Compressed sensing channel estimation for STBC‐SM based hybrid MIMO‐OFDM system for visible light communication

This paper presents the idea of sparse channel estimation using compressed sensing (CS) method for space–time block coding (STBC), and spatially multiplexing (SM) derived hybrid multiple‐input multiple‐output (MIMO) Asymmetrically clipped optical‐orthogonal frequency division multiplexing (ACO‐OFDM) optical wireless communication system. This hybrid system accounts multiplexing gain of SM and diversity gain of STBC technique. We present a new variant of sparsity adaptive matching pursuit (SaMP) algorithm called dynamic step‐size SaMP (DSS‐SaMP) algorithm. It makes use of the inherent and implicit structure of SaMP, along with dynamic adaptivity of step‐size feature which is compatible with the energy of the input signal, thus the name dynamic step size. Existing CS‐based recovery algorithms like orthogonal matching pursuit, SaMP, adaptive step‐size SaMP, and proposed DSS‐SaMP were compared for hybrid MIMO‐ACO‐OFDM visible light communication system. The performance analysis is demonstrated through simulation results with respect to bit error rate, symbol error rate, mean square error, computational complexity, and peak‐to‐average power ratio. Simulation results show that the proposed technique gives improved performance and lesser computational complexity in comparison with conventional estimation algorithms.     

Improved linear group detection for combined spatial multiplexing/STBC systems

Wireless architectures combining the use of spatial division multiplexing (SDM) and space-time block coding (STBC) have begun to appear in the latest communications standards. In recent years, different detection strategies for these schemes have been proposed which can be broadly categorised as groupbased or direct-detection techniques. While the former class aims at separating the different STBC streams and then apply conventional simple Alamouti decoding, the latter type directly estimates the transmitted symbols without fully exploiting the Alamouti structure. Previous publications have shown that directdetection outperform group-based detectors with the penalty of a higher computational complexity. This letter presents a unified view of the detection strategies for hybrid SDM/STBC systems and presents two new families of group-based detectors. It is shown that linear group detection, when properly implemented, attains the same performance as linear direct detectors at a significant lower complexity.     

Iterative cyclic prefix reconstruction and channel estimation for a STBC OFDM system

In order to prevent a loss of spectral efficiency due to the use of a cyclic prefix (CP), a CP reconstruction (CPR) method, such as residual intersymbol interference cancellation (RISIC), has recently been developed. In this letter, we apply the RISIC method to an Alamouti space-time block coded (STBC) orthogonal frequency division multiplexing (OFDM) system with insufficient CP. It is shown that in the STBC OFDM, tail cancellation as well as cyclic restoration of the RISIC should be repeated. An iterative channel estimation method for the STBC OFDM system with CPR is proposed. The performance of the proposed method is evaluated by computer simulation in a multipath fading environment.     

Performance investigation of STBC-OFDM with code-division multiplexing in time-varying channels

Orthogonal frequency division multiplexing (OFDM) is sensitive to carrier frequency offset and channel estimation error, which destroy the subcarrier orthogonality and give rise to intercarrier interference (ICI). This paper addresses the performance degradation due to imperfect parameters in an orthogonal frequency and code division multiplexing (OFDM-CDM) system with multiple transmit antennas. For the performance evaluation, the average bit error rate (BER) impairment due to imperfect channel information and frequency offset is investigated taking into account the effect of time-varying channels. Derived results show that a space-time block coded (STBC) OFDM-CDM system experiences a severe BER degradation regardless of the number of users.     

Minimum symbol error rate multiuser detection using an effective invasive weed optimization for MIMO/SDMA–OFDM system

Space division multiple access–orthogonal frequency division multiplexing system has become a potential wireless communication system by offering high spectral efficiency, performance and capacity. This article deals with minimum symbol error rate (MSER)‐based multiuser detection (MUD) technique for the space division multiple access–orthogonal frequency division multiplexing system using an efficient invasive weed optimization (IWO) algorithm. The IWO algorithm is used for finding optimal weights such that the probability of error is directly minimized rather than minimizing the mean square error. Because of this, the MSER MUD is able to detect users even in overload scenario, where the number of users is more than the number of receiving antennas, unlike several classical detection techniques. The IWO is inspired from the nature of invasive colonization of weeds and relatively simple compared with other optimization techniques. The bit error rate performance of the proposed IWO‐aided MSER MUD is found to be better than minimum means square error and differential evolution algorithm‐aided MSER MUDs. Simulation results show that the proposed IWO MSER achieves faster convergence and lower complexity compared with the differential evolution MSER MUD. Copyright © 2013 John Wiley & Sons, Ltd.     

Genetic Grey Wolf Optimizer Based Channel Estimation in Wireless Communication System

Various methods are available for channel estimation in the orthogonal frequency division multiplexing and orthogonal frequency and code division multiplexing (OFCDM) based wireless communication schemes. Along with this, the most utilized techniques are namely the minimum mean square error (MMSE) and least square (LS). The process of LS channel estimation method is simple but it occupies a very high mean square error. On the other hand, the performance of MMSE is better than LS in terms of SNR, though it shows high computational complexity. Compared to MMSE and LS based techniques, the combination of MMSE and LS techniques using evolutionary programming reduces the error significantly to receive exact signal. In this study, we propose a hybrid method namely GGWO that includes grey wolf optimization (GWO) and genetic algorithms (GA) for estimate the channel in MIMO–OFCDM schemes. At first, the best channel is estimated using GWO and afterwards, the MMSE and LS are hybridized through GA for calculating the best channel to decrease error. Overall, the GWO and GA contribute in fine tuning the obtained channel scheme so that the channel model is derived further to correlate with the ideal scheme. Our results demonstrate that the proposed scheme is superior to conventional MMSE and LS in terms of BER and SNR.     

Low complexity channel estimation for space-time coded wideband OFDM systems

In this article, channel estimation for space-time coded orthogonal-frequency division multiplexing (OFDM) systems is considered. By assuming that the channel frequency response is quasi-static over two consecutive OFDM symbols, we develop channel parameter estimators based on the use of space-time block coded (STBC) training blocks. Using an STBC training pattern, a low-rank Wiener filter-based channel estimator with a significant complexity reduction is proposed. A simplified approach for the optimal low-rank estimator is also proposed to further reduce the estimator complexity while retaining an accurate frequency domain channel estimation. Numerical results are provided to demonstrate the performance of the proposed low complexity channel estimators for space-time trellis coded OFDM systems.     

A new subspace method for blind estimation of selective MIMO‐STBC channels

In this paper, a new technique for the blind estimation of frequency and/or time‐selective multiple‐input multiple‐output (MIMO) channels under space‐time block coding (STBC) transmissions is presented. The proposed method relies on a basis expansion model (BEM) of the MIMO channel, which reduces the number of parameters to be estimated, and includes many practical STBC‐based transmission scenarios, such as STBC‐orthogonal frequency division multiplexing (OFDM), space‐frequency block coding (SFBC), time‐reversal STBC, and time‐varying STBC encoded systems. Inspired by the unconstrained blind maximum likelihood (UML) decoder, the proposed criterion is a subspace method that efficiently exploits all the information provided by the STBC structure, as well as by the reduced‐rank representation of the MIMO channel. The method, which is independent of the specific signal constellation, is able to blindly recover the MIMO channel within a small number of available blocks at the receiver side. In fact, for some particular cases of interest such as orthogonal STBC‐OFDM schemes, the proposed technique blindly identifies the channel using just one data block. The complexity of the proposed approach reduces to the solution of a generalized eigenvalue (GEV) problem and its computational cost is linear in the number of sub‐channels. An identifiability analysis and some numerical examples illustrating the performance of the proposed algorithm are also provided. Copyright © 2009 John Wiley & Sons, Ltd.     

ML‐Based Estimation Algorithm of Frequency Offset for 2×2 STBC‐OFDM Systems

In this letter, we propose a novel frequency offset estimation algorithm for space‐time block code (STBC) orthogonal frequency division multiplexing systems. The algorithm mainly exploits the specific construction of STBC so that it does not need any additional pilots or sequences in the data field. The estimator is derived on the basis of the maximum likelihood theory. Simulation results show that this method can provide a significant performance improvement in terms of the estimation accuracy of the frequency offset.     

Soft-Information Assisted Near-Optimum Nonlinear Detection for BLAST-type Space Division Multiplexing OFDM Systems

In this contribution, a nonlinear hybrid detection scheme based on a novel soft-information assisted genetic algorithm (GA) is proposed for a turbo convolutional (TC) coded space division multiplexing (SDM) aided orthogonal frequency division multiplexing (OFDM) system. Our numerical results show that the performance of the currently known GA-assisted system can be improved by about 2 dB with the aid of the GA's population-based soft solution, approaching the optimum performance of the soft-information assisted maximum likelihood (ML) detection, while exhibiting a lower complexity, especially in high-throughput scenarios. Furthermore, the proposed scheme is capable of achieving a good performance even in the so-called overloaded systems, where the number of transmit antennas is higher than the number of receiver antennas     

MC‐CDMA MIMO systems with quasi‐orthogonal space–time block codes: Channel estimation and multiuser detection

This paper investigates blind channel estimation and multiuser detection for quasi‐synchronous multi‐carrier code‐division multiple‐access (MC‐CDMA) multiple‐input multiple‐output (MIMO) systems with quasi‐orthogonal space–time block codes (QO‐STBC). Subspace‐based blind channel estimation is proposed by considering a QO‐STBC scheme that involves four transmit antennas and multiple receive antennas. Based on the first‐order perturbation theory, the mean square error of the channel estimation is derived. With the estimated channel coefficients, we employ minimum output energy and eigenspace receivers for symbol detection. Using the QO‐STBC coding property, the weight analyses are performed to reduce the computational complexity of the system. In addition, the forward–backward averaging technique is presented to enhance the performance of multiuser detection. Numerical simulations are given to demonstrate the superiority of the proposed channel estimation methods and symbol detection techniques. Copyright © 2011 John Wiley & Sons, Ltd.     

室内可见光通信中的自适应STBC MIMO-OFDM系统设计

提出基于空时块码（STBC）调制的自适应多输入多输出正交频分复用（MIMO-OFDM）可见光通信（VLC）系统,该系统能够克服MIMO信道相关性并实现可靠通信。同时,引入功率比特分配（PBA）与OFDM相结合,以适应信道传输条件从而有效提高频谱效率。通过搭建一个22的MIMOOFDM VLC演示系统,实现了80 cm距离的传输,实验中测得的误码率始终保持在7%的前向纠错阈值3.810-3之下。实验结果表明,STBC MIMO-OFDM系统对MIMO信道相关性鲁棒,且PBA的应用能够大幅度提高数据速率。     

A general scheme for equalization of space-time block-coded systems with unknown CSI

Recently, space-time block codes (STBCs) have gained much attention as an effective transmit diversity technique to increase the capacity of wireless communication systems. In this paper, we consider a general technique for direct equalization of space-time block-coding systems with unknown channel state information (CSI). This technique is suitable for several existing hybrid STBC schemes, such as STBC/orthogonal-frequency-division multiplexing (STBC-OFDM) and STBC/code-division multiple access (STBC-CDMA). We show that by exploiting the redundancy in the structure of STBC, a zero-forcing equalizer can be constructed without channel estimation. The conditions for the identifiability of the zero-forcing equalizer are also derived to ensure correct equalization. To further improve the performance of the proposed method, a new iterative algorithm is developed by incorporating the finite alphabet property of information symbols. Simulation results show that the proposed algorithms can outperform comparative schemes in most simulation conditions.     

Hybrid iterative multiuser detection for channel coded space division multiple access OFDM systems

Space division multiple access (SDMA) aided orthogonal frequency division multiplexing (OFDM) systems assisted by efficient multiuser detection (MUD) techniques have recently attracted intensive research interests. The maximum likelihood detection (MLD) arrangement was found to attain the best performance, although this was achieved at the cost of a computational complexity, which increases exponentially both with the number of users and with the number of bits per symbol transmitted by higher order modulation schemes. By contrast, the minimum mean-square error (MMSE) SDMA-MUD exhibits a lower complexity at the cost of a performance loss. Forward error correction (FEC) schemes such as, for example, turbo trellis coded modulation (TTCM), may be efficiently combined with SDMA-OFDM systems for the sake of improving the achievable performance. Genetic algorithm (GA) based multiuser detection techniques have been shown to provide a good performance in MUD-aided code division multiple access (CDMA) systems. In this contribution, a GA-aided MMSE MUD is proposed for employment in a TTCM-assisted SDMA-OFDM system, which is capable of achieving a similar performance to that attained by its optimum MLD-aided counterpart at a significantly lower complexity, especially at high user loads. Moreover, when the proposed biased Q-function based mutation (BQM) assisted iterative GA (IGA) MUD is employed, the GA-aided system's performance can be further improved, for example, by reducing the bit error ratio (BER) measured at 3 dB by about five orders of magnitude in comparison to the TTCM-assisted MMSE-SDMA-OFDM benchmarker system, while still maintaining modest complexity.     

FBG传感网络技术研究

系统分析和总结了现有 FBG传感技术的复用结构和系统性能 ,并依据复用方式的频域、时域和空间特性 ,从网络拓扑结构的角度 ,提出把 FBG传感网络划分为波分复用网络、时分复用网络、空分复用网络、频分复用网络和混合复用网络 ,在此基础上比较了各种复用方式的优缺点 ,并针对网络实用化开发提出了自己的观点 ,从而为 FBO传感网络拓扑结构及性能的研究提供了参考。     

OFDM‐Based STBC with Low End‐to‐End Delay for Full‐Duplex Asynchronous Cooperative Systems

We propose a new space‐time block coding (STBC) for asynchronous cooperative systems in full‐duplex mode. The orthogonal frequency division multiplexing (OFDM) transmission technique is used to combat the timing errors from the relay nodes. At the relay nodes, only one OFDM time slot is required to delay for a pair‐wise symbol swap operation. The decoding complexity is lower for this new STBC than for the traditional quasi‐orthogonal STBC. Simulation results show that the proposed scheme achieves excellent performances.     

Semidefinite programming detection for OvHDM signal

Non-orthogonal time-frequency division multiplexing （NTFDM） transmission scheme has been proposed to further improve the bandwidth efficiency and overcome the drawbacks of the conventional orthogonal frequency division multiplexing （OFDM） method. Based on such approach, the fast signal detection algorithm, semidefinite programming （SDP） detection, has been studied. As the coefficient matrix tends to be ill conditioned, the modified SDP algorithm combined with successive interference cancellation （SIC） has been developed. The improved algorithm is a good tradeoff between performance and detection complexity. Simulation results show that the proposed algorithm can achieve better performance than cutting plane aided SDP method.     

A Low-BER Clipping Scheme for PAPR Reduction in STBC MIMO-OFDM Systems

Clipping is a simple scheme to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. Further, it can be extended to space-time block coding (STBC) multiple-input multiple-output (MIMO) OFDM systems for the PAPR reduction. In the conventional clipping schemes for STBC MIMO-OFDM systems, the input symbols are first encoded and then clipped. In this paper, a new scheme is proposed, where the clipping operation is performed before space-time block coding. We theoretically prove that the proposed scheme has better bit-error rate (BER) performance while maintaining the same PAPR reduction as the conventional schemes. Additionally, we derive the symbol-error rate (SER) and BER expressions for the new scheme over multipath fading channels. The simulation results show a good match with our analysis.     

Delay-time=code division multi-access in frequency-selective channel

A new hybrid multi-access technique, called delay-time/code division multi-access (DT/CDMA), is proposed. Multiplexing is accomplished in both the delay-time domain and code domain. Delay-time division multiplexing is achieved by assigning different cyclic time delays to the same spreading sequence. At a receiver, frequency-domain equalisation, despreading, and demultiplexing are performed simultaneously in the frequency domain. The bit error rate performance when using the proposed DT/CDMA in a frequency-selective Rayleigh fading channel is evaluated by computer simulation.     

SAGE Algorithm for Semi-Blind Channel Estimation and Symbol Detection for STBC MIMO OFDM Systems

In this paper, a novel semi-blind channel estimation and symbol detection based on space-alternating generalized expectation-maximization (SAGE) algorithm are proposed for space time block coded (STBC) multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) systems. At the receiver, the transmitted signals of all users are added. By utilization of SAGE algorithm, this superimposed received signals are decomposed into their signal components. Then, the channel is estimated at pilot positions. After that, the channel estimation in the other positions are obtained by interpolation and symbol detection is done by utilizing the estimated channel. In SAGE algorithm, selection of initial value is very important for the convergence. We discuss the appropriate range for selection of initial value in this paper. Simulation results show that by increasing the number of users, the slope of bit error rate curve increases with same initial value and the channel estimation becomes worse by increasing the iteration of SAGE algorithm if the initial value is not selected in the proposed rang.