Frequency-nonselective Channels Research Articles

Spectral Efficiency of Wireless Relay Network in Frequency Non-Selective Channel

Introduction. A wireless communication system based on a relay network where a link between a source and a destination is carried out through a network of relay nodes have been considered. Relay networks operate according with amplifier-and-forward protocol where each relay node performs reception, amplifying, phase shifting and retranslation of a signal to the destination node. As a result a task of powers and phases optimization in the relay nodes (i.e. the complex weighted coefficients optimization) becomes actual. Complex weighted coefficients of the relay nodes are optimized in such a way as to ensure the maximum signal to noise ratio at the receiver while limiting a power emitted by the relay nodes. In the paper, optimization of spatial processing with different a priori channel state information (i.e. instantaneous channel state information and the second order statistics) have been considered. Aim. Spectral efficiency analysis of a relay network in a multipath channel where the relay network was optimized by using of two types a priori information: an instantaneous channel state information and second order statistics. Materials and methods. Optimization of spatial signal processing in the relay network was based on methods of statistical theory and optimization using analytics of linear algebra and methods of mathematical programming. Performances of the relay network were analyzed using Monte Carlo simulation. The simulation was performed in MATLAB program environment using CVX toolbox for solving convex optimization task. Results. In the paper optimal solutions for spatial signal processing in the relay network were presented. The solutions were based on maximum of signal to noise ratio while limiting total relay power and individual power of relay nodes. Monte Carlo simulation was performed to provide performances of the relay network for different types of channel state information and channel parameters. Mean capacities versus mean source power, a budget of relay nodes power and a ratio between random and deterministic power of the channel were gained for the Rayleigh model of multipath channel. Conclusions. The results have a practical application. Thus, the use of the second order statistics is possible in relay networks when direct visibility with a low level of background from local objects is provided. In urban areas, where shading and multipath propagation of signals occur, it is possible to use only an approach based on the knowledge of channel instantaneous state.

On the BEP analysis of M-QAM and R-QAM under cascaded double η-μ, κ-μ or α-μ fading channels

This article presents new expressions for the bit error probability (BEP) of the M-ary quadrature amplitude modulation (M-QAM) and rectangular QAM (R-QAM) scheme in a frequency non-selective channel model with cascaded double fading, characterized in the links by the distributions η-μ, κ-μ or α-μ. In this paper, the probability density function (PDF) of the signal-to-noise ratio (SNR) of the double link is written in terms of an integral of the product of the PDF of the envelope of the first link by the PDF of the SNR of the second link. With this procedure, expressions for the BEP of the M-QAM and R-QAM scheme, with and without a maximum ratio combiner (MRC) associated, are obtained by means of the general expressions of Cho and Yoon, calculated with the help of the Gauss-Laguerre quadrature. The new expressions are written in terms of the integral of the moment generating function (MGF) of the instantaneous SNR from the second link, which are simpler than using expressions involving infinite series in terms of special functions like hypergeometric functions and the Meijer G-function. In addition, insights regarding the expressions obtained and comments about their complexity are provided. Several curves for the BEP are presented, for different parameters that characterize the channel and different numbers of branches of the MRC. All analytical expressions obtained in this paper are corroborated by computer simulations performed with the Monte Carlo method.

Practical Evaluations of SEFDM: Timing Offset and Multipath Impairments

The non-orthogonal signal waveform spectrally efficient frequency division multiplexing (SEFDM) improves spectral efficiency at the cost of self-created inter carrier interference (ICI). As the orthogonal property, similar to orthogonal frequency division multiplexing (OFDM), no longer exists, the robustness of SEFDM in realistic wireless environments might be weakened. This work aims to evaluate the sensitivity of SEFDM to practical channel distortions using a professional experiment testbed. First, timing offset is studied in a bypass channel to locate the imperfection of the testbed and its impact on SEFDM signals. Then, the joint effect of a multipath frequency selective channel and additive white Gaussian noise (AWGN) is investigated in the testbed. Through practical experiments, we demonstrate the performance of SEFDM in realistic radio frequency (RF) environments and verify two compensation methods for SEFDM. Our results show first frequency-domain compensation works well in frequency non-selective channel conditions while time-domain compensation method is suitable for frequency selective channel conditions. This work paves the way for the application of SEFDM in different channel scenarios.

Soft-Decision Decoding of Permutation Block Codes in AWGN and Rayleigh Fading Channels

We introduce an efficient soft-decision decoder for permutation block codes when assuming AWGN and slow fading frequency-nonselective channels. The decoder implements a combination of the Hungarian algorithm and Murty's algorithm for the kth assignment. Results show that the proposed algorithm achieves better coding gain when compared with hard-decision decoding. A salient feature of the decoder is that it operates within practical computational complexity, especially for large codebooks.

Optimization of Cyclic-Delay Diversity Aided Frequency-Selective Scheduling in OFDMA Downlink Systems

Cyclic-delay diversity (CDD)-aided frequency-selective scheduling has been known as an effective technique for increasing the system capacity of orthogonal frequency-division multiple-access (OFDMA) systems. By increasing the channels' frequency selectivity with different cyclic delays applied on transmit antennas, the merit of multiuser diversity can be more effectively exploited in the system. In this paper, we aim to optimize the design of the CDD-aided frequency-selective scheduling for OFDMA downlink systems. Optimization is done from two aspects: cyclic-delay search and scheduling-information feedback. In the optimization of cyclic-delay search, three new methods are proposed, including antenna-wise sequential search for frequency-nonselective (AWSS-FNS) channels, AWSS for frequency-selective (AWSS-FS) channels, and a genetic-algorithm-based search (GAS). AWSS-FNS achieves the optimal performance if a channel is frequency nonselective over a subchannel, whereas AWSS-FS and GAS provide good tradeoffs between system performance and complexity for channels that are frequency selective over a subchannel. In the optimization of scheduling-information feedback, a new channel-dependent feedback (CDF) method is proposed, where the feedback resource is allocated to users according to their frequency-selectivity rates and a principle of proportional fairness. The optimized scheduler is shown to provide significant improvement in both system sum rate and user fairness over previous methods under fixed feedback overhead.

Modulation-Mode Assignment for SVD-assisted Downlink Multiuser MIMO Transmission Schemes with Iterative Detection

Modulation-Mode Assignment for SVD-assisted Downlink Multiuser MIMO Transmission Schemes with Iterative DetectionSince the capacity of multiple-input multiple-output (MIMO) systems increases linearly with the minimum number of antennas at both, the transmitter as well as the receiver side, MIMO systems have attracted a lot of attention for both frequency and non-frequency selective channels and reached a state of maturity. By contrast, MIMO-aided multiple-user systems require substantial further research. In comparison to zero-forcing (ZF) multiuser transmission techniques, where all users are treated jointly, the investigated singular value decomposition (SVD) assisted DL multiuser MIMO solution takes the individual user's channel characteristics into account. In analogy to bit-interleaved coded irregular modulation, we introduce a MIMO-BICM scheme, where different user-specific signal constellations and mapping arrangement were used within a single codeword. Extrinsic information transfer (EXIT) charts are used for analyzing and optimizing the convergence behaviour of the iterative demapping and decoding. Our results show that in order to achieve the best bit-error rate, not necessarily all user-specific MIMO layers have to be activated.

Cooperative Filter-and-Forward Beamforming for Frequency-Selective Channels with Equalization

Most of the existing literature on cooperative relay networks has focused on frequency-nonselective channels or frequency-selective channels with multi-carrier transmission. However, several practical systems employ single-carrier transmission over frequency-selective channels and the design of corresponding relaying schemes is a largely under-explored topic. In this paper, we investigate filter-and-forward beamforming (FF-BF) for relay networks employing single-carrier transmission over frequency-selective channels. In contrast to prior work, we assume that the destination node is equipped with a simple linear or decision feedback equalizer. The FF-BF filters at the relays are optimized for maximization of the signal-to-noise ratio at the equalizer output under a joint relay power constraint. For infinite impulse response (IIR) FF-BF filters, we derive a unified expression for the filter frequency response valid for linear equalization, decision feedback equalization, and an idealized matched filter receiver. A numerical algorithm with guaranteed convergence is developed for optimization of the power allocation factor included in the expression for the IIR FF-BF filter frequency response. We also provide an efficient gradient algorithm for recursive calculation of near-optimal finite impulse response (FIR) FF-BF filters. Simulation results show that, in general, short FIR FF-BF filters are sufficient to closely approach the performance of IIR FF-BF filters even in severely frequency-selective channels and that the proposed FF-BF scheme with equalization at the destination achieves substantial performance gains compared to a previously proposed FF-BF scheme without equalization.

Super-Orthogonal Block Codes with Multichannel Equalisation and OFDM in Frequency Selective Fading

Super-orthogonal block codes in space-time domain (i.e., Super-orthogonal space-time trellis codes (SOSTTCs)) were initially designed for frequency nonselective (FNS) channels but in frequency selective (FS) channels these super-orthogonal block codes suffer performance degradation due to signal interference. To combat the effects of signal interference caused by the frequency selectivity of the fading channel, the authors employ two methods in this paper, namely, multichannel equalization (ME) and orthogonal frequency division multiplexing (OFDM). In spite of the increase complexity of the SOSTTC-ME optimum receiver design scheme, the SOSTTC-ME scheme maintains the same diversity advantage as compared to the SOSTTC scheme in FNS channel. In OFDM environments, the authors consider two forms of the super-orthogonal block codes, namely, super-orthogonal space-time trellis-coded OFDM and super-orthogonal space-frequency trellis-coded OFDM. The simulation results reveal that super-orthogonal space-frequency trellis-coded OFDM outperforms super-orthogonal space-time trellis-coded OFDM under various channel delay spreads.

Coded noncoherent communication with amplitude/phase modulation: from shannon theory to practical architectures

We develop bandwidth efficient radio transceivers, using amplitude/phase modulations, for frequency non-selective channels whose time variations are typical of outdoor mobile wireless systems. The transceiver is noncoherent, neither requiring pilots for channel estimation and tracking nor assuming prior channel knowledge on the part of the receiver. Serial concatenation of a binary outer channel code with an inner differential modulation code provides a turbo structure that, along with the channel memory, is exploited for joint iterative channel and data estimation. While prior work on noncoherent communication mainly focuses on PSK alphabets, we consider a moderate to high SNR regime in which amplitude/phase constellations are more efficient. First, the complexity of block noncoherent demodulation is reduced to a level that is comparable to coherent receivers. Then, a tool for choosing the constellation and bit-to-symbol mapping is developed by adapting extrinsic information transfer (EXIT) charts for noncoherent demodulation. The recommended constellations differ significantly from standard coherent channel constellations, and from prior recommendations for uncoded noncoherent systems. The analysis shows that standard convolutional codes are nearly optimal when paired with differential amplitude/phase modulation.

Finite-state Markov modeling of fading channels - a survey of principles and applications

This article's goal is to provide an in-depth understanding of the principles of FSMC modeling of fading channels with its applications in wireless communication systems. While the emphasis is on frequency nonselective or flat-fading channels, this understanding will be useful for future generalizations of FSMC models for frequency-selective fading channels. The target audience of this article include both theory- and practice-oriented researchers who would like to design accurate channel models for evaluating the performance of wireless communication systems in the physical or media access control layers, or those who would like to develop more efficient and reliable transceivers that take advantage of the inherent memory in fading channels. Both FSMC models and flat-fading channels will be formally introduced. FSMC models are particulary suitable to represent and estimate the relatively fast flat-fading channel gain in each subcarrier.

FPGA Implementation of Block Parallel DF-MPIC Detectors for DS-CDMA Systems in Frequency-Nonselective Channels

Multistage parallel interference cancellation- (MPIC-) based detectors allow to mitigate multiple-access interference in direct-sequence code-division multiple-access (DS-CDMA) systems. They are considered serious candidates for practical implementation showing a good tradeoff between performance and complexity. Better performance is obtained when decision feedback (DF) is employed. Although MPIC and DF-MPIC have the same arithmetic complexity, DF-MPIC needs much more FPGA resources when compared to MPIC without decision feedback. In this letter, FPGA implementation of block parallel DF-MPIC (BP-DF-MPIC) is proposed allowing better tradeoff between performance and FPGA area occupancy. To reach an uncoded bit-error rate of10−3, BP-DF-MPIC shows a 1.5 dB improvement over the MPIC without decision feedback with only 8% increase in FPGA resources compared to 69% for DF-MPIC.

Optimized Distributed Space-Time Filtering

Distributed space-time filtering (DSTF) facilitates node cooperation and achieves a diversity gain while not requiring node coordination, i.e., cooperating nodes do not have to be aware of their partners. This makes DSTF attractive for application in future sensor, ad hoc, and wireless networks. In this paper, we derive a novel cost function and related practical algorithms for optimization of the signature filter vectors (SFVs) used in DSTF. We extend DSTF to frequency-selective fading channels and compare its performance with that of delay diversity transmission with co-located antennas. For the special case of frequency-nonselective channels we show that SFV optimization is closely related to the signature sequence design for code-division multiple access (CDMA) systems and the design of Grassmannian frames. Numerical and simulation results show that the novel SFV designs perform significantly better than previously proposed designs even if non-ideal effects such as suboptimum equalization, imperfect channel estimation, and imperfect timing synchronization are taken into account

Blind channel estimation and detection for space–time coded CDMA in ISI channels

In order to accommodate high data-rate applications, there is a significant interest in extending space-time (ST) coding, originally proposed for frequency-nonselective channels assumed known to the receiver, to unknown ISI (inter-symbol interference) channels. In this paper, we consider the problem of blind channel identification and linear multiuser receiver design for ST-coded CDMA (code-division multiple-access) systems operating in frequency-selective fading channels. We investigate the identifiability conditions and present a subspace-based blind channel estimator for such systems. To assess the performance of the proposed and other potential estimators, we derive an unconditional Cramer-Rao bound (CRB) which, similarly to the proposed blind channel estimator, does not assume the knowledge of the transmitted information symbols. We discuss various linear multiuser detection schemes that can be used in conjunction with the proposed channel estimator for symbol demodulation in ST-coded CDMA systems. Numerical examples are presented to illustrate the performance of the proposed channel estimator and linear detectors in multipath Rayleigh fading channels.

Blind multiuser detection of STBC signals using hybrid constant modulus and subspace methods

A blind multi-user detection algorithm for space-time block coded (STBC) signals is proposed based upon subspace methods and constant modulus algorithms for multiple antenna based receivers. The subspace method is used to determine blindly the multiuser detector required for the mitigation of multi-user interference and to coherently combine multipath and multiple receive antenna signals. It is shown that this subspace method for STBC could however, solve this problem only up to a matrix ambiguity and the structure of this ambiguity matrix looks like a channel matrix obtained for a single user STBC system with frequency non-selective channel. Hence, the remaining problem is shown to be equivalent to blind equalisation of a single user STBC in flat fading channels. A tap constrained constant modulus algorithm is proposed to solve the remaining problem. In this blind method, only the signature waveform of the user of interest is assumed to be known, and the signature waveforms of all other users in addition to the channel impulse responses of all the users are assumed to be unknown. Simulation results confirm superior performance of the proposed method over conventional non-blind matched filters.

Signal Mappings of 8-Ary Constellations for BICM-ID Systems over a Rayleigh Fading Channel

It is known that in a bit-interleaved coded-modulation with iterative decoding (BICM-ID), signal constellation and mapping strongly influence the system's error performance. This letter presents good mappings of various 8-ary constellations for BICM-ID systems operating over a frequency non-selective block Rayleigh fading channel. Simulation results for the error performance of different constellations/mappings are also provided and discussed.

Low-complexity bit-interleaved coded DAPSK for Rayleigh-fading channels

We analyze the achievable performance of bit-interleaved coded differential amplitude and phase-shift keying (DAPSK) systems over frequency nonselective Rayleigh-fading channels with suboptimal differential detection assuming an ideal bit interleaving. The suboptimal differential detection in this work refers to the bit metric calculation based only on the difference between two consecutive symbols, in contrast to more complex maximum-likelihood (ML)-based differential detection, which makes use of all the observed consecutive symbols for its metric calculation in channel decoding. As benchmarks of coded system performance, we analyze the average mutual information (AMI) and cutoff rate of this system. Exact probability density functions of the suboptimal differential detector outputs are derived for this purpose. Comparative studies suggest that the performance loss of the suboptimal approach is in fact noticeable. Therefore, we also develop a low-complexity receiver structure in the framework of suboptimal differential detection that can approach the performance of ML-based system by suitably incorporating the amplitude statistics of received symbols. The theoretical framework developed in this paper is also confirmed by simulations using convolutional and turbo codes.

Block-error rate model for DPSK in Rayleigh- and sub-Rayleigh-fading channels

Wireless transmission channels experience substantial signal degradation due to irregularities in the propagation path that impact synchronization between two communicating ends. Consequently, differential and noncoherent techniques that require no such synchronization have gained wide popularity. However, the transmitted data are still highly prone to transmission errors and appropriate error-control mechanisms have to be employed. Knowledge of the error statistics becomes crucial when designing these systems. A block-error rate (BLER) characteristic defines the error distribution within data blocks and in this paper, we propose a finite-state Markov chain-based method to evaluate BLERs in slow frequency-nonselective Rayleigh-fading channels for differential phase-shift keying. The methodology is then extended to the cases of faster Rayleigh and sub-Rayleigh fading. The model is verified by means of numerical simulation. The proposed model possesses many advantages of the well-developed Markov theory and may be easily extended to other modulation schemes such as noncoherent frequency shift keying.

Improved Channel Estimation for OFDM Systems with Multiple Transmit Antennas over Time-Varying Multipath Fading Channels

By using multiple transmit antennas, wireless systems have a large capacity in time-varying multipath fading channels. Space-time block code (STBC), space-frequency block code (SFBC), and space-time-frequency (STF) block code are well-known techniques in transmitter diversity schemes. While the SFBC (or the STF block coded) system gives full diversity at frequency-nonselective channels, it breaks down when used in a frequency-selective environment. This is because the SFBC (or the STF block code) scheme disregards frequency selectivity of the channel by assuming that channel frequency responses (CFRs) at adjacent subcamers are the same. In this paper, we propose efficient channel estimation and symbol decoding methods, which consider the difference between CFRs at the adjacent subcamers of the SFBC (or the STF block coded) orthogonal frequency division multiplexing (OFDM) system in multipath fading channels. The proposed method gives initial channel information by designing a simple training symbol, and the CFRs at all the subcarriers and the differences between the CFRs are easily calculated by using an interpolation method or a discrete Fourier transform (DFT) operation.

Frequency-Domain Pre-Equalization Transmit Diversity for MC-CDMA Uplink Transmission

Recently, multi-carrier code division multiple access (MC-CDMA) has been attracting much attention for the broadband wireless access in the next generation mobile communications systems. In the case of uplink transmissions, the orthogonality among users' signals is lost since each user's signal goes through different fading channel and hence, multi-access interference (MAI) is produced, thereby significantly degrading the transmission performance compared to the downlink case. The use of frequency-domain equalization at the receiver cannot sufficiently suppress the MAI. In this paper, we propose frequency-domain pre-equalization transmit diversity (FPTD), which employs pre-equalization using multiple transmit antennas with transmit power constraint, in order to transform a frequency-selective channel seen at a receiver close to the frequency-nonselective channel. We theoretically analyze the bit error rate (BER) performance achievable with the proposed FPTD and the analysis is confirmed by computer simulation.

SVM Multiregression for Nonlinear Channel Estimation in Multiple-Input Multiple-Output Systems

This paper addresses the problem of multiple-input multiple-output (MIMO) frequency nonselective channel estimation. We develop a new method for multiple variable regression estimation based on Support Vector Machines (SVMs): a state-of-the-art technique within the machine learning community for regression estimation. We show how this new method, which we call M-SVR, can be efficiently applied. The proposed regression method is evaluated in a MIMO system under a channel estimation scenario, showing its benefits in comparison to previous proposals when nonlinearities are present in either the transmitter or the receiver sides of the MIMO system.