An Efficient Cyclic Prefix Reconstruction Technique for MIMO Single-Carrier Frequency-Domain Equalization

In this letter, an efficient cyclic prefix reconstruction (CPR) technique with turbo equalization is developed for a multi-input multi-output (MIMO) single-carrier frequency-domain equalization (SC-FDE) system. The proposed method consists of pre-processing estimation (PPE) and residual inter-carrier interference suppression (RICIS). The PPE is employed to compute initial values of MIMO turbo equalization, and the RICIS is used to mitigate residual intercarrier interference (ICI) after each iteration of the CPR. By applying the proposed method to MIMO SC-FDE system with insufficient cyclic prefix (CP), we can significantly improve its error performance, obtaining both the benefits of multiplexing gain and spectral efficiency gain     

MIMO-OFDM系统载波间干扰的时域消除

由于载波间干扰（ICI）是影响正交频分复用（OFDM）系统性能的重要因素,一般消除ICI时作信道估计都采用辅助导频的方法,需要使用大量的离散导频,为此提出了一种对MIMO信道中ICI消除的时域均衡方法。该方法利用MIMO空时检测后的数据信息迭代回去,当作导频符号估计出时变信道的冲激响应,然后通过时域均衡消除ICI,而无需额外的导频,在改善系统误误码率的同时,提高了频谱的利用率。     

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.     

An MIMO-OFDM technique for high-speed mobile channels

In this letter, a new orthogonal frequency-division multiplexing (OFDM) technique for multiple-input multiple output (MIMO) channels is proposed to reduce interchannel interference (ICI) caused by high-speed mobiles in cellular environments. After analyzing the ICI caused by high-speed mobile channels using a simple curve fitting technique, the weighting factor for group transmission is optimized. Then, a new MIMO-OFDM technique, based on the weighting factor optimization, is proposed for reducing ICI caused by time-varying channels. Performances of the proposed technique are verified by using the I-METRA channel, proposed for an MIMO channel to 3GPP, and a MIMO-OFDM simulator designed for macrocellular mobile communication. It is shown by computer simulation that the proposed MIMO-OFDM technique is effective in reducing ICI and noise as well as in obtaining diversity gain even under highly-correlated fast fading channels, compared with the conventional MIMO-OFDM schemes.     

Finite-SNR Diversity–Multiplexing Tradeoff for Correlated Rayleigh and Rician MIMO Channels

A nonasymptotic framework is presented to analyze the diversity-multiplexing tradeoff of a multiple-input-multiple-output (MIMO) wireless system at finite signal-to-noise ratios (SNRs). The target data rate at each SNR is proportional to the capacity of an additive white Gaussian noise (AWGN) channel with an array gain. The proportionality constant, which can be interpreted as a finite-SNR spatial multiplexing gain, dictates the sensitivity of the rate adaptation policy to SNR. The diversity gain as a function of SNR for a fixed multiplexing gain is defined by the negative slope of the outage probability versus SNR curve on a log-log scale. The finite-SNR diversity gain provides an estimate of the additional power required to decrease the outage probability by a target amount. For general MIMO systems, lower bounds on the outage probabilities in correlated Rayleigh fading and Rician fading are used to estimate the diversity gain as a function of multiplexing gain and SNR. In addition, exact diversity gain expressions are determined for orthogonal space-time block codes (OSTBC). Spatial correlation significantly lowers the achievable diversity gain at finite SNR when compared to high-SNR asymptotic values. The presence of line-of-sight (LOS) components in Rician fading yields diversity gains higher than high-SNR asymptotic values at some SNRs and multiplexing gains while resulting in diversity gains near zero for multiplexing gains larger than unity. Furthermore, as the multiplexing gain approaches zero, the normalized limiting diversity gain, which can be interpreted in terms of the wideband slope and the high-SNR slope of spectral efficiency, exhibits slow convergence with SNR to the high-SNR asymptotic value. This finite-SNR framework for the diversity-multiplexing tradeoff is useful in MIMO system design for realistic SNRs and propagation environments     

基于V-BLAST的特征波束形成技术在大规模MIMO中的运用

为了获得较高的频谱效率及较低的误码率,提出了大规模MIMO系统中基于V-BLAST的特征波束形成技术(the Eigen-Beamforming combined with V-BLAST,V-BLAST&E-BF),利用大规模天线形成多个特征波束,在这些特征波束上传输多个码流,既可以获得阵列增益又可以获得复用增益.仿真结果表明:提出的方案较MF预编码和传统特征波束形成技术具有较好的性能,并且在接收端无须进行传统V-BLAST的检测算法(如MMSE检测、ZF检测)亦可分离出信号.     

一种基于MIMO空时分集的HARQ方法及其性能分析

通常,MIMO利用降低空时编码效率的方法,在复用增益与分集增益乘积一定的约束条件下,实现二者折衷;该文则从系统设计的角度,通过分析VBLAST,HARQ,STBC内在的联系,构造一种新型的混合重传方法,实现编码与HARQ二者平滑对接,在保持空时码满的复用增益前提条件下,以HARQ时延换取信噪比的提升,额外引入分集增益。同时,该文对信道相关性、重传时间与空间分集之间的重要关系作了系统深入的理论分析。仿真结果表明,所提方法对不同信道环境下的系统性能均有较大的提升。     

Multiplexing Gain of Variable-Rate MIMO Systems

Multiple-input multiple-output (MIMO) systems with adaptive modulation and coding (AMC) have a potential to approach the channel capacity. We analyze the multiplexing gain of variable-rate (VR) MIMO systems in a generic framework and derive a sufficient and necessary condition for a VR MIMO system to achieve any given multiplexing gain. A pragmatic way of predicting the multiplexing gain of a VR MIMO system based on the component modulation modes is obtained     

Transmitter Precoding for ICI Reduction in Closed-Loop MIMO OFDM Systems

The mitigation of intercarrier interference (ICI) in closed-loop single-input-single-output (SISO) and multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) is considered. The authors show that the ICI coefficient matrix is approximately unitary and exploit this property to design a nonlinear Tomlinson-Harashima precoder for the reduction of ICI in closed-loop SISO OFDM and orthogonal space-time block-coded (OSTBC) MIMO OFDM. With the proposed design, the transmitter does not need to know the frequency offsets, and hence, their impact on the bit error rate (BER) is significantly reduced. Moreover, for spatially correlated MIMO channels, the precoder and OSTBC OFDM perform with a negligible BER-performance loss     

Quality of Service Aware Inter Carrier Interference Mitigation and Antenna Selection Schemes for Beyond 4G Systems

Future broadband wireless communication systems demand high quality of service (QoS) for anytime anywhere multimedia applications. The standards which use orthogonal frequency division multiplexing (OFDM) coupled with multi input multi output (MIMO) are expected to rule the future wireless world. Time selective nature of the channel introduces inter carrier interference (ICI), which is the major performance limiting parameter in OFDM based systems. ICI causes loss in spectral efficiency and results in poor bit error rate (BER) performance, affecting the QoS of MIMO-OFDM systems. The conventional single input single output (SISO)-OFDM-flexible subcarrier spacing (FSS) system offers better performance than the fixed subcarrier spacing systems in terms of ICI mitigation. But BER and spectral efficiency performance of SISO-OFDM-FSS is not good enough to satisfy the requirements of future wireless broadband services. To improve the BER performance, SISO-OFDM system is replaced by space frequency block coded (SFBC)-OFDM system, which adds spatial and frequency diversity benefits to the conventional system. More number of antennas in the MIMO scheme increases the hardware cost, computational complexity and percentage of overhead. In the present study, to improve the spectral efficiency and to reduce the complexity and cost, optimal transmit antenna selection (OTAS) is combined with the SFBC-OFDM-FSS scheme. The simulation results prove that the proposed SFBC-OFDM-FSS-OTAS scheme offers better QoS than the conventional SISO-OFDM-FSS scheme.     

Channel Magnifying Limited Feedback Technique for FDD Massive MIMO Systems

A limited feedback system, so-called, channel magnifying (CM) is proposed for a downlink (DL) frequency-division duplex (FDD) massive multiple-input multiple-output (MIMO) system. Although massive MIMO system has received significant research interest as a key technology for beyond 4G wireless communications systems, it has a number of issues that needs to be technically addressed. Among such issues is the difficulty of acquiring channel state information at transmitter for an FDD massive MIMO system which cannot exploit channel reciprocity as in a time-division duplex system. The proposed CM technique makes it possible to support a few user equipments in DL FDD massive MIMO system by finding a balance between spatial resources and channel quantization error (CQE). By choosing a subchannel with low CQE, CM can secure multiplexing gain at high SNR based on a fixed size codebook. Two types of subchannel indicator alignment (SIA) schemes are introduced for efficient interference nulling for the proposed CM technique. Specifically, we discuss how to maximize the sumrate of CM through genie added SIA and minimum CQE based SIA. Simulation results show that the sum rate of the proposed CM has a higher multiplexing gain than that of random vector quantization, especially when the number of transmit antennas is sufficiently large.     

Semi-Blind Time-Domain Equalization for MIMO-OFDM Systems

In this paper, a semi-blind time-domain equalization technique is proposed for general multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. The received OFDM symbols are shifted by more than or equal to the cyclic prefix (CP) length, and a blind equalizer is designed to completely suppress both intercarrier interference (ICI) and intersymbol interference (ISI) using second-order statistics of the shifted received OFDM symbols. Only a one-tap equalizer is needed to detect the time-domain signals from the blind equalizer output, and one pilot OFDM symbol is utilized to estimate the required channel state information for the design of the one-tap equalizer. The technique is applicable irrespective of whether the CP length is longer than, equal to, or shorter than the channel length. Computer simulations show that the proposed technique outperforms the existing techniques, and it is robust against the number of shifts in excess of the CP length.     

Efficient DMT/OFDM transmission with insufficient cyclic prefix

In this letter, we describe a new method to eliminate intersymbol interference (ISI) and interchannel interference (ICI) for discrete multitone/orthogonal frequency division multiplexing (DMT/OFDM) systems with insufficient cyclic prefix (CP). The proposed structure prevents ICI with a preprocessing method that utilizes redundancy in the frequency domain at the transmitter and removes ISI with a simple cancellation method at the receiver. Analytical and simulation results show that the proposed method can reduce computational complexity, while providing the same spectral efficiency as the frequency-domain equalizer (Trautmann and Fleige, 2002), which utilizes redundancy in the frequency domain at the receiver, in the presence of additive white Gaussian noise.     

Diversity‐multiplexing tradeoff over correlated Rayleigh fading channels: a non‐asymptotic analysis

In this paper, we present a finite‐signal‐to‐noise ratio (finite‐SNR) framework to establish tight bounds on the diversity‐multiplexing tradeoff of a multiple input multiple output (MIMO) system. We focus on a more realistic propagation environment where MIMO channel fading coefficients are correlated and where SNR values are finite. The impact of spatial correlation on the fundamental diversity‐multiplexing tradeoff is investigated. We present tight lower bounds on the outage probability of both spatially uncorrelated and correlated MIMO channels. Using these lower bounds, accurate finite‐SNR estimates of the diversity‐multiplexing tradeoff are derived. These estimates allow to gain insight on the impact of spatial correlation on the diversity‐multiplexing tradeoff at finite‐SNR. As expected, the diversity‐multiplexing tradeoff is severely degraded as the spatial correlation increases. For example, a MIMO system operating at a spectral efficiency of R bps/Hz and at an SNR of 5 dB in a moderately correlated channel, achieves a better diversity gain than a system operating at the same spectral efficiency and at an SNR of 10 dB in a highly correlated channel, when the multiplexing gain r is greater than 0.8. Another interesting point is that provided that the spatial correlation channel matrix is of full rank, the maximum diversity gain is not affected by the spatial correlation. Copyright © 2009 John Wiley & Sons, Ltd.     

几种接收机在MIMO信道下的性能比较

多入多出（MIMO）无线信道具有空间复用增益和分集增益特性,因此MIMO系统和单入单出（SISO）无线系统相比能够获得更高的频谱效率。本文在不同天线组合下分析了几种MIMO空时信号处理算法的性能,仿真结果和理论分析表明：空间复用增益和分集增益不能同时获得最大,因此在设计MIMO通信系统时可根据实际情况选择天线数,即不仅考虑系统抵抗信道衰落的分集增益,还要考虑能够提供更高的数据传输速率,通过折衷考虑空间复杂增益和分集增益,从更全面的观点评估系统的性能。     

Turbo编码MIMO OFDM系统多径干扰抵消

正交频分复用(OFDM)利用保护间隔(GI)消除因多径时延而产生的符号间干扰(ISI)和载波间干扰(ICI),但是太长的GI会占用过多的信道资源。为此,在保持一定GI长度的情况下,提出了利用减法抵消法消除ISI和加法抵消法抵消ICI的多径干扰抵消(MPIC)方法,使得当多径时延大于GI时,仍然可以保持系统的低误码率以及较强的抗多径干扰的优异性能。蒙特卡洛仿真结果表明,有多径干扰的情况下,在Turbo码编码的MIMO OFDM系统中使用提出的多径干扰抵消方法可以更有效地减少ISI和ICI。     

Time-Domain Block and Per-Tone Equalization for MIMO–OFDM in Shallow Underwater Acoustic Communication

Shallow underwater acoustic (UWA) channel exhibits rapid temporal variations, extensive multipath spreads, and severe frequency-dependent attenuations. So, high data rate communication with high spectral efficiency in this challenging medium requires efficient system design. Multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO–OFDM) is a promising solution for reliable transmission over highly dispersive channels. In this paper, we study the equalization of shallow UWA channels when a MIMO–OFDM transmission scheme is used. We address simultaneously the long multipath spread and rapid temporal variations of the channel. These features lead to interblock interference (IBI) along with intercarrier interference (ICI), thereby degrading the system performance. We describe the underwater channel using a general basis expansion model (BEM), and propose time-domain block equalization techniques to jointly eliminate the IBI and ICI. The block equalizers are derived based on minimum mean-square error and zero-forcing criteria. We also develop a novel approach to design two time-domain per-tone equalizers, which minimize bit error rate or mean-square error in each subcarrier. We simulate a typical shallow UWA channel to demonstrate the desirable performance of the proposed equalization techniques in Rayleigh and Rician fading channels.     

Polynomial Estimation of Time-Varying Multipath Gains With Intercarrier Interference Mitigation in OFDM Systems

In this paper, we consider the case of a high-speed mobile receiver operating in an orthogonal frequency-division multiplexing (OFDM) communication system. We present an iterative algorithm for estimating multipath complex gains with intersubcarrier interference (ICI) mitigation (using comb-type pilots). Each complex gain variation is approximated by a polynomial representation within several OFDM symbols. Assuming knowledge of delay-related information, polynomial coefficients are obtained from time-averaged gain values, which are estimated using the least-square (LS) criterion. The channel matrix is easily computed, and the ICI is reduced by using successive interference suppression (SIS) during data symbol detection. The algorithm's performance is further enhanced by an iterative procedure, performing channel estimation and ICI mitigation at each iteration. Theoretical analysis and simulation results for a Rayleigh fading channel show that the proposed algorithm has low computational complexity and good performance in the presence of high normalized Doppler spread.     

Joint Space-Frequency Iterative Interference Suppression in Multiuser MIMO-OFDM Systems

In uplink multiuser multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems, inter-user interference includes co-channel interference (CCI) due to spatial multiplexing and inter-carrier interference (ICI) due to carrier-frequency offset (CFO). In this letter, a space-frequency joint processing scheme is proposed to suppress the two-dimensional interference alternatively and iteratively. Moreover, by the studies of convergence behavior of iterations, a sufficient condition for convergence is presented. The theoretical analysis and simulation results both show that this iterative scheme can effectively suppress inter-user interference with low complexity.     

MIMO系统中线性弥散空时码的优化设计

MIMO系统的大部分空时编码方法在设计时单一强调对传输分集性能或传输复用性能的提高,为了联合提高分集性能和复用性能,在LDC编码基础上提出了线性正交弥散空时码(OLDC),LDC编码的优化中利用遗传算法来构造OLDC的空时正交调制基矩阵.优化的编码方法设计简单,便于调制解调.仿真结果表明,OLDC码比传统的LDC编码在误码性能、分集性能、复用性能等方面均有大幅度提高.