Performance of Alamouti transmit diversity over time-varying Rayleigh-fading channels

We analyze the impact of a time-varying Rayleigh-fading channel on the performance of an Alamouti transmit-diversity scheme. We propose several optimal and suboptimal detection strategies for mitigating the effects of a time-varying channel, and derive expressions for their bit-error probability as a function of the channel correlation coefficient /spl rho/. We find that the maximum-likelihood detector that optimally compensates for the time-varying channel is very tolerant to time-varying fading, attaining full diversity order even for the extreme case of /spl rho/=0. In contrast, although lower in complexity, the suboptimal schemes suffer a diversity penalty and are thus suitable only for slowly fading channels.     

ICI cancellation for OFDM communication systems in time-varying multipath fading channels

In orthogonal frequency division multiplexing (OFDM) systems, time-varying multipath fading leads to the loss of subcarrier orthogonality and the occurrence of intercarrier interference (ICI). In this study, an efficient ICI suppression with less noise enhancement for multicarrier equalization is presented by using a parallel canceling scheme via frequency-domain equalization techniques, with the assumption that the channel impulse response (CIR) varies linearly during a block period. In order to avoid performance deterioration due to unreliable initial estimations in the parallel cancellation scheme, a cost function with proper weighting factor is introduced to improve the performance of the proposed equalizer. The proposed equalizer consists of a set of prefilters and a set of ICI cancellation filters, with two stages to perform different functions to achieve minimum mean square error (MMSE) equalization. The prefilters compensate for the multiplicative distortion at the first stage, and the ICI cancellation filters remove the effects of ICI by a parallel cancellation scheme at the second stage. Finally, the performance of the proposed equalizer is analyzed and compared with that of other equalizers, indicating significant performance improvement.     

Iterative receivers for space-time block-coded OFDM systems indispersive fading channels

We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations     

Combined transmit and receive antenna diversity for WCDMA in multipath fading channels

We analyze the performance of various receiver antenna diversity combining schemes in combination with transmit diversity on the downlink of a WCDMA system. The analytical framework developed can handle an arbitrary number of transmit and receive antennas, as well as the study on the effect of dissimilar received signal strengths at the mobile handset on the average symbol error rate performance of a multitude of digital modulation schemes.     

Estimation of time-varying channels for pilot-assisted OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems encounter performance degradations because of the time-varying (TV) channels common in wireless environments. The channel variations within one OFDM symbol introduce intercarrier interference. In this case, the frequency domain channel matrix is no longer diagonal, hence the corresponding channel estimation is challenging. In this article, two novel TV channel estimation approaches are proposed for the pilot-assisted OFDM systems, where the channel is approximated by the high-order linear model or the piece-wise linear model in time domain. The least square estimation is derived for the two kinds of channel approximations. The simulation is performed based on realistic TV channels with a fairly high Doppler spread. The results show the significant decreasing of the estimation mean square error using the proposed approaches.     

Maximum multipath diversity with linear equalization in precoded OFDM systems

In wireless communications, the fading multipath channel attenuates and distorts the transmitted signal. To decode the transmitted symbols and take advantage of the full multipath diversity that the channel has to offer, computationally complex maximum-likelihood (ML) decoding is often employed. We show that a linear equalizer followed by a hard decision is capable of benefiting from maximum multipath diversity in linearly precoded orthogonal frequency-division multiplexing (OFDM) systems, where the information symbols are mapped through a matrix transformation before the inverse fast Fourier transform (IFFT) at the OFDM transmitter. As far as we are aware, this is the first proof of a linear equalization scheme achieving maximum multipath diversity over single-input single-output wireless links. We can conclude from this result that at sufficiently high signal-to-noise ratios (SNR), precoded OFDM systems will perform better over channels with more taps even with linear equalization, due to the increase in diversity order.     

莱斯多径衰落信道下OFDM系统的容量方差分析

方差是系统容量的一个重要参数,可以用来估计通信系统的中断容量。该文研究了正交频分复用（OFDM）系统在莱斯衰落信道下的容量方差。首先该文建立了多径莱斯信道的模型并且定义了多径莱斯信道的莱斯因子,基于此信道模型推出了一个OFDM系统容量方差新的数学表达式,此表达式以OFDM系统的子载波数、信噪比、信道的多径时延等为参数。基于此表达式,计算机仿真和数值计算研究了信噪比、多径数目、莱斯因子对OFDM系统容量方差的影响。结果表明:计算机仿真和数值计算基本吻合,验证了所推导数学表达式的正确性;系统容量方差与信噪比成正比,与莱斯因子和信道的多径数目成反比。另外,该文以积分的形式给出了任意两个相关莱斯随机变量的联合概率密度函数。      

An error probability analysis of the optimum noncoherent multiuser detector for multipath and multiantenna diversity communications over Rayleigh-fading channels

The optimum noncoherent multiuser detector is obtained for generalized diversity symbol-synchronous communication systems that employ nonorthogonal multipulse modulation. A unified approach is adopted to simultaneously address various forms of diversity such as time, frequency, multipath, and/or receiver-amenna diversity. Upper and lower bounds on the average bit-error probability of the optimum noncoherent detector are derived. While these bounds are numerically computable, they are too complicated to give insights about the relative influence of system parameters on the essential behavior of the bit-error rate. To address this issue, an asymptotic (low noise) analysis of the bit-error probability is undertaken. It is shown that the upper and lower bounds are indeed asymptotically convergent. A formula for the asymptotic efficiency of the optimum noncoherent detector is thereby derived. Interestingly, the asymptotic efficiency is found to be positive, and independent of the signal strengths of the interfering users.     

Nondata-aided channel estimation for OFDM systems with space-frequency transmit diversity

This paper proposes a computationally efficient nondata-aided maximum a posteriori (MAP) channel-estimation algorithm focusing on the space-frequency (SF) transmit diversity orthogonal frequency division multiplexing (OFDM) transmission through frequency-selective channels. The proposed algorithm properly averages out the data sequence and requires a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve (KL) orthogonal expansion and estimates the complex channel parameters of each subcarrier iteratively, using the expectation maximization (EM) method. To further reduce the computational complexity of the proposed MAP algorithm, the optimal truncation property of the KL expansion is exploited. The performance of the MAP channel estimator is studied based on the evaluation of the modified Cramer-Rao bound (CRB). Simulation results confirm the proposed theoretical analysis and illustrate that the proposed algorithm is capable of tracking fast fading and improving overall performance.     

Efficient transmit diversity scheme for synchronisation channel in OFDM cellular systems

Transmit diversity is a well-known technique that improves receiving performance by mitigating the amplitude variation of received signal. A number of diversity techniques have been investigated for data and control channels in OFDM cellular systems [1]. However, the diversity schemes for the synchronisation channel have not yet been fully investigated. Because of the inherent characteristics of a synchronisation channel, FSTD and TSTD can be considered as diversity schemes [2]. For data and control channels, it is important to increase the average detection probability for reliable system operation. For the synchronisation channel, however, the searching time is more important than the average detection probability. It implies that the transmission scheme increasing the average detection probability after multiple detection trials can be more beneficial even if it degrades the average detection performance at a first detection trial.     

Analysis of pilot-assisted channel estimators for OFDM systems with transmit diversity

In this paper, we analyze the performance of pilot-assisted least square (LS) and minimum mean squared error (MMSE) channel estimators for orthogonal frequency division multiplexing (OFDM) systems with transmit antenna diversity. We first provide a design of orthogonal pilot sequences to simplify the estimators. We then analyze the mean squared error (MSE) performance, and study the leakage effect. When a channel tap is not sample-spaced, our analysis shows that the power of the channel tap will leak to not only other taps of the same antenna, but also taps belonging to other antennas. The leakage across antennas is mainly determined by the phase separation between pilot sequences, which is further related to the ratio between the number of pilots and number of antennas. We demonstrate that the MSE performance can be improved if more pilots are used, or fewer channels are estimated simultaneously.     

适用于多径衰落信道的OFDM同步方法

针对正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)系统在多径衰落信道中容易受到同步误差的影响,本文提出了两种OFDM符号同步方法.该方法利用重复共轭对称的训练序列和改进的定时测度与判决方法完成符号定时;利用序列的一致性与频域差分编码进行频偏估计.分析与仿真结果表明,在多径衰落信道中与传统方法相比,本文提出的方法具有更好的同步性能和较低的计算复杂度.     

Blind adaptive MIMO receivers for space-time block-coded DS-CDMA systems in multipath channels using the constant modulus criterion

We propose blind adaptive multi-input multi-output (MIMO) linear receivers for DS-CDMA systems using multiple transmit antennas and space-time block codes (STBC) in multipath channels. A space-time code-constrained constant modulus (CCM) design criterion based on constrained optimization techniques is considered and recursive least squares (RLS) adaptive algorithms are developed for estimating the parameters of the linear receivers. A blind space-time channel estimation method for MIMO DS-CDMA systems with STBC based on a subspace approach is also proposed along with an efficient RLS algorithm. Simulations for a downlink scenario assess the proposed algorithms in several situations against existing methods.     

Performance analysis of scheduled transmit beamforming systems under non-identically delayed feedback channels

Wireless Networks - In this paper, we investigate the performance analysis of a scheduled transmit beamforming system under non-identically delayed feedback channels. This paper derives the exact...     

空时发射分集在LAS-CDMA中的性能分析

发射分集是克服信道衰落的有效方式,而LAS-CDMA由于特有的智慧编码技术而具有优异的特性.对三种发射分集方式(OTD,STTD,STS)与LAS-CDMA结合的性能进行了研究,从理论上推导了采用这几种方式后的信干比表达式,并且在LAS-CDMA中仿真了其性能,并与传统CDMA进行了比较.表明发射分集与LAS-CDMA相结合能充分的发挥两者的优越特性.     

Comb type pilot aided channel estimation in OFDM systems with transmit diversity

Transmit diversity can be applied to OFDM systems by adopting space time code. Since the received signal is the overlapped signals transmitted from different transmit antennas, channel estimation is a rather challenging task for space time coded OFDM (ST-OFDM) systems. Pilot structure can help the receiver to effectively separate the overlapped signals and perform accurate channel estimation. In this paper, we propose three different channel estimation algorithms based on specially designed comb type pilots inserted in frequency domain. One of our proposed algorithms is performed in frequency domain and the other two are performed in time domain. Such comb type pilot based algorithms can provide higher bandwidth efficiency than common significant-tap-catching algorithm using training block pilots. Numerical analyzes and computational simulation show that our proposed estimation schemes have the same good performance while the time domain methods have relatively simple structure.     

Antenna diversity combining schemes for WCDMA systems in fading multipath channels

In this paper, we analyze the performance of various receive antenna diversity schemes for use in combination with transmit diversity on the downlink of wideband code-division multiple-access third-generation systems. We consider open loop and closed loop versions of both maximal ratio combining and selection diversity, and study the impact of channel correlation on the performance of these schemes. We also present an analysis of the asymptotic performance of coded systems. The analytical results are compared with simulation results obtained in typical channels.     

双发射天线选择正交空时码分集系统性能分析

针对任意Nakagami衰落信道,分析了多输入多输出(MIMO)系统中采用双发射天线选择正交空时码的平均误码率(ASER)性能。利用矩生成函数方法,推导出采用相干检测的二进制相移键控/二进制频移键控(BPSK/BFSK)、多进制相移键控(MPSK)和多进制正交幅度调制(MQAM)的精确ASER闭合解析式,对不同条件下的系统性能做了数值仿真,验证了分析结果的正确性。     

Performance analysis of combined transmit selection diversity and receive generalized selection combining in Rayleigh fading channels

We analyze the average symbol error rate (SER) of M-PSK and M-QAM modulations with transmit antenna selection diversity (SD) and receive generalized selection combining (GSC) in Rayleigh fading channels. SER formulas are derived in closed form, and numerical results show that transmit SD and receive GSC are flexible to tradeoff performance for complexity.     

BER analysis of QAM on fading channels with transmit diversity

In this letter, we derive analytical expressions for the bit error rate (BER) of space-time block codes (STBC) from complex orthogonal designs (COD) using quadrature amplitude modulation (QAM) on Rayleigh fading channels. We take a bit log-likelihood ratio (LLR) based approach to derive the BER expressions. The approach presented here can be used in the BER analysis of any STBC from COD with linear processing for any value of M in an M-QAM system. Here, we present the BER analysis and results for a 16-QAM system with i) (2-Tx, L-Rx) antennas using Alamouti code (rate-1 STBC), ii) (3-Tx, L-Rx) antennas using a rate-1/2 STBC, and iii) (5-Tx, L-Rx) antennas using a rate-7/11 STBC. In addition to being used in the BER analysis, the LLRs derived can also be used as soft inputs to decoders for various coded QAM schemes, including turbo coded QAM with space-time coding as in high speed downlink packet access (HSDPA) in 3G.