Abstract The author addresses the question, under which conditions an active jammer will be successful in corrupting a wireless communication link, when both the desired link and the jamming link are subject to multipath signal fading. To this end, a simple closed‐form analytical expression is derived for the resulting block‐error probability (BLEP) under Nakagami‐ fading with in general unequal fading parameters for the desired and the jamming link. As a by‐product, a novel expression for the corresponding probability of a capacity outage is obtained. An asymptotic analysis and numerical evaluation of the BLEP expression show that in the single‐antenna case the jammer will indeed be successful in corrupting the wireless link, unless there is a rather high signal‐to‐noise ratio (SNR) advantage for the desired link. This even holds true, when signal fading for the jamming link is significantly more severe than for the desired link. Finally, the case of multiple antennas is considered and the benefits of array processing and diversity reception at the desired receiver are explored. It is found that receive array processing can significantly improve the resulting BLEP, provided that the desired link is characterized by favorable fading conditions. On the other hand, diversity reception can significantly improve the BLEP, if the desired link has favorable SNR conditions.

We propose a novel space-time waveform coding (STWC) scheme for joint radar and wireless communications (RadCom). In particular, we consider a frequency-modulated continuous-wave (FMCW) multiple-input multiple-output (MIMO) radar for near-range to medium-range radar applications. In order to establish the required orthogonal transmit signals, up- and down-chirp FMCW waveforms are combined with Alamouti space-time coding across two transmit antennas. We demonstrate that, from a radar perspective, the cross-ambiguity function is improved significantly by the orthogonal Alamouti code, while the FMCW waveforms ensure robustness in the presence of Doppler shifts. Regarding the communications part, the generic orthogonality of the Alamouti code allows us to embed random information symbols within the radar signal, enabling wireless communications in the 100 kb/s regime. To this end, we propose a suitable communications receiver algorithm for multiple antennas with corresponding maximum-ratio combining, which is able to extract the embedded information symbols while acquiring spatial diversity gains.

Multiple-input multiple-output (MIMO) radars have been shown to improve target detection for surveillance applications thanks to their proven high-performance properties. In this paper, the design, implementation, and results of a complete 3-D imaging frequency-modulated continuous-wave MIMO radar demonstrator are presented. The radar sensor working frequency range spans between 16 and 17 GHz, and the proposed solution is based on a 24-transmitter and 24-receiver MIMO radar architecture, implemented by time-division multiplexing of the transmit signals. A modular approach based on conventional low-cost printed circuit boards is used for the transmit and receive systems. Using digital beamforming algorithms and radar processing techniques on the received signals, a high-resolution 3-D sensing of the range, azimuth, and elevation can be calculated. With the current antenna configuration, an angular resolution of 2.9° can be reached. Furthermore, by taking advantage of the 1-GHz bandwidth of the system, a range resolution of 0.5 m is achieved. The radio-frequency front-end, digital system and radar signal processing units are here presented. The medium-range surveillance potential and the high-resolution capabilities of the MIMO radar are proved with results in the form of radar images captured from the field measurements.

In this paper, a multiuser two-way filter-and- forward relaying scheme for wireless communication over wideband channels is considered. We propose pre/post-rake processing in conjunction with optimized filtering at the relay to reduce the signal processing burden at the source and destination nodes. Two relay filter design problem formulations are introduced, namely (a) a convex optimization problem formulation with closed-form solutions and (b) the more general case, which is a non-convex problem solvable via an alternating optimization algorithm. For both design alternatives widely linear formulations are devised. The presented numerical results demonstrate the capability of the proposed designs to establish reliable two-way communication links between nodes with limited signal processing power and in the absence of a direct link.

In this paper, we propose multiuser two-way relaying strategies for pairwise internode communication in a network consisting of ultrawideband transceivers with limited signal processing capability, via a central relay unit. We propose reducing the complexity associated with the design of filters at the relay by using pre/post-rake processing in conjunction with optimized filtering at the relay. Two relaying strategies relevant to multipath fading channels, namely, detect-and-forward and filter-and-forward with self-interference cancelation, are considered. For both methods, we start with a convex optimization problem formulation with closed-form solutions, then extend the design to the more general case, which is a nonconvex problem, and use an alternating optimization algorithm to solve the design problems. Furthermore, for both design schemes, widely linear design formulations are devised. The presented numerical results demonstrate the capability of the proposed design schemes in establishing a reliable communication link between nodes with limited signal processing power and in the absence of a direct link.

In this paper, we propose a novel optimization-based pre-equalization filter (PEF) design framework for direct-sequence ultra-wideband (DS-UWB) systems with pre-Rake combining. The key feature in our design is that we explicitly take into account the spectral mask constraints that are usually imposed in practice by the telecommunications regulation and standardization bodies. This avoids the need for an inefficient power back-off, which is necessary for the existing pre-Rake-based transmitter structures designed solely based on average transmit power constraints. We consider two PEF design structures. In the first structure, the PEF is placed before the up-sampling unit at the transmitter. In the second structure, the PEF is placed after the up-sampling unit. We show that these two structures are significantly different in terms of their capabilities in reducing the residual inter-symbol interference at the receiver and also in following the spectral mask at the transmitter. This introduces a trade-off such that one design structure outperforms the other one, depending on the system parameters. In this regard, we show that if the spreading factor is large enough, the second design structure has a superior performance. Simulation results confirm that both of the proposed PEF structures lead to significant performance gains over PEF designs without explicit spectral mask considerations. In addition, our PEF designs have the capability of adhering to spectral masks with arbitrary shapes.

In this paper, we propose a novel multi-hop relaying scheme to improve the performance and coverage of impulse-radio-based ultra-wideband (IR-UWB) systems. With regard to a simple practical realization, we focus on a non-coherent system setup in conjunction with amplify-and-forward (A&F) relaying. In particular, we propose to employ a multiple-differential encoding scheme at the source node and single differential decoding at each relay and at the destination node, respectively, so as to efficiently limit intersymbol-interference effects at the destination node. For a dual-hop system we derive a closed-form expression for the signal-to-noise ratio (SNR) at the destination node, and for the general multi-hop case we provide a simple recursive formula for SNR calculation. Based on these SNR results, we obtain a closed-form expression for the optimal transmit power allocation to the source node and the relay for a dual-hop system and a simple recursive suboptimal power allocation scheme for the multi-hop case, which permits a semi-distributed implementation with limited feedback between nodes. Simulation results illustrate the excellent performance of the proposed multiple-differential encoding scheme with A&F relaying for both uncoded and coded transmission compared to various alternative coherent and non-coherent schemes based on A&F relaying and decode-and-forward (D&F) relaying. Furthermore, our simulations confirm the (near-)optimal performance of the proposed power allocation solutions.

In this paper, we propose a novel optimization-based pre-equalization filter (PEF) design for direct-sequence ultra-wideband (DS-UWB) systems with pre-Rake combining. The key feature in our design is that we explicitly take into account the spectral mask constraints that are usually imposed by the telecommunication regulation bodies. This avoids the need for an inefficient power back-off, which is necessary for transmit structures designed solely based on average transmit power constraints. Simulation results confirm that the proposed PEF design leads to significant performance gains over UWB PEF structures without explicit spectral mask considerations.

In this paper, we propose to improve the performance of impulse-radio ultra-wideband (IR-UWB) systems by means of cooperative relaying. With regard to a simple practical realization, we focus on a non-coherent system setup in conjunction with amplify-and-forward (A & F) relaying. In particular, considering a two-hop scenario we propose to employ a double-differential encoding scheme at the source node and single differential decoding at the relay and the destination node, respectively, so as to efficiently limit intersymbol-interference (ISI) effects. A thorough performance analysis of the proposed scheme is provided, along with a closed-form optimization of the transmit power allocation between source and relay. Simulation results illustrate the excellent performance of the proposed scheme, which is compared to alternative coherent and non-coherent schemes based on A & F relaying and decode-and-forward (D & F) relaying.

The generalized κ-fading model, characterized by two parameters, κ and m, is a very versatile model and was recently shown to accurately capture the effects of composite shadowing and multipath fading in wireless communication systems. Furthermore, it can be used to model cascade multipath fading, which is relevant in, e.g., mobile-to-mobile communication scenarios. In this paper, we derive closed-form expressions for the bit error probability of two non-coherent transmission schemes over L diversity branches being subject to generalized κ-fading. Specifically, focus is on binary differential phase-shift keying (DPSK) and binary non-coherent frequency-shift keying (FSK) modulation with (post-detection) equal-gain combining at the receiver. We also discuss the extension of our results to M-ary modulation schemes. Considering both independent and correlated fading across the L branches, we derive expressions for the asymptotic diversity order, which reveal an interesting interplay between the two fading parameters κ and m. Moreover, we show that the diversity order of the considered non-coherent transmission schemes is the same as in the case of a coherent transmission scheme. Finally, numerical performance results are presented, and our analytical results are corroborated by means of Monte-Carlo simulations.

The generalized K-fading model, characterized by two parameters, k and m, was recently shown to accurately capture the effects of composite shadowing and multipath fading in wireless communication systems. In this paper, we derive closed-form expressions for the bit error probability of two non-coherent transmission schemes over L diversity branches being subject to generalized K-fading. Specifically, focus is on binary differential phase-shift keying (DPSK) and binary non-coherent frequency-shift keying (FSK) modulation with equal-gain combining at the receiver. We also derive expressions for the asymptotic diversity order, which reveal an interesting interplay between the two fading parameters k and m. Moreover, we show that the diversity order of the considered non-coherent transmission schemes is the same as in the case of coherent transmission. Finally, numerical performance results are presented, and our analytical results are corroborated by means of Monte-Carlo simulations.

In this paper, we consider a relay-assisted wideband cognitive-radio (CR) system under the assumption that the frequency band chosen by the CR relay network for unlicensed spectrum usage overlaps with one or more bands dedicated to primary (e.g., licensed) narrowband links. Our objective is to optimize the performance of the CR system while limiting the interference in direction of the primary receivers, without requiring any adaptation of the transmitted signal spectra at the cognitive nodes. To this end, we study appropriate transmit power allocation (TPA) strategies among the cognitive relays. We first investigate the optimal centralized (OC) TPA solution and show that it can be formulated as a linear program. Since the OC-TPA solution requires a considerable amount of information exchange between the cognitive nodes, we develop two distributed TPA schemes, namely (i) a fully decentralized (FD) TPA scheme and (ii) a distributed feedback-assisted (DFA) TPA scheme. The FD-TPA scheme aims at maximizing the output signal-to-interference- plus-noise ratio (SINR) at the destination node of the CR network according to a best-effort strategy. It requires neither feedback information from the destination node nor an exchange of channel state information between the cognitive relays. The DFA-TPA scheme, on the other hand, utilizes feedback information from the destination node, in order to achieve a predefined target output SINR value, while minimizing the overall transmit power spent by the relays. Analytical and simulation-based performance results illustrate that notable performance improvements compared to non-cooperative transmission (i.e., without relay assistance) are achieved by the proposed schemes, especially when more than two hops are considered. In particular, the proposed distributed TPA schemes typically perform close to the OC-TPA solution.

In this paper, we propose two novel pre-equalization schemes for multiple-input single-output (MISO) direct-sequence ultra-wideband (DS-UWB) systems with pre-Rake combining and symbol-by-symbol detection. The first scheme employs one pre-equalization filter (PEF) per transmit antenna, whereas in the second scheme, the simplified PEF (S-PEF) scheme, all transmit antennas share the same PEF. For both schemes the optimum finite impulse response (FIR) and infinite impulse response (IIR) PEFs are calculated based on the minimum mean squared error (MMSE) criterion. Our approach is sufficiently general to include also reduced-complexity versions of pre-Rake combining that employ a limited number of Rake fingers. We show that under certain conditions the S-PEF scheme achieves the same performance as the more complex PEF scheme. We also demonstrate that a single-input multiple-output (SIMO) DS-UWB system with post-Rake combining and MMSE post-equalization is the dual system to the considered MISO DS-UWB system with pre-Rake combining and MMSE pre-equalization. This uplink-downlink duality can be exploited for efficient calculation of the PEFs and for complexity reduction. Our simulation results show that the proposed PEF schemes achieve significant performance gains over pre-Rake combining without equalization, even if only short PEFs are employed.

We analyze the performance of a fully decentralized (FD) transmit power allocation (TPA) scheme for relay-assisted cognitive-radio (CR) systems. In particular, we assume that the frequency band chosen by the CR relay network for unlicensed spectrum usage overlaps with an active primary narrowband link. The considered FD-TPA scheme maximizes the signal-to-noise-plus-interference ratio at the destination node of the CR network according to a best-effort strategy, while limiting the interference experienced by the primary receiver. Numerical performance results show substantial improvements compared to non-cooperative transmission. Moreover, the performance of the FD-TPA scheme is close to that of the optimal centralized power allocation solution.

In this paper, we propose two novel pre-equalization schemes for multiple-input single-output (MISO) direct-sequence ultra-wideband (DS-UWB) systems with pre-Rake combining and symbol-by-symbol detection. The first pre-equalization filter (PEF) scheme employs one PEF per transmit antenna, whereas in the second, simplified PEF (S-PEF) scheme all transmit antennas share the same PEF. For both schemes, the optimum finite impulse response (FIR) and infinite impulse response (IIR) PEFs are calculated based on the minimum mean squared error (MMSE) criterion. Our approach is sufficiently general to include also complexity-reduced versions of pre-Rake combining that employ a limited number of Rake fingers. We show that under certain conditions the S-PEF scheme achieves the same performance as the more complex PEF scheme. Moreover, our simulation results show that the proposed PEF schemes achieve significant performance gains over pure pre-Rake combining without equalization, even if only short PEFs are employed.

We discuss recent results on discrete-time models for triply-selective multiple-input multiple-output (MIMO) Rayleigh fading channels. Our key finding is that a previously proposed model (Xiao et al., 2004), which allows for efficient computer simulations, is sufficiently accurate for a wide range of practical scenarios.

Error performance of distributed space–time codes used for mobile broadcasting systems consisting of multiple base stations is analysed. The base stations are assumed to operate in a simulcast mode, that is they simultaneously transmit the same message using the same carrier frequency. Mobile users within the intersection of the coverage areas, thus, enjoy a small probability of shadowing and a high probability of at least one line-of-sight (LOS) link. In effect, the base stations establish a virtual multiple-antenna system. The use of a distributed space-time code offers an additional spatial diversity gain. Considering a single user with a fixed (random) position, the impact of shadowing and LOS components on the error performance of the system is analysed and compared with a conventional multiple-antenna system with co-located transmit antennas. Specifically, the analysis shows that already a single LOS link significantly improves the overall system performance. In the case of shadowing, huge diversity gains are obtained in the distributed system. In a system with co-located transmitters, however, the performance improvements compared with a single-antenna system are rather small, when shadowing is taken into account. Altogether, it can be concluded that systems with distributed transmitters are typically superior to conventional multiple-antenna systems (due to macroscopic diversity), and that distributed space–time codes are superior to conventional simulcasting (due to microscopic diversity).

We analyze the correlation properties of the discrete-time triply-selective multiple-input multiple-output (MIMO) Rayleigh fading channel model, while triply-selective refers to selectivity in time, frequency, and space. A rigorous analysis of these correlation properties is indispensable to assess the performance of multiple-antenna techniques in realistic environments. To the best of the authors' knowledge, a similar analysis has not yet been presented in the literature.

Sparse intersymbol-interference (ISI) channels are encountered in a variety of high-data-rate communication systems. Such channels have a large channel memory length, but only a small number of significant channel coefficients. In this paper, trellis-based equalization of sparse ISI channels is revisited. Due to the large channel memory length, the complexity of maximum-likelihood detection, e.g., by means of the Viterbi algorithm (VA), is normally prohibitive. In the first part of the paper, a unified framework based on factor graphs is presented for complexity reduction without loss of optimality. In this new context, two known reduced-complexity algorithms for sparse ISI channels are recapitulated: The multi-trellis VA (M-VA) and the parallel-trellis VA (P-VA). It is shown that the M-VA, although claimed, does not lead to a reduced computational complexity. The P-VA, on the other hand, leads to a significant complexity reduction, but can only be applied for a certain class of sparse channels. In the second part of the paper, a unified approach is investigated to tackle general sparse channels: it is shown that the use of a linear filter at the receiver renders the application of standard reduced-state trellis-based equalizer algorithms feasible, without significant loss of optimality. Numerical results verify the efficiency of the proposed receiver structure

In this paper, typical uplink scenarios in a cellular system are considered, where two cooperating mobile stations (serving, for example, as mobile relays) are transmitting the some information to a base station by using a distributed space-time coding scheme. Due to the distributed nature of the system, the transmitted signals are typically subject to different average path losses. For fixed distances between the mobile stations and the base station, the error performance of the distributed space-time coding scheme is determined analytically. Then, based on considerations concerning the spatial distribution of the mobile stations, analytical expressions for the distribution of the average path losses are derived and verified by means of simulations. These results are then used in order to compute the expected error performance of the system. It is shown that in most scenarios the average performance loss compared to a conventional multiple-antenna system with colocated antennas is less than 2 dB at a bit error rate of 10/sup -3/. The most significant performance losses occur for a large path-loss exponent.

On basis of the GSM/GPRS system, we investigate the application of the delay diversity scheme. The delay diversity scheme is a simple special case of a space-time trellis code (STTC). Delay diversity may be offered by network operators even in existing systems, since the standard is not affected at all. The performance improvements obtainable by means of this technique are demonstrated both on basis of analytical and simulation results. A lower bound on the bit error probability is derived and an optimization of the intrinsic delay parameter is considered.

On the basis of the GSM/EDGE system, the application of space-time coding techniques is investigated, especially with regard to aspects of compatibility with current specifications. In this paper, simple special cases of space-time trellis codes and space-time block codes are considered, namely the delay-diversity and the Alamouti scheme. The performance improvements obtainable by means of these techniques in the case of a typical urban (TU) wireless channel are demonstrated both on the basis of analytical and simulation results. For the delay-diversity scheme, a lower bound on the bit-error probability is derived and an optimization of the intrinsic delay parameter is considered. For the Alamouti scheme, a novel trellis-based soft-output equalization and detection algorithm is presented, which is of same complexity as in the single transmit antenna case. Moreover, in the case of the Alamouti scheme, appropriate training sequence pairs are required. In this context, optimized partner sequences with respect to the eight GSM sequences are introduced yielding pairs that significantly outperform any pair of GSM sequences. Furthermore, the sensitivity of the delay-diversity and the Alamouti scheme to a time-variant TU channel is addressed as well as to nonperfect knowledge of the channel coefficients at the receiver.