Frequency-selective Radio Channels Research Articles

Analytical model OFDM-MIMO signal in a radio channel with frequency-time selective fading

One of the main problems in modern wireless electronic communication networks is the transmission and reception of signals in multipath propagation. Due to the imperfect impulse response of the communication channel, frequency-selective fading of the transmitted signal occurs. This can cause waveform distortion and inter-symbol interference, which can seriously degrade the performance of the communication system. Thus, increasing the noise immunity of wireless electronic communication networks in real-world conditions is one of the most important tasks of modern communication technologies. Orthogonal frequency division multiplexing is known for its resistance to multipath fading and is used to reduce the impact of inter-symbol interference. Multi-antenna systems are a key technology for increasing bandwidth and improving communication reliability. The combination of these two technologies in wireless electronic communication networks can reduce the impact of negative phenomena on the transmitted signal in difficult interference conditions. An analytical model of the signal transmitted by a nonstationary frequency-selective radio channel in a wireless electronic communication network with orthogonal frequency multiplexing and a multi-antenna system is proposed. Analytical expressions are presented that allow us to jointly take into account the influence of interchannel and inter-symbol interference (the effect of multipath signal propagation and Doppler shift on the transmitted signal) against the background of fluctuating noise. Taking into account the factors affecting the transmitted signal makes it possible to bring the model closer to the real processes that occur in the vast majority of radio channels of wireless electronic communication networks.

Joint CFO and channel estimation using pilot aided interpolation for high performance MIMO-OFDM

ABSTRACT Multiple Input Multiple Output (MIMO) system with orthogonal frequency division multiplexing (OFDM) is a promising physical layer candidate for the current and future wireless standards. MIMO increases system throughput while OFDM maximizes spectrum efficiency. To reduce the impacts of frequency selective fading and increase the capacity, MIMO requires precise channel state information (CSI). The presence of carrier frequency offset (CFO) introduces intercarrier interference (ICI) among the subcarriers which degrades the system performance. MIMO channel estimation is done under the assumption of perfect frequency synchronisation between the transmitter and receiver, or zero CFO. However, such an assumption is incorrect for time-varying and frequency-selective wireless channels, and the presence of CFO has an impact on channel estimation accuracy. In the existing methods, CFO and channel estimations are carried out independently using separate training sequences which reduces the spectral efficiency. As a result, the pilot aided interpolation approach is proposed in this paper for the joint estimate and compensation of CFO and channel effects using the same set of pilot tones. Theresults show that the proposed method offers precise CFO and channel estimation which improves the bit error rate performance and carrier to interference power ratio (CIR) of MIMO-OFDM system.

Semi-Supervised Extreme Learning Machine Channel Estimator and Equalizer for Vehicle to Vehicle Communications

Wireless vehicular communications are a promising technology. Most applications related to vehicular communications aim to improve road safety and have special requirements concerning latency and reliability. The traditional channel estimation techniques used in the IEEE 802.11 standard do not properly perform over vehicular channels. This is because vehicular communications are subject to non-stationary, time-varying, frequency-selective wireless channels. Therefore, the main goal of this work is the introduction of a new channel estimation and equalization technique based on a Semi-supervised Extreme Learning Machine (SS-ELM) in order to address the harsh characteristics of the vehicular channel and improve the performance of the communication link. The performance of the proposed technique is compared with traditional estimators, as well as state-of-the-art machine-learning-based algorithms over an urban scenario setup in terms of bit error rate. The proposed SS-ELM scheme outperformed the extreme learning machine and the fully complex extreme learning machine algorithms for the evaluated scenarios. Compared to traditional techniques, the proposed SS-ELM scheme has a very similar performance. It is also observed that, although the SS-ELM scheme requires the largest operation time among the evaluated techniques, its execution time is still far away from the latency requirements specified by the standard for safety applications.

Shaping duo binary turbo-coded BICM scheme for JPWL image transmission using a link adaptation strategy over wireless channels

In order to guarantee a robust transmission of JPWL (JPEG Wireless: Joint Photographic Experts Group Wireless) images through time and frequency selective wireless channels, an efficient adaptive communication strategy is proposed. It is based on an optimization of a closed-loop adaptive multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) scheme associated with a shaping BICM (bit-interleaved coded modulation) technique composed of a duo binary turbo code (DBTC), high-order modulations such as 64–256 QAM (Quadrature Amplitude Modulation) and a shaping code. According to the CSI (channel state information) knowledge at the transmitter side, an algorithm based on unequal error protection (UEP) and unequal power allocation (UPA) is used to select the transmitter key parameters (source/channel encoder rate, modulation order, power, number of quality layers and number of iterations of the Turbo decoder) to achieve the target Quality of Service (QoS). The proposed DBTC-shaping BICM scheme reaches a shaping gain of 1.2 dB for a 256 QAM modulation over a SISO Gaussian channel, whereas only 0.7 dB of shaping gain can be achieved in a scheme that uses the LDPC shaping BICM scheme for the same modulation order. Based on a DBTC shaping BICM scheme and an adaptive algorithm, the proposed MIMO-OFDM strategy achieves better performance compared to a strategy using an iterative process between an RS (Reed-Solomon) and arithmetic decoders. As a result, and on the one hand, a gain of 5.38 dB can be achieved in terms of PSNR (peak signal-to-noise ratio). On the other hand, a gain of 78% in terms of power consumption is obtained for the same QoS level. Moreover, the adaptive number of iterations in the proposed strategy can minimize the computational complexity of the turbo decoding compared to a scheme using four iterations whatever the channel conditions.

Stochastic Geometry Analysis of Multi-User Asynchronous OFDM Wireless Networks

This letter develops an analytical framework for the analysis of asynchronous orthogonal frequency division multiplexing (OFDM)-based wireless networks. We assume that the transmitters have different timing misalignments with respect to a typical receiver, which results in a loss of orthogonality between different OFDM sub-carriers. Moreover, the position of a used sub-carrier, within the available spectrum, is taken into consideration in order to make the analysis more precise. A frequency selective wireless channel is considered, in order to appropriately characterize the impact of misaligned multi-user interference on the coverage probability of a typical user. The analysis is conducted by using the mathematical tool of stochastic geometry and by considering a social interaction-based connectivity model. With the aid of the proposed analytical framework, the impact of the considered connectivity model is studied and discussed. The developed approach is validated via Monte Carlo simulations.

Aerial Channel Prediction and User Scheduling in Mobile Drone Hotspots

In this paper, we investigate the aerial wireless channel, where a moving drone is deployed to stream content to a set of mobile clients on the ground over a small cell size. Experimental traces collected over more than twenty flights with multiple clients suggest that drone mobility in lateral or vertical path leads to time-selective and frequency-selective wireless channel for a low-altitude drone. The resulting aerial wireless channel can be predicted reasonably well when we model the channel based on the constructive and destructive interference patterns between the line-of-sight path and other propagation paths via nearby reflectors. We propose a novel channel prediction approach to predict the subcarrier SNRs for all clients as drone moves and a novel scheduling approach to select the subset of clients that maximize the network utility using the predicted SNRs. We have implemented the proposed approach on a commodity 802.11n chipset and evaluated in the field over twenty flights, each serving up to 17 live clients. Experiments demonstrate, for the first time, the feasibility of tracking and predicting the aerial Wi-Fi channel, resulting in up to a 56% increase in overall throughput as compared to the conventional 802.11n hotspot, while maintaining fairness across clients.

Robust MIMO-OFDM System for Frequency-Selective Mobile Wireless Channels

This paper presents a novel technique to enhance the robustness of space–frequency block coded (SFBC) orthogonal frequency-division multiplexing (OFDM) systems. The proposed system shapes the channel matrix of a frequency-selective fading channel into a piecewise flat fading over each block of two adjacent subcarriers. Analytical and simulation results show that the proposed scheme substantially outperforms the conventional SFBC in frequency-selective fading channels, which assumes that the channel parameters are equal over the duration of the codeword. Moreover, we demonstrate that the proposed system offers a superior performance in fast time-varying channels where the interference that results from the channel variations acts only as an additive noise. In addition to its superior performance, the proposed system has low computational complexity, because it is based on the short-block-length Walsh–Hadamard transform.

Performance of opportunistic fixed-beam spatial multiplexing in OFDMA uplink

Adaptive spatial multiplexing (SM) Multiple Input Multiple Output (MIMO) techniques enhance the spectral efficiency of wideband wireless communications in favorable radio channel conditions. In this study, we show the benefits of combining a traditional fixed-beam scheme and multiuser opportunistic radio resource allocation in space-time-frequency domains. This combining is feasible in wideband Orthogonal Frequency Domain Multiple Access (OFDMA) systems such as the Universal Mobile Telecommunications System (UMTS) Long Term Evolution (LTE). This study brings novel knowledge which indicates that an orthogonal fixed beam approach benefits from opportunistic radio resource allocation clearly more than a conventional antenna domain approach in frequency-selective radio channels. It was found out that the fixed beam radio link performance is enhanced in wideband adaptive radio links due to the fact that orthogonal beams reduce correlation between the MIMO channels in the allocated sub-bands. Moreover, beamforming gains bring higher Eigenvalues of the MIMO channel matrix in the opportunistically selected sub-bands. It is shown that the fixed beam deployment changes the distribution of the MIMO channel correlation values and Eigenvalues in a manner which enhances opportunistic multiuser gains in wideband time-frequency-selective MIMO radio channels. Simulation results with 2 × 2 spatial multiplexing in an OFDMA uplink indicate that the proposed beam domain scheme gives up to 80% data throughput gain over the corresponding antenna domain scheme in a pedestrian microcell environment.

A wide band QAM receiver based on DFE to reject ISI in wireless time varying fading channel

Quadrature Amplitude Modulation (QAM) system with high data rate suffers from Inter-Symbol-Interference (ISI) in frequency-selective wireless channel. To reduce this ISI signal, Decision Feedback Equalizer (DFE) is used. When the channel impulse response is time varying, the channel's coefficients need to be adaptively estimated. Adaptive implementation of the forward filter on the conventional DFE can track the time variation of the channel coefficients, but its tracking capability depends on the covariance matrix of the receiver vector and the speed of channel variation. In this paper, wide band QAM receiver is proposed with new arrangement of the DFE filters. Data diversity at the receiver is achieved by using N antennas at its input. The forward filter of the DFE will be used as a Maximum-Ratio-Combiner (MRC) as well as a channel compensator. By first removing the ISI signal from the received vector, the tracking performance of the forward filter will be enhanced. The new performance of the proposed receiver will be compared with the conventional DFE receiver. The tracking performance of the forward filter using LMS and RLS algorithms in the new arrangement is compared with that of the conventional DFE receiver.

Minimum-Error-Probability CFO Estimation for Muti-User MIMO OFDM Systems

We consider carrier frequency offset (CFO) estimation in the context of multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems over noisy frequency-selective wireless channels with both single- and multiuser scenarios. We conceived a new approach for parameter estimation by discretizing the continuous-valued CFO parameter into a discrete set of bins and then invoked detection theory, analogous to the minimum-bit-error-ratio optimization framework for detecting the finite-alphabet received signal. Using this radical approach, we propose a novel CFO estimation method and study its performance using both analytical results and Monte Carlo simulations. We obtain expressions for the variance of the CFO estimation error and the resultant BER degradation with the single-user scenario. Our simulations demonstrate that the overall BER performance of a MIMO-OFDM system using the proposed method is substantially improved for all the modulation schemes considered, albeit this is achieved at increased complexity.

Joint Transceiver Design for Iterative FDE

This paper considers the wideband single-carrier transmission through frequency-selective wireless channels with a quality-of-service (QoS) requirement. Power allocation at the transmitter and iterative frequency domain equalization (FDE) at the receiver are jointly designed to achieve the target QoS with minimum transmission power consumption. The joint transceiver design is based on a novel evolutional analysis framework for iterative FDE without the aid of decoding during the iterative processing. It introduces a set of QoS and quality-of-convergence (QoC) constraints, which recast the design problem into a convex and structured joint QoS/QoC constrained power allocation problem. Thanks to the underlying sparsity in the mathematical structure of the problem, a specific interior-point method (IPM) solver is developed, which significantly reduces the computational complexity, as compared to the generic solvers. A duality gap analysis is then applied to show that the performance gap between the numerical algorithms and the theoretical optimum can be controlled by adjusting its key parameters, which, in turn, determines the convergence behavior of the iterative solver. Extensive simulation results are given to support the mathematical development of the problem.

A learning approach in link adaptation for MIMO-OFDM systems

We propose a neural network (NN)-based adaptive modulation and coding (AMC) for link adaptation in MIMO-OFDM systems. The AMC optimizes the best modulation and coding scheme (MCS) under a packet error rate (PER) constraint. In our approach, a NN with a multilayer perceptron (MLP) structure is applied for the AMC and its performance is compared with the k-nearest neighbor (k-NN) algorithm under the frequency-flat (1-tap) and frequency-selective (4-tap) wireless channel conditions. The simulation results show that the NN classifier outperforms the k-NN algorithm, especially in terms of the PER, due to the fact that the MLP guarantees a MCS with a lower data rate by way of the selection of a class label with a lower index number. It has a slightly worse spectral efficiency performance compared to the k-NN. Thus, the MLP approach provides higher communication robustness over the k-NN. It can be concluded from the results that the selection of the AMC classifier depends on a trade-off between the PER and the spectral efficiency, relying on the user's requirements.

Frequency-Domain Equalization for Orthogonal and Quasi-Orthogonal STBCs over Frequency-Selective Wireless and Power-Line Channels

In recent years, combination of single-carrier frequency-domain equalization (SC-FDE) and space-time block coding/code (STBC) techniques to exploit the advantages of both, has received a great attention. In this paper we propose new techniques for combining SC-FDE with orthogonal and Quasi-orthogonal STBCs applicable to any number of transmit antennas. For Quasi-orthogonal STBC we first propose a new structure for codes with four transmit antennas and then extend it to higher numbers. We convert Quasi-orthogonal system to two equivalent orthogonal subsystems and equalize and decode these subsystems based on our proposed procedure for orthogonal codes. Finally, we present our simulation results for different frequency-selective wireless and power-line channels and show that a significant SNR gain is achieved when SC-FDE is combined with diversity techniques.

Performance of MUTP-aided MIMO systems over correlated frequency-selective wireless communication channels: a multi-cell perspective

In this article, we investigate the performance of multiuser transmitter preprocessing (MUTP)-aided multiple-input multiple-output (MIMO) systems in a multi-cell multiuser setting where co-channel interference (CCI) is the major channel impairment, for both uplink (UL) and downlink (DL) transmissions. CCI can considerably reduce data rates resulting in outages in cellular systems, particularly at the cell edges in DL transmission. The MUTP considered in this article is based on singular value decomposition (SVD), which exploits the channel state information (CSI) of all the users at the base stations (BSs) with the aid of BS cooperation, and only the individual users' CSI at the mobile stations (MSs) for both UL and DL transmissions. In particular, in this article, we study the effects of three types of delay spread distributions coupled with different interferer configurations over correlated and uncorrelated frequency-selective channels. Our simulation study shows that SVD-aided MUTP perfectly eliminates CCI with lesser detection complexity under perfect CSI. Also, we provide performance comparisons of SVD-aided MUTP with various precoding techniques widely addressed in literature, and the results show that it provides better achievable symbol error rate (SER) by mitigating multi-stream interference (MSI) and CCI. Further, simulation results demonstrate that compared to equal CCI, the presence of a dominant interferer can lead to more degradation in the system performance in terms of achievable SER while, further degradation results when noise is dominant. Furthermore, this study confirms that imperfect CSI as well as imperfect power control can lead to degradation in the system performance.

Cooperative multiple relay system for frequency selective environment: outage analysis and power allocation

AbstractIn this paper, we propose a novel cooperative relaying scheme with multiple relays for frequency selective wireless environment. In our proposed scheme, the frequency selective wireless channel is divided into flat fading sub‐channels. Cooperative relaying is then employed over each sub‐channel to improve the system diversity order. We then investigate the asymptotic behavior of the outage probability and show that the proposed scheme achieves full diversity order in both amplify and forward (AF) and adaptive decode and forward (ADF) relaying scenarios. Furthermore, we propose a power allocation strategy to minimize the system outage probability. Simulation results confirm our analysis and show that the proposed power allocation method outperforms uniform power allocation. Copyright © 2010 John Wiley & Sons, Ltd.

Physical layer security of MIMO–OFDM systems by beamforming and artificial noise generation

In this paper we address physical layer security in multiple-input-multiple-output (MIMO) frequency selective wireless channels in the presence of a passive eavesdropper, i.e., the associated channel is unknown to the transmitter. Signalling is based on orthogonal frequency division multiplexing (OFDM). Spatial beamforming and artificial noise broadcasting are chosen as the strategy for secure transmission. The contribution of channel frequency selectivity to improve secrecy is presented by performance and probabilistic analysis. Moreover, we investigate the capability of the eavesdropper to jeopardize the security of the system (defined as the SNR difference between the intended receiver and the eavesdropper) by mitigating the interfering effect of the artificial noise using zero forcing as a receive beamforming strategy. The results show that although zero forcing is not the optimal strategy to maximize the SNR, it offers (from the eavesdropper’s perspective) a better performance than MMSE for MIMO frequency selective channels and thus threatens the overall security of the system.

Performance of MMSE Receiver based Multi Input Multi Output-Interleave Division Multiple-Access System with Multi-user Detection over Frequency Selective Wireless Communication Channel

Problem statement: This study presents the performance analysis of turbo assisted Interleave Division Multiple-Access (IDMA) system with Multi Input Multi Output (MIMO) support for multi user scenario over correlated frequency selective and uncorrelated frequency selective channel. Approach: The key principle of IDMA is that interleaver unique which distinguishes the users in contrast to spreading sequence in Code Division Multiple Access System (CDMA). Results: In this work, we assume that Interleavers are generated independently and randomly. At the receiver, we employed Ordered SIC (OSIC) technique using ZF and MMSE criterion to combat Inter Antenna Interference (IAI) and Multi User Interference (MUI) problem along with iterative decoding to improve the performance in terms of BER. The performance of system has been discussed for different channel conditions with realistic channel model using extensive simulation runs based on Monte Carlo simulation trials. We have exhibited the flexibility and robustness provided by MIMO-IDMA. Conclusion/Recommendations: It has been proved from the results that IDMA principle can be applied to realize many potential performance gains highlighted by information theory, including coding gain multiplexing gain and multiuser gain. Simulation results presented to demonstrate the benefits of IDMA with MUD and iterative decoding. It is discerned that IDMA performs better than CDMA in frequency selective channel for high load conditions which is assessed through computer simulation results.

Performance Analysis of Multi-user Multi Input Multi Output- Interleave-Division Multiple-Access System Employing Turbo Coding with Multi-User Detection over Frequency-Selective Wireless Communication Channel

Problem statement: This study presents the performance analysis of multi-user Multi Input Multi Output (MIMO) assisted interleave based multiple-access system. In IDMA, different interleavers are used to distinguish users as against different signature sequence in a conventional code-Division Multiple-Access (CDMA) scheme. Approach: The basic principle of IDMA is that the interleaver is unique for the users. Results: In this study, we consider that Interleavers are generated independently and randomly. Also the IDMA technique is extended to multi user MIMO IDMA with multi-user detection. At the receiver, OSIC detector is realized using ZF for frequency fading channel to combat MAI and MUI problem. The performance of the system is analyzed for different channel conditions using extensive simulation runs based on Monte Carlo simulation trials. Conclusion: It is shown that the IDMA scheme can achieve near single user performance in situations with very large numbers of users while maintaining very low receiver complexity. It is discerned from the computer simulation results that IDMA outperforms CDMA in frequency selective channel for high load conditions.

A Gaussian Mixture Approach to Blind Equalization of Block-Oriented Wireless Communications

We consider blind equalization for block transmissions over the frequency selective Rayleigh fading channel. In the absence of pilot symbols, the receiver must be able to perform joint equalization and blind channel identification. Relying on a mixed discretecontinuous state-space representation of the communication system, we introduce a blind Bayesian equalization algorithm based on a Gaussian mixture parameterization of the a posteriori probability density function (pdf) of the transmitted data and the channel. The performances of the proposed algorithm are compared with existing blind equalization techniques using numerical simulations for quasi-static and time-varying frequency selective wireless channels.

Application of Sequential Estimation Algorithm Based on Branch Metric to Frequency-Selective Channel Equalization

In this work, a sequential estimation algorithm based on branch metric is used as channel equalizer to combat intersymbol interference in frequency-selective wireless communication channels. The bit error rate (BER) and computational complexity of the algorithm are compared with those of the maximum likelihood sequence estimation (MLSE), the recursive least squares (RLS) algorithm, the Fano sequential algorithm, the stack sequential algorithm, list-type MAP equalizer, soft-output sequential algorithm (SOSA) and maximum-likelihood soft-decision sequential decoding algorithm (MLSDA). The BER results have shown that whilst the sequential estimation algorithm has a close performance to the MLSE using the Viterbi algorithm, its performance is better than the other algorithms. Beside, the sequential estimation algorithm is the best in terms of computational complexity among the algorithms mentioned above, so it performs the channel equalization faster. Especially in M-ary modulated systems, the equalization speed of the algorithm increases exponentially when compared to those of the other algorithms.