda}}$ worse compared to the upper bound because of the uncertainty in the nodes' location. In this work, nodes know their location and employ power control. The expected throughput-progress is analyzed for a nearest neighbor forwarding strategy, which benefits from power control by reducing spatial contention. By this, the connectivity-throughput trade-off is resolved and the upper bound is achieved.

Acknowledgements: The author would like to thank Dr.-Ing. H. Jäkel and Prof. F. K. Jondral for having supervised this work.
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Hanns-Ulrich Dehner, Holger Jäkel, Friedrich K. Jondral:
On the modified Teager-Kaiser Energy Operator regarding Narrowband Interference
IEEE Wireless Telecommunications Symposium (WTS), New York City, New York, USA, 13-15 April 2011

Abstract:
This paper analyzes a modified Teager-Kaiser energy operator under narrowband interference within a subband of a multiband impulse radio UWB system. It is shown that a simple weighting of the Teager-Kaiser operator results in a significant frequency dependent energy distribution of the received signal. Hence, the modified Teager-Kaiser operator is not only a DC shifter. In comparison to the traditional Teager-Kaiser operator, a considerable increase of the amplitude spectrum around twice the subband's carrier frequency occurs at its output, which can improve the detection performance in absence or in presence of narrowband interference.
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Maximilian Hauske, Felix Robel, Friedrich K. Jondral, Marc Ihle:
Real-Time Realization of Interference Suppression for Robust Wireless Optical Communication
IEEE Signal Processing Society 14th DSP Workshop & 6th SPE Workshop, Sedona, Arizona, USA, 4.-7. Januar 2011

Abstract:
A robust short-range wireless optical communication system with direct sequence spread spectrum modulation and additional interference suppression is presented. The real-time system is implemented on a DSP/FPGA development board. The intensity modulation and direct detection system is designed to operate in the wireless optical indoor channel under strong non-Gaussian interference. An interference model with a combination of narrowband and pulsed wide-band components is assumed. Multiple stages of interference suppression based on non-linear signal processing and adaptive filtering are applied. A soft limiter with robust estimation of limiting thresholds is employed. The coefficients of an adaptive transversal filter are computed by means of parametric spectral estimation with Burg’s method. The real-time system is implemented on a rapid prototyping platform using model based design and high level code synthesis. Algorithms, simulation results as well as implementation and final measurements are presented.
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Maximilian Hauske, Friedrich K. Jondral:
Characterization of Wireless Optical Indoor Channels
Photonics Global Conference (PCG), Singapur, 14.-16. Dezember 2010

Abstract:
Wireless optical links with intensity modulation and direct detection are widely used for short-range communication. For indoor applications, the main source of interference is artificial lighting. The emitted optical power of fluorescent lamps with reactive and electronic control gears as well as light emitting diodes is measured and analyzed in time and frequency domain using Welch power spectral density estimate and spectrogram with short-time Fourier transform. Driven by the need for energy efficient lighting, the optical interference environment undergoes a radical change from low-frequency incandescent lamps and conventional fluorescent lamps to fluorescent lamps with high efficient, high-frequency electronic control gears and LEDs with pulse-width modulation dimming. Based on the interference measurements, a model of the wireless optical indoor channel is presented.
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Ralph Tanbourgi, Holger Jäkel, Friedrich K. Jondral:
Increasing the One-Hop Progress of Nearest Neighbor Forwarding
IEEE Communications Letters, Vol. 15, No. 1, January 2011, pp. 64-66

Abstract:
Nearest neighbor forwarding (NNF) intends to maximize throughput in wireless networks. However, NNF suffers from low one-hop progress and may therefore significantly increase end-to-end delay. The spatial efficiency (SE), i.e., the expectation of the ratio of progress to interference area associated with one hop, is introduced in order to quantify this trade-off. The problem of low progress is addressed by maximizing the one-hop SE, subject to the central angle γ, determining the forwarding area. By this, the optimal balance between minimizing the interference area and maximizing progress is found. Then, this analysis is extended by considering a Poisson point process, driven by some traffic intensity, on the interference area. Furthermore, the traffic aware γ*-NNF strategy is proposed which adapts γ to the traffic intensity in order to maximize SE. Simulation results show a significant reduction of the end-to-end delay if γ*-NNF is used.
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Ralph Tanbourgi, Jens P. Elsner, Friedrich K. Jondral:
Lowering Outage Probability in Ad Hoc Networks by Nearest Neighbor FDMA Scheduling
2011 Proceedings IEEE Wireless Communications & Networking Conference 2011, Cancun, Mexico,

Abstract:
This paper investigates the performance of a wireless ad hoc network capable of dynamically coordinating a set of orthogonal frequency channels such that every receiver and its nearest interferer operate in different channels. The distribution of the ratio of the interference powers of the second and first interferer is derived to show that the nearest interferer has dominant character, particularly in the absence of fading. Lower and upper bounds on outage probability are derived for the pathloss model. For Rayleigh fading, an approximation is presented. Simulation results show that nearest neighbor scheduling is beneficial, thereby avoiding interference effectively. In contrast to other techniques such as successive interference cancellation, the complexity of nearest neighbor scheduling remains comparatively small.
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Stefan Nagel, Michael Schwall, Friedrich K. Jondral:
Porting of Waveform: Principles and Implementation
FREQUENZ, Band 64, Heft 11-12, November/ Dezember 2010, S. 218-223

Abstract:
Waveform Portability is one of the major topics in present Software Defined Radio research. Porting a waveform from one platform architecture to another, requires a waveform development, that is independent from the underlying SDR platform. Although there are several development flows and software architectures, they are not well suited for the high demanding physical layer implementation. In this paper, a development flow is proposed, based on the Model Driven Architecture. For the evaluation of that development flow an OFDM waveform, based on IEEE 802.11g was implemented and then ported from the GPP dominated USRP to a heterogeneous DSP/FPGA platform with the SFF SDR. Finally, results are presented and performance drawbacks for platform independent development and code generation for GPPs and DSPs are summarized.
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Fabrizio Massaro, Jens Elsner, Florio Dalla Vedova, Friedrich K. Jondral:
Components for Software Radio Wideband Receivers: A space segment survey
SDR'10 Technical Conference, Washington DC, USA, Nov 30 - Dec 3, 2010

Abstract:
Many interesting scenarios are emerging on the horizon of satellite communication: communication system flexibility is a keyword for revenue enhancement and Software Defined Radio (SDR) is a means for achieving it. However, its development on the satellite side is slowed down by several constraints that the harsh space environment imposes on the satellite technology. This paper outlines the main scenarios for space-borne SDR and digs into the details of some application requirements. The goal consists in highlighting the capabilities of the current technology for SDR receivers, through a survey that compares space with ground market for two of the main receiver components: ADC and FPGA.
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Jens P. Elsner, Ralph Tanbourgi, Friedrich K. Jondral:
A Note on Local Receive Channel Scheduling versus Transmit Channel Scheduling in Wireless Multi-Channel Ad Hoc Networks
ICCIA'10, Tianjin, China, Dec 3-5 2010

Abstract:
Two strategies for FDMA channel assignment in multi-channel ad hoc networks, where transmissions occur over a maximum distance rmax, are compared: local transmit channel and local receive channel FDMA orthogonalization. While transmit scheduling yields higher gains, it also requires a contention resolution mechanism on the MAC layer. Receive scheduling does not necessarily require contention resolution, but performs worse in the relevant low outage regime.
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Markus Mueck, Marco Di Renzo, Merrouane Debbah, Tobias Renk:
Combination of Centralized & Decentralized Database and Terminal-Based Spectrum Sensing for Secondary Spectrum Access  
IEEE International Conference on Wireless Information Technology and Systems, 28. August - 3. September, Honolulu, Hawaii, USA, 2010

Abstract:
The general opportunistic spectrum usage approach has been broadly studied and published in the scientific literature. Recently, the concept of opportunistic relay was introduced. The authors outline the advantages and possible gains of using opportunistic relay nodes in the context of a cellular communication network. More specifically, if TV White Space frequencies in the range of 470 MHz to 790 MHz are available, they can be exploited with the following potential advantages: i) lighter infrastructure deployment, ii) increased spectral efficiency through less propagation losses and iii) increased spectral efficiency through extended macro diversity. This paper further illustrates how the approach of opportunistic relay is aligned to the regulatory requirements. In particular, the paper focuses on problems related to the detection of white spaces, by jointly exploiting information available from central & distributed databases and from distributed spectrum sensing algorithms at the relay nodes.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral:
Incremental Relaying with Imperfect Feedback in Wireless Cooperative Networks
IEEE 72nd Vehicular Technology Conference (VTC-Fall 2010), 6.-9. September, Ottawa, Canada, 2010

Abstract:
We present the outage capacity in the low signal-to-noise ratio (SNR) regime of an incremental relaying network. The network consists of one source, one destination, and two half-duplex relays. The one-bit feedback from the destination node is considered to be imperfect and is modeled as binary symmetric channel. The basic idea is that in each block only one terminal transmits depending on the feedback information in order to achieve a better use of the degrees of freedom of the channel. We derive a closed-form expression of the outage capacity and show that the quality of the feedback link has a great influence on the outage capacity of the network. We further examine an extension to the “usual” incremental relaying protocol by allowing the relays to overhear the transmissions of each other in order to increase their own decoding probability. This novel incremental relaying protocol performs close to the cut-set bound (within 0.5 bit/s/Hz).
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Stephan Sand, Christian Mensing, Roland Raulefs, Ralph Tanbourgi:
Position-aided mobile communications
IET Electronic Letters, Volume 46, Issue 17, p.1232-1234

Abstract:
We present capacity limits of novel position aware adaptive mobile communication systems exploiting channel state information (CSI) at the transmitter (CSIT). The position aware adaptive communication systems are benchmarked against state-of-the-art adaptive modulation and coding (AMC) systems that use only feedback CSI or additionally predict CSIT. Numerical results show that position aware adaptive systems achieve large capacity gains compared to state-of-the-art AMC systems for medium to large feedback delays.
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Jens P. Elsner, Ralph Tanbourgi, Friedrich K. Jondral:
On the Transmission Capacity of Wireless Multi-Channel Ad Hoc Networks with local FDMA scheduling
International Conference on Ultra Modern Telecommunications (ICUMT), Moscow, Russia, October 2010

Abstract:
The problem of the capacity of multi-channel interference-limited ad hoc networks with local frequency planning is examined in the transmission capacity framework. Bounds on outage and transmission capacity are given. If local frequency planning, i.e., frequency domain orthogonalization of neighbors in the communication range, is employed, transmission capacity can be significantly improved. The gain on the transmission capacity depends on the node density and is found to be a factor of 30 - 110 for a path loss attenuation = 4 and outage probability greater than 0.01. A design insight is, that if frequency planning in the communication range of nodes is employed, low node densities call for lower spectral efficiencies, while at high node densities higher spectral efficiencies increase the capacity of the network.
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Hanns-Ulrich Dehner, Holger Jäkel, Dennis Burgkhardt, Friedrich Jondral:
The Teager-Kaiser Operator in Presence of Multiple Narrowband Interference
IEEE Communications Letters, Vol. 14, No. 8, August 2010, pp. 716-718

Abstract:
As the analog front-end of an energy detector is highly susceptible to narrowband interference a modified energy detection scheme based on the Teager-Kaiser operator and high-pass filtering is considered. This letter analyzes the Teager-Kaiser operator output for an arbitrary number of narrowband band-pass signals having different amplitudes, center frequencies and bandwidths. The analysis reveals that additional cross components occur at the operator’s output in case of multiple narrowband interference. Hence, a simple narrowband interference suppression with Teager-Kaiser operator and highpass filtering can be insufficient.
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Martin Braun, Christian Sturm, Friedrich K. Jondral:
On the Frame Design for Joint OFDM Radar and Communications
Proceedings of the 15th International OFDM-Workshop 2010, Hamburg

Abstract:
We discuss how to design OFDM signals which are suitable for both radar and communications using high-bandwidth OFDM signals. A list of constraints which the signal configuration must comply with is defined. We present a solution which satisfies the requirements for radar and allows for reliable communications.
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Stephan Sand, Christian Mensing, Ronald Raulefs, Ralph Tanbourgi:
Position Aware Communication with Noisy Position Measurements
Advances in Positioning and Location-Enabled Communications (Workshop), PIRMC 2010, Istanbul, 26.-29. September 2010

Abstract:
This paper presents capacity limits of novel position aware adaptive cellular communication systems exploiting channel state information (CSI) at the transmitter (CSIT). The position aware adaptive communication systems are benchmarked against state-of-the-art adaptive modulation and coding (AMC) systems that use only feedback or predict CSIT in addition to the feedback CSI. We analyze the position aware adaptive communication systems both with perfect and imperfect position information. Numerical results demonstrate that position aware adaptive systems operate close to the channel capacity over a large range of feedback delays and channel variability with perfect position information. Further, position aware adaptive communication systems can outperform state-of-the-art AMC systems for Gaussian distributed position errors with standard deviations smaller than 2.5 m by more than 0.6 bits/s/Hz.
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Friedrich K. Jondral:
Software Defined Radios - Ein Überblick
ntz, Heft 5/2010, S. 34 - 37

Abstract:
Der Begriff Software Radio (SR) wurde zum ersten Mal vor fast 20 Jahren von Joseph Mitola III benutzt. Die zum Software Defined Radio (SDR), der realisierbaren Variante eines SRs, führende Entwicklung beginnt jedoch eigentlich mit der Veröffentlichung der Maxwellschen Gleichungen im Jahr 1873. Dieser Beitrag zeichnet diese Entwicklung nach, erklärt die wichtigsten Charakteristika eines SDRs und versucht darzustellen, wie und wo SDR heute im Mobilfunk, in der militärischen Kommunikation sowie in Forschung und Lehre verwendet werden.
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Sebastian Koslowski, Martin Braun, Jens Elsner, Friedrich K. Jondral:
Wireless Networks In-the-Loop: Emulating an RF front-end in GNU Radio (Slides)
SDR Forum 2010 European Reconfigurable Radio Technologies Workshop, Mainz, June 25, 2010

Abstract:
The objective of this work is to emulate the behavior of the Universal Software Radio Peripheral as an example of an RF front-end hardware for software radios. The model includes digital and analog signal processing. The emulator is implemented in GNU Radio and is intended to be used as part of a wireless network simulator.
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Ralph Tanbourgi, Maximilian Hauske, Friedrich K. Jondral:
Interference Suppression in Visible Light Communication
Advanced Photonics: OSA Optics & Photonics Congress; Signal Processing in Photonic Communications (SPPCom), Karlsruhe, 21.-24. Juni 2010

Abstract:
Visible light communication is highly exposed to interference. We therefore propose the use of interference suppression techniques known from radio communication. Analytic and simulative results show a significant increase in link performance in terms of bit error rate.
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Ralph Tanbourgi, Stephan Sand:
Evaluation of Handover Rates and Outage Probability for the Macrocellular Environment
2010 Proceedings IEEE Wireless Communications & Networking Conference 2010, Sydney, Australia

Abstract:
Statistical models for handover scenarios are necessary in order to identify key parameters and to allow proper benchmarking for handover techniques. This is quite a difficult task, since many assumptions have to be made, resulting in a loss of generality. Efforts towards the generation of such models were already made. Although these models show insights into trade offs, they leave some open questions. It is our purpose to extend previous work by presenting a new model for the macro cellular environment which is based on relative received signal strength technique employing a hysteresis margin and exponential smoothing. This model also incorporates implicitly the effect of the velocity of the mobile terminal on the handover system performance as well as the statistical dependency of consecutive signal strength measurements. We evaluate handover performance parameters such as handover probability, handover delay, expected number of handovers and outage probability. The outcome of this analysis is presented through numerical evaluations of the proposed model.
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Hanns-Ulrich Dehner, Yves Koch, Holger Jäkel, Dennis Burgkhardt, Friedrich K. Jondral, Rainer Moorfeld, Adolf Finger: 
Narrow-band Interference Robustness for Energy Detection in OOK/PPM
IEEE International Conference on Communications (ICC), Cape Town, South Africa, 23-27 May, 2010

Abstract:
This paper investigates robustness of a non-coherent multi-band impulse radio ultra-wideband communication system with respect to narrow-band interference. A statistical analysis of interference at the energy detector output gives insight into its impact and system dependency. Furthermore, the potential of interference mitigation is shown and verified with respect to the processing gain.
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Hanns-Ulrich Dehner, Holger Jäkel, Dennis Burgkhardt, Friedrich K. Jondral, Rainer Moorfeld, Adolf Finger: 
Treatment of Temporary Narrowband Interference in Non-coherent Multiband Impulse Radio UWB
The 15th IEEE Mediterranean Electrotechnical Conference (Melecon), Valletta, Malta, 25-28 April, 2010

Abstract:
This paper considers low complex interference handling with regard to temporary narrowband interference occurrence within a non-coherent On-Off Keying based multiband impulse radio UWB system. The approach allows a precise and reliable sub-band specific on-line detection of narrowband interference's presence or absence within a packet's data phase. Due to the system's instability after detection of interference absence, a decision threshold replacement is applied to correctly demodulate the data packet's remaining possibly not interfered data bits. Hence, an increased flexibility with respect to efficient interference handling as well as a significant performance gain can be achieved.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral, Andrea Goldsmith:
Do Decode-and-Forward Relaying Protocols beat Transmit Diversity?
16th European Wireless Conference, 12.-15. April, Lucca, Italy, 2010

Abstract:
This paper addresses the question whether decode-and-forward relaying protocols can beat transmit diversity with respect to outage capacity. We derive expressions of outage capacity for transmit diversity, decode-and-forward, selective decode-and-forward, and incremental decode-and-forward. We demonstrate that the relaying protocols outperform transmit diversity in certain regions. Decode-and-forward is beneficial over transmit diversity when the relay is located close to the source. For selective decode-and-forward to beat transmit diversity, the relay should be placed between the source and the destination, but not too close to the source. The best performance is achieved by incremental decode-and-forward. The region in which this scheme is beneficial over transmit diversity is large. Moreover, as the region becomes smaller, the outage probability for the source-destination link becomes larger.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral
Outage Regions of Cooperative Networks for different Coding Schemes
4th International Symposium on Communications, Control and Signal Processing (ISCCSP), 3.-5. March, Limassol, Cyprus, 2010

Abstract:
In this paper we present outage regions for two cooperation strategies, namely multi-routing and adaptive multi-routing, if two different coding schemes are used. The first coding scheme is repetition coding and the second one is parallel channel coding. We show that for one cooperation strategy, the outage region for parallel channel coding is a strict subset of the outage probability of repetition coding, thus, leading to a smaller outage probability. We demonstrate that adaptive multi-routing in conjunction with parallel channel coding achieves by far the best performance.
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Micha Linde, Gewinner des IEEE SB Karlsruhe Student Paper Contest 2009:
A Simple Stochastic Channel Simulator for Car-to-Car Communication at 24 GHz
Beitrag zum Finale des Region 8 Student Paper Contest, MELECON, Malta, 2010

Abstract:
A channel model, based on stochastics and ray tracing, is described for a communication system, in which moving participants communicate in the 24 GHz band. A realistic behaviour of propagation effects, both short-term and long-term, is modeled. This allows to simulate and design future vehicle-to-vehicle systems.
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Friedrich K. Jondral, Hanns-Ulrich Dehner, Michael Mühlhaus (Editors):
Proceedings of the 6th Karlsruhe Workshop on Software Radios
Karlsruhe, Germany, March 2010

Preface:
On March 29, 2000, I had the honor to open the 1st Karlsruhe Workshop on Software Radios. This is now almost ten years ago and I am very proud to welcome you today to the 6th workshop in this series. This time we are going to have 30 presentations from academia as well as from industry.

In 2000 the development of software radios was at its starting point. Today, there is a well established market for Software Defined Radios and the focus of research projects is on cognitive radio networking, adaptive resource allocation, or overlay systems, just to name a few topical fields. At least in Western Europe, we see that the research in mobile radio changes from big infrastructure systems to self organizing ad hoc networks that are themselves again connected to IP-based infrastructure networks. New interesting challenges are waiting for us!

The program of the 6th Karlsruhe Workshop on Software Defined Radios is divided into the sessions

• Advances in Signal Processing for Cognitive Radio –
  A special session presenting results from the European Network of Excellence in Wireless Communications, known as NEWCom++
• Hardware
• GNU Radio Implementations
• Scheduling and Partitioning
• Applications I and II
• Spectrum Sensing and Resource Allocation

A special welcome goes to my colleagues from the Special Interest Group on Radio Systems (Fachausschuss 7.2) of the Information Technology Society (ITG) within Germany’s VDE (Verband der Elektrotechnik Elektronik Informationstechnik e.V.). The Special Interest Group on Radio Systems acts as a co-host of this workshop and I very much appreciate the support of its chairman, Professor Matthias Hein from the Ilmenau University of Technology.

I want to express my sincere thanks to Gabriele Kuntermann, Beate Mast, and Angelika Olbrich who did the major part of the organizational work. Reiner Linnenkohl acted as the workshop’s web page administrator. Hanns-Ulrich Dehner and Michael Mühlhaus managed the abstract review process, put together the workshop’s program, edited and prepared the proceedings for publication. The staff of KIT’s Communications Engineering Lab is prepared to make the workshop a success for all participants.

Karlsruhe, March 3, 2010 Friedrich K. Jondral
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Martin Braun, Christian Sturm, Friedrich K. Jondral:
Maximum Likelihood Speed and Distance Estimation for OFDM Radar
2010 IEEE Radar Conference, Washington D.C.

Abstract:
A distance and speed estimation algorithm for OFDM based radar is analysed statistically. The maximum likelihood estimator is derived and compared to previous results. A connection to spectral analysis is drawn, simplifying the analysis of the estimator in question. Finally, a method to evaluate the performance of the algorithm is presented and conclusions for the structure of the OFDM signals are drawn. It is shown that the estimation algorithm works well above an SNR threshold, but rapidly degrades below.
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Stephan Sand, Ralph Tanbourgi, Christian Mensing, Ronald Raulefs
Position Aware Adaptive Communication Systems
2009 Proceedings Asilomar Conference on Signals, Systems and Computers, Pacific Grove, CA, USA

Abstract:
This paper presents capacity limits of novel position aware adaptive cellular communication systems exploiting channel state information (CSI) at the transmitter (CSIT). The position aware adaptive communication systems are benchmarked against state-of-the-art adaptive modulation and coding (AMC) systems that use only feedback or predict CSIT in addition to the feedback CSI. Numerical results demonstrate that position aware adaptive systems operate close to the channel capacity over a large range of feedback delays and channel variability. Further, position aware adaptive communication systems achieve large capacity gains compared to state-of-the-art AMC systems for medium to large feedback delays.
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Johannes Fink, Sandra Roger, Alberto Gonzalez, Vicenc Almenar, Victor M. Garcia:
Complexity assessment of sphere decoding methods for MIMO detection
2009 IEEE International Symposium on Signal Processing and Information Technology, Ajman, UAE, Dec 14-17, 2009

Abstract:
Sphere Decoding (SD) algorithms have been shown to provide maximum likelihood (ML) detection over Gaussian multiple input-multiple output (MIMO) channels with lower complexity than the exhaustive search. These methods are based on a closest lattice point search over a limited search space (hypersphere). There exist several implementations of these algorithms pursuiting different search strategies and working either within a set of real numbers, thus called Real Sphere Decoders (RSD), or performing the search directly within a set of complex numbers, commonly known as Complex Sphere Decoders (CSD). In this paper, a performance comparison between the real and the complex version of the Schnorr-Euchner (SE) sphere decoder has been carried out in order to find out which algorithm is the most suitable depending on the application. Furthermore a recently appeared fixed-complexity version of the SE decoder (FSD) has been evaluated both in terms of complexity and performance and the results have been compared with the original version. In contrast to yet existing complexity analyses, not only the number of visited nodes has been investigated but also the total number of operations.
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Jens Elsner, Martin Braun, Stefan Nagel, Kshama Nagaraj, Friedrich K. Jondral:
Wireless Networks In-the-Loop: Software Radio as the Enabler (Slides)
Software Defined Radio Forum Technical Conference, Washington DC, December 1-4, 2009

Abstract:
A software architecture to rapidly develop and test radio networks in real and physical environments is proposed. Radio network terminals are developed in software and run on generic hardware to maximize reconfigurability. Due to the software nature of the radio terminals, radio networks can be simulated in a virtual environment, where physical channels are emulated by software entities. Without any changes to the code base, the same waveform can also be run in a real, physical environment. This feature is used to rapidly switch between real and virtual networks, thus bridging the gap between simulation and physical reality. Aspects of the proposed system are implemented and demonstrated with the GNU Software Radio framework.
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Martin Braun:
Weniger ist mehr: Fusion von Radar- und Kommunikationssystemen
Proceedings des KIT-Workshop "Fahrzeugkommunikation", November 2009

Abstract:
Radar und Kommunikation sind fur die Verkehrssicherheit sehr nutzliche Hilfsmittel. Es wird ein Projekt vorgestellt, in welchem die Fusion der beiden Systeme untersucht wird. Anhand von verwandten Forschungsprojekten wird gezeigt, wie durch Ausnutzen von Synergieeffekten die einzelnen Komponenten sogar verbessert werden können.
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Volker Blaschke, Tobias Renk, Friedrich K. Jondral:
A Cognitive Radio Receiver Supporting Wide-Band Sensing
Wireless Sensor Networks, Scientific Research Publishing, Vol. 1, No. 3, pp. 123-131, October 2009

Abstract:
The specification of IEEE 802.22 defines the world-wide first cognitive radio (CR) standard. Within a range of 40 MHz to 910 MHz CR systems are allowed to allocate spectrum besides the currently established radio services like radio and TV broadcasting. In order to fulfill the regulative guidelines of interference limitations, a capable spectral sensing and user detection has to be provided. Due to the wide frequency range specified in IEEE 802.22 and the high dynamic range of signals allocated in this band there are high demands on the CR receiver’s front-end. Especially the performance requirements on analog-to-digital converters increase significantly compared to current wireless systems. Based on measurements taken in this frequency range requirements to CR’s ADCs are figured out. Furthermore, the measurement results are analyzed regarding expectable allocation scenarios and their impacts to spectral sensing. Derived from this results and a comparison of general spectral sensing mechanisms an approach for a CR receiver supporting wide-band sensing is presented. Considering the a priori information resulting from scenario analysis and including adapted information processing in the terminal the ADC’s performance requirements can be reduced.
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Tobias Renk, Holger Jäkel, Clemens Kloeck, Friedrich K. Jondral:
General expression of outage probability in cooperative networks
European Transactions on Telecommunications, Vol. 20, 2009, pp. 537-548

Abstract:
A general expression of outage probability in a wireless relay network with an arbitrary number of relays and selection combining at the destination is derived. This formula is then adopted to independent but not identically distributed channel coefficients and lower and upper bounds are developed. We study two relay schemes: the one-relay case and the two-relay case. The latter can further be divided into a two-phase and a three-phase transmission scheme which depend on the cooperation among the relays. For all considered cases, outage probabilities in Rayleigh fading environments are derived and analysed and it is demonstrated by numerical simulations that all relay protocols achieve full order of diversity. We further elaborate the impact of relay locations on outage probability. Finally, we discuss the required signal-to-noise ratio in order to achieve a predefined outage probability. With respect to the mentioned evaluation metrics the two-phase relaying scheme achieves by far the best performance, however, with the drawback of increased receiver complexity.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral:
Outage Regions and Optimal Power Allocation for Wireless Relay Networks
IEEE Information Theory Workshop (ITW), 11.-16. Oktober, Taormina, Sizilien, Italien, 2009

Abstract:
We study outage regions for energy-constrained multi-hop and adaptive multi-route networks with an arbitrary number of relay nodes. Optimal power allocation strategies in the sense that outage probability is minimized are derived depending on the distances between the transmit nodes. We further investigate the rate gain of adaptive multi-route and multi-hop over direct transmission. It is shown that a combined strategy of direct transmission and adaptive multi-route outperforms multi-hop for all values of rate R. It can be stated that cooperation strategies are beneficial for low-rate systems where the main goal is a very low outage probability of the network. As the rate is increased, direct transmission becomes more and more attractive.
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Jens Elsner, Nico Grove, Dennis Burgkhardt, Friedrich K. Jondral, Arnold Picot:
Convergence of Mobile Communications and Broadcasting: A Long-Term Perspective (Slides) 
8th Conference on Applied Infrastructure Research, Berlin, October 9-10, 2009

Abstract:
Terrestrial unidirectional broadcasting is currently being substituted by wired bidirectional IP-based services. In practical day-to-day use, user mobility for the ‘last mile’ is supported by wireless local area networks with massively growing data rates. Terrestrial bidirectional mobile communications, on the other hand, is currently moving into its fourth technological generation. With the new UMTS Long Term Evolution (LTE) standards, data rates required for television broadcasts can be achieved with mobile bidirectional IP-based services. To increase technological and economic efficiency, unidirectional terrestrial broadcasting will, in the long run, have to make room for bidirectional mobile communications. This has consequences for policy makers and technologist alike, as public broadcasting has to be integrated into these networks and hence the regulatory framework needs to be adjusted accordingly. From a technological viewpoint, a bidirectional network technology is needed which allows for efficient incorporation of broadcasting. From a regulatory point of view, policy makers need to revise regulatory thinking towards frequency allocation principles, too. A unified network approach can be more efficient, both technologically and economically. Technological aspects include higher spatial reuse factor of frequencies, lower infrastructure costs in sparsely populated rural areas due to lower frequencies and much higher supportable data rates: the current amount allocated to mobile communications in the VHF ranges would be more than doubled. Also, such a centralized approach will make it easier to integrate dynamic spectrum access methods currently under research. Economic efficiency is increased on the one hand by the possibility of eliminating redundant broadcasting infrastructure and by increasing social welfare from higher capacities available. Regulatory issues to be solved mainly concern the relationship between public broadcasting and private mobile communications corporations. As opposed to the situation of state monopolies in wired communications at the end of the 20th century, which moved towards supervised competition on a single infrastructure, mobile communications networks are largely run by private companies. In the long run, all these activities have to be integrated into one multidimensional infrastructure network system. A first step in this process was the reassignment of spectrum formerly allocated to terrestrial broadcasting during the digital switchover, also known as “Digital Dividend”. In most countries of the EU, and especially in Germany, this resulted in conflicts of interest between political entities responsible for public broadcasting and mobile communication companies. A second long-term Digital Dividend, beneficial to both broadcasting and mobile communications, will have to be based on a common infrastructure.
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Hanns-Ulrich Dehner, Rainer Moorfeld, Holger Jäkel, Dennis Burgkhardt, Adolf Finger, Friedrich K. Jondral:
Multi-band Impulse Radio – An Alternative Physical Layer for High Data Rate UWB Communication
FREQUENZ, Band 63, Heft 9-10, September/Oktober 2009, S. 200-205

Abstract:
This article overviews current research results within the UKoLoS sub-project MIRA (Physical Layer Optimization for a Multi-band Impulse Radio UWB Architecture).
A multi-band impulse radio UWB system operating in the frequency range between 3.1 GHz to 10.6 GHz is presented. To highlight its potential to be a suitable, alternative solution for future low complexity, power efficient and high data rate wireless communication over short distances, two aspects are pointed out.
The first aspect concentrates on optimized multilevel pulse amplitude modulation, enabling higher data rates and flexible frequency usage.
The second one deals with various approaches aiming at an efficient broad- or narrow-band interference handling.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral, Deniz Gündüz, Andrea Goldsmith:
Outage Capacity of Incremental Relaying for Low Signal-to-Noise Ratios
IEEE 70th Vehicular Technology Conference (VTC-Fall), 20.-23. September, Anchorage, AK, USA, 2009

Abstract:
We present the ε-outage capacity of incremental relaying at low signal-to-noise ratios (SNR) in a wireless cooperative network with slow Rayleigh fading channels. The relay performs decode-and-forward and repetition coding is employed in the network, which is optimal in the low SNR regime. We derive an expression on the optimal relay location that maximizes the ε-outage capacity. It is shown that this location is independent of the outage probability and SNR but only depends on the channel conditions represented by a path-loss factor. We compare our results to the ε-outage capacity of the cut-set bound and demonstrate that the ratio between the ε-outage capacity of incremental relaying and the cut-set bound lies within 1/√2 and 1. Furthermore, we derive lower bounds on the ε-outage capacity for the case of K relays.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral:
Comparing the Outage Capacity of Transmit Diversity and Incremental Relaying
IEEE 70th Vehicular Technology Conference (VTC-Fall), 20.-23. September, Anchorage, AK, USA, 2009

Abstract:
We derive the ε-outage capacity of a wireless relay network where relaying only takes place if the destination has not been able to decode the source message. We refer to this scheme as incremental relaying. The relays use space-time block coding and perform decode-and-forward. We compare incremental relaying to transmit diversity in terms of ε-outage capacity and signal-to-noise ratio gain. It is demonstrated that incremental relaying outperforms transmit diversity only in certain regions that determine the relay locations.
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Tobias Renk, Holger Jäkel, Martin Braun, Friedrich K. Jondral:
A Novel Combining Receiver for a Dual-Diversity Wireless Relay Network
The 12th International Symposium on Wireless Personal Multimedia Communications (WPMC), 7.-10. September, Sendai, Japan, 2009

Abstract:
We present a simple combining receiver for a dual-diversity wireless relay network. The main concern of the paper is to face the trade-off between performance and complexity. The receiver focuses on signal-to-noise ratio (SNR) monitoring and selects dynamically between selection combining (SC) and equal gain combining (EGC) depending on the SNR ratio of the two received branches. It is shown that SC suffers no SNR degradation compared to a single branch communications system if the two receive branches are unbalanced, wheres EGC suffers a loss of 3 dB. Error performance with respect to branch unbalance is considered as well and limiting values for a high degree of branch unbalance are derived.
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Hanns-Ulrich Dehner, Andrés Romero, Holger Jäkel, Dennis Burgkhardt, Rainer Moorfeld, Friedrich K. Jondral, Adolf Finger: 
Iterative coexistence approaches for non-coherent multi-band impulse radio UWB
2009 IEEE International Conference on Ultra-Wideband (ICUWB), Vancouver, Canada, September 9-11, 2009

Abstract:
This paper presents two easy to realize iterative coexistence approaches for non-coherent multi-band impulse radio ultra wide band systems with respect to multiple interference. Thereby, a global coexistence approach is extended to an iterative detection of possible interfered time-frequency gaps by means of an adaptive two-dimensional band plan. The proposed coexistence approaches can be applied for various periodic or constant operating broad- or narrow-band interfering signals. The iterative coexistence approaches outperform the global coexistence approach and hence contribute to more reliable interference detection under realistic dynamic interference scenarios.
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Rainer Moorfeld, Adolf Finger, Hanns-Ulrich Dehner, Holger Jäkel, Friedrich K. Jondral:
Performance of a high flexible non-coherent multiband impulse radio UWB system
Proceedings of 9th IASTED International Conference on Wireless and Optical Communications (WOC09), Juli 2009

Abstract:
This paper demonstrates the performance of a multiband impulse radio architecture with a non-coherent energy detection receiver in the IEEE802.15.3a channel model. Frequency dependent attenuation due to free space path loss leads to a significant performance degradation of any UWB system. The flexibility of the multiband impulse radio architecture in conjunction with a dynamic subband deactivation algorithm allows to fight this performance degradation. Furthermore an approximated log-likelihood ratio calculation for a non-coherent energy detector is introduced. Link budget calculations show the performance of different configurations of the proposed architecture.
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Martin Braun, Christian Sturm, Andreas Niethammer and Friedrich K. Jondral:
Parametrization of Joint OFDM-based Radar and Communication Systems for Vehicular Applications
20th IEEE Symposium on Personal Indoor and Mobile Radio Communications PIMRC 2009, Tokyo, Japan 13-16 Sept 2009

Abstract:
The design process of a joint radar and communication system employing OFDM is analyzed. In particular, the question on how to choose OFDM modulation parameters such as carrier distance, guard interval length, frame length or pilot design is studied. Such systems use the same OFDM frames to transmit data and image the surroundings, so both radar and communication are affected by the choice of these parameters. The main design criteria for the radar component are accuracy of distance and speed measurements, whereas the communication subsystem needs to be designed such that it transmits robustly through a mobile communication channel. As these design criteria often are antipodal, arguments for choosing a particular set of modulation parameters are discussed. The presented design process is applied to the case of a vehicular (car-to-car) communication, in which moving participants communicate in the 24 GHz range.
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Arnold Picot, Friedrich K. Jondral, Jens Elsner, Nico Grove:
Why the Digital Dividend will not close the Digital Divide
Fourth International Conference on Economics and Management of Networks, Sarajevo, Sep. 3 - 5, 2009

Abstract:
The changeover from analogue to digital terrestrial TV made a wide range of frequency spectrum available for redistribution. This fact is known as the "Digital Dividend". Many options on how to make best use of these frequencies are being discussed and have to be evaluated. Spectrum re-distribution is further complicated by the need to provide adequate protection or alternative solutions for incumbent users affected by changing spectrum allocation. Moreover, many stakeholders are en-gaged in the Digital Dividend discussion, with quite contrary positions. Broadcasters see new opportu-nities for terrestrial broadcasting and would like to offer new services such as HDTV, which requires much more bandwidth. Mobile network operators argue that these frequencies are desperately needed to satisfy the rising demand of mobile data services. Many additional interest groups, like wireless mi-crophone manufacturers, WiMAX deploying companies and others are claiming their rights on these frequencies as well. A major political goal, which is driving the Digital Dividend discussion particu-larly in Germany, the USA and Australia, is the “Digital Divide”. The term “Digital Divide” refers to the absence of high performance broadband connections in remote rural areas. It is envisioned espe-cially by politicians to use the spectrum made available by the Digital Dividend to close this divide and deliver high-performance internet access by wireless broadband deployment in rural areas. This ap-proach is apparently advantageous, as it seems to solve an urgent socioeconomic problem. However, it suffers from a major issue, inherent to wireless communications: a physically shared medium. The na-ture of the shared medium and the trade-off between reach and speed significantly limit scalability of the network. Given current growth rates of transmission demands, a wireless broadband access network covering large areas will be likely outdated before being deployed. A case study analysis of a pilot pro-ject in Germany backs the finding that the Digital Dividend cannot adequately solve the Digital Divide problem. From a policy viewpoint these issues should be treated separately. The Digital Dividend can improve mobile access to the internet but will not be able to scale with the development of wired net-works – the Digital Divide will remain. An alternative approach of using the Digital Dividend for par-tially a) basic mobile online access (which will as proven not be broadband under any current or future definition) and b) for short range communication under a general authorization is introduced. Due to the specific physics of electromagnetic wave propagation this has the potential to significantly improve the "last mile" problem of network access.
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Jens Elsner, Stefan Taing, Nico Grove:
Getting the next 5 billion online: Counteracting Digital Divide in Asia
Harvard Project for Asian and International Relations Academic Conference, Seoul, Aug. 14 - 17, 2009

Abstract:
Asia, the world’s most populous region, has a per capita penetration of internet access of 20 percent. In the developing nations of China and India, internet access is avail-able to, respectively, around 20 percent and 10 percent of the population. In industri-alized countries such as Japan, Germany or the USA, this figure is much higher and lies at about 70 percent. Today, access to the global information networks means access to the world’s infor-mation resources: a vast opportunity to learn, to share and to take part in social life. The difference in opportunities available to internet users and non-users has become known as the “Digital Divide”. While most industrialized countries have deployed Internet access to their citizens, bringing, e.g., China’s and India’s rural population online is a major challenge. In developing regions, infrastructure development should include improving access to the internet, as economic and social development is closely related to the availability of region-wide internet access. This problem has been recognized by most governments and national efforts are underway. E.g., China is working on NGN deployment in the country. In addition to accessibility problems and affordability of access devices, soft factors such as acceptance and skills play an important role. The paper analyses the possibilities and obstacles to improve internet access in developing countries using China as an example. It follows a twofold approach. At first, possibilities of getting urban and rural regions online are discussed from a technologi-cal and financial perspective. Secondly, soft factors like acceptance and education are analyzed and their impact on infrastructure deployment decisions is quantified.
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Maximilian Hauske, Dayong Shi, Marc Ihle, Friedrich K. Jondral:
Artificial Lighting Interference on Free Space Photoelectric Systems
2009 International Symposium on Electromagnetic Compatibility 20.-24. Juli, 2009, Kyoto, Japan

Abstract:
Artificial lighting is a main source of interference on free space photoelectric systems for communication applications. This work presents digital measurements and numerical analysis of the time and frequency characteristics of artificial lighting interference using Welch Power Spectral Density estimate and spectrogram with Short Time Fourier Transform. Measurements of time waveforms and spectra of incandescent lamps, tubular and compact fluorescent lamps with conventional and electronic ballasts, as well as Light Emitting Diodes are discussed and compared to other works.
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Tobias Renk, Friedrich K. Jondral:
Location-dependent Performance of Cooperative Networks with Power Allocation
IEEE Symposium on Computers and Communications (ISCC), 5.-8. Juli, Sousse, Tunesien, 2009

Abstract:
In this paper we investigate a cooperative mobile network with respect to error performance. Power allocation is applied between source and relay. The relay cooperates after having received a request from the destination which decides upon successful or non-successful transmission based on a threshold criterion. We show that with respect to the relay’s location a suitable power allocation technique can increase performance enormously. Our results reveal that a practical implementation strategy for cooperative networks first proposed in [1] can easily be adopted to cognitive networks where a relay senses its environment for feedback signals from the destination. Depending on the applied services, however, it may be beneficial to always cooperate, since this creates spatial diversity at the destination.
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Dennis Burgkhardt, Ivan Cosovic, Gunther Auer, Friedrich K. Jondral:
Reducing the probability of network overload by spectrum trading
Proceedings Canadian Conference on Electrical and Computer Engineering CCECE ’09, Seiten 363– 367, May 3-6, 2009

Abstract:
In this paper dynamic spectrum trading is addressed, where time-frequency resources are traded between network operators on a short-term basis to meet current demands. Specifically, we derive the probability of system overload for the entire network as well as for a single operator. Numerical results demonstrate that by dynamically adjusting the resource assignment to operators on a short-term basis, per-operator and system overload probabilities can be significantly reduced at given offered traffic.
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Virgilio Rodriguez, Friedrich K. Jondral, Rudolf Mathar: 
Power allocation through revenue-maximising pricing on a CDMA reverse link shared by energy-constrained and energy-sufficient heterogeneous data terminals
9th IEEE Vehicular Technology Conference (VTC-Spring), Barcelona, Spain, 26.-29. April, 2009

Abstract:
We perform market-oriented management of the reverse link of a CDMA cell populated by data terminals, each with its own data rate, channel gain, willingness to pay (wtp), and link-layer configuration, and with energy supplies that are limited for some, and inexhaustible for others. For both types, appropriate performance indices are specified. A terminal pays in proportion to its fraction of the total power at the receiver, which directly determines its signal-to-interference ratio (SIR), and hence its performance. Hence, a terminal can individually choose its optimal power fraction without knowing the choices made by others. The network sets for each terminal an individual price that drives the terminal to the “revenue per Watt” maximiser. An optimal link-layer configuration can be identified through this analysis. Distinguishing features of our model are: (i) the simultaneous consideration of both limited and unlimited energy supplies, (ii) the performance metrics utilised (one for each type of energy supply), (iii) the generality of our physical model, and (iv) our focus on the fraction of total power at the receiver allocated to a terminal.
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Jens P. Elsner, Christian Körner, Friedrich K. Jondral:
Centralized modeling of the communication space for spectral awareness in cognitive radio networks
ACM SIGMOBILE Mobile Computing and Communications Review Volume 13 , Issue 2 (April 2009)

Abstract:
The communication space is five dimensional: its degrees of freedom are frequency, time and space. The use of the electromagnetic spectrum depends on these parameters. With future applications such as opportunistic overlay access or distributed spectrum monitoring in mind, it is important to estimate the state of the communication space on the basis of incomplete or imprecise information. A promising approach are technology centric Cognitive Radio networks. In these networks, nodes cooperate to infer information on spectral occupancy. This conceptual paper proposes a novel approach for centralized modeling of the communication space with emphasis on spatial dependencies through the use of a regression model. The modeling approach is verified with practical measurements.
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Hanns-Ulrich Dehner, Micha Linde, Rainer Moorfeld, Holger Jäkel, Adolf Finger, Friedrich K. Jondral:
A low complex and efficient coexistence approach for non-coherent multiband impulse radio UWB
IEEE 2009 Sarnoff Symposium, Princeton, New Jersey, March 30, 31 & April 1, 2009

Abstract:
When bringing a high data rate multiband impulse radio UWB system to the market it has to coexist with other already existing UWB technologies such as multiband OFDM UWB. This paper analyzes the interference impact of a multiband OFDM UWB system on a non-coherent multiband impulse radio UWB system. It is shown that the impact can be so dominant that communication within the impulse based multiband UWB system gets worse. For this reason a static as well as a dynamic coexistence approach are considered. The dynamic approach uses an efficient, robust and easy to realize pixel based interference detection algorithm in conjunction with an adapted bandplan array. In comparison to the static approach, the dynamic coexistence approach allows higher data rates during data transmission phases without significant losses in bit error rate performance.
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Tobias Renk, Friedrich K. Jondral:
Upper and Lower Bound on Signal-to-Noise Ratio Gains for Cooperative Relay Networks
43rd Annual Conference on Information Sciences and Systems (CISS), Baltimore, Maryland, 18.-20. März, 2009

Abstract:
Cooperative networking as a means of creating spatial diversity is used in order to mitigate the adverse effect of fading in a wireless channel and increase reliability of communications. We investigate signal-to-noise ratio (SNR) gain in wireless cooperative networks. We show that the differential SNR gain in the high data rate regime, which we refer to as SNR gain exponent ζ∞ , is independent of the relaying strategy and only depends on the number of transmission phases used for communication. Furthermore, a straight-line upper and lower bound is derived based on geometric considerations. It is shown that the approximation error of the upper bound with respect to the exact SNR gain tends to zero for R → ∞. For the lower bound, the approximation error tends asymptotically to a constant factor δ for R → ∞. Both bounds are the best possible straight-line bounds with respect to absolute error.
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Tobias Renk , Dimitar Iankov , Friedrich K. Jondral:
Adaptive Resource Allocation in Wireless Relay Networks
69th IEEE Vehicular Technology Conference (VTC-Spring), Barcelona, Spain, 26.-29. April, 2009

Abstract:
We consider a wireless relay network in a Rayleigh fading scenario where transmission power as well as transmission time per user are optimized. The criterion of optimization is the capacity. We apply an optimization algorithm based on Brent’s method. Hence, we employ parabolic interpolation for finding the optimum whenever possible in order to achieve a faster convergence. If parabolic interpolation is not suitable, we use golden section search which is a robust root-finding method. Optimization takes place in two steps. First, we optimize power allocation and consider time as parameter. Second, time is optimized. We demonstrate that, depending on the cooperation strategy, remarkable capacity gains over direct transmission can be achieved by multi-routing. However, for a high overall transmission power, direct transmission outperforms multi-hopping. Generally, capacity gains increase with decreasing overall system power, which demonstrates that relaying is beneficial for low overall transmission powers.
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Martin Braun, Jens P. Elsner, Friedrich K. Jondral:
Signal Detection for Cognitive Radios with Smashed Filtering
69th IEEE Vehicular Technology Conference (VTC-Spring), Barcelona, Spain, 26.-29. April, 2009

Abstract:
Compressed Sensing and the related recently intro duced Smashed Filter are novel signal processing methods, which allow for low-complexity parameter estimation by projecting the signal under analysis on a random subspace. In this paper the Smashed Filter of Davenport et al. is applied to a principal problem of digital communications: pilot-based time offset and frequency offset estimation. An application, motivated by current Cognitive Radio research, is wide-band detection of a narrow-band signal, e.g. to synchronize terminals without prior channel or frequency allocation. Smashed Filter estimation and maximum likelihood-based, uncompressed estimation for a signal corrupted by additive white Gaussian noise (Matched Filter estimation) are compared. Smashed Filtering adds a degree of freedom to signal detection and estimation problems, which effectively allows to trade signal-to-noise ratio against processing bandwidth for arbitrary signals.
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Michael Mühlhaus, Oliver Braz, Friedrich K. Jondral:
The Impact of Relay Stations on MIMO Systems
FREQUENZ, Band 62, Heft 11-12, November/Dezember 2008, S. 278-280

Abstract:
This paper investigates the impact of a relay station onto a MIMO transmission system. Performed simulations as well as real measurements indicate that a MIMO transmission via a conventional relay station is feasible. With respect to a comparable SISO system it has been shown that the MIMO coding gain degrades only slightly.
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Tobias Renk, Clemens Klöck, Dennis Burgkhardt, Friedrich K. Jondral, David Grandblaise, Sophie Gault, Jean C. Dunat:
Bio-Inspired Algorithms for Dynamic Resource Allocation in Cognitive Wireless Networks
Mobile Networks and Applications, Vol. 13, pp. 431-441, 2008

Abstract:
Regulation will experience enormous changes in the near future resulting in seamless connectivity by spectrum borders. A promising approach in this context is dynamic spectrum allocation which leads to a more flexible access to spectral resources by employing intelligent radio devices called cognitive radios. This paper is concerned with bio-inspired approaches that exploit distribution in multi-radio environments where many users have to share a finite resource harmoniously. Three applications of bio-inspired techniques are described. The first one deals with the detection of spectrum holes whereas the second one describes resource allocation in orthogonal frequency division multiple access based systems. The third one is concerned with distributed resource auctioning.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral: 
Signal-to-Noise Ratio Gains for Cooperative Relaying Strategies
Proceedings of IEEE International Symposium on Wireless Communication Systems ISWCS 2008, Reykjavik, Oktober 21-24, 2008

Abstract:
We study signal-to-noise ratio (SNR) gains for two different cooperation strategies that are applied to networks consisting of one source, one destination, and one or two relays, respectively. The first strategy is called orthogonal decode-and-forward where all transmit terminals transmit interference free in orthogonal time slots. The second strategy is called nonorthogonal decode-and-forward where all relays transmit simultaneously by applying space-time block coding. We demonstrate that the SNR gain Δγ is proportional to (1/p^K_out)1/(K+1) with K representing the number of relays. We further define the SNR gain exponent as a new criterion to evaluate the asymptotic behavior of SNR gain for high-rate regimes. It will be demonstrated that the SNR gain exponent only depends on the number of transmission phases.
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Maximilian Hauske, Holger Jäkel, Hanns-Ulrich Dehner, Friedrich K. Jondral: 
Interference Mitigation for Energy Detection in an Multiband Impulse Radio UWB System
Proceedings of IEEE Vehicular Technology Conference VTC2008-Fall, Calgary, September 22-25, 2008

Abstract:
On-Off-Keying with energy detection is a promising candidate when aiming at simple receiver concepts, due to the fact that simple energy detection reduces receiver complexity. On the other hand, data rates are low and interference mitigation by correlation is no longer possible. This paper proposes and analyzes a method for mitigating asynchronous interferers in energy detection receivers by adapting the maximum likelihood decision rule. It is shown that detection in the presence of interferers is improved and detection in absence of interferers is not declined.
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Ulrich Berthold, Fangwen Fu, Mihaela van der Schaar, Friedrich K. Jondral: 
Detection of Spectral Resources in Cognitive Radios Using Reinforcement Learning
Proceedings of IEEE 3rd International Symposium on New Frontiers in Dynamic Spectrum Access Networks, DySPAN 2008, Chicago, IL, USA, 14-17 October, 2008

Abstract:
Overlay systems are a feasible approach to increase the efficiency of spectrum use. An independent secondary user (SU) system operates in the same frequency band as a primary user (PU) system. In order to avoid mutual interference, an important task of every SU system is to periodically monitor the PU system's allocation. In this paper, we investigate the question how many SUs have to be deployed in the system area of an ad hoc network in order to meet a given network wide detection probability and how this effects the false alarm probability (and thus the performance of the SU system). This is achieved with the help of geometric random graph theory and results from a distributed detection approach. In our investigations we consider a scenario with an infinite system area, and therefore neglecting border effects, as well as a scenario including border effects.
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Hanns-Ulrich Dehner, Rainer Moorfeld, Holger Jäkel, Adolf Finger, Friedrich K. Jondral:
A simple and fast detect and avoid algorithm for non-coherent multiband impulse radio UWB
ISSSTA2008, August 25-28, Bologna, Italy, 2008

Abstract:
In this paper we present a simple and fast detect and avoid (DAA) algorithm for non-coherent multiband impulse radio UWB. By independent parallelized interception of all subbands, this algorithm shows a very robust and reliable behavior in the detection of narrowband interference such as the IEEE 802.11a wireless LAN (WLAN) standard. Thereby, narrowband interference detection is possible during an initialization mode but also during data transmission. The detection is based on a static threshold which guarantees a maximum bit error rate in each subband. If a narrowband interferer is present the transmission is avoided on the corresponding subband and a receiver known initialization sequence is transmitted. Due to the fact that there is a loss in data rate, this approach retains the possibility of using all subbands when no interferer is present.
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Tobias Renk, Volker Blaschke, Friedrich K. Jondral:
Time-dependent Statistical Analysis of Measurements for the Evaluation of Vacant Spectrum Bands
XXIX. U.R.S.I. General Assembly, 7. - 17. August, Chicago, Illinois, 2008

Abstract:
The demand for more bandwidth due to multi-media applications and the inherently finite resource ‘frequency’ require a more efficient use of the radio spectrum in future wireless systems. These requirements pave the way for the use of cognitive radios. This paper describes measurements that have been performed during the Soccer World Cup 2006 in Germany. Data is analyzed to obtain a suitable means to evaluate the vacancy of spectrum bands due to timedependent statistics. This includes average channel allocation, average run length (sequence of ‘1’s in a binary allocation matrix), and amount of runs.
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Stefan Nagel, Volker Blaschke, Friedrich K. Jondral, Eric Nicollet, Dominique Ragot:
A European System for Interoperability in Public and Governmental Security Networks
Safety and Security Systems in Europe 2008, Potsdam, June 19-20, 2008 CD-ROM

Abstract:
1. INTRODUCTION
In light of natural or manmade disasters like hurricane Katrina in US or terrorist bombing attacks in Madrid and London the demand for an efficient disaster and crises management became apparent again. Due to the heterogeneous structure of communication systems involved in such scenarios a huge effort needs to be spent for organization and coordination of a basic communication between the partners, itself. Therefore, a flexible, adaptable and cost effective radio architecture providing interoperability needs to be established in future crises management communication systems. Based on the analysis of current solutions the Wireless Interoperability for Security project WINTSEC envisions future P&GS “seamless” interoperable communication systems be addressed by a “system of system” approach, that will be presented.

2. STATE OF THE ART IN CRISES MANAGEMENT
The state of the art in today’s crises management systems is a very heterogeneous radio communications structure due to different physical layer specification and a variety of transmission protocols. There are several proposals to handle this issue like the use of conventional multi-mode radios or the distribution of one-type terminals, but all these solutions do not address the problem at whole, they are inflexible or cost-intensive. The requirements for future systems are to provide interoperability at a cost-efficient and application-oriented level.

3. INTEROPERABILIY ON SYSTEM LEVEL
Extending the scope of interoperability from network to system level offers flexibility necessary for crises management. Below the application layer that supports several services the core network layer handles the interoperable backbone between the different networks. So, a service can be provided to all users independent of the radio access technology they are associated to.

4. TECHNOLOGICAL APPROACH FOR INTEROPERABILITY
From a technological point of view a wide range of heterogeneous reconfigurable radio equipment ranging from mono channel terminals to multi channel base stations need to be covered. Therefore, the definition of a SDR Architectural Framework is required. This framework consists of the SDR Platform and the Software Applications. The platform provides all hardware resources required for physical layer tasks as well as data input/output, and security features and programmable computing resources. The platform’s resources are determined in order to support specified Software Applications. These provide the waveform specific data processing and configuration of the platform in order to support dedicated radio-communication standards. In the presentation, this SDR architectural framework supporting wireless interoperability will be described in more detail. Due to the enhanced interoperability of this framework an increased flexibility for first responder’s radio communication equipment can be supported.
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Uwe Kiencke, Holger Jäkel:
Signale und Systeme
4. überarbeitete Auflage, Oldenbourg Wissenschaftsverlag, ISBN: 3-486-57811-1

Abstract:
"Signale und Systeme" wendet sich nicht nur an Studenten der Fachrichtung Elektrotechnik an wissenschaftlichen Hochschulen, sondern auch an Ingenieure und Naturwissenschaftler, die einen Einblick in dieses Gebiet gewinnen wollen.

Die Signale und Systeme werden zuerst im zeitkontinuierlichen und anschließend im zeitdiskreten Zeitbereich betrachtet. Das notwendige mathematische Handwerkszeug wie Grundbegriffe der Wahrscheinlichkeits- und Funktionentheorie, eine Einführung in die Theorie linearer Räume und Operatoren sowie die Fourier-, Laplace- und z-Transformation wird dem Leser in leicht verständlicher Form bereitgestellt.
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Takefumi Yamada, Dennis Burgkhardt, Ivan Cosovic and Friedrich K. Jondral:
Resource Distribution Approaches in Spectrum Sharing Systems
EURASIP Journal on Wireless Communications and Networking/, vol. 2008, Article ID 598080, 15 pages, 2008

Abstract:
It is increasingly difficult to satisfy growing demands for spectrum with the conventional policy of fixed spectrum allocation. To overcome this problem, flexible/dynamic spectrum sharing methods that can significantly improve spectrum utilization of the spectrum have gained increasing interest recently. This paper presents two dynamic spectrumsharing approaches, a centralized and a decentralized one. The centralized approach is based on hierarchical trading. Each level of hierarchy is composed of “markets” that are associated with a certain spatial area and trading occurrence frequency, whereas area size and trading occurrence frequency depend on the hierarchy level. The decentralized approach is based on game-theory. There, it is assumed that the operators are averse to unequal payoffs and act unselfishly, enabling a stable and sustainable community. Numerical results show that, in the observed scenario, both proposals outperform the reference case of fixed resource allocation significantly in terms of utilized bandwidth. Whereas, negotiation costs for spectrum brokerage appear in the centralized approach, nonnegligible amounts of spectrum are lost in the decentralized approach due to collisions. Thus, a hybrid of centralized and decentralized approach that exploits the benefits of both is also considered.
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Piotr Rykaczewsi, Mikko Valkama, Markku Renfors: 
On the Connection of I/Q Imbalance and Channel Equalization in Direct-Conversion Transceivers
IEEE Transactions on Vehicular Technology, Vol. 57, No. 3, May 2008, pp. 1630-1636

Abstract:
Due to its simple structure and reduced requirements for radio frequency filtering, the direct-conversion principle has recently gained a lot of interest in radio transceiver design. This paper considers the so-called inphase/quadrature (I/Q) imbalance problem in such structures under the assumption of frequencyselective transmission channel requiring equalization on the receiver side. It will be shown that I/Q imbalance and channel equalization are strongly interdependent, and thus, I/Q mismatch should be taken into account in the equalizer design, and vice versa, particularly with high-order symbol alphabets. Most importantly, the amount of signal distortion caused by transmitter and receiver I/Q imbalances to the equalized received signal will be analytically analyzed through signal-to-interference ratio (SIR) analysis. This gives a quantitative measure to analyze and judge the role of I/Q imbalances in practical communication system design, without having to resort to lengthy data simulations. Furthermore, the analysis shows that imbalance levels that are tolerable in additive white Gaussian noise channels can turn into a serious problem in multipath channels requiring equalization. This gives further motivation and invaluable insight in the overall receiver and system design and in developing digital signal processing-based techniques to mitigate the I/Q imbalance effects on the receiver side. In general, the analytical outcomes and results are verified using extensive computer simulations.
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Friedrich K. Jondral:
From Maxwell's Equations to Cognitive Radio
Proceedings of the 3rd International Conference on Cognitive Radio Oriented Wireless Networks and Communications, CrownCom 2008, Singapore, May 15-17, 2008

Abstract:
Maxwell's equations that were published around 1865 caused Heinrich Hertz to prove the existence of electromagnetic waves. He succeeded in 1887/88. Around 1900, Marconi established the first long distance radio communication connections. During the following five decades, analog radio communication was brought to perfection. A big change in radio development was launched by Shannon with the publication of the sampling theorem in 1949. Together with the invention of the transistor and integrated circuits this led to the establishment of digital signal processing and, emerging from this, to the overwhelming success of digital cellular mobile radio. The demand for radio systems optimally adapted to different applications (personal, office, home, car, urban, rural environments) together with economic necessities resulted in the definition of a variety of standards (DECT, GSM, UMTS, IEEE 802.11x, Bluetooth, ZigBee, …), a development that called for SDRs. The next step in radio evolution will create devices that will help to make highest efficient use of the radio spectrum. In this scenario radios have to be aware of their location and to supervise their electromagnetic environment in order to be able to optimally adapt their transmission methods. This contribution tries to trace radio development from its origins to cognitive radio as well as to classify future developments.
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Dennis Burgkhardt, Ivan Cosovic, Friedrich K. Jondral:
Dynamic Spectrum Allocation by Hierarchical Resource Trading
Proceedings of IEEE Vehicular Technology Conference VTC2008-Spring, Singapore, May 11-14, 2008

Abstract:
Dynamic spectrum allocation is a key technique to face spectrum scarcity and inefficient pre-allocation of resources in an environment of rapidly growing demands for mobile communications. By allowing operators to adaptively exchange transmission resources of their communication systems with each other by way of trading, short-term traffic variations can be addressed based on local actual demand. Concatenating trading areas and coarsening the time scale, pre-allocation of resources can be adapted with respect to long-term estimations of traffic, thereby building a hierarchic market framework for trading transmission resources. Numerical simulations show, that our approach can lead to a significant increase in resource usage.
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Ulrich Berthold, Friedrich K. Jondral:
Distributed Detection in OFDM based Ad Hoc Overlay Systems
Proceedings of IEEE Vehicular Technology Conference VTC2008-Spring, Singapore, May 11-14, 2008

Abstract:
Overlay systems are a feasible approach to increase the efficiency of spectrum use. An independent secondary user (SU) system operates in the same frequency band as a primary user (PU) system. In order to avoid mutual interference, an important task of every SU system is to periodically monitor the PU system's allocation. In this paper, we investigate the question how many SUs have to be deployed in the system area of an ad hoc network in order to meet a given network wide detection probability and how this effects the false alarm probability (and thus the performance of the SU system). This is achieved with the help of geometric random graph theory and results from a distributed detection approach. In our investigations we consider a scenario with an infinite system area, and therefore neglecting border effects, as well as a scenario including border effects.
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Tobias Renk, Clemens Kloeck, Holger Jäkel, Friedrich K. Jondral 
Error Performance of Multi-hopping Half-duplex Relay Networks
IEEE International Symposium on Wireless Pervasive Computing (ISWPC), 7. - 9. Mai 2008, Santorini, Griechenland, 2008

Abstract:
We derive a compact description of error performance of a multi-hopping half-duplex relay network, where each relay performs decoding. For this, we introduce the event of consecutive error which describes that a decoding error might not be corrected by chance at a subsequent relay node. Furthermore, two tight upper bounds are derived. The first one is based on the union bound method and the second one describes the probability that there occurs at least one decoding error in the network. General expressions on error probability in slow, flat fading Rayleigh channels are developed as well and adopted to the chosen transmission scenarios to ensure fair comparison with respect to the amount of transmitted data and system energy. It is shown and evaluated by simulations that relaying achieves better results on error performance even if no spatial diversity is employed. Gains are evaluated by a new metric called asymptotic probability gain.
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Tobias Renk, Holger Jäkel, Friedrich K. Jondral:
Relay-Netzwerke - Kooperative Kommunikation unter mobilen Teilnehmern
Telekommunikation Aktuell, 61. Jahrgang, Heft 08-09, August – September 2007, S. 1-19

Abstract:
In den letzten Jahren hat sich ein immer größeres Interesse an Relay-Netzwerken und kooperativer Kommunikation in drahtlosen Netzwerken entwickelt. Hauptgrund hierfür ist der zunehmende Bedarf an höheren Datenraten, der durch Multimedia-Applikationen hervorgerufen wird. Dabei zeigen Relay-Netzwerke gegenüber einer gewöhnlichen direkten Übertragung ein wesentlich verbessertes Übertragungsverhalten. Nicht nur höhere Datenraten können erzielt werden, auch die Sicherheit einer Übertragung wird wesentlich gesteigert. Der vorliegende Artikel bietet eine Einführung in die theoretischen und praktischen Grundlagen von kooperativer Kommunikation. Neben der Beschreibung von Relay-Netzwerken und den gebräuchlichen Relay-Strategien wird der Fokus auf die Analyse der Leistungsfähigkeit gelegt. Hier wird vor allem auf das Ausfallverhalten der Relay-Netzwerke eingegangen.
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Volker Blaschke, Tobias Renk, Friedrich K. Jondral: 
A Cognitive Radio Receiver Supporting Wide-Band Sensing
Second IEEE Workshop CogNet: Towards Cognition in Wireless Networks, IEEE ICC Workshop, 19. Mai 2008, Beijing, China

Abstract:
IEEE 802.22 defines the world-wide first cognitive radio (CR) standard. In the range between 41MHz and 910MHz CR overlay-systems can be installed besides licensed radio services such as radio and TV broadcasting. In order to fulfill the regulative guidelines for interference limitations, adequate spectral sensing and user detection has to be supported by the CR terminals. The wide frequency range specified in IEEE 802.22 and the high dynamic range of signals in this band lead to high demands on the CR receiver's front-end. Especially the performance requirements on analog-to-digital converters (ADCs) increase significantly compared to current wireless systems. Based on measurements taken in the frequency range between 41 MHz and 910 MHz requirements to CR's ADCs are figured out. Furthermore, the measurement results are analyzed regarding expectable allocation scenarios and their impacts on spectral sensing. Derived from these results and a comparison of general spectral sensing mechanisms, an approach for a CR receiver enabling wide-band sensing is presented. By combining a-priori information resulting from scenario analysis with adapted information processing in the CR terminal, the ADC's performance requirements can be reduced.
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Volker Blaschke, Clemens Kloeck, Jan Weiss, Tobias Renk, Friedrich K. Jondral:
Opportunistic WiMAX-GSM Coexistence
IET Communication Magazine, Special Issue on Cognitive Spectrum Access, 2008

Abstract:
In future multi-media applications dynamic access to spectral resources will play an important role. However, regulation of spectrum will only change very slowly and spectrum rights holders might not be willing to share their resources without gaining something out of it. Therefore, we analyze the suitability of WiMAX as an overlay system on GSM where available resources will be auctioned among several competitors. In our vision we design a cross-layer approach to facilitate dynamic resource allocation. It is demonstrated that with minor changes of the original WiMAX frame and a small signalling overhead currently unused GSM resources can be allocated efficiently without decreasing system performance. Thus, this adapted WiMAX system used in opportunistic coexistence to GSM offers a suitable alternative to broadband service access.
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Volker Blaschke, Friedrich K. Jondral:
Modelling of Spectral Utilization using Times Series
MPRG Symposium on Wireless Communications, 4.-6.Juni 2008 Blacksburg, USA

Abstract:
One of the main challenges to overcome spectrum scarcity is the increase of spectral utilization. In most of the frequency ranges allocated to mobile communication services the overall spectral utilization is less than 20 percent. The installation of so-called overlay systems allocating the remaining radio transmission resources will help to establish dynamic spectrum allocation. In order to adapt the resource allocation of the Cognitive Radio terminals in the overlay system high sensing effort is necessary. Since the processing capabilities are limited in mobile terminals, efficient and adaptable sensing procedures observing only the most potential frequency bands are essential. Forecasting the available resources in the observed frequency bands will help to optimize the sensing process.
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Tobias Renk, Holger Jäkel, Clemens Klöck, Friedrich K. Jondral:
Signal-to-Noise Ratio Requirements for Wireless Relay Networks
3rd International Symposium on Communications, Control and Signal Processing (ISCCSP), 12. - 14. März, St. Julians, Malta, 2008

Abstract:
We develop and analyze expressions on the required instantaneous signal-to-noise ratios for wireless relay networks in this paper. Two basic schemes are studied, namely the one-relay case and the two-relay case. The latter can further be divided into a two-phase and a three-phase scheme, respectively. In the two-phase scheme both relays are only allowed to transmit with half the power in order to ensure fair comparison between the different schemes. Moreover, for the one-relay case and the two-phase scheme, each terminal must serve double the target rate, whereas in the three-phase scheme, each terminal has to serve three times the target rate to ensure that the amount of transmitted information remains constant. We show the following correspondences when decode-and-forward relaying is applied in Rayleigh fading environments: For the one-relay case SNR is proportional to 1/sqrt(p_out) and for both two-relay schemes SNR is proportional to 1/sqrt[3](p_out). We further show that an increase in data rate of 1 bit/s/Hz results in a required increase of signal-to-noise ratio of 6 dB for the one-relay case and the two-phase two-relay scheme, and an increase of 9 dB for the three-phase two-relay scheme.
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Ihan Martoyo, Friedrich K. Jondral:
Frequency domain equalisation in CDMA detection
European Transactions on Telecommunications, Vol. 19, 2008, pp. 123-135

Abstract:
The present paper introduces frequency domain equalisation (FDE) as an efficient means for single-user detection (SUD) in universal mobile telecommunication sys-tems (UMTS) handhelds. Such an SUD is, at least partially, able to remove multiple access interference (MAI) like a multi-user detector (MUD), however with several im-portant advantages

• the operation principle of an FDE-SUD is simple and easy to understand,
• the FDE-SUD shows low numerical complexity and therefore,
• it is not power hungry like an MUD.

The problem of applying FDE without zero-padding or cyclic-prefix insertion is solved by using the overlapcut method. In this paper we also prove that the performance of the FDE-SUD is practically identical to that of linear MUD for a fully loaded UMTS cell. Furthermore, armed with the idea of FDE for code division multiple access (CDMA), we can construct a common receiver structure for CDMA, time division mul-tiple access (TDMA), orthogonal frequency division multiplexing (OFDM) and multi-carrier CDMA (MC-CDMA) systems. This is an extremely useful idea in enabling a software-defined radio (SDR) which can operate seamlessly in several environments such as UMTS, the IEEE 802.11a and WiMaX. FDE based receivers could be one important building-block for the next generation mobile communications.
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Friedrich K. Jondral, Hanns-Ulrich Dehner, Christian Körner (Editors):
Proceedings of the 5th Karlsruhe Workshop on Software Radios
Karlsruhe, Germany, March 2008

Preface:
Dear participants,

we cordially welcome you to the 5th Karlsruhe Workshop on Software Radios.

Compared to our previous workshops you will find a somewhat different environment in our university. First of all, we have to mention that in October 2006 the Institut für Nachrichtentechnik (INT) moved from its previous location on the campus to a build-ing located near the city center. Please feel free to visit us in Kreuzstraße 11.

More important, also in October 2006, the Wissenschaftsrat, installed by Germany‘s Federal Government, published his decision to establish the Universität Karlsruhe (TH) as one of Germany’s first elite universities. This privilege is closely related to the foundation of the Karlsruhe Institute of Technology (KIT), the innovative merger of the Forschungszentrum Karlsruhe with the Universität Karlsruhe (TH). Inaugurated in an official ceremony on February 22, 2008, KIT is on its way to become a world-wide leading institution for research and education in natural sciences, engineering and economics. Keeping this actual development in mind, you are for the first time par-ticipants of a Karlsruhe Workshop on Software Radios at KIT.

According to the ambitious goals of KIT, we are proud to present a comprehensive program of 29 contributions presented by authors from eleven European countries and the US. The presentations pick up all Software Radio topics currently under in-vestigation. We start with session 1 on SDR Baseband consisting of papers on signal processing and hardware. Sessions 2 and 3 are dedicated to Cognitive Radio, i.e. a radio technology where the devices are able to monitor their environment and to act according to their findings. System related concepts as well as steps in direction to practical implementations are presented. SDR Architectures is the topic of sessions 4 and 5 where implementation aspects, waveform development, reconfigurability of hardware and portability of applications are in the focus of interest. Session 6 deals with reports on research that different groups performed on GNU Radio. Finally, ses-sion 7 is devoted to the rapidly growing field of Dynamic Spectrum Access, a tech-nology that is going to challenge politicians, lawyers, administrators and engineers simultaneously.

On behalf of the organizing committee and the staff of INT, we wish you an interest-ing workshop with many individual discussions at KIT as well as a pleasant stay in Karlsruhe.

Friedrich K. Jondral
Hanns-Ulrich Dehner
Christian Körner
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Jianming Pan, Peter Slanina, Tobias Renk, Mugdim Bublin, Iavar Kambourov:
A cognitive pilot channel system design approach
Proceedings of the 5th Karlsruhe Work-shop on Software Radios, Karlsruhe, Germany, March 2008, pp. 55- 64

Abstract:
To cope with a future heterogeneous wireless environment which applies dynamic spectrum allocation, a mobile wireless terminal without prior network information has to scan a huge frequency range in order to find a suitable network and service. This process appears time consuming and power inefficient. The scanning process can be sped up, as proposed by several cognitive pilot channel (CPC) concepts, which are basically based on a harmonised frequency band, on which network information is conveyed. Agreement on a harmonised frequency band requires knowledge on the desired frequency range and necessary bandwidth, where economical aspects and cost constraints of such a system have to be taken into account. In this paper, we first analyse the bandwidth demands, access time, and coverage issues of the hierarchical CPC concept. Then we propose a system design taking into account multipath mitigation, Doppler effect and fading protection. Finally simulation is performed to verify feasibility of the proposed system.
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Prasanjit Chakravarty, Tobias Renk, Friedrich K. Jondral:
Combining Receiver for Wireless Relay Networks based on Signal-to-Noise Criterion
Proceedings of The 5th Karlsruhe Workshop on Software Radios, Karlsruhe, Germany, March 2008, pp. 89-93

Abstract:
In this paper, we utilize a user cooperation system with a symmetric network structure in order to design and implement a receiver which not only implements the three combining techniques selection combining (SC), equal gain combining (EGC), and maximal ratio combining (MRC), but is also capable of choosing between them based on the signal-to-noise ratio (SNR) criterion. The relative error performance of the three techniques is shown. We develop a receiver structure that uses EGC if the ratio between the two mean SNR values of the incoming paths lies between -5.4 dB and 5.4 dB. If the absolute value of the ratio is higher than 5.4 dB, SC is used. MRC is totally skipped for complexity reasons.
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Jens P. Elsner, Christian Koerner, Friedrich K. Jondral: 
Non-parametric Modeling with Gaussian Processes for Spatial Radio-Scene Analysis
5th IEEE Consumer Communications and Networking Conference (CCNC), 2008, Las Vegas, 10 - 12 January 2008

Abstract:
To increase area spectral efficiency, overlay use of spectrum already assigned to a primary system is considered in the context of Cognitive Radio research. Minimizing interference with the primary system is a key requirement and equivalent to the problem of transmission opportunity identification. Transmission opportunities are unused slots in the communication space which is spanned by time, frequency, and location. In this paper we describe a method to identify transmission opportunities by modeling the spatial power distribution with Gaussian processes. This approach to distributed sensing is verified with real-world measurements using the GSM downlink band as the primary system and contrasted with traditional wave propagation modeling approaches.
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Piotr Rykaczewski, Friedrich K. Jondral: 
Blind I/Q Imbalance Compensation in Multipath Environments
IEEE International Symposium on Circuits and Systems, ISCAS 2007, New Orleans (Louisiana), 27-30 May 2007, pp.29 - 32

Abstract:
Mismatches of the amplitudes and phases of the I and Q signal branches in quadrature receivers result in insufficient rejection of the image frequency band. Particular sensitiveness of modern re-configurable transceivers reinforces the need for efficient mitigation of I/Q imbalance. This paper introduces two algorithms for I/Q imbalance compensation: blind source separation and a novel low order statistics method. A signal-to-noise-and-distortion ratio based measure to evaluate the performance of the algorithms is used. It is observed that the results of I/Q imbalance compensation are highly coupled with the dependencies between the mismatches in the transmitter and the receiver. Therefore a known frequency offset in the transmitter is introduced. Due to this operation, the mismatches may be treated as two independent impairment sources, and the compensation is performed in two stages. Results of a theoretical study are verified by extensive Monte Carlo simulations for multipath fading environments.
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Ulrich Berthold, Sinja Brandes, Michael Schnell and Friedrich K. Jondral: 
OFDM-Based Overlay Systems: A Promising Approach for Enhancing Spectral Efficiency
IEEE Communications Magazine, Vol. 45 No. 12, December 2007, pp. 52-58

Abstract:
Recently, several approaches for increasing spectral efficiency have been proposed to overcome the scarcity of spectral resources and to ensure that the ever-growing demand for high-data rate mobile communications can be met in the future. In this article, we focus on OFDM-based overlay systems that share a frequency band with already existing systems without degrading their performance. This is achieved by temporarily using those parts of the common frequency band that are currently not used by the licensed system. In particular, we focus on design issues for the physical layer as well as the medium access control layer and propose appropriate solutions for enabling coexistence. As a result, our investigations show that OFDM-based overlay systems are a promising approach for enhancing spectral efficiency.
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Virgilio Rodriguez, Friedrich K. Jondral: 
Simple, adaptively-prioritised, spatially-reusable medium access control through the Dutch auction, with decentralised implementation for synchronised terminals
14th IEEE Symposium on Communications and Vehicular Technology (SCVT), November 15, 2007, Delft University of Technology, Delft, The Netherlands, CD-ROM

Abstract:
The Dutch auction (the price progressively falls until a buyer “takes” the object) is proposed as a foundation for decentralised medium-access control. Common auction formats are well-understood, relatively simple mechanism which have long been used for allocating an indivisible good to the party that values it the most, for such reasons as speed of allocation, discovery of the true “value” of the object, and fraud prevention. Various auction schemes have been proposed for the allocation of telecommunication resources, including medium access control (MAC). But previously proposals require a controller, and, to receive the bids, an alternate protocol which could waste resources, or miss important bids. For MAC, the Dutch auction has several major virtues: (i) a bid-processing protocol that automatically and simply prioritises the highest bid(s); (ii) possibility of distributive (auctioneer-free) implementation for synchronised terminals; (iii) confirmation of transmitter-receiver pairs at auction time, with smooth continuation if the pair is infeasible; (iv) exceptional signalling economy (the only strictly necessary signal is the winning bid). Secure software inside each terminal may record transactions for eventual payment collection, or the auction can be used as a prioritised-access algorithm, without real money exchange. Below we evaluate qualitatively the MAC potential of this auction, emphasising the distributed version, which can control access with spatial reuse in a wireless adhoc environment.
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Volker Blaschke, Friedrich K. Jondral, Stefan Nagel, Eric Nicollet, Dominique Ragot:
Wireless Interoperability for Security - WINTSEC
Proceedings of the SDR Forum Technical Conference 2007, Denver (Colorado), November 5-9, 2007, CD-ROM

Abstract:
Crises management for homeland security, emergency and peace keeping operations is a communication intensive process that requires the involvement of many agencies and actors. Generally, each organization operates a specific communication system. Unfortunately, in major disaster scenarios these heterogeneous and often incompatible radio communication systems form a serious barrier for efficient and mission-oriented communications. Therefore, there is a strong demand for investigations and developments of enhanced interoperability in public and governmental security (P&GS) systems. Based on the analysis of current solutions, the WINTSEC project envisions future P&GS “seamless” interoperable communication systems implementing a “system of systems” approach that is discussed in this paper.
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Christian R. Berger, Michael Eisenacher, Shengli Zhou, Friedrich K. Jondral: 
Improving the UWB Pulseshaper Design Using Nonconstant Upper Bounds in Semidefinite Programming
IEEE Journal of Selected Areas in Signal Processing, Vol. 1, No. 3, October 2007, pp. 396-404

Abstract:
A critical obstacle for ultra-wideband (UWB) communications is conformity to restrictions set on the allowed interference level to other wireless devices. To this end, UWB signals have to comply with stringent constraints on their emitted power, defined by the Federal Communications Commission spectral mask. Different UWB pulseshaper designs have been studied to meet the spectral mask, out of which an approach based on digital finite impulse response filter design via semidefinite programming has stood out. However, so far this approach has assumed an ideal basic analog pulse to use pice-wise constant constraints for the digital filter design. Since any practical analog pulse does not have a flat spectrum, using piece-wise constant constraints leads to considerable power loss. Avoiding such a loss has motivated us to implement the exact constraints through nonconstant piece-wise continuous bounds. Relative to the design assuming an ideal basic analog pulse, our design examples show that the transmission power can be enhanced considerably while obeying the spectral mask. Such an improvement comes with no extra cost of implementation complexity.
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Friedrich K. Jondral: 
Cognitive Radio: A Communications Engineering View
IEEE Wireless Communications, August 2007, pp. 28-33

Abstract:
Cognitive radio (CR) is an emerging technology that leads to the flexible development, construction, production, shipping and deployment of highly adaptive radios that are built upon software defined radio (SDR) technology. This contribution starts with a brief section that underlines the paramount importance of the mobile radio communications channel. After that, spectrum issues are discussed to emphasize the reasons of spectrum scarcity as well as the importance of dynamic spectrum allocation. Some remarks about the development from digital radio to software defined radio lead to a discussion of the most important engineering aspects of CR, e.g. location and spectrum awareness, transmission power control and signal analysis. Since usually papers about CR are somewhat visionary, we try to describe first practical steps to an implementation of helpful CR properties into mobile communication base station as well as user terminal equipment. The paper concludes with a short summary.
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Friedrich K. Jondral, Volker Blaschke:
Evolution of Digital Radios – From Analog to Cognitive Features
In: F.H.P. Fitzek, M.D. Katz (Editors): Cognitive Wireless Nezworks. Dordrecht (The Netherlands), Springer, 2007, pp. 635-655 

Summary:
Starting with some historic remarks about radio communications, this chapter first of all highlights the limitations on mobile communications, induced by the physics of electromagnetic wave propagation, and the importance of standards to meet technical as well as commercial goals. The development from past analog over digital and software defined to future cognitive radios is reviewed. Special attention is dedicated to the parameterization of software defined radios. The software communications architecture as a framework for the reconfigurability of radio hardware and the portability of waveforms is briefly described. Cognitive radio features in current and future communication systems are discussed.
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S. Brandes, U. Berthold, M. Schnell and F.K. Jondral: 
OFDM Based Overlay Systems - Design Challenges and Solutions
Invited Paper, in 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Athens, Greece, September 2007

Abstract:
With respect to the ever-growing demand for mobile communications on the one hand and spectral scarcity on the other hand it is necessary to increase spectral efficiency. In this paper, we focus on OFDM (Orthogonal Frequency-Division Multiplex) based overlay systems, which share a frequency band with already existing systems by adaptively filling the spectral holes. In order to enable such a co-existence, on the physical (PHY) layer promising methods for minimising mutual interference between the overlay system and the licensed systems are presented. The medium-access control (MAC) layer protocols are designed such as to be able to efficiently signal the available subcarriers as well as to handle the changing available bandwidth. A third issue is the joint optimisation of PHY and MAC parameters which is essential in order to optimally exploit the available spectrum with minimum signalling overhead.
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Christian Körner, Friedrich K. Jondral:
Spread Spectrum Sequence Estimation for the UMTS Downlink Using Eigenvectors
FREQUENZ, Volume 61, July/August 2007, pp. 157-161

Abstract:
This article describes the spread spectrum sequence estimation for the noncooperative reception of a synchronous CDMA system at the example of the UMTS FDD downlink. We propose an approach calculating the eigenvectors of the received signal’s autocorrelation matrix cascaded by an automatic phase rotation of the eigenvectors and an elementwise decision. We assume that synchronization has been reached. The paper presents simulation results for the BER and the probability of an incorrect code sequence estimation.
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Volker Blaschke, Holger Jäkel, Tobias Renk, Clemens Kloeck, Friedrich K. Jondral:
Occupation Measurements Supporting Dynamic Spectrum Allocation for Cognitive Radio Design
Proceedings on the 2007 2nd International Conference on Cognitive Radio Oriented Wireless Networks and Communications (CrownCom), Orlando FL, USA, 1.-3. August 2007

Abstract:
Efficient use of radio spectrum is a necessity for future wireless systems. This can, for example, be accomplished by cognitive radios. An important point for designing future systems is to gain knowledge about the occupation of the frequency bands which may be used by the cognitive radio. This includes the general characterization of the traffic density at the frequency bands as well as a detailed analysis of the temporal frequency occupation. Thus, this paper describes measurements taken in the GSM network at three different scenarios. Furthermore, a detailed analysis of the measurement results is presented including statistical as well as spectral occupation analysis.
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T. Renk, C. Kloeck, D. Burgkhardt, F. K. Jondral , D. Grandblaise, S. Gault, J. C. Dunat:
Bio-Inspired Algorithms for Dynamic Resource Allocation in Cognitive Wireless Networks
Second International Conference on Cognitive Radio Oriented Wireless Networks and Communications (CrownCom), 31 July - 3 August, Orlando, Florida, 2007

Abstract:
Regulation will experience enormous changes in the near future resulting in seamless connectivity by bluring spectrum borders. A promising approach in this context is dynamic spectrum allocation which leads to a more flexible access to spectral resources by employing intelligent radio devices called cognitive radios. This paper is concerned with bio-inspired approaches that exploit distribution in multi-radio environments where many users have to share a finite resource harmoniously. Three applications of bio-inspired techniques are described. The first one deals with the detection of spectrum holes and the second one describes resource allocation in orthogonal frequency division multiple access based systems. The third one is concerned with distributed resource auctioning.
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Tobias Renk, Clemens Kloeck, Dennis Burgkhardt und Friedrich K. Jondral:
Cooperative Communications in Wireless Networks – A Requested Relaying Protocol
16th IST Mobile & Wireless Communications Summit, July 1-5, 2007, Budapest, Hungary

Abstract:

In this paper we present a requested relaying protocol that helps mitigating multipath and shadowing effects in wireless networks. In our approach it is assumed that the destined mobile station has not been able to receive information from the base station and sends a request to surrounding terminals. Those terminals able to receive the request act as relays and help to transmit information from the base station to the destination. Performance analysis with respect to probability of decoding error and outage probability have been made with the results that both can be enormously improved by exploiting diversity. Additionally, order of diversity is defined and applied to our relaying approach as well.
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Tobias Renk, Clemens Kloeck, Friedrich K. Jondral, Pascal Cordier, Oliver Holland und Fernando Negredo:
Spectrum Measurements supporting Reconfiguration in Heterogeneous Networks
16th IST Mobile & Wireless Communications Summit, July 1-5, 2007, Budapest, Hungary

Abstract:

This paper represents spectrum occupation measurements performed during the Soccer World Cup 2006 in Germany. The measurements cover the range from 400 MHz up to 2600 MHz including among other communication standards GSM 900, DCS 1800, UMTS, and WiFi. The intention of the paper is to compare spectrum occupation during a mega-event to spectrum allocation in a “normal usage” situation and to investigate correlation between the received power levels and specific events, e.g., scoring within a match. These results form the basis for a metric that may be assigned for dynamic spectrum access in heterogeneous radio environments. Measurement results are deeply investigated with respect to time-dependent and peak power statistics as well as quality of service considerations. Quality of service in that case is represented in terms of succeeded voice calls, data calls and data transfers as well as average throughput.
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Volker Blaschke, Tobias Renk, Friedrich K. Jondral:
Analysis of Occupation Measurements Concerning Overlay Systems
Proceedings of the 17th Annual MPRG Symposium on Wireless Personal Communications, June 6.-8., 2007, CD-ROM

Abstract:

During the last years several spectral measurements were presented. In these studies considering a frequency range between 30MHz and 6 GHz it was shown that the average spectral utilization is significantly below the theoretical limit. Providing flexible and reasonable allocation mechanisms the spectral usage can be intensified. Overlay systems seem to be a suitable approach for secondary usage of already allocated frequency bands. Providing a capable adaptation of a secondary system to the established system a dynamic allocation of remaining resources by the overlay system can be supported. In this paper measurements taken in two characteristic scenarios are analyzed in order to figure requirements and challenges that have to be discussed concerning dynamic spectrum allocation in overlay systems.
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U. Berthold, H. Heimpel and F. K. Jondral: 
Coordination of Allocation Measurements in OFDM Based Ad Hoc Overlay Systems
IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP 2007), April 15-20, 2007, Honolulu, USA

Abstract:
With an increasing demand for mobile communications an efficient usage of the available spectral resources gains importance, since suitable frequency bands are limited. Although nearly all frequency bands are assigned to specified services and users causing a frequency shortage, measurements show that nevertheless there are a lot of unused spectrum holes in the time frequency domain, leading to an overall low spectral efficiency. In this paper we consider an overlay scenario with an OFDM (orthogonal frequency division multiple access) based additional ad hoc overlay system operating in the same frequency band as the licensed system and exploiting the spectrum holes. Since spectrum sensing is a key issue for the coexistence of both systems we propose a method in the MAC (medium access control) layer for frame synchronization, enabling the coordination of allocation measurements. Furthermore, we point out some signal processing aspects regarding overlay systems.
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J. P. Elsner, P. Rykaczewski, C. Körner and F. K. Jondral: 
Orthogonal Complex Hadamard Spreading Codes for I/Q Imbalance Mitigation in MC-CDMA Systems
IEEE Vehicular Technology Conference (VTC), April 22-25, 2007, Dublin, Ireland

Abstract:
Zero-IF (direct conversion) receivers are very sensitive to non-linearities of analog hardware. A phase and/or amplitude imbalance between the inphase and quadrature signal branch results in a base band signal deterioration known as I/Q imbalance. Especially OFDM and the OFDM-based MC-CDMA modulation technique are affected by this error. This paper proposes a novel orthogonal quaternary MC-CDMA spreading code scheme that uses the inherent mathematical symmetry to mitigate I/Q imbalance without additional computational cost.
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Virgilio Rodriguez, Friedrich K. Jondral: 
Technical-Economic impact of UWB personal area networks on a UMTS cell: Market-driven dynamic spectrum allocation revisited
IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks, DySPAN 2007, Dublin, Ireland, April 17-20, 2007, CD-ROM, 1-4244-0663-3/07/$20.00 ©2007 IEEE

Abstract:
Recently, ultra-wide-band (UWB) technology has been approved for communication applications, utilising spectrum in use by other technologies. Recent studies have evaluated the technological impact of UWB on UMTS, but not the corresponding economic impact. This is of paramount importance, given huge investments already made on UMTS. Those studies also target "traditional" spectrum allocation. Dynamic spectrum allocation (DSA) exploits temporal/spatial variations in the "loads" of various networks. A recent proposal implements DSA by periodically selling or auctioning short-term spectrum licenses, and solves the problem of a participating CDMA cell populated by delay-tolerant terminals with dissimilar "willingness to pay" operating at various data rates, on the downlink. In the present work, UWB personal/body area networks (PAN/BAN) are introduced in the aforementioned DSA environment. The UWB networks utilise the spectrum for free, and do not compete for customers against the CDMA networks, but do increase the noise experienced by the CDMA terminals. The present work studies the economic impact of UWB on a CDMA cell, and its terminals. Before considering DSA, the techno-economic management of a CDMA cell with a fixed spectrum allocation (relevant to present day networks) is studied. The analysis reveals that neither the service priorities nor the technological operating point of the data terminals is affected by UWB. Consequently, a served terminal~s contribution to network revenues remain unchanged. The network bears the brunt of UWB interference, because the network can fit fewer terminals in a given spectrum allocation, yet receives an unchanged amount of revenue from each served terminal.
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O. Holland, P. Cordier, M. Muck, L. Mazet, C. Klöck, T. Renk: 
Spectrum Power Measurements in 2G and 3G Cellular Phone bands during the 2006 Football World Cup in Germany
IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 17.-20. April, 2007, Dublin, Ireland

Abstract:
This contribution presents a series of spectrum occupancy measurements which have been performed during the football World-Cup Germany 2006 in the cities of Kaiserslautern and Dortmund. The measurements cover the cumulative power of 2G bands 900MHz and 1800MHz as well as the 3G band 2000MHz - 2600MHz for the day prior to a match and during the match. The intent is to use a change in power measurements as a metric for user activity and to subsequently relate this metric to specific events, such as i) the start of the match, ii) the half-time and iii) the end of the match. Finally, a discussion on these results follows including an outlook on how the measurements may be used in order to predict the need for increased network capacity at similar events, which can in particular be provided by deploying WiMAX, WiFi, etc. access points in the context of a future heterogeneous network.
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O. Holland, C. Klöck, T. Renk et al.: 
Development of a Radio Enabler for Reconfiguration Management within the IEEE P1900.B Study Group
IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 17.-20. April, 2007, Dublin, Ireland

Abstract:
An important emerging capability is for mobile terminals to be dynamically reconfigured. Through ongoing advances in technology such as software defined radio, reconfiguration of mobile terminals will in the near future be achievable across all layers of the protocol stack. However, with the capability for such wide-ranging reconfiguration comes the need to manage reconfiguration procedures, in order to coordinate reconfigurations, to ensure that there are no negative effects (e.g. interference to other RATs) as a result of reconfiguration, and to maximize the potential benefits of reconfiguration and ensuing technologies such as those involving dynamic spectrum access. The IEEE P1900.B study group is therefore defining three building blocks for reconfiguration management: Network Reconfiguration Management (NRM), Terminal Reconfiguration Management (TRM), and a radio enabler to provide connectivity between the NRM and TRMs. In this paper we concentrate on aspects of the radio enabler, highlighting its relevance in heterogeneous radio access technology scenarios, its advantages, and some aspects of its technical realization.
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D. Grandblaise, C. Klöck, T. Renk et al.: 
Microeconomics Inspired Mechanisms to Manage Dynamic Spectrum Access
IEEE Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN), 17.-20. April, 2007, Dublin, Ireland

Abstract:
Trying to achieve higher usage efficiency for spectrum has been on the research agenda for some time now. More efficient transmission technologies are being developed, but they alone will not solve the problem of spatially and temporally underused spectrum and radio resources. Mechanisms to optimize spectrum access over space and time are required. This paper describes schemes, based on multiple agents that collaborate to find more efficient allocation patterns in a defined coverage area. Leveraging on microeconomics inspired mechanisms, the paper describes and analyses schemes based on collaborating agents (that can be either whole operator, or BS or end user terminal) that negotiate with each other to find the most optimized allocation pattern for a given area and allocation duration. The optimization strategies investigated include both bargaining as well as auction based mechanisms. Both allow the negotiation of spectrum and radio resources, based on market driven incentives. The auction types investigated support dynamic allocations on different timescales, ranging from short to medium and long term allocation scenarios. While auctions are discussed to be used for the longer term allocations, a MAC based rental protocol is evaluated for shorter term allocations when operated either at the BS or end user terminal level. Finally, the paper discusses how the MAC based rental protocol between BSs can be implemented.
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Mengüç Öner, Friedrich K. Jondral: 
On the Extraction of Channel Allocation Information in Spectrum Pooling Systems
IEEE Journal on Selected Areas in Communications, Vol. 25, No. 3, April 2007, pp. 558-565

Abstract:
The spectrum pooling strategy allows a license owner to share a part of his licensed spectrum with a secondary wireless system (the rental system, RS) during its idle times. The coexistence of two mobile systems in the same frequency band poses many new challenges, one of which is the reliable extraction of the channel allocation information (CAI), i.e. the channel occupation of the licensed system (LS). This paper presents a strategy for the extraction of the CAI based on exploiting the distinct cyclostationary characteristics of the LS and RS signals and demonstrates, via simulation, its application on a specific spectrum pooling scenario, where the LS is a GSM network and the RS is an OFDM based WLAN system.
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Mengüç Öner, Friedrich K. Jondral:
Air interface identification for Software Radio systems
AEÜ International Journal of Electronics and Communication, Vol. 61, Issue 2, February 2007, pp. 104-117

Abstract:
Reconfigurable Software Radio equipment is considered as the next evolutionary step in the mobile communications. One of the most crucial properties of a Software Radio terminal is that it is capable of using a wide range of air interface standards, providing a seamless interoperability between different standards and an enhanced roaming capability, paving way to a more flexible and efficient use of spectral resources. This multimode operation has to be supported by a number of key functionalities, one of which is the air interface identification. A Software Radio terminal, when switched on, has to be able to locate and identify the air interfaces available in the frequency environment, and while connected to a network, it has to monitor the presence of alternative air interfaces to perform inter-standard handover if necessary. In our work, we propose exploiting the distinct cyclostationary properties of signals from different air interfaces as features for air interface identification.
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C. Kloeck, H. Jäkel, Friedrich K. Jondral:
Multi-Agent Radio Resource Allocation
Mobile Networks and Applications, Springer Netherlands, ISSN 1383-469X (Print) 1572-8153 (Online), Volume 11, Number 6, December, 2006, pages 813-824

Abstract:
This paper presents a spatially distributed and dynamic billing, pricing and allocation mechanism for which a user terminal requires Cognitive Radio abilities. That is, the Cognitive Radio abilities will be applied to the economical environment. The radio resource goods are allocated to the users by a multi-unit sealed-bid auction. Intelligent entities like the bidding strategy have to represent operator's and users' behavior and to make decisions for them in order to fulfill the preferences and QoS. The main functionalities, used to execute the dynamic auction sequence and located in the MAC are described, assuming the entities possess Cognitive Radio abilities.
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V. Blaschke, S. Nagel, Friedrich K. Jondral:
Mechanisms for the Adaptation of the Physical Layer in a Cognitive Radio
Proceedings of the 9th European Conference on Wireless Technology, Manchester (UK), September 10-12, 2006, pp. 43 - 46

Abstract:
During the last years a modification of the spectrum allocation policy has been discussed. The desired increase of spectral utilization can be realized using overlay systems. These systems allocate the temporarily unused frequency bands of established licensed systems. In order to minimize the interference to the primary system an adaptable and flexible mobile terminal is required. Due to the ongoing research of software defined radio technology, different wave forms can already be supported by the same hardware. In order to adapt the secondary user to the offered frequency bands a spectral monitoring is required. This enhancement to the classical software defined radio concept was introduced as a Cognitive Radio feature. In this paper we describe the additional demands to the physical layer of a Cognitive Radio. Furthermore, we present a mechanism for supporting QoS under different channel conditions and radio access technologies (RAT).
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Z. Boufidis, N. Alonistioti, M. Stamatelatos, J. Vogler, U. Lücking, C. Kloeck, D. Grandblaise, D. Bourse: 
End-to-End Architecture for Adaptive Communication Systems
2006 IEEE 64th Vehicular Technology Conference, 25 - 28 September, 2006, Montreal, Canada

Abstract:
Reconfigurability is considered the collection of software and cognitive radio technologies that aim to differentiate user perception in volatile radio conditions while optimizing the use of network resources. The realization of equipment reconfiguration through software download and the efficient utilisation of spectrum require coordinated distribution of functionality among the user, control, and management planes. This contribution presents the end-to-end architecture of future adaptive communication systems based on the Reconfiguration Management Plane model, which includes intelligence for policy-based context-aware decision making, negotiation control, software download, and dynamic spectrum management. The paper describes the RMP model, maps its modules to end-to-end physical configurations, and describes the next steps towards autonomously-managed end-user equipment and network elements.
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Clemens Klöck, Holger Jäkel, Friedrich K. Jondral: 
Introduction of an analytical Figure of Merit for Quality of Service Measurement
2006 IEEE 64th Vehicular Technology Conference, 25 - 28 September, 2006, Montreal, Canada

Abstract:
Investigations performed for future applications in various research areas will allow users to communicate over more than one radio access technology with one single device. Thus, user terminals need methods to specify the quality of service provided by a radio access technology in order to decide about the radio access technology being used for a certain service or even to consider a possible change of an operator. In this paper a quality of service measure is proposed that allows the evaluation of services by certain service parameters, as, for example, a data transmission may be judged by the bit error rate, the overall network data rate or the delay. The measure defined in this paper bases on simple observation of frames resp. packets and can be applied for the assessment of any service property whose validity can be characterized by a simple decision whether or not a given quality of service was fulfilled.
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Tobias Renk, Clemens Klöck, Holger Jäkel, Friedrich K. Jondral:
A New Cognitive Pilot Channel Concept based on Multi-Hop Networks
Wireless World Research Forum 17 (WWRF17), 15.-17. November, 2006, Heidelberg, Deutschland

Abstract:
In contrast to wired systems, where additional bandwidth can easily be obtained by simply adding new hardware to the network, wireless communication systems share an inherently finite resource -- the radio frequency spectrum. Therefore, it is inevitable to create new concepts for accessing resources such as bandwidth, time-slots, codes, and all thinkable combinations of them. In this paper we present a cognitive pilot channel that conveys information about the current resource allocation to mobile stations. In order to ensure reliable communication, we introduce a new relay principle called requested relaying and a new protocol named requested relay protocol. The case of three mobile stations where two act as relays for a third one is considered. Thereby, considerations in Rayleigh fading channels are done and formulas for signal-to-noise ratios, probability of decoding error, and outage probability are derived.
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Clemens Klöck, Jijun Luo, Tobias Renk, Holger Jäkel, Volker Blaschke:
On Self-reconfigurability of Cognitive Radios
Wireless World Research Forum 17 (WWRF17), 15.-17. November, 2006, Heidelberg, Deutschland

Abstract:
Cognitive Radio behavior often is discussed and explained with respect to the Cognition Cycle introduced by Mitola VII. In this paper we present a modified version of the Cognition Cycle which includes all features of Mitola~s Cognition Cycle and additionally takes self-estimation and self-reconfiguration into account. This Cognition Cycle will be applied to dynamic resource allocation and 3 GPP LTE.
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Volker Blaschke, Friedrich K. Jondral:
An Approach for Providing QoS in Cognitive Radio Terminals
FREQUENZ, Band 60, Heft 9-10, September/Oktober 2006, S. 194- 198

Abstract:
Due to the demand for more radio transmission resources the dynamic spectrum management has been identified as a necessary modification in the spectrum allocation policy. A dynamic and temporal assignment of the spectrum to different logical networks or user groups requires very flexible mobile radio terminals as well as base stations. Moreover, the classical assignment of service classes and predefined wave forms cannot be used in such heterogeneous systems. Therefore, it is necessary to design a mobile radio which offers a high adaptability to the current spectral conditions but also supports QoS for a set of predefined service classes. Within this paper we describe the basic components of such a flexible mobile terminal based on the Cognitive Radio design. Furthermore, we present a method for disintegrating the fixed mapping of the service classes to the standard specific transmission parameters of the present radio access technologies. This will offer the possibility to estimate the feasibility of a service class using the available transmission chan-nels. In case of a number of alternative transmission channels an approach for an evaluation with respect to the characteristic parameters, like channel bandwidth or required E_b/N₀ is described.
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Christian R. Berger, Michael Eisenacher, Holger Jäkel and Friedrich K. Jondral:
Pulseshaping in UWB Systems Using Semidefinite Programming with Non-Constant Upper Bounds
Proceedings of the 17th International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC '06), Helsinki (Finland), September 11-14, 2006, CD-ROM

Abstract:
Multiple Access Interference (MAI) arises to be a severe problem for the mass deployment of low-complex Ultra Wideband (UWB) devices. This holds true especially for non-coherent receivers which are quite sensitive to MAI. In this paper a simple interference avoidance technique for non-coherent UWB devices is proposed, that does not require any information about the potential UWB interferer at all. It is shown that this technique can improve the performance in case of interferers without lowering the performance in the absence of interferers.
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Piotr Rykaczewski, Mikko Valkama, Markku Renfors, Friedrich K. Jondral: 
Non-Data-Aided I/Q-Imbalance Compensation Using Measured Receiver Front-End Signals
Proceedings of the 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Communications, September 11.-14., 2006

Abstract:
The direct-conversion or homodyne receiver principle has received lots of interest in recent years for building flexible and compact radios. The homodyne architecture is, however, found to be rather sensitive to certain non-idealities of the analog front-end components. One big concern here is related to the amplitude and phase mismatches of the I and Q signal branches, resulting in insufficient rejection of the mirror frequency band. This paper presents an efficient low-order statistics based method (stat) for digital I/Q mismatch compensation, and compares the proposed technique with the blind signal separation (BSS) and hard decision (HD) based compensators proposed earlier by the authors. In addition to algorithm developments, big emphasis in the paper is on performance measurements and verifications carried out using measured true-world receiver front-end signals obtained from a laboratory homodyne measurement system. In general, the obtained results indicate that I/Q imbalance can be efficiently compensated using the proposed ideas. Even with really high-order modulation techniques, such as 64-QAM, close-to additive noise bound compensator performance is demonstrated using the measured signals.
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Ulrich Berthold, Friedrich K. Jondral, Sinja Brandes, Michael Schnell:
A Framework for Crosslayer Optimization in OFDM Based Overlay Systems
Proceedings of the 11^th International OFDM-Workshop, Hamburg, August 30.-31., 2006, pp. 253 - 257

Abstract:
One approach for increasing spectral efficiency is the coexistence of OFDM based overlay systems with an already existing licensed system in the same frequency band. An important issue in overlay systems is to use the changing available bandwidth as efficiently as possible. As the dynamic spectrum allocation influences not only the physical (PHY) layer but also the medium access control (MAC) layer, we present a general framework for crosslayer optimization designed for the special requirements in overlay systems. In the presented crosslayer design not only information about the currently available spectrum is considered, but also further parameters, providing a basis for a joint optimization. As an example, the optimization of throughput is investigated, based on the combined knowledge of the allocation vector and the signal-to-noise ratio.
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Tobias Renk, Clemens Klöck, Christian Körner, Friedrich K. Jondral:
Increasing Spectral Efficiency by Managing Adjacent Channel Interference
Proceedings of the First International Workshop on Technology and Policy for Accessing Spectrum, August 8., 2006, Boston, USA

Abstract:
Due to the ever-increasing demands in wireless communications, spectrum seems to be a scarce resource today. Hence, new techniques that deal with the issue of spectrum management have to be developed. In this paper, possibilities to reduce the guard band between consecutive channels dynamically are investigated, thus improving spectral efficiency at the expense of an increasing amount of adjacent channel interference. In a first consideration, the strategy of shifting carrier frequencies is investigated. A second approach is the adaptive adjustment of roll-off factors when root raised cosine filters are used for pulse shaping as it is the case in 3G mobile communication systems. Moreover, an algorithm for dynamic guard band adaptation is developed. A simulation environment is set up and the simulation results are presented and discussed as well.
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Piotr Rykaczewski, Friedrich K. Jondral:
Impact of Dynamic DC Offset on 3G and WLAN within a Software Defined Radio
Proceedings of the 16th Annual MPRG Symposium on Wireless Personal Communications, June 7.-9., 2006, CD-ROM

Abstract:
The problem of dynamic DC offset in a homodyne receiver supporting 3G and WLAN standards is addressed in this paper. Comparisons between 3G (UTRA FDD) and WLAN (IEEE 802.11a) are done with the emphasis placed on the particular way dynamic DC offset distorts the signal of the OFDM based WLAN. Possibility of deploying a decision directed compensation method observing the particular demands of the software defined radio environment is discussed. The theoretical outcomes are presented and backed by simulations. Finally, a modified hardware architecture addressing the problem of DC offset is proposed.
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Ulrich Berthold, Sinja Brandes, Michael Schnell, Friedrich K. Jondral:
On Focus: OFDM Based Overlay Scenarios
IST Mobile & Wireless Communication Summit, June 04-08, 2006, Myconos, Greece

Abstract:
Regarding the ever-growing demand for mobile communications it is necessary to consider different approaches for increasing spectral efficiency, since spectral resources for wireless applications are limited. In this paper, we focus on OFDM (Orthogonal Frequency-Division Multiplex) based overlay systems, which share a frequency band with already existing systems without degrading their performance. An emphasis is put on three overlay scenarios in different frequency ranges which serve as realistic examples for the challenges for OFDM based overlay techniques. These three scenarios show that overlay systems are a promising approach for enhancing spectral efficiency by implementing additional systems in existing frequency bands.
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Clemens Kloeck, Holger Jäkel, Friedrich K. Jondral:
Multi-Agent Wireless System for Dynamic and Local Combined Pricing, Allocation and Billing
Journal of Communications, vol. 1, no. 1, ISSN 1796-2021, April 2006

Abstract:
In established communication systems prices are determined in a quasi-static way and for a large area according to a fixed price model. Additionally, the main decision criteria of resource allocation is the arrival time of resource requests. This system approach is not able to respond to the spatially distributed and dynamic users' demand and willingness-to-pay. In this paper a system will be proposed to combine pricing, allocation and billing in order to react dynamically and locally to the market assuming that the user terminal has cognitive radio abilities and multiple interfaces.
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Michael Eisenacher, Holger Jäkel, Friedrich K. Jondral:
Low-Complexity Interference Suppression for Non-Coherent Ultra-Wideband Systems
IST Mobile & Wireless Communication Summit, June 04-08, 2006, Myconos, Greece

Abstract:
Multiple Access Interference (MAI) arises to be a severe problem for the mass deployment of low-complex Ultra Wideband (UWB) devices. This holds true especially for non-coherent receivers which are quite sensitive to MAI. In this paper a simple interference avoidance technique for non-coherent UWB devices is proposed, that does not require any information about the potential UWB interferer at all. It is shown that this technique can improve the performance in case of interferers without lowering the performance in the absence of interferers.
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Dennis Burgkhardt, Clemens Klöck, Holger Jäkel, Friedrich K. Jondral:
Fast Bidding Strategy for Auction-Based Radio Resource Allocation Mechanisms
IST Mobile & Wireless Communication Summit, June 04-08, 2006, Myconos, Greece

Abstract:
For allocation and pricing systems based on sequential auctions, the bidding strategy is the central part of user terminal's software agent. It determines optimal bids with respect to the user's quality-of-service needs and his economical circumstances and preferences. This paper proposes a MAC-layer system model and introduces a bidding strategy which saves memory and computing efforts. In simulation it is compared to a more complex statistics-based strategy and will turn out to be an equivalent alternative.
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P. Cordier, C. Klöck, et. al.:
E2R Cognitive Pilot Channel concept
IST Mobile & Wireless Communication Summit, June 04-08, 2006, Myconos, Greece

Abstract:
The E2R project is pursuing research on the definition of the most appropriate distribution of intelligence and functionalities between end-users terminals and network, in the context of B3G heterogeneous environment. E2R is currently developing concepts and solutions for a Cognitive Pilot Channel (CPC), encompassing both in-band/out-band and downlink/uplink functionalities. The out-band CPC should provide sufficient information to a mobile terminal so as it can initiate a communication session in an optimal way, optimised to time, situation and location. The CPC is required to broadcast relevant information with regard to frequency bands, RATs, services, load situation, etc. in the terminal location and in a possible future case of DSA context.
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Oriol Sallent, Jordi Perez-Romero, Ramon Agusti, Lorenza Giupponi, Clemens Kloeck, Ihan Martoyo, Stefan Klett, Jijun Luo:
Resource Auctioning Mechanisms in Heterogeneous Wireless Access Networks
2006 IEEE 63rd Vehicular Technology Conference, 7 - 10 May, 2006, Melbourne

Abstract:
The vision of the wireless communications beyond 3G is characterized by flexibility. Future communication networks should be able to flexibly allocate resources and radio access technologies (RATs) to maintain high quality of communication and efficient use of radio resources. This should be the task of the joint radio resource management (JRRM) algorithms. Furthermore, this flexibility should also cover the pricing mechanism which should be able to react to the instantaneous users~ needs and resource availability. This will be tackled by a real time spectrum auction system. This paper draws an architecture which embodies the two mechanisms: JRRM and spectrum auction, in order to create highly efficient wireless systems.
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Klaus Moessner, Jijun Luo, David Grandblais, Clemens Kloeck, Ihan Martoyo, et al:
Functional Architecture of End-to-End Reconfigurable Systems
2006 IEEE 63rd Vehicular Technology Conference, 7 - 10 May, 2006, Melbourne

Abstract:
Adaptive networks are envisaged to play a significant part in the future, where the time and space variations in the traffic pattern will necessitate the ability to continuously amend the Radio Access Technologies~ (RATs~) operating parameters. Reconfiguration of communications systems is a facilitator towards this convergence. However, such concept involves many different working areas, each of which represents an answer to a different problem, such as Dynamic Network Planning and Management (DNPM), Advanced Spectrum Management (ASM) and Joint Radio Resource Management (JRRM). Consequently, there is need of combining all working areas in the form of a Functional Architecture (FA), where each module would represent a concept, aiming at forming part of the global end-to-end reconfigurability architecture. This paper includes a detailed analysis of the Reconfigurability FA, along with a description of the functionality of each of the modules included therein.
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Clemens Kloeck, David Grandblaise, Jijun Luo, George Dimitrakopoulos: 
Multi-Level Spectrum Auction Through Radio Access
The 13th IEEE Mediterranean Electrotechnical Conference, May 16-19, 2006, Benalmádena (Málaga), Spain

Abstract:
This paper describes the resource exchange between all important interfaces of a communication system using an auction sequence. Differences between the different mechanisms are shown and implementation concepts are presented.
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Jijun Luo, David Grandblaise, Clemens Kloeck, George Dimitrakopoulos, Klaus Moessner: 
Reconfigurability Impacts to Resource Management in Future Wireless Communication Systems
The 13th IEEE Mediterranean Electrotechnical Conference, May 16-19, 2006, Benalmádena (Málaga), Spain

Abstract:
This paper illustrates the essential impacts from the reconfigurability to the resource management and spectrum management aspects in the wireless communication system. Since reconfigurability provides potential inter-operability, varies system operational parameters and radio system characteristic, the management functions in the communication system such as radio resource management, spectrum management and even network planning and management are facing an evolutionary opportunity. In order to study those three issues in a systematic manner, we take the multi-loop control circuits as a reference model for the overall comprehensive system. Each resource management mechanism contributes to the overall multi-loop control structure. Some selected typical algorithms allocated in each management category providing significant system capacity gains are as well studied.
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Volker Blaschke, Friedrich K. Jondral:
A Concept for a standard-independent Cognitive Radio Terminal
12th European Wireless Conference, Athens, April 2006

Abstract:
During the last years the dynamic spectrum management has been classified as a necessary modification in the spectrum allocation policy. Within the huge amount of publications dealing with this topic two main approaches can be pointed out. On the one hand the overlay system concept which allocate temporary unused frequency bands of established licensed systems. On the other hand the auction-based spectrum allocation schemes providing abstract radio resource units to the user. These units have to be transmitted using the available Radio Access Technologies (RAT). Both concepts have in common that the mobile terminal has to be adapted to the used frequency bands or radio transmission standards, respectively. Therefore, the Cognitive Radio concept is considered to provide the required flexibility. In order to support a high adaptability of the mobile terminal we present a concept for the physical layer structure of a Cognitive Radio. In addition to the classical parts of a mobile transmitter the Cognitive Radio specific functions like spectral observation and knowledge processing will be included.
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Holger Jäkel, Michael Eisenacher:
Impulsbasierte UWB-Systeme - ein Überblick
Telekommunikation Aktuell, 60. Jahrgang, Heft 01-03, Januar-März 2006, S. 1-19

Abstract:
Ultrabreitbandige Funktechnik (Ultra-Wideband, UWB) steht zunehmend im Blickpunkt des Interesses. Die Anwendung der ultrabreitbandigen Technik verspricht verbesserte Leistungsfähigkeit bei geringem Leistungsverbrauch und gleichzeitiger Lokalisierungsfähigkeit. Seitens einiger etablierter Funkdienste bestehen Vorbehalte gegenüber UWB-Technologien. Der vorliegende Artikel bietet einen Überblick über die Funktionsweise des impulsbasierten UWB und möchte dadurch das Wissen um die Funktionsprinzipien impulsbasierter UWB-Technik leicht zugänglich machen. Die Darstellung konzentriert sich auf die physikalische Schicht von impulsbasierten UWB-Systemen, deren Sender- und Empfängerkonzepte und die daraus resultierenden Spektraleigenschaften.
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Friedrich K. Jondral:
Cognitive Radio - A Necessity for Spectrum Pooling
Proceedings of the Journées Scientifique "Vers des Radiocommunications Reconfigurables et Cognitives", Comité National Français de Radioélectricité Scientifique, Section française de l' U.R.S.I., Paris (France), March 28/29, 2006

Abstract:
The demand for wireless communication capacities is continuously growing. This is especially true for cellular and also for wireless local area networks (WLANs). The options for increasing capacity are to apply advanced signal processing technologies in "traditional" frequency regions or to open up higher frequency ranges. The second alternative, however, is somewhat difficult in mobile applications because the radio wave propagation becomes nastier with increasing frequency.

Recent measurements (e.g. in New York City [1]) indicate that the average spectrum utilization for frequencies below 6 GHz is in the order of 15%. That is that a huge capacity gain for these frequencies should be possible with the implementation of advanced signal processing technologies. One of them, spectrum pooling, is the starting point of this talk.

This talk discusses the spectrum pooling approach [2] that enables public access to already licensed frequency bands. The notion spectrum pool basically represents the idea of merging spectral ranges from different spectrum owners (military, trunked radio, etc.) into a common pool. It reflects the need for a completely new way of spectrum allocation. From this common spectrum pool hosted by the so-called licensed system, public rental users may temporarily rent spectral resources during idle periods of licensed users. The basic proposition is that the licensed system does not need to be changed. The installed hardware can be operated like there was no other system present in the same frequency range. This approach kills two birds with one stone. Rental users obtain access to spectral ranges they have not yet been allowed to use, and the actual license owners can tap new sources of revenue for a good they have not been using intensively anyway. A multitude of juridical and economic consequences occurs when implementing the idea of spectrum pooling in a real system. Concerning the regulatory aspects of spectrum pooling, one must say that regulators are well aware of the fact that public mobile radio spectrum is falling short, and considerations toward secondary use of already licensed frequency bands are going on. After all, it is a political question whether this new concept will be admitted. However, once the technical obstacles are overcome and the feasibility of spectrum pooling is proven, politics cannot refuse this idea. The economic questions that must be answered are currently subject to scientific investigations in research projects funded by the European Union involving a variety of topnotch industrial partners and leading edge research institutions. Despite all the interesting juridical and economic aspects, this talk focuses on the technical challenges spectrum pooling implies. First, a short introduction to the general structure of a spectrum pooling transceiver and the utilized orthogonal frequency-division multiplexing (OFDM) modulation scheme on the rental user side is given, which is essential to understanding the remainder of this contribution. After investigations of spectrum pooling, specific tasks in the physical layer, and problems and their solutions concerning the MAC layer of the rental system are presented.

Obviously, receivers in a spectrum pooling rental system must be able to detect temporarily idle frequencies very fast. This is a typical task for a cognitive radio that comes into the game when considering what follows: Today spectrum is regulated by governmental agencies like the Federal Communications Commission (FCC) in the US or the Bundesnetzagentur (BNetzA) in Germany. "Spectrum" is assigned to users or licensed to them on a long term basis normally for huge regions like whole countries. Doing so, resources are wasted. It is our vision that spectrum within a large frequency range (that has to be defined by governments) becomes free for open access such that resources are assigned only as long as they are needed by the user. Moreover, spectrum is assigned on a regional basis obeying that stations using identical frequencies cannot interfere. Already existing examples for self regulation of networks are WLANs (IEEE 802.11) or ultra wide band (UWB) systems.

A cognitive radio (CR) is a software defined radio (SDR) that additionally senses its environment, tracks changes and reacts upon its findings. A CR is an autonomous unit in a communications environment. In order to use the spectral resource most efficiently, it has to

• be aware of its location
• be interference sensitive
• comply with some communications etiquette
• be fair against other users
• keep its owner informed

In order to handle its tasks, a CR carries location sensors (i.e. GPS or Galileo) in order to determine its own location. It has to monitor its spectral environment, e.g. by employing a broadband fast Fourier transform (FFT). To track its location or the spectral environment's development, it has to use appropriate learning and reasoning algorithms. Most important, CRs should be polite to other spectrum users. I.e. it has to compromise its own demands with the demands of other users, most probably making decisions in a competitive environment using the results of game theory. Last but not least, a CR should contact his owner via a highly sophisticated man-machine-interface. So the CR's user may be kept informed about its actions.

This talk is intended review past investigations on spectrum pooling and to present the work concerning CR and advanced spectrum usage currently under way at the Institut für Nachrichtentechnik of the Universität Karlsruhe (TH).
[1] Mark McHenry, Dan McCloskey: New York City Spectrum Occupancy Measurements September 2004. Shared Spectrum Company, 8133 Leesburg Pike, Suite 220, Vienna, VA 22182, 2004
[2] Timo A. Weiss, Friedrich K. Jondral: Spectrum Pooling: An Innovative Strategy for the Enhancement of Spectrum Efficiency. IEEE Communications Magazine, March 2004, Radio Communications Supplement, pp. S8 - S14
[3] Friedrich K. Jondral: Software Defined-Radio - Basics and Evolution to Cognitive Radio. Invited paper, EURASIP Journal on Wireless Communications and Networking, 2005, No. 3, pp. 275 - 283
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Proceedings of the 4th Karlsruhe Workshop on Software Radios
Karlsruhe (Germany), March 22/23, 2006, ISSN 1616 - 6019

Editorial:
Since the term of Software Radio was coined by Joseph Mitola roughly 15 years ago, the idea of multi-band, multi-standard and multi-mode transceivers has emerged from a niche to mark a milestone in the development of modern system strategies for truly universal mobile telecommunications. While visionaries and managers continue to think about business models and try to predict the implications of globalization for the future of mobile radio, researchers and developing engineers pick specific problems of transceiver design. There is a tremendous effort to technically solve Software Radio problems, to improve existing techniques, to propose new ones.

The vivid, future-oriented character of this research field was reflected in previous Karlsruhe Workshops on Software Radios. Established in 2000, our Workshop has evolved to a biennial event, providing a platform for expert discussions and resulting in many established research and business contacts reaching far beyond the framework of the Workshop.

Today, the Communications Engineering Lab of the Universität Karlsruhe (TH) is glad to welcome you to the Fourth Karlsruhe Workshop on Software Radios. 26 contributions await you. The presenters of 11 papers come from the industry and 15 represent academia. The contributions cover an area as extensive as a two days of Workshop allow and are centred around seven main topics:

• Dynamic Resource Allocation
• Software Radio Architectures
• Signal Processing Algorithms
• Hardware/Software Co-Design
• Cognitive Radio
• Reconfigurable Hardware
• Applications

Authors from Finland, France, Greece, Italy, Serbia and Montenegro, Singapore, Spain, the United Kingdom, the United States and Germany maintain the truly international character of our Workshop.
Friedrich K. Jondral
Christian Körner
Piotr Rykaczewski
Karlsruhe, March 22, 2006
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Piotr Rykaczewski, Mikko Valkama, Markku Renfors: 
Analytical Approach to I/Q Imbalance in OFDM, CDMA and MC-CDMA Based Systems
Proceedings of the IEEE Radio and Wireless Symposium (RWS 2006), 17.-19. Januar 2006, San Diego (CA), USA, pp. 555-558

Abstract:
Growing requirements set upon communications transceivers lead to high sensitivity to non-idealities of analog front-end components. Mismatches in the amplitudes and phases of the I and Q signal branches is one of the major concerns resulting in insufficient rejection of the image frequency band. This paper presents a novel analytical model of I/Q imbalance in OFDM, CDMA and MC-CDMA based communications systems. According to the obtained results, I/Q imbalance effects depend heavily on the applied spreading code class and cell load in case of CDMA and MC-CDMA, which should be taken into account in the system and receiver design. Outcomes of the theoretical studies are verified by extensive system simulations.
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Friedrich K. Jondral:
Signalverarbeitung in der Funktechnik - vom Digitalen Empfänger zum Cognitive Radio
Telekommunikation Aktuell, 59. Jahrgang, Heft 05-06, Mai-Juni 2005, S. 1-19

Abstract:
Die Funkübertragung fasziniert die Menschen seit der Entdeckung der elektromagnetischen Wellen in den achtziger Jahren des neunzehnten Jahrhunderts. Ausgehend von der Analogtechnik hat in der Folge der von Shannon's Arbeiten angeregten Entwicklungen in der zweiten Hälfte des zwanzigsten Jahrhunderts die Digitaltechnik Einzug in die drahtlose Kommunikation gehalten; der Durchbruch ist ihr mit der Einführung der digitalen Mobilfunksysteme in den neunziger Jahren gelungen. Der vorliegende Übersichtsartikel beschreibt, wie auf der Grundlage des digitalen Radios im einundzwanzigsten Jahrhundert die Entwicklung zu in höchstem Maße adaptiven Funkgeräten, hier als Cognitive Radios bezeichnet, und Funksystemen weitergehen kann.
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Pascal Codier, Didier Bourse, David Grandblaise, Klaus Moessner, Jijun Luo, Clemens Kloeck et. al.:
Cognitive Pilot Channel
Proccedings of WWRF15, Paris, 8.- 9.12.2005

Abstract:
The E2R project is pursuing research on the definition of the most appropriate distribution of intelligence and functionalities between end-users terminals and network, in the context of B3G heterogeneous environment. E2R is currently developing concepts and solutions for a Cognitive Pilot Channel (CPC), encompassing both in-band / out-band and downlink/uplink functionalities. In particular, the out-band CPC should provide sufficient information to a mobile terminal so as it can initiate a communication session in an optimal way, optimized to time, situation and location. The CPC is required to broadcast relevant information with regard to frequency bands, RATs, services, load situation, etc. in the terminal location. In particular after 'power on the mobile does not know which RAT may be the most appropriate or in which frequency bands potential RATs are operating. Thus the reconfigurable terminal alone will have difficulties to properly assume a configuration that fulfils the communication needs. In addition, this situation will be even more critical in the long term when new regulatory approaches to spectrum usage and the expected establishment of a secondary spectrum market will allow the implementation of Dynamic Spectrum Allocation (DSA) and Flexible Spectrum Management (FSM) (including Spectrum Pooling). In this case, the mobile terminal will have to initiate a communication in a spectrum context which is completely unknown due to dynamic reallocation mechanisms.
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Oriol Sallent, J. Perez-Romoero, R. Augusti, Kloeck Clemens, Ihan Martoyo, Jijun Luo:
Extending Auction Principles to Multi-RAT Scenarios
Proccedings of WWRF15, Paris, 8.- 9.12.2005

Abstract:
The introduction of new JRRM (Joint Radio Resource Management) algorithms operating from a common perspective that takes into account the overall amount of resources offered by the available RATs opens new potentials in system's flexibility and efficiency. On the other hand, auctioning radio resource goods of a cell changes the market philosophy in comparison to the fixed price model and both the operator and the users gain from the auction. The target of this paper is to focus on extending auctioning concepts into a heterogeneous RATs scenario. In particular, the interactions between JRRM entity and the auctioning mechanisms are further elaborated.
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Clemens Kloeck, Jijun Luo, David Grandblaise, Oriol Sallent:
Comaprison between ASM and Established Rseource Allocation Mechanism
Proccedings of WWRF15, Paris, 8.- 9.12.2005

Abstract:
Advanced Spectrum Management liberalizes the spectrum trading and allocation. This allows a local and dynamic resource assignment depending on the varying demand in space and time. The gained freedom offers different opportunities. These differences will be highlighted in the paper in comparison to the established systems.
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Michael Schmidt, Holger Jäkel, Friedrich K. Jondral:
Suppression of Multi User Interference Between Unsynchronized Ultra Wideband Systems
Proceedings of 2005 International Workshop on UWB Technologies, Yokosuka, Japan, 08.-10.12.2005

Abstract:
Ultra wideband (UWB) systems work unlicensed as an overlay system over several GHz and thus face interference from other UWB devices. This interference is often modelled as additive white Gaussian noise (AWGN) which is only reasonable for a very large number of users. We present an analytical approach to predict the interference impact that uses the exact statistical properties of UWB signals. It is shown that the resulting bit error rate is completely different from the one in case of AWGN interference. From this prediction we provide correlation templates which are optimized to UWB interference instead to AWGN. It is shown that these templates show a significant better performance without being synchronized to the interferer.
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Michael Schmidt, Holger Jäkel, Friedrich K. Jondral: 
Influence of the Amplitude Distribution to the Interference of UWB Signals on Radio Receivers
Proceedings of IEEE Globecom 2005, St. Louis, MO, USA, 28.11.-02.12.2005

Abstract:
The interference of Ultra Wideband (UWB) signals on conventional radio systems, called victims, is still an open issue. In this paper we show that not only the interference power but also the amplitude statistics have significant influence on the bit error rate (BER) of the victim. The amplitude probability distribution (APD) of UWB signals at the channel and inside the victim's receiver is studied analytically and verified by simulation. It is shown under which boundary conditions the UWB signal looses its pulsed behavior inside the victim's receiver. From this an interference model for the additional BER caused by UWB is proposed. The theoretical model is evaluated by simulations with a WLAN and a UMTS victim.
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Michael Schmidt, Holger Jäkel, Friedrich K. Jondral: 
Spectral Properties of Ultra Wideband Signals with General Time-Hopping Codes
Proceedings of IEEE EUROCON, Belgrade, Serbia & Montenegro, 21.-24.11.2005

Abstract:
Spectra of Ultra Wideband (UWB) signals are restricted to frequency masks defined by regulation authorities like the Federal Communications Commission (FCC) in the United States or the Regulierungsbehörde für Telekommunikation und Post (RegTP) in Germany. To make optimum use of these frequency masks, technologies that smooth the peaks in the signals' power spectral density (PSD) should be implemented. This contribution investigates the influences of various modulation schemes as well as different time hopping codes with general probability density functions on the PSDs of impulse radio UWB signals.
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Clemens Kloeck, Holger Jäkel, Friedrich K. Jondral:
Auction Sequence as a New Resource Allocation Mechanism
Proceedings of VTC'05 Fall, Dallas, 25.-28.09.2005

Abstract:
This paper presents a spatially distributed and dynamic billing, pricing and allocation mechanism for which a user terminal requires Cognitive Radio abilities. That is, the Cognitive Radio abilities will be applied to the economical environment. The radio resource goods are allocated to the users by a multi-unit sealed-bid auction.Intelligent entities like the bidding strategy have to represent operator's and users' behavior and make decisions for them in order to fulfill the preferences and QoS. The main functionalities, used to execute the dynamic auction sequence and located in the MAC are described, assuming the entities are aware of Cognitive Radio abilities.
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Uwe Kiencke, Holger Jäkel:
Signale und Systeme
3. überarbeitete Auflage, Oldenbourg Wissenschaftsverlag, ISBN: 3-486-57811-1

Abstract:
"Signale und Systeme" wendet sich nicht nur an Studenten der Fachrichtung Elektrotechnik an wissenschaftlichen Hochschulen, sondern auch an Ingenieure und Naturwissenschaftler, die einen Einblick in dieses Gebiet gewinnen wollen.

Die Signale und Systeme werden zuerst im zeitkontinuierlichen und anschließend im zeitdiskreten Zeitbereich betrachtet. Das notwendige mathematische Handwerkszeug wie Grundbegriffe der Wahrscheinlichkeits- und Funktionentheorie, eine Einführung in die Theorie linearer Räume und Operatoren sowie die Fourier-, Laplace- und z-Transformation wird dem Leser in leicht verständlicher Form bereitgestellt.
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David Grandblaise, Klaus Moessner, Jijun Luo, Clemens Kloeck, George Dimitrakopoulos, Ramon Agusti, Jorge Luis Rodriguez:
Radio Resource Management for Reconfigurable Systems - An Overview of EER Research Project Outcomes
Proceedings of EVEREST Workshop, Barcelona, Spain, 16. November 2005

Abstract:
The heterogeneity of the radio access networks environment provides considerable obstacles when aiming for best possible usage of radio resources. The different mechanisms and radio resource management approaches of broadcast, cellular or WLAN networks have been initially optimized for their individual systems, but more efficient performance can be expected when looking at the overall radio resource and spectrum usage. This paper describes the E2R projects approach towards a hybrid radio and spectrum resources management scheme capable to support the dynamic allocation of radio resources in a composite radio environment where operators can collaborate. The paper provides an overview of the project research outcomes for different RRM research topics encompassing Dynamic Network Planning and Management (DNPM), Advanced Spectrum Management (ASM) and Joint Radio Resource Management (JRRM) areas.
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Clemens Kloeck, Ihan Martoyo, David Grandblaise, Jijun Luo, Klaus Moessner, Oriol Sallent,George Dimitrakopoulos, Panagiotis Demestichas:
Functional Architecture of Reconfiugurable Systems
Proccedings of WWRF14, San Diego, 7.-8. July 2005

Abstract:
Cognitive networks are envisaged to play a significant part in the future, where the time and space variations in the traffic pattern will necessitate the ability to continuously amend the Radio Access Technologies' (RATs') operating parameters. Reconfiguration of communications systems is a facilitator towards this convergence. However, such concept involves many different working areas, each of which represents an answer to a different problem, such as Dynamic Network Planning and Management (DNPM), Advanced Spectrum Management (ASM) and Joint Radio Resource Management (JRRM). Consequently, there is need of putting things all-together in the form of a Functional Architecture (FA), where each module represents a concept, aiming at forming part of the global end-to-end reconfigurability architecture. This paper includes exactly a detailed analysis of the Reconfigurability FA, along with a description of the functionality of each of the modules included therein.
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Clemens Kloeck, Holger Jäkel, Friedrich K. Jondral: 
Dynamic and Local Combined Pricing, Allocation and Billing System with Cognitive Radios
Proceedings of the IEEE 1st International Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN '05), 8.-11. November 2005, Baltimore (MD), USA,p. 73-81

Abstract:
In established communication systems the price is determined quasi-static and for a large area according to a fixed price model. Additionally, the main decision criteria of resource allocation is the arrival time of resource requests. This system approach is not able to respond to the spatial distributed and dynamic users' demand and willingness-to-pay. In this paper a system will be proposed to combine pricing, allocation and billing in order to react dynamically and locally to the market assuming that the user terminal has cognitive radio abilities and multiple interfaces.
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David Grandblaise, Clemens Kloeck, Klaus Moessner, Virgilio Rodriguez, Eiman Mohyeldin, Maran Kumar, Pereirasamy, Jijun Luo, Ihan Martoyo: 
Techno-Economic of Collaboratove based Secondary Usage - E2R Research Project Outcomes Overview
Proceedings of the IEEE 1st International Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN '05), 8.-11. November 2005, Baltimore (MD), USA,p. 318-327

Abstract:
The under-use of spectrum offers new perspective for a secondary usage of the spectrum. However, this can be achieved at the expense of a tight spectrum sharing control and management between the different involved parties (operators and users). This paper presents the latest research achievements of a major European research initiative (E2R Project) in the field of flexible spectrum management. Both the economic and technical aspects are addressed in the specific case of collaborative based secondary spectrum usage. The paper approaches the spectrum sharing problem with a distributed control and management solution perspective, paving the way towards first insights of a cognitive radio based solution. Firstly, two approaches for a market based spectrum sharing, including the bidding and pricing incentives, are presented. Secondly, the achievable capacity gains and the underlying spectrum sharing coordination processes to manage the co-channel interference are presented. For each of the economic and technical aspects, concepts, related results and analysis are included. Finally, an outlook on the objectives of the phase 2 (2006-2007) of the project in the field of flexible spectrum management including the spectrum measurement campaign is presented.
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Jijun Luo, Clemens Kloeck, Klaus Moessner,David Grandblaise, George Dimitrakopoulos, Panagiotis Demestichas, Eiman Mohyeldin, Oriol Sallent, Pascal Codier:
Inter-Operator Flexible Spectrum Management vs. Roaming
Proceedings of the 2005 Software Defined Radio Forum Technical Conference (SDR '05), 14.-17. November 2005, Orange County (CA), USA, CDROM

Abstract:
This paper illustrates the current vision on Flexible Spectrum Management (FSM), dimensions the technical mechanism involved in FSM from time, space, function, entity and ownership and in addition gives comprehensive comparisons with the state of the art in inter-operator spectrum resource management presented by the roaming technology. Comparisons show the attractive potentials given by FSM compared to the roaming technology and motivate the research activities towards a set a high efficient FSM solutions.
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Klaus Moessner, George Dimitrakopoulos, Panagiotis Demestichas, Jijun Luo, David Grandblaise, Oriol Sallent, Clemens Kloeck:
Dynamic Radio Resource Allocation Strategies and Time Scales
Proceedings of the 2005 Software Defined Radio Forum Technical Conference (SDR '05), 14.-17. November 2005, Orange County (CA), USA, CDROM

Abstract:
The heterogeneity of the radio environment provides considerable obstacles when aiming for best possible usage of radio resources. The different mechanisms and radio resource management approaches of broadcast, cellular or even ad hoc networks have been optimized for their individual systems, but do not provide a sufficiently efficient performance when looking at the overall radio resource and spectrum usage. This paper describes the E2R projects approach towards a hybrid radio and spectrum resources management scheme capable to support the dynamic allocation of radio resources in a composite radio environment. The paper describes the individual approaches, matches them to their applicability-time-scale and introduces the overall hybrid scheme and its mechanisms.
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Ulrich Berthold, Friedrich K. Jondral:
Generalized System Model of an Overlay Environment
Proceedings of the 2005 Software Defined Radio Forum Technical Conference (SDR '05), 14.-17. November 2005, Orange County (CA), USA, CDROM

Abstract:
Due to the natural limitation of the resource spectrum and an increasing need for wireless communications, it is indispensable to increase the efficiency of the spectrums utilization. One approach to achieve this goal is the concept of spectrum pooling which allows the coexistence of two independent systems in the same frequency band, also called spectrum pool. The overlay system may only use the time-frequency gaps that are not used by the licensed system. Since the licensed systems allocation is time variant, situations can occur where there are not enough resources left for the overlay system to operate efficiently. In this paper we assume that there are several independent spectrum pools with different allocation characteristics. The overlay system now has to observe the spectrum pools and choose the most suitable pool for its transmission.
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Ulrich Berthold, Friedrich K. Jondral: 
Guidelines for Designing OFDM Overlay Systems
Proceedings of the IEEE 1st International Symposium on New Frontiers in Dynamic Spectrum Access Networks (DySPAN '05), 8.-11. November 2005, Baltimore (MD), USA, pp. 626-629

Abstract:
Due to the increasing demand in mobile communications it is necessary to use the available spectrum as efficiently as possible. In this paper we focus on the concept of overlay scenarios where two different systems operate independently in the same frequency band. We describe a general set of characteristic parameters for licensed systems using FDMA (frequency division multiple access). In the context of these licensed systems especially OFDM (orthogonal frequency-division multiplexing) is a suitable technique to operate an additional system in a coexistence mode. Based on the parameter sets of both systems we derive constraints and guidelines on the design of OFDM overlay systems.
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Ulrich Berthold, Friedrich K. Jondral, Sinja Brandes, Michael Schnell:
Performance of OFDM-CDMA Overlay Systems considering Inaccurate Allocation Vectors
Proceedings of the 10^th International OFDM-Workshop, Hamburg, August 31/September 1, 2005, pp. 230 - 234

Abstract:
In this paper, we investigate an OFDMCDMA overlay system. It has to continuously monitor the licensed system's frequency allocations and broadcast the used subchannels to all overlay terminals. In order to reduce the signaling overhead we study the effects on system performance when the receiver is using an inaccurate allocation vector. From these results we derive criteria by which the transmitter can decide whether or not it is necessary to broadcast an update of the allocation vector.
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Friedrich K. Jondral:
Software-Defined Radio - Basics and Evolution to Cognitive Radio
Invited paper, EURASIP Journal on Wireless Communications and Networking, 2005, No. 3, pp. 275-283

Abstract:
We provide a brief overview over the development of software-defined or reconfigurable radio systems. The need for softwaredefined radios is underlined and the most important notions used for such reconfigurable transceivers are thoroughly defined. The role of standards in radio development is emphasized and the usage of transmission mode parameters in the construction of software-defined radios is described. The software communications architecture is introduced as an example for a framework that allows an object-oriented development of software-defined radios. Cognitive radios are introduced as the next step in radio systems' evolution. The need for cognitive radios is exemplified by a comparison of present and advanced spectrum management strategies.
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Arnd-Ragnar Rhiemeier:
Modular Software-Defined Radio
EURASIP Journal on Wireless Communications and Networking, 2005, No. 3, pp. 333- 342

Abstract:
In view of the technical and commercial boundary conditions for software-defined radio (SDR), it is suggestive to reconsider the concept anew from an unconventional point of view. The organizational principles of signal processing (rather than the signal processing algorithms themselves) are the main focus of this work on modular software-defined radio. Modularity and flexibility are just two key characteristics of the SDR environment which extend smoothly into the modeling of hardware and software. In particular, the proposed model of signal processing software includes irregular, connected, directed, acyclic graphs with random node weights and random edges. Several approaches for mapping such software to a given hardware are discussed. Taking into account previous findings as well as new results from system simulations presented here, the paper finally concludes with the utility of pipelining as a general design guideline for modular software-defined radio.
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Michael Schmidt, Djordje Simic, Rainer Moorfeld:
Low Complexity Low Data Rate UWB Devices - Architecture and Performance Comparison
Proceedings of 14th IST Mobile & Wireless Communications Summit 2005, 19-23 June, Dresden, Germany, CD-ROM

Abstract:
In this work the performance of low data rate Ultra-Wideband (UWB) devices is studied. The most important design constraint is low complexity and low power consumption. Four different architectures of different complexity are built in a modular approach from functional blocks. The trade-offs of these architectures between performance and complexity are shown.
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David Grandblaise, Clemens Kloeck, Klaus Moessner, Eiman Mohyeldin, Maran Kumar Pereirasamy, Jijun Luo, Ihan Martoyo:
Towards a Cognitive Radio based Distributed Spectrum Management
Proceedings of 14th IST Mobile & Wireless Communications Summit 2005, 19-23 June, Dresden, Germany, CD-ROM

Abstract:
The under-use of spectrum offers new perspective for a secondary usage of the spectrum. The flexibility provided by reconfigurable transceivers (terminals and base stations) allows exploiting this additional spectrum to enhance capacity and QoS of the networks. However, this can be achieved at the expense of a tight spectrum sharing control and management between the different involved parties (operators and users). Firstly, this paper looks at two approaches for a brokerage based spectrum sharing including the bidding and pricing incentives. Secondly, the paper looks at the achievable capacity gains and the underlying spectrum sharing coordination processes to manage the interference. For both of the economic and technical aspects, concepts and related simulations results are presented.
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Clemens Kloeck, Holger Jäkel, Friedrich K. Jondral:
Auctions Sequence as a New Spectrum Allocation Mechanism
Proceedings of 14th IST Mobile & Wireless Communications Summit 2005, 19-23 June, Dresden, Germany, CD-ROM

Abstract:
Based on the cognitive radio approach, auctions sequence as a potential spectrum allocation mechanism is introduced. The auctions will be periodically repeated in short-term within seconds or even milliseconds. Because of this hard time constraint, agents located in the MAC-layer have to arrange the auction automatically. A special class of auctions, the multi-unit sealed-bid auctions, saves signaling effort in comparison to the multi-unit open auction and the multi-unit sequential auction and thus is suitable for communication systems. This proposed auction sequence possesses two important advantages: First, an auction instantaneously reacts on the market and customers' demand. Second, as shown in a simulation, the operator's gain is almost always higher than for the billing mechanism in established communication systems. A simulative comparison between a Vickrey auction and a uniform-price auction shows both the economical and technical behavior of this highly dynamical stochastic auction process for an OFDMA/TDD system like IEEE 802.16. Furthermore, an optimal bidding strategy for both auctions is analytically deduced maximizing spectrum allocation. implementation.
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Piotr Rykaczewski, Friedrich K. Jondral:
Open Source SCA as a Reconfigurable Platform for WLAN and 3G
Proceedings of the 15th Annual MPRG Symposium on Wireless Personal Communications, June 8.-10., 2005, CD-ROM

Abstract:
An implementation strategy of the IEEE 802.11a standard within the framework of the Software Communications Architecture (SCA) realization OSSIE provided by Virginia Tech's Mobile and Portable Radio Research Group is presented in this paper. Integration of 3G communication standards and reconfiguration aspects are discussed. Theoretical outcomes are backed by results of practical implementation.
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Jörg Hillenbrand, Timo A. Weiss, Friedrich K. Jondral: 
Calculation of Detection and False Alarm Probabilities in Spectrum Pooling Systems
IEEE Communications Letters, April 2005, Vol. 9, No. 4, pp. 349 - 351

Abstract:
The innovative new strategy of spectrum pooling enables public access to spectral ranges of already licensed yet rarely used frequency bands by overlaying a secondary mobile radio system (the rental system, RS) to an existing one (the licensed system, LS). Coexistence of both systems is realized by filling the idle time-frequency gaps of the LS. A key issue in spectrum pooling is the reliable and efficient detection of those spectral ranges that are currently accessed by the LS as those ranges have to be spared from the RS's transmission power. In this letter, formulas for the calculation of the detection and false alarm probability are derived for the general case of an arbitrary measurement covariance matrix, allowing for a maximum exploitation of the proposed distributed detection approach.
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Friedrich K. Jondral:
Flexible Spectrum Allocation and Cognitive Radio
Proceedings of the XI National Symposium of Radio Science (URSI 2005), Poznan (Poland), April 7-8, 2005, pp. 27-32

Abstract:
Mobile radio communications continues to be one of the fastest growing markets worldwide. This corresponds to an increasing demand for radio transmission resources and for the application of advanced spectrum management strategies. In order to not affect existing systems more than necessary, the latest digital transmission technologies need to be employed. This request leads to the conclusion that the development of intelligent networks and cognitive radios becomes necessary. The present letter is intended to pave the way for the first implementation of a cognitive radio.
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Piotr Rykaczewski, Dariusz Pienkowski, Radu Circa, Bernd Steinke: 
Signal Path Optimization in Software Defined Radio System
IEEE Transactions on Microwave Theory and Techniques, vol. 53, no. 3, March 2005, Special Issue on Multifunction Systems, pp. 1056 - 1064

Abstract:
Growing requirements set upon communication transceivers lead to high sensitivity to nonidealities of analog components, especially in case of software-defined radio (SDR) systems. This paper deals with disturbances, mismatches, and the rejection of images that are caused by the front-end processing. A hybrid architecture comprehending advantages of homodyne and heterodyne receivers is proposed for an SDR system. Theoretical outcomes are discussed and backed by simulations. A fast Fourier transform (FFT)-based algorithm, the phase increment Kay algorithm, and two autocorrelation algorithms, i.e., Morelli Mengali and Crozier Moreland, used for frequency offset correction, are compared. The FFT algorithm turns out to be the best solution, both in terms of performance and ease of implementation. DC correction is then discussed and in-phase/quadrature imbalance compensation by means of a blind-source separation (BSS) algorithm and a hard-decision algorithm is performed. The latter algorithm displays implementation advantages, while BSS performs better.
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Ihan Martoyo:
Transforming the Secondary Education in Indonesia
Proceedings of the International Conference on Toward Indonesia 2020, 5/6 März 2005, Hamburg

Abstract:
At age 16, Albert Einstein imagined what it would be like to ride on a beam of light. "Could I travel faster than light?" Einstein asked himself. At age 12 the young Thomas Alva Edison stymied his parents, as he asked them to explain those things written in Isaac Newton's Principia. No doubt that the teenage years can shape an individual's life substantially. However, the secondary education system in Indonesia (Sekolah Menengah Pertama and Sekolah Menengah Atas), which is supposed to play a crucial role in forming the next generation, is often trapped by a rigid and narrow curriculum. It has become a place where knowledge is treated only as data to be transferred. In this paper, a road map towards a better secondary education system will be proposed. The dream is to develop a system, which accommodates the generation and exploration of knowledge, rather than a mere knowledge transmission. The strategy consists of reforming the education process, so that it focuses on teaching how to think and learning to learn, and not memorizing data. Of course, this requires some freedom for schools to innovate and specialize in certain fields. It is only possible, if the governmental education offices position themselves not as controllers and monitors, but as resource providers focusing on support for academic improvement. Only if the transformation is anchored at the school level, can it have an overreaching impact and a greater chance to succeed.
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Thorben Detert (TU Darmstadt), Gunther Sessler:
A fast converging PE detector using Lanczos method
2004 Workshop on Signal Processing Advances in Wireless Communications, July, 11-14, 2004, pp. 199 - 203

Abstract:
The polynomial expansion (PE) multiuser detector (MUD) is a well known iterative approach to approximate the decorrelating or the minimum meansquared error (MMSE) detectors. The computational complexity for the direct matrix inversion involved with the decorrelating and the MMSE detector is 0(K3), where K is the number of users.Due to the iterative approximation, the computational complexity of the PE detector can be 0(K²) instead. The text at hand presents a new method for a PE detector based on the estimation of the eigenvalues of the channel correlation matrix by the implicitly restarted Lanczos method (IRLM). The tight estimate of the eigenvalues results in faster convergence compared to the MMSE detector. Also, a better near-far resistence is achieved. As an application, the PE detectors presented here are tested in a UMTS Terrestrial Radio Access (UTRA) frequency division duplex (FDD) uplink multiuser environment, where the multiple users are assumed to be asynchronous.
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K. Tsagkaris, P. Demestichas, G. Dimitrakopoulos, A. Saatsakis, G. Vivier, D. Grandblaise, J. Luo, C. Kloeck:
Radio Access Technology Selection in a B3G, Reconfigurable Radio, Context
Proceedings of the Wireless World Research Forum, 12nd Meeting, WG6, Toronto Canada, 03-05.11.04

Abstract:
Wireless communications migrate towards the era of ÈBeyond the 3rd GenerationÉ (B3G) communications. A fundamental concept for B3G is reconfigurability, which is needed for better handling the offered demand and improved resource utilization. Many new requirements are posed with respect to the efficient management of the whole reconfiguration process. To this effect, the paper starts from the business case that justifies the need for reconfigurability and then continues with the management functionality for reconfiguration decisions, which are targeted to the selection of RATs and the corresponding configuration of the required resources. Particularly, the paper focuses on the RAT selection problem in B3G reconfigurable radio context. The problem is described and mathematically formulated. Finally, a heuristic based solution method is proposed and results are presented.
[ Abstract ]

David Grandblaise, Klaus Moessner, Clemens Kloeck, Ramon Agusti, Oriol Sallent:
Spectrum Scenarios in Composite Radio Environments
Proceedings of the Wireless World Research Forum, 12nd Meeting, WG6, Toronto Canada, 03-05.11.04

Abstract:
Although providing a number of very powerful concepts, it has been a challenge to find sufficient economical reason for the research and development of SDR technology. Applying the SDR concepts in the end-to-end reconfigurability context, and in particular with view on the possibilities reconfigurability offers to more efficient use of radio spectrum, the technology offers the means to facilitate concepts like flexible spectrum management (FSM) as well as joint radio resource management (JRRM). The paper outlines and describes four basic application scenarios that are being investigated in the E2R project to evaluate the efficiency gain of both JRRM as well as FSM.
[ Abstract ]

Arnd-Ragnar Rhiemeier, Timo Weiss, Friedrich K. Jondral:
A Simple and Efficient Solution to Half-frame Pipelining for Modular Software Defined Radio
Proceedings of the 2004 Software Defined Radio Technical Conference (SDR '04). November 16-18, Scottsdale (AZ), Volume A, pp. A119-A125

Abstract:
The understanding of fundamental approaches to designing and operating Modular Software Defined Radio (Mod-SDR) are of immediate relevance for offering flexible services to mobile users, by means of a single communication device. We review Half-Frame Pipelining (HFP) as a way of operating any software defined PHY layer on a specific multiprocessor hardware architecture. In this paper HFP is improved against former results with respect to the runtime efficiency of its partitioning concept and the achievable speedup. A comparison to another approach, Graph Duplication Pipelining (GDP), reveals HFP's advantages and disadvantages. PHY layer signal processing for both circuit-switched and packet-switched services is discussed in detail. Finally, we conclude on the utility of HFP for application in Mod-SDR terminals.
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Fatih Capar, Friedrich K. Jondral:
Spectrum Pricing for Excess Bandwidth: Competing Radio Networks
VDE-Kongress 2004, Ambient Intelligence, 18.-20. Oktober 2004, Berlin

Abstract:
In diesem Beitrag stellen wir ein Konzept zur Vermarktung des Spektrums in Sekundärmärkten vor. Wir betrachten den Fall, dass ein Netzbetreiber seine überschüssige Bandbreite, durch die Vergabe von Kurzzeitlizenzen, vermietet. Wir unterscheiden zwischen einem Stamm- (SN) und Mietnutzer (MN) System. Das SN-System besitzt die primäre Lizenz, wogegen MN nur über Kurzzeitlizenzen das Recht erwerben können, das Spektrum des SNs zu nutzen. Wir stellen ein Verkehrs- und ein Marktmodell vor, um die Vermarktung der Kurzzeitlizenzen zu modellieren. Die Vermarktung werden wir am Beispiel der Konkurrenz zwischen zwei SN-Systemen um die gleichen MN-Kunden untersuchen. Für die Analyse der strategischen Preisgestaltung der beiden Netzbetreiber bedienen wir uns der Spieltheorie.
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Piotr Rykaczewski, Jörg Brakensiek, Friedrich K. Jondral: 
Decision Directed Methods of I/Q Imbalance Compensation in OFDM Systems
Proceedings of the IEEE 60th Vehicular Technology Conference (VTC F'04), 26.-29. September 2004, Los Angeles (CA), USA

Abstract:
Nonlinearities of analog hardware result in phase and amplitude imbalance in case of zero-IF receivers, which imply direct down-conversion of the RF signal. The compensation of undesirable effects must be performed in the digital baseband. Particularly OFDM based systems are likely to suffer from imbalances due to distortions introduced by the time-to-frequency transformation of the filtered signal in case of any I/Q imbalance. This paper introduces two decision directed compensation methods: a Hard Decision based and a Blind Source Separation based, operating in the time and frequency domain, respectively. Time and frequency domain compensation methods are compared, and corresponding analytical models analyzed. Special emphasis is placed on applicability of the compensation methods to OFDM communication systems. The theoretical study is accompanied by a presentation of simulation results involving an IEEE 802.11a system.
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Piotr Rykaczewski, Ihan Martoyo, Zhenhua Liu, Friedrich K. Jondral: 
Multimode Detector and I/Q Imbalance Compensator in a Software Defined Radio
Proceedings of the IEEE 2004 Radio and Wireless Conference (RAWCON 2004), 19.-22. September 2004, Atlanta (GA), USA

Abstract:
Selected components of a software defined radio (SDR) supporting UMTS (UTRA FDD) and WLAN (IEEE 802.11a) are presented in this paper. The novelty of the approach discussed here is the idea of unifying an I/Q imbalance compensation module and a multimode detector based on the frequency domain equalization principle. Both functionalities are designed and implemented based on the FFT operation and make the RAKE receiver obsolete. Two I/Q imbalance compensation methods allowing the re-use of hardware components, hard decision based (HD) and preamble based (Pre) algorithms, are analyzed and a comparison of their performances and resources demands is drawn. Theoretical results of the study are valid for a whole range of SDR systems, not only for the one investigated in this paper.
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Mengüc Öner, Friedrich K. Jondral 
Cyclostationarity-Based Air Interface Recognition For Software Radio Systems
IEEE Radio and Wireless Conference, RAWCON 2004, Atlanta, USA 19-22 Sept. 2004.

Abstract:
Reconfigurable Software Radio equipment is seen as the next evolutionary step in the mobile communications. One of the most important properties of a Software Radio terminal is that it is capable of using a wide range of air interface standards, providing a seamless interoperability between different air interface standards and an enhanced roaming capability. This multimode operation has to be supported by a number of key functionalities, one of which is the air interface recognition. A Software Radio terminal has to be able to detect, recognize and monitor the air interfaces available in the frequency environment. In our work, we propose exploiting the distinct cyclostationary properties of signals from different air interfaces as features for air interface recognition
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Mengüc Öner, Friedrich K. Jondral 
Cyclostationarity-Based Methods for the Extraction of the Channel Allocation Information in a Spectrum Pooling System
IEEE Radio and Wireless Conference, RAWCON 2004, Atlanta, USA 19-22 Sept. 2004.

Abstract:
Spectrum pooling can be considered as a first step towards a fully dynamic spectrum allocation strategy. It allows a license owner to share a sporadically used part of his licensed spectrum with a renter system, until he needs it himself. For a smooth operation of a spectrum pooling scheme, the renter system has to monitor the channel and extract the channel allocation information (CAI), i.e. it has to detect, which parts of the shared spectrum the owner system accesses to, in order to immediately vacate the frequency bands being required by the license owner and to gain access to the frequency bands, which the license owner has stopped using. This paper presents and compares two methods based on exploiting the cyclostationary properties of the spectrum owner signal for the extraction of the CAI in a specific spectrum pooling scenario, where the license owner is a GSM network and the spectrum renter is an OFDM based WLAN system.
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Timo Weiss, Stephan Maas, Friedrich K. Jondral:
Bandwidth-based Handoff Algorithms Spectrum Pooling Systems
Proceedings of the IEEE International Symposium on Wireless Communication Systems (ISWCS) 2004, 20-22 September, Port Louis, Mauritius

Abstract:
Public cellular networks have experienced exponential increase in service demand in recent years. Hence, public mobile radio spectrum has become a scarce resource while other spectral ranges are only rarely used. Here, a new strategy called spectrum pooling is considered. It aims at enabling public access to rarely used spectral ranges without sacrificing the transmission quality of the actual license owners. In this paper, we investigate the case of a handoff between two spectrum pools, which becomes necessary if the available bandwidth in one spectrum pool becomes insufficient due to increased traffic of the license owners. The interpool handoff decision is based on algorithms that are adopted and extended from the conventional intercell handoff case. We derive optimal smoothing filter types and lengths for the filtering of short-term components as well as optimal hysteresis values and drop timers. A new method using variable hysteresis is presented and shown to perform better than conventional hysteresis techniques.
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Clemens Kloeck, Wilhelm Fries, Friedrich K. Jondral:
Kombinierte Infrastruktur WLAN/Mobilfunk ermöglicht ortsunabhäniges mobiles Lernen
Proceedings der DeLFI 2004: Die 2. e-Learning Fachtagung Informatik, 6.-8. September, Paderborn, Deutschland

Abstract:
Im Rahmen des BMBF-Projekts Notebook Universität Karlsruhe (TH) (NUKATH) soll den Studierenden über beide Kommunikationssysteme, GPRS und WLAN IEEE 802.11b, der Internetzugang und die Datenübertragung untereinander ermöglicht werden. Dazu werden in diesem Beitrag die Verknüpfung beider Systeme und die Systemaspekte für die Universität Karlsruhe dargestellt. Das vorteilhafte Konzept, die lose Kopplung, wurde auf dem Campus implementiert und dessen Eignung durch Messungen untermauert.
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Mengüc Öner, Friedrich K. Jondral
Extracting the Channel Allocation Information in a Spectrum Pooling System Using Cyclic Feature Detection
12th European Signal Processing Conference, EUSIPCO 2004, Vienna, Austria 6-10 Sept 2004

Abstract:
Spectrum pooling is a resource sharing strategy, which allows a license owner to share a sporadically used part of his licensed spectrum with a renter system, until he needs it himself. For a frictionless operation of a spectrum pooling system, the license owner has to have the absolute priority to access the shared spectrum. This means, the renter system has to monitor the channel and extract the channel allocation information (CAI), i.e. it has to detect, which parts of the shared spectrum the owner system accesses to, in order to immediately vacate the frequency bands being required by the license owner and to gain access to the frequency bands, which the license owner has stopped using. This paper proposes using cyclic feature detection for the extraction of the CAI in a specific spectrum pooling scenario, where the license owner is a GSM network and the spectrum renter is an OFDM based WLAN system.

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Mengüc Öner, Friedrich K. Jondral
Extracting the Channel Allocation Information in a Spectrum Pooling System Exploiting Cyclostationarity
15th IEEE Symposium on Personal Indoor and Mobile Radio Communications PIMRC 2004, Barcelona, Spain 5-8 Sept 2004.

Abstract:
Spectrum pooling concept allows the license owner of a spectral band to share a sporadically used part of his licensed spectrum with a renter system, until he needs it himself. For a smooth operation of a spectrum pooling system, the license owner has to have the absolute priority to access the shared spectrum. This means, the renter system has to monitor the channel and extract the channel allocation information (CAI), i.e. it has to detect, which parts of the shared spectrum the owner system accesses to, in order to immediately vacate the frequency bands being required by the license owner and to gain access to the frequency bands, which the license owner has stopped using. This paper proposes exploiting the cyclostationary properties of the spectrum owner signal for the extraction of the CAI in a specific spectrum pooling scenario, where the license owner is a GSM network and the spectrum renter is an OFDM based WLAN system.

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Mengüc Öner, Friedrich K. Jondral 
Air Interface Recognition For a Software Radio System Exploiting Cyclostationarity
15th IEEE Symposium on Personal Indoor and Mobile Radio Communications PIMRC 2004, Barcelona, Spain 5-8 Sept 2004.

Abstract:
Reconfigurable Software Radio equipment is seen as the key technology in the evolution of mobile communications. One of the most important properties of a Software Radio terminal is that it is capable of using a wide range of air interface standards, providing a seamless interoperability between different standards and an enhanced roaming capability. One of the key functions supporting this multimode operation is the air interface recognition. A Software Radio terminal has to be capable of detecting, recognizing and monitoring the air interfaces available in the frequency environment. In our paper, we propose exploiting the distinct cyclostationary properties of different air interface signals as features for air interface recognition.

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Timo Weiss, Marcus Spiering, Friedrich K. Jondral:
Quality of Service in Spectrum Pooling Systems
Proceedings of the 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC) 2004, 5-8 September, Barcelona, Spain

Abstract:
Public cellular networks have experienced exponential increase in service demand in recent years. Hence, public mobile radio spectrum has become a scarce resource while other spectral ranges are only rarely used. Here, a new strategy called spectrum pooling is considered. It aims at enabling public access to rarely used spectral ranges without sacrificing the transmission quality of the actual license owners. In this paper, the quality of service boundaries in such a system are investigated as demanding wireless multimedia applications produce more and more interest among wireless users. Simulations have been conducted in a spectrum pooling specifically modified HIPERLAN/2 testbed revealing achievable service guarantees with respect to throughput and delay under different wireless weighted fair scheduling policies. Simulation results show that service guarantees can be met across a wide range of activity (arrival rate) of the accessing license owners. However, if certain activity thresholds provided in this paper are exceeded, service guarantees - especially delay guarantees - have to be renegotiated. Furthermore, performance evaluations concerning the different scheduling policies in a spectrum pooling system are given.
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Fatih Capar, Friedrich K. Jondral: 
Spectrum Pricing for Excess Bandwidth in Radio Networks
Proceedings of the 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC) 2004, 5-8 September, Barcelona, Spain

Abstract:
Radio networks are designed to guarantee Quality of Service (QoS). This QoS guarantee is offered for customers with time varying load characteristics. Since the peak to average load of a network is usually very high the network utilizes the maximum capacity only for a fraction of time. Radio spectrum is a scarce resource and radio network providers pay huge amounts of money for licenses. For this reason applications with high QoS requirements are usually very expensive. For a better subscriber satisfaction the price for QoS should be as low as possible. From an economic perspective the excess band-width could be disposed of in a secondary market. This would mean additional profit and by lowering the price for primary users the user satisfaction will increase. Surely there will be an optimum pricing strategy to maximize the profit. As a matter of fact, the QoS which can be offered in the secondary market is lower than the QoS which is offered in the primary market. In this paper we will model a network which offers high QoS ser-vices to primary users and low QoS services for secondary users. The primary user applications are not queued whereas the secondary user applications are queued. Our focus is on a spectrum band at a dedicated frequency which is shared using a TDMA access scheme. We will model the traffic as Poisson arrival processes. Since the achievable QoS depends on the queue state of the network a state dependent pricing scheme for services in the secondary market will be applied. An acceptance function will characterize the behavior of the users in the secondary market.
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Arnd-Ragnar Rhiemeier, Timo Weiss, Friedrich K. Jondral: 
Half-Frame Pipelining for Modular Software Defined Radio
Proceedings of the 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC) 2004, 5-8 September, Barcelona, Spain, vol. 3, pp. 1664 - 1668

Abstract:
The understanding of design approaches to Modular Software Defined Radio (Mod-SDR) are of immediate relevance for offering flexible services to mobile users, by means of a single communication device. Both the potential of modular hardware/software for short time to market and the capability of mastering diverse data transmission modes under a unified multiprocessing framework are the motivation for research efforts in this field. In the present paper we continue to study Half-Frame Pipelining (HFP) as a method for scheduling software modules on a particular two-processor hardware. We compare the properties of HFP to those of Graph Duplication Pipelining (GDP) for both circuit-switched and packet-switched services. Based on computer simulations of both a generalized SDR environment and the example of an IEEE 802.11a WLAN communication system, we conclude on the utility of these pipelining methods for operating Mod-SDR terminals.
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Michael Schmidt, Ihan Martoyo, Holger Jäkel, Friedrich K. Jondral: 
Theoretical Boundaries for Combined Information Estimation and Non Data-Aided Channel Estimation in UWB
8th IEEE International Symposium on Spread Spectrum Techniques and Applications (ISSSTA) 2004, CD-ROM, 30. Aug. - 2. Sept. 2004, Sydney, Australia

Abstract:
One of the main advantages of Ultra Wide Band (UWB) systems is the extreme time resolution due to the huge bandwidth of several GHz. This allows the distinction between many different propagation paths in multipath environments and thus transmit with very low power. In order to benefit from the advantages of UWB, accurate estimations of the current channel realization are necessary. In this paper we derive theoretical boundaries for pulse transmission for the expected BER, the accuracy of the channel estimation and the interaction between these two problems. As an example, the popular Time-Hopping Pulse-Position-Modulation UWB (TH-PPM UWB) is used to show the trade-off between information and channel estimation.
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Ihan Martoyo, Michael Schmidt, Friedrich K. Jondral: 
FDE-RAKE: A PIC Multiuser Detector for CDMA Downlink
8th IEEE International Symposium on Spread Spectrum Techniques and Applications (ISSSTA) 2004, CD-ROM, 30. Aug. - 2. Sept. 2004, Sydney, Australia

Abstract:
This paper introduces a two stage multiuser detector for CDMA downlink systems. Frequency domain equalization (FDE) is used at the first stage as a singleuser detector. A RAKE receiver is then employed at the second stage, following a parallel interference cancellation process. The performance of the FDE-RAKE detector is evaluated in a UTRA-TDD system, and compared with the linear multiuser detector. The FDE-RAKE demonstrates a slightly better performance than the linear multiuser detector, while retaining the simplicity of the employed algorithm. It also offers the important advantage, that the detection quality of the first stage is significantly better than the detection of the RAKE receiver, which is usually used as the first processing stage in most CDMA multiuser detection strategies. Hence, the second stage of the FDE-RAKE detector can be optionally added if better detection is required.
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Friedrich K. Jondral, Timo A. Weiss:
On the Role of Software Defined Radios in Advanced Spectrum Management
Proceedings of the 2004 International Symposium on Signals, Systems, and Electronics (ISSSE 2004), CD-ROM, Linz (Austria), August 10-13, 2004

Abstract:
This paper starts with a discussion about the necessity for the introduction of advanced spectrum management strategies. Then an introduction to spectrum pooling is given and results of this new field are presented. Finally, the major role that software defined radios are going to play in future mobile communications systems is emphasized.
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Ihan Martoyo, Friedrich K. Jondral:
Multimode Receiver with Frequency Domain Equalizer
4th International Workshop on ITS Telecommunications (ITST) 2004, 8-9 July, Singapore

Abstract:
New mobile and wireless technologies offer new possibilities to get connected. Some technologies provide low cost high datarate connections just like the wireless LAN technology, others specialize in providing the mobile capability such as the 3G mobile communication systems. However, the future intelligent transport system may demand not just one but all of these special properties. This underlines the need for a transceiver structure that can operate across several mobile/wireless air interface technologies. This paper presents the application of the frequency domain equalization (FDE) for a receiver structure that can work with several air interface standards. The FDE has been shown to be a better receiver than the RAKE detector for CDMA downlink system in terms of multiple access interference (MAI) suppression. The FFT operation which is implied in the FDE calculation can be used at the same time to detect signal from multicarrier systems (e.g. OFDM or MC-CDMA). Hence, the same structure can be employed to receive signals from several air interface technologies. In this paper, a particular implementation of a multimode receiver for UTRA and WLAN standards is considered.
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Ihan Martoyo, Friedrich K. Jondral:
Spectrum Pooling: Physical Layer Issues
13th IST Mobile & Wireless Communications Summit 2004, 27-30 June, Lyon, France

Abstract:
This paper presents the concept of spectrum pooling, a new paradigm to use the wireless spectrum efficiently. The idea is to pool licensed spectra that are not used extensively and to offer them for rental use. The rental users can access these pooled spectra, but have to release these frequencies as soon as the spectrum owner needs their resources back. By doing this, the under-utilized licensed frequencies can be freed to be used by the rental users, hence increasing the efficiency of the overall spectrum usage. This paper will focus on the physical layer issues and will try to answer the question: What kind of air access technology should be used to enable spectrum pooling.
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Friedrich K. Jondral (Herausgeber):
FREQUENZ Sonderheft zum Software Defined Radio
Band 58, Heft 5-6, Mai/Juni 2004

Editorial:
The roots of Software Radios are found at the beginning of digital signal processing. With his seminal paper [1] Claude E. Shannon made the sampling theorem applicable to information technology. In the 1970s the first books on digital signal processing were published [2, 3]. During the 1980s the digital signal processor and analog-to-digital converter technologies arrived at a stage that allowed their application to high frequency devices. The first Digital Radios were developed for short wave applications. Finally, the application of digital signal processing to communications experienced its breakthrough around 1990 when the first cellular networks of second generation mobile phones were launched successfully.

Coming from the field of military communications the notion of Software Radio (SR) was coined in 1992 by Joseph Mitola [4]. A broader community of scientists and technicians became familiar with SRs in 1995 when a special issue of the IEEE Communications Magazine on this topic appeared. This has been the starting point for a multitude of publications on various aspects of Software Radios. In the meantime, important developments of Software Radio are summarized in textbooks, e.g. [5].

Taking into account that an ideal SR digitizes the received signal directly at the antenna – which is not feasible today due to technological restrictions – practical developments are concerned with Software Defined Radios (SDRs). Within SDRs sampling is done at a suitable intermediate frequency or, after direct conversion, in the complex baseband.

The most prominent sponsor of SDR is the SDR Forum, which is supported by many industrial and academic partners who are interested in the development of SDR. The SDR Forum's activities do not only refer to base stations or mobile transceivers but include overall system (e.g. software download), frequency allocation and regulatory aspects. Several Integrated Projects (IPs) within the 6^th Research Framework Program of the European Union are concerned with or based upon SDR. For example, the IP E²R investigates End-to-End Reconfigurability of future mobile communication systems which can obviously not be reached without the application of SDRs.

In 2000, the Institut für Nachrichtentechnik of the Universität Karlsruhe (TH) decided to organize a Workshop on Software Radios. Driven by the success of this initial event, a second Workshop was held in 2002. The present issue of the Frequenz contains nine out of 25 contributions selected from the 3^rd Karlsruhe Workshop on Software Radios [6] continuing the series of biennial events on March 17/18, 2004.

• In the first paper PAPR Analysis of a Multiplex of Modulated Carriers in a Software Radio Context Jacques Palicot and Sidkieta Zabre study the Peak to Average Power Ratio (PAPR) of signals containing different standards, like it is expected for SR. First, the authors modify the definition of PAPR with respect to the requirements of SR signals. Then it is shown that the PAPR properties of SR signals are similar to those of OFDM signals.
• In his contribution A Comparison of Scheduling Approaches in Modular Software Defined Radio Arnd-Ragnar Rhiemeier continues to explore the design principles of Modular (Mod) SDR. A novelty in the modeling of SDR software is the use of random graphs. Moreover, Half-Frame Pipelining is introduced as a method for scheduling software modules on a particular two-processor hardware. By comparison to Graph Duplication Pipelining the author concludes on the utility of these scheduling approaches for circuit-switched services.
• The subject of Network Functions Supporting Reconfiguration in B3G Environments by Markus Dillinger and his co-authors is a concept for mobile networks to provide reconfiguration functions in a very flexible way. The paper predicts that, in the same way as terminals have evolved into reconfigurable devices, mobile networks will evolve into a new type of infrastructure, ready to support multiple radio technologies, software download and terminal reconfiguration. The principal aspects of such a network concept are identified, and a reference model for its resource management plane is proposed.
• Stoytcho Gultchev, Klaus Moessner and Rahim Tafazolli provide a solution for the discovery of reconfiguration support services, entities in local but also global reconfiguration support domains, in their contribution Provisioning of Reconfiguration Services between Different Access Networks. The mechanism for establishing a secure context between home and visited reconfiguration support infrastructure is presented using the Reconfiguration Management Infrastructure as basis.
• In order to improve the efficiency in frequency utilization Klaus Moessner and his co-authors investigate the concept of flexible spectrum management in their paper Reconfiguration Techniques to Enhance Network Efficiency on the basis of Spectrum Pooling and Dynamic Network Planning. The work presented here is being carried out within the E²R Project. The paper reports on approaches and initial investigations, outlining fundamental principles as well as assumptions about the underlying systems.
• An automotive broadcast receiver for both FM audio and DVB-T video is described in the contribution A Software Defined Multistandard Tuner Platform for Automotive Applications by Thomas Müller, Maja Sliskovic et al. The three logical parts of the multi-standard receiver (analog front-end, analog-to-digital converter, digital signal processing) are explained in detail. Since the software radio structure is modular and all digital parts are controlled by software, the resulting platform is highly adaptive for performance optimization and accommodation of different standards.
• The paper Implementation Alternatives for a Flexible Wireless LAN Transceiver by Roel Schiphorst, Fokke Hoeksema and Kees Slump presents three implementation alternatives for an SDR testbed providing Bluetooth and HiperLAN/2. The implementation variants are evaluated with respect to computational power requirements, electric power consumption and manufacturing costs.
• A few selected hardware/software partitioning issues in a multi-antenna system are covered by Edmund Coersmeier, Ernst Zielinski and Klaus-Peter Wachsmann in their paper Software Pre-Equalization for a Multi-Antenna Transmitter. In multi-antenna systems involving low-cost analog front-ends, Inphase/Quadrature (I/Q) imperfections have to be removed in the baseband. This may be done by an I/Q sample estimation combined with a pre-equalizing filter algorithm.
• Finally, Jean-Philippe Delahaye and his co-authors report on Software Radio and Dynamic Reconfiguration on a DSP/FPGA Platform. A sample implementation of baseband signal processing for mobile communications is discussed. The basic design approach is Parameter Controlled (PaC) SDR. The method of dynamic FPGA reconfiguration as well as the design flow are of particular interest. Regarding the practical implementation it turns out that both on- and off-chip data transfers are serious design issues in SDR devices. This observation calls for further research into a combination of Mod-SDR and PaC-SDR design paradigms.

I want to thank all staff of the Institut für Nachrichtentechnik of the Universität Karlsruhe (TH) for their support in the preparation and the realization of our Workshop. My special thanks go to Piotr Rykaczewski and Michael Schmidt who also did a great job in assisting the editorial work of this special issue of the Frequenz.

[1] C.E. Shannon: "Communications in the Presence of Noise". Proc. IRE, Vol. 37, 1949, pp. 10-21
[2] H.W. Schüßler: Digitale Systeme zur Signalverarbeitung. Berlin – Heidelberg – New York, Springer-Verlag, 1973
[3] L.R. Rabiner, B. Gold: Theory and Application of Digital Signal Processing. Englewood Cliffs (New Jersey), Prentice-Hall Inc. 1975
[4] Joseph Mitola: "Software Radios". National Telesystems Conference, May 19-20, 1992, George Washington University, Virginia Campus Washington, D.C. (USA)
[5] Jeffrey H. Reed: Software Radio – A Modern Approach to Radio Engineering. Upper Saddle River (New Jersey), Prentice Hall PTR, 2002
[6] Proceedings of the 3^rd Karlsruhe Workshop on Software Radios. Universität Karlsruhe (TH), Institut für Nachrichtentechnik, 2004, ISSN 1616 - 6019
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Arnd-Ragnar Rhiemeier:
A Comparison of Scheduling Approaches in Modular Software Defined Radio
Frequenz, Zeitschrift für Telekommunikation, Band 58, Heft 5-6, Mai/Juni 2004, S. 115 - 120

Abstract:
The understanding of design approaches and operating conditions for Modular Software Defined Radio (Mod-SDR) are of immediate relevance for offering flexible services to mobile users, by means of a single communication device. Both the potential of modular hardware/software for short time to market and the capability of mastering diverse data transmission modes under a unified multiprocessing framework are the motivation for research efforts in this field. The paper in hand introduces Half-Frame Pipelining (HFP) as a method for scheduling software modules on a particular two-processor hardware. HFP requires partitioning of directed acyclic graphs capturing the logical structures of a Mod-SDR. A novel way of determining edge cost allows to solve the problem by spectral partitioning. We compare the properties of HFP with those of Graph Duplication Pipelining (GDP), which we have found to be an optimal scheduling method with respect to speedup and delay for circuit-switched services. Based on computer simulations we conclude on the utility of these scheduling methods under different operating conditions for Modular Software Defined Radio.

Reprint aus: Proceedings of the 3rd Karlsruhe Workshop on Software Radios, Karlsruhe (Germany), March 17/18, 2004
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Piotr Rykaczewski, Jörg Brakensiek, Friedrich K. Jondral: 
Towards an Analytical Model of I/Q Imbalance in OFDM Based Direct Conversion Receivers
IEEE Vehicular Technology Conference, 17.-19. Mai 2004. CD-ROM. VTC Spring 2004, Milan, Italy

Abstract:
Architectural simplicity, possibility of monolithic integration as well as the lack of image filter problems make homodyne receivers very popular. However, their I/Q architecture is sensitive to impairments. Particularly OFDM based systems are likely to suffer from imbalances due to distortions introduced by the time-to-frequency transformation of the filtered signal in case of any mismatch between I and Q branches. The focus of this paper is the clarification of how phase and amplitude imbalances influence the baseband signal processing of an OFDM based homodyne receiver. A novel analytical impairments model is presented, discussed and compared with simulations of an IEEE 802.11a system.
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Ihan Martoyo, GMA Sessler, Jaroslaw Luber, Friedrich K. Jondral: 
Comparing Equalizers and Multiuser Detectors for CDMA Downlink Systems
IEEE Vehicular Technology Conference, 17.-19. Mai 2004. CD-ROM. VTC Spring 2004, Milan, Italy

Abstract:
In CDMA systems, the multiuser detector has been recognized as a solution to the Multiple Access Interference (MAI) problem. The multiuser detector uses the information about multiple users jointly to better detect each individual user. On the other hand, a channel equalizer which simply equalizes the channel without any information from the other users can also effectively suppress the MAI, especially in CDMA downlink. The equalizer displays an excellent performance even in a heavily loaded CDMA system. This paper presents a performance comparison between equalizers and linear multiuser detectors in a CDMA downlink system. It is found, that the equalizers are almost as good as the linear multiuser detectors. In this paper, it is shown that the performance of the equalizer (ZF or MMSE) is approximately equal to the performance of the corresponding linear multiuser detector for any multipath channel in a fully loaded CDMA downlink system when orthogonal spreading codes are used.
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Fatih Capar, Friedrich K. Jondral: 
A Rayleigh Fading Channel Model for Multicarrier Systems: A Tapped Delay Line Model
IEEE Vehicular Technology Conference, 17.-19. Mai 2004. CD-ROM. VTC Spring 2004, Milan, Italy

Abstract:
There are different approaches in the literature to model the Rayleigh fading channel. We are focusing on a model with low complexity to capture the channel effects which are relevant to simulate a wireless network. We use an algorithm which exploits the Toeplitz form of the channel covariance matrices to identify the coefficients of a coefficient matrix to generate random variates which have the correlation properties defined by the correlation functions in time and frequency. The correlation in time is given by the Doppler spectrum and the correlation in frequency is given by the power delay profile of the wireless channel. We demonstrate the properties of the simulator and show quantitative quality measures of the random variates. Up to now such an approach has not been presented for modeling of fading channels. The advantage of such a model is, that the computational complexity may be reduced depending on the application by incorporating less taps.
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Ulrich Berthold, Arnd-Ragnar Rhiemeier, Friedrich K. Jondral: 
Spectral Partitioning for Modular Software Defined Radio
IEEE Vehicular Technology Conference, 17.-19. Mai 2004. CD-ROM. VTC Spring 2004, Milan, Italy, vol. 2, pp.1218 - 1222

Abstract:
Guidelines for designing and operating Modular Software Defined Radio (Mod-SDR) devices gain in importance as the interworking of different radio technologies emerges as a potential solution to achieving short time-to-market and to offering flexible services to mobile users. In this paper we continue to study the partitioning and scheduling of software modules in multiprocessor devices. In particular, we focus on Spectral Partitioning which is known to yield good partitions for general-purpose parallel computing applications. We discuss a spectral partitioning algorithm in the context of Mod-SDR, comparing it to implicit partitioning results produced by a variant of Hu's algorithm under realistic operating conditions. Test settings include two identical processors connected through either one or two data buses, and pipelining as a signal processing option for delay-insensitive transmission modes. Furthermore, we show how close both approaches come to the theoretical upper bound on two-processor speedup. Based on computer simulations we can conclude on an enhanced set of guidelines for operating a Modular Software Defined Radio.
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Timo Weiss, Joerg Hillenbrand, Albert Krohn, Friedrich K. Jondral: 
Mutual Interference in OFDM-based Spectrum Pooling Systems
IEEE Vehicular Technology Conference, 17.-19. Mai 2004. CD-ROM. VTC Spring 2004, Milan, Italy

Abstract:
Public mobile radio spectrum has become a scarce resource while wide spectral ranges are only rarely used. Here, a new strategy called \emph{spectrum pooling} is considered. It aims at enabling public access to these spectral ranges without sacrificing the transmission quality of the actual license owners. Unfortunately, using OFDM modulation in a spectrum pooling system has some drawbacks. There is an interaction between the licensed system and the OFDM based rental system due to the non-orthogonality of their respective transmit signals. In this paper, this interaction is described mathematically providing a quantitative evaluation of the mutual interference that leads to an SNR loss in both systems. However, this interference can be mitigated by windowing the OFDM signal in the time domain or by the adaptive deactivation of adjacent subcarriers providing flexible guard bands between licensed and rental system. It is obvious that both approaches sacrifice bandwidth of the rental system. A quantitative comparison of both approaches is given as a tradeoff between interference reduction and throughput in the rental system.
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Thorben Detert, Waldemar Haak - TU Darmstadt, Ihan Martoyo - Uni. Karlsruhe: 
Complete Complementary Codes Applied to UTRA FDD Asynchronous Uplink
IEEE 12th Mediterranean Electrotechnical Conference (MELECON) 2004, Dubrovnik, Croatia, May 12-15, 2004

Abstract:
In this paper, a performance comparison of orthogonal variable spreading factor (OVSF) codes and complete complementary (CC) channelization codes in Rayleigh fading channels is presented in the uplink. The bit error rate (BER) is simulated to provide a means of comparison for different spreading factors (SF) and different numbers of users. The system is totally asynchronous, and a special simulation strategy is applied to make the simulation of long scrambling codes possible. An introduction to the correlation functions utilized in the computation of the multiple access interference (MAI) seen by each user is given. Except the application of CC-codes, the simulation strictly sticks to the UTRA FDD standard.
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Ulrich Berthold, Arnd-Ragnar Rhiemeier, Friedrich K. Jondral: 
A Pipelining Approach to Operating Modular Software Defined Radio
IEEE Sarnoff Symposium, 26./27. April 2004, Princeton, NJ, USA, pp. 201 - 204

Abstract:
Interworking between different radio technologies emerges as a potential solution to offering flexible services to mobile users. Modular Software Defined Radio (Mod-SDR) devices are key to achieving short time-to-market under these conditions. Hence, guidelines for designing and operating such systems gain in importance. In this paper we continue to study the partitioning and scheduling of software modules on system-on-chip hardware for Mod-SDR. In particular, we focus on an efficient pipelining method, which we have named Graph Duplication Pipelining (GDP). We discuss the GDP approach in the context of Mod-SDR, taking a UTRA FDD 64kbps uplink transmitter as an example. We compare GDP to both non-pipelined and extremely pipelined signal processing, while observing realistic operating conditions. We show how close these approaches come to the theoretical upper bound on two-processor speedup. Furthermore, we examine the resulting depth of the radio frame pipeline which is important for delay-sensitive applications, where we discuss both circuit-switched and packet-switched services. Based on computer simulations we conclude on guidelines for operating Modular Software Defined Radio.
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Wojciech Kuropatwinski, Frank Lillie, Norbert Geng, Friedrich K. Jondral:
Low-Cost MIMO Real-Time Demonstrator: Concept and First Results
ITG Workshop on Smart Antennas, 18./19. März 2004, Technische Universität München

Abstract:
The majority of previous publications dealing with MIMO (multiple-input multiple-output) transmission concentrate on information theory, link-level simulations, signal processing, or channel sounding/modeling. Of course, there are increasing activities on MIMO demonstrators to show feasibility of MIMO transmission under real-world propagation conditions. However, most such work is performed for WCDMA or OFDM, both adding significant complexity to the system. In our work, we would like to gain experience on real-world problems of MIMO transmission (e.g., antenna coupling, effects of analog/digital filter accuracy, power consumption, timing, DC offset, channel estimation, TX/RX processing requirements) with a hardware demonstrator as simple as possible. In addition to a quick design and implementation, low overall size is a major goal. To achieve both, we use existing small and low-cost GSM/EDGE modules for the MIMO transmitter and receiver.
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Timo A. Weiss, Friedrich K. Jondral: 
Spectrum Pooling: An Innovative Strategy for the Enhancement of Spectrum Efficiency
IEEE Communications Magazine, March 2004, Radio Communications Supplement, pp. S8 - S14

Abstract:
This paper describes the technical challenges that have to be met when implementing the interesting new technology of spectrum pooling. This notion represents the coexistence of two mobile radio systems within the same frequency range. It enables the secondary utilization of already licensed frequency bands as aimed at by several regulatory authorities worldwide. The goal of spectrum pooling is to enhance the spectral efficiency by overlaying a new mobile radio system to an existing one without requiring any changes to the actual licensed system. Several demanding tasks originate from this idea. Some of them have been solved in recent research projects. Others are subject to ongoing investigations. Here, the state of the art in spectrum pooling is presented.
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Arnd-Ragnar Rhiemeier:
A Comparison of Scheduling Approaches in Modular Software Defined Radio
Proceedings of the 3rd Karlsruhe Workshop on Software Radios, Karlsruhe (Germany), March 17/18, 2004, ISSN 1616-6019, pp. 33 - 38

Abstract:
The understanding of design approaches and operating conditions for Modular Software Defined Radio (Mod-SDR) are of immediate relevance for offering flexible services to mobile users, by means of a single communication device. Both the potential of modular hardware/software for short time to market and the capability of mastering diverse data transmission modes under a unified multiprocessing framework are the motivation for research efforts in this field. The paper in hand introduces Half-Frame Pipelining (HFP) as a method for scheduling software modules on a particular two-processor hardware. HFP requires partitioning of directed acyclic graphs capturing the logical structures of a Mod-SDR. A novel way of determining edge cost allows to solve the problem by spectral partitioning. We compare the properties of HFP with those of Graph Duplication Pipelining (GDP), which we have found to be an optimal scheduling method with respect to speedup and delay for circuit-switched services. Based on computer simulations we conclude on the utility of these scheduling methods under different operating conditions for Modular Software Defined Radio.
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Arnd-Ragnar Rhiemeier, Friedrich K. Jondral:
Software Partitioning and Hardware Architecture for Modular SDR Systems
Proceedings of the 2003 Software Defined Radio Technical Conference (SDR '03), Orlando (FL), November 18-19, 2003, vol. 2, pp. 9-15

Abstract:
In this paper we investigate the relationship between logical and physical structures of a Modular Software Defined Radio (Mod-SDR) system, based on multi-processor hardware. The logical interdependence of software modules and functions in the signal processing chain is captured in a directed graph where both input/output data and inter- mediate results flow along directed arcs and where nodes represent software modules or entire communication functions. While the need for data exchange is obvious on a logical level, its mapping to physical resources is not. Computation and data communication require synchro- nization such that the SDR terminal can provide best performance to its user. Hence, our object of interest is partitioning and scheduling for software defined physical layers, which is a core component of any SDR operating system firmware. We briefly review a partitioning approach [1] which we have developed for application in the SDR domain. Subsequent scheduling is treated in greater detail. Taking the graph of an IMT-2000 W-CDMA 64kbps uplink as an example we explore hardware architecture options when using an application-specific co-processor, and we discuss the achievable system speedup. Finally, we arrive at a conclusion regarding subsystem pipelining and dynamic power dissipation in CMOS hardware implementations.
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Timo Weiss, Joerg Hillenbrand, Albert Krohn, Friedrich K. Jondral:
Efficient Signaling of Spectral Resources in Spectrum Pooling Systems
Proc. of the 10th Symposium on Communications and Vehicular Technology SCVT 2003, Eindhoven, Netherlands

Abstract:
Public mobile radio spectrum is a scarce resource while wide spectral ranges are only rarely used. Here, we consider a new strategy called Spectrum Pooling enabling public access to these new spectral ranges without sacrificing the transmission quality of the actual license owners. By temporarily leasing their spectrum during idle periods the license owners could tap new sources of revenue. We favor a modified wireless LAN as a rental system. Especially OFDM based WLANs like IEEE802.11a and HIPERLAN/2 are suitable for an overlay system like spectrum pooling as they allow a very flexible frequency management on a carrier-by-carrier basis. The modifications of these standards comprise many areas of the physical and MAC layer. Idle frequency subbands can be detected by spectral power measurements conducted in each single participating mobile terminal. Hence, the volume of measurement data can become very high. Signaling this overhead in ordinary data frames would leave only few resources left for useful data and would be very error-prone. In this paper, we show that the signaling of spectral resources can be performed in the physical layer that has been harmonized for both considered standards. The basic idea is the superposition of emitted radio power for the signaling instead of creating new higher layer frames for the measured data. Furthermore, a robust method to broadcast information on spectral resources back to the mobile terminals is presented.
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Arnd-Ragnar Rhiemeier, Friedrich K. Jondral:
A Software Partitioning Algorithm for Modular Software Defined Radio
The 6th International Symposium on Wireless Personal Multimedia Communications WMPC'03, October 19-22, 2003, Yokosuka, Kanagawa, Japan, pp. 42-46

Abstract:
Since its introduction in the 1990s [1][2] the Software Radio concept has been discussed numerous times [3][4][5]. Meanwhile it has turned out that Software Defined Radio (SDR) techniques, up to a certain degree of reconfigurability, can indeed establish a rewarding compromise between speed, flexibility and power consumption of general purpose processors, as compared to dedicated communication devices. In the present paper we continue to study the properties of Modular Software Defined Radio (Mod-SDR) where logically dependent software modules are mapped to multiple interconnected processors. This mapping is achieved under realistic resource constraints by applying an algorithm which we have adapted and enhanced from the fundamental approach to graph partitioning by Kernighan and Lin [6]. In particular, we introduce a single node transfer procedure in addition to node pair exchanges. Partitioning is necessarily complemented by scheduling. Algorithm variants for scheduling and different starting configurations for partitioning are discussed using the practical example of an IMT-2000 W-CDMA transmitter operating in the uplink. Throughout the experimental section we emulate the application of our algorithms in an environment where software module runtimes are not known a priori. To do so, we make use of a stochastic SDR model, and we interpret its parameters in their technical context. Based on computer simulations we derive guidelines for operating a Mod-SDR, using our proposed algorithm.
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Michael Schmidt und Friedrich K. Jondral:
Verallgemeinertes UWB-OFDM
Frequenz, Band 57, Heft 9-10, 2003, S. 179-184

Abstract:
Ultra Wideband (UWB)-Kommunikationssysteme waren in den letzten Jahren Gegenstand umfangreicher Forschungs- und Entwicklungsprojekte. Diese Projekte konzentrierten sich genauso wie der Großteil der wissenschaftlichen Veröffentlichungen auf Ba­sis­band­über­tra­gung mit sehr kurzen Impulsen. UWB-Signale werden jedoch über ihre extreme Bandbreite definiert und sind deswegen nicht auf Basisbandübertragung beschränkt. In dieser Arbeit schlagen wir ein alternatives, auf Mehrträgerverfahren basierendes Konzept vor. Es handelt sich dabei um ein hybrides System, das Orthogonal Frequency Division Multiplex (OFDM) mit klassischer UWB-Impulstechnik verbindet.

During the last years Ultra Wideband (UWB) communication systems have been subject of extensive research and development projects. These projects as well as most scientific publications focused on base band transmission using very short pulses. However, UWB signals are defined by their extreme bandwidth and are therefore not limited to base band transmission. In this paper we propose an alternative concept based on multicarrier transmission. It acts as an hybrid system combining Orthogonal Frequency Division Multiplex (OFDM) and classic UWB impulse radio.
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Fatih Capar, Ihan Martoyo, Timo Weiss, Friedrich K. Jondral: 
Analysis of Coexistence Strategies for Cellular and Wireless Local Area Networks
Proceedings of IEEE VTC2003-Fall (ISBN 0-7803-7955-1), Orlando (FL), October 6-9, 2003

Abstract:
The demand for ubiquitous communications necessitates the development of wireless communication systems which are able to offer high bandwidths for different types of traffic. The technological developments allow sophisticated and flexible systems with 'anywhere, anytime' capabilities. The bottleneck of such systems is the electromagnetic spectrum, which is scarce in nature. Measurements campaigns show that large parts of the licensed spectrum are not exploited or not utilized very efficiently. A dynamic frequency sharing strategy would allow systems with different radio access technologies (RAT) to use the same part of the spectrum. In this paper we describe and analyze two systems with different RATs which share the same spectrum. In our approach, frequencies belonging to different RATs are gathered in a spectrum pool without restrictions of the availability of the spectra for their owners and are shared according to a strategy to which we further refer as Spectrum Pooling (SP) [1], [2] and [3]. The Spectrum Pooling system has access to all frequencies within the pool and is allowed to access frequencies which are not used by the licensee. After the licensee accesses frequencies which are used by the SP system, the SP system has to release the frequencies which are affected. We develop a simulation model to capture the characteristics of such a system.
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Mengüc Öner, Friedrich K. Jondral: 
Extracting the Channel Allocation Information in a Spectrum Pooling System Using a Prefilter Delay and Multiply Nonlinearity
Proceedings of the 2003 IEEE Workshop on Statistical Signal Processing, September 28 - October 1, 2003, St. Louis, MO, USA

Abstract:
The spectrum pooling concept is a resource sharing strategy, which allows a license owner to share a sporadically used part of his licensed spectrum with a renter system, until he needs it himself. In order to make such a resource sharing plausible for a license owner, the impact of this strategy on the operation of the license owner system must be kept at a minimum. This means the renter system must extract the channel allocation information (CAI) from the channel, i.e. it has to detect, which parts of the shared spectrum the owner system accesses to, in order to immediately vacate the frequency bands being required by the owner and to gain access to the frequency bands, which the owner has stopped using. In this paper we propose a method for the extraction of the CAI using a prefilter delay and multiply nonlinearity in a specific spectrum pooling scenario, where the license owner is a GSM network and the spectrum renter is an OFDM based WLAN system
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Ihan Martoyo, Timo Weiss, Fatih Capar, Friedrich K. Jondral: 
Low Complexity CDMA Downlink Receiver Based on Fequency Domain Equalization
Proceedings of IEEE VTC2003-Fall (ISBN 0-7803-7955-1), Orlando (FL), October 6-9, 2003

Abstract:
This paper proposes a new approach to apply frequency domain equalization (FDE) in the downlink of broadband CDMA cellular systems. Equalization has been recognized as a better receiving method than the RAKE receiver for CDMA downlink systems, especially in cells with a high number of users. By performing the equalization in the frequency domain, the complexity of the equalization algorithm can be significantly reduced with the help of the FFT operation. Three methods of implementing FDE in the CDMA downlink will be described: the cyclic prefix method, the zero padding method, and the overlap-cut method. In addition to its simplicity and good performance, the FDE also offers the possibility to build a multimode receiver for singlecarrier and multicarrier signals. Simulation results are presented in a TD-CDMA system with parameters taken from the UTRA-TDD standard.
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Georg Boeck, Dariusz Pienkowski, Radu Circa, Marius Otte, Benjamin Heyne, Piotr Rykaczewski, Reimund Wittmann, Ralf Kakerow:
RF Front-end Technology for Reconfigurable Mobile Systems
Proceedings of the International Microwave and Optoelectronics Conference (IMOC 2003), September 2003, Brasil

Abstract:
This paper provides an overview on RF-front-end architectures and technologies of future reconfigurable mobile systems (4G-systems). Aspects of the historic evolution of mobile communication systems are given at the beginning. With respect to reconfigurability three different system approaches are introduced and compared: switchable systems, systems with reusable components and systems with multifunctional components. Furthermore, front-end key components like low noise amplifiers, voltage controlled oscillators and mixers are treated, particularly with regard to reconfigurable systems.
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Piotr Rykaczewski, Volker Blaschke, Friedrich K. Jondral:
I/Q Imbalance Compensation for Software Defined Radio OFDM Based Direct Conversion Receivers
Proceedings of the 8th International OFDM-Workshop, TU Hamburg-Harburg, September 24/25, 2003, pp. 279-283

Abstract:
Direct conversion receivers are widely utilized in today's communication systems. However, their I/Q architecture is sensitive to phase and amplitude imbalances. The present paper deals with the compensation of imbalances and the rejection of images that are caused by the analog front-end processing. A \emph{standard independent approach} is presented, which enables an implementation of the compensation algorithms in any OFDM system with respect to the Software Defined Radio (SDR) principles [1]. Implementation of the compensation algorithms in non-OFDM systems is also discussed.
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Timo Weiss, Joerg Hillenbrand, Friedrich K. Jondral:
A Diversity Approach for the Detection of Idle Spectral Resources in Spectrum Pooling Systems
48. Internationales Wissenschaftliches Kolloquium, TU Ilmenau, 22.-25.September 2003, Executive Summary: Tagungsband S. 37/38, Full Paper: CD-ROM

Abstract:
Public mobile radio spectrum is a scarce resource while wide spectral ranges are only rarely used. Here, we consider a new strategy called Spectrum Pooling enabling public access to these new spectral ranges without sacrificing the transmission quality of the actual license owners. By temporarily leasing their spectrum during idle periods the license owners could tap new sources of revenue. We favor a modified wireless LAN as a rental system. Especially OFDM based WLANs like IEEE802.11a and HIPERLAN/2 are suitable for an overlay system like spectrum pooling as they allow a very flexible frequency management on a carrier-by-carrier basis. The modifications of these standards comprise many areas of the physical and MAC layer. Idle frequency subbands can be detected by spectral power measurements conducted in each single participating mobile terminal. In this paper, it is investigated how the detection of a licensed user can be performed in a mathematically optimal fashion. The proposed detector is derived from the Neyman-Pearson critereon. Simulation results show how a trade-off between detection duration and detection accuracy can be performed.
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Arnd-Ragnar Rhiemeier, Friedrich K. Jondral:
Enhanced Resource Utilization in Software Defined Radio Terminals
Internationales Wissenschaftliches Kolloquium, TU Ilmenau, 22.-25. September 2003, Executive Summary: Tagungsband S. 39/40, Full Paper: CD-ROM

Abstract:
Software Radio has been proposed in the 1990s as a solution to flexible transceiver design for future wireless systems. Meanwhile, however, it has become evident that this software radio ideal is not feasible, neither technologically nor commercially. Software Defined Radio (SDR) compromizes the ideal by some means or other, in order to achieve an implementation on real, available hardware. A considerable number of publications on SDR exists, and the majority among them does focus on one particular aspect of the signal processing chain. Especially, contributions to baseband processing are centered around algorithms. In the present paper we describe our perspective on SDR systems, including a novel way of modelling its particular characteristics. Furthermore, for the example of a direct sequence spread spectrum (DSSS) system such as UMTS we point out a beneficial way of operating a software defined physical (PHY) layer. This leads to enhanced resource utilisation while retaining the average sustained data throughput across the PHY layer.
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Arnd-Ragnar Rhiemeier, Friedrich K. Jondral:
On the Design of Modular Software Defined Radio Systems
IEE Colloquium on DSPenabledRadio, Institute for System Level Integration, Alba Campus, Scotland, UK, September 22/23, 2003

Abstract:
Software Radio and its enabling technologies have been discussed extensively in the past. The focus of most work is either on a specific hardware part of the transceiver's signal processing chain, or on algorithm design for the digital baseband. However, a methodology for managing open, Software Defined Radio systems is still in demand. In this paper, the implementation of a software defined physical layer is perceived as a real-time embedded system design problem on multiprocessor hardware, using pieces of software which are unknown a priori. We propose a new way of modeling this design situation. Granularity G is introduced to describe the degree of modularity in software defined communication functions. The speedup s is used to assess the quality of modular implementations. Computer simulations lead us to first observations under the premises of the new model. A mathematical analysis complements these experiments and reveals how to adjust the simulation parameters for interpretable results. The speedup behavior of a simple graph structure is predicted from this analysis. A more complicated structure is then reassessed by means of simulation, finally leading to enhanced guidelines for the design of Modular Software Defined Radio systems.
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Timo Weiss, Albert Krohn, Fatih Capar, Ihan Martoyo, Friedrich K. Jondral
Synchronization Algorithms and Preamble Concepts for Spectrum Pooling Systems
Proceedings of the IST - Mobile & Wireless Communications Summit 2003, June 15-18, 2003, Aveiro (Portugal), Volume 2, pp. 788-792

Abstract:Public mobile radio spectrum has become a scarce resource while wide spectral ranges are only rarely used. Here, we consider a new strategy called Spectrum Pooling (SP) enabling public access to these spectral ranges without sacrificing the transmission quality of the actual license owners. By temporarily leasing their spectrum during idle periods the license owners could tap new sources of revenue. We favor a modified wireless LAN as rental system. Especially OFDM based WLANs like IEEE802.11a and HIPERLAN/2 are suitable for an overlay system like SP as they allow a very flexible frequency management on a carrier-by-carrier basis. The modifications of these standards comprise many areas of the physical and MAC layer. In this paper, we show by simulation results that the existing preambles and synchronization methods are insufficient for SP. We derive new concepts that fit the special needs of an SP scenario. We focus on frequency and frame synchronization in the presence of multiple narrow-band interferers which is an appropriate transmission model for the licensed users in our context. Simulation results show that the proposed preamble and synchronization concepts perform well.
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Nikolaus Lehmann, Alexander M. Haimovich:
New Approach to Control the Power Spectral Density of a Time Hopping UWB Signal
Proceedings of the 2003 Conference on Information Sciences and Systems, The Johns Hopkins University, Baltimore (MD), March 12-14, 2003

Abstract:
In this publication the power spectral density (PSD) of some common time hopping impulse radio signals is discussed. Both analytical expressions and simulatin results are provided. The impact of the PSD on the transmitted power is analyzed in view of the Federal Committee on Communications regulations on UWB emissions. A new approach is introduced that can be applied to control the PSD of UWB signals. A significant gain is demonstrated over classical time hopping schemes.
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Ihan Martoyo, Friedrich K. Jondral:
Frequency Domain Equalization for the Downlink of CDMA Mobile Radio Systems
Proceedings of the IEEE International Syposium on Advances in Wireless Communications, Victoria (Canada), September 23-24, 2002, pp. 97 - 98

Abstract:
It has been recognized that a RAKE receiver performs rather poorly in CDMA systems because of the Multiple Access Interference (MAI). This MAI arises through the multipath propagation which destroys the orthogonality between the spreading codes. Recently, it has been discovered that a simple time domain equalizer can outperform a RAKE detector in CDMA downlink systems. Here, this equalizer approach will be further pursued, but this time in the frequency domain. A frequency domain equalizer (FDE) which takes advantage of the FFT structure will be presented. We simulate the FDE receiver with the typical urban and the rural channel, and show that the FDE outperforms the RAKE receiver. Another feature of the FDE is its striking similarity to an OFDM system structure. This similarity opens up the possibility to construct a software radio structure, which encompasses several air interface standards.
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Ihan Martoyo, Henrik Schober, Friedrich K. Jondral: 
CDMA versus OFDM, A Performance Comparison in Selective Fading Channels
Proceedings of IEEE ISSSTA 2002, Prague, September 2-5, 2002, Vol.1, pp. 139 - 143

Abstract:
CDMA (Code Division Multiple Access) and OFDM (Orthogonal Frequency Division Multiplexing) are two well known air interface technologies in modern mobile communication. In this paper, a performance comparison of CDMA and OFDM systems in selective fading channels is presented. The comparison proceeds in two steps. First, simple OFDM and CDMA systems with the same bandwidth will be considered. A comparable bit rate will be transmitted over both systems and the bit error rate in selective fading channels will be observed. Second, two existing standards (DVB and UTRA FDD) will be compared. We concentrate on the downlink and simulate the two systems in the rural and the typical urban channel. Simulations with a very high mobile velocity (350 km/h) are also conducted on both systems. We use a RAKE receiver for the CDMA system and compare its performance with a coherent and differential OFDM detector. We conclude that OFDM achieves better BER results than CDMA using a RAKE receiver for the same bandwith efficiency.
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Fatih Capar, Ihan Martoyo, Timo Weiss, Friedrich K. Jondral:
Comparison of Bandwidth Utilization for Controlled and Uncontrolled Channel Assignment in a Spectrum Pooling System
Proceedings of the IEEE 55th Vehicular Technology Conference VTC Spring 2002, Birmingham (AL), May 6-10, 2002

Abstract:
Spectrum for mobile networks is becoming scarce and on the other side plenty of sporadicly used frequencies exist. The spectrum utilization of such spectra is very low, which is a reason to think about a way to make them available for commercial purpose without a drawback for the license owners. In this paper we will recommend a strategy called Spectrum Pooling which is based on this idea. The notion Spectrum Pool was first mentioned in [6]. In a Spectrum Pooling System the license owner of the spectrum allows priorly specified candidate renters to use his spectrum until he needs his spectrum himself. The renters of the spectrum may be treated in two different ways. One way is that communication processes of spectrum renters can persist as long as there are channels for the processes of the license owners. The other way is that the owner has no knowledge whether the channels are occupied by renters or not and treat the channels used by renters like free channels. In both cases the renters have to measure the interference level in the channel after dedicated time intervals and have to leave the channel within the time interval Tp as soon as the interference exceeds a priorly specified threshold. In this paper we will compare the spectrum utilization, blocking probability and forced termination probability for these two different channel access schemes.
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Gunther Sessler, Ihan Martoyo, Friedrich K. Jondral:
RBF Based Multiuser Detectors for UTRA-TDD
Proceedings of the IEEE 54th Vehicular Technology Conference VTC Fall 2001, Atlantic City (NJ), October 7, 2001

Abstract:
Two radial-basis function (RBF) based detectors which are used as multiuser detectors (MUD) for the UTRA-TDD mode of IMT-2000, will be presented in this paper. One of the RBF detectors is chip level based (CLB) and the other is preprocessesing based (PPB). A detailed performance evaluation, based on new simulation results of the RBF detectors in the UTRA-TDD system, is presented. These results illustrate that the detectors achieve low BERs, even for time-variant multipath propagation channels (Pedestrian, Vehicular test environment). Furthermore, it is shown that the BER of the RBF based detector is virtually independent of the number of users. On the down side, the complexity of the detectors is significantly affected by the number of users.
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