A linear receiver for coded multiuser CDMA

We consider a CDMA system with error-control coding. Optimal joint decoding is prohibitively complex. Instead, we propose a sequential approach for handling multiple-access interference and error-control decoding. Error-control decoding is implemented via single-user soft-input decoders utilizing metrics generated by linear algebraic multiuser metric generators. The decorrelator, and a new scheme termed the projection receiver, are utilized as metric generators. For a synchronous system, the coded performance of the projection receiver metric is shown to be superior to the decorrelator even though they are equally complex. Also, the theoretical degradation relative to the single user bound is derived     

基于均方误差最小的图像错误隐藏内插算法

以图像像素间的强相关性为基础,本文以ＭＰＥＧ－２的视频信号为例,对于以条为单位的信息丢失,提出了一种内插算法,并导出了内插像素的显式表达式,从而可以快速地执行实时处理。仿真实验表明应用该算法实现丢失像素的内插,效果是令人满意的。     

Asynchronous block-based minimum mean square error (B-MMSE) CDMA multi-user detection

This paper introduces a novel asynchronous CDMA multi-user detector, the block-based MMSE (B-MMSE) multi-user detector, in which the data stream is segmented into blocks by inserting zero bits and detection takes place block-by-block without compromising the MMSE detection efficiency. The BER performance of the B-MMSE detector is studied and the results are compared with those of the decorrelating detector. It is shown that the B-MMSE detector offers promising detection efficiency at a much lower implementation complexity, which is linear in the product of number of users and block size, than that of the traditional MMSE detector. © 1998 John Wiley & Sons, Ltd.     

A maximum-likelihood bound approaching minimum bit error rate linear multiuser detector for frequency-selective DS-CDMA systems

In this letter, a minimum bit error rate (MBER) linear multiuser detector (MUD) is considered for direct-sequence code-division multiple-access (DS-CDMA) communication systems, distorted by time-varying and frequency-selective multipath fading channels. Based on the approach for finding filter coefficients of the proposed MBER MUD, an efficient Newton method with a barrier parameter is developed. The BER performance of the MBER MUD is compared to other conventional detectors. The study finds that the proposed MBER MUD has more than 2 dB gain over the linear minimum mean-squared error (LMMSE) detector. Furthermore, in the high SNR region, the BER performance of the proposed MBER MUD approaches the performance of the maximum-likelihood (ML) detector.     

毫米波MIMO系统中迭代最小均方误差混合波束成形算法

混合波束成形结构能有效解决毫米波MIMO系统中射频链路受限的问题,但要设计性能较优的混合波束成形算法仍然存在困难.为了实现更高的频谱利用率,提出了一种性能较优的迭代最小均方误差(Alt-MMSE)混合波束成形算法.该算法利用数字矩阵的正交特性,首先进行初始数字矩阵设计,然后通过最小化发送信号与接收信号的均方误差不断迭代更新数字矩阵,在每一次迭代过程中,通过更新后的数字矩阵得到模拟矩阵的相位信息.仿真结果表明,与OMP混合波束成形算法和基于矩阵分解的混合处理方案相比,该算法具有更优的性能且更接近于纯数字波束成形.     

Analysis of the error performance of trellis-coded modulations inRayleigh-fading channels

This work presents an exact expression for the pairwise error event probability of trellis-coded modulation (TCM) transmitted over Rayleigh-fading channels. It includes phase shift keying (PSK) and multilevel quadrature amplitude modulation (QAM) codes, as well as coherent and partially coherent (e.g. differential, pilot tone, etc.) detection. Due to the form of the exact pairwise error event probabilities, this calculation technique cannot be used with the transfer function technique to obtain an upper (union) bound on the overall bit error probability. For this reason, the authors estimate the bit error probability by considering only a small number of short error events. Through simulations, they found that the estimation is usually very accurate at high signal-to-noise ratios but not as accurate at lower signal-to-noise ratios. They study several coded modulation schemes this way. Among the results are the fact that TCM provides significant improvement in the error floor when detected differentially, and an asymmetry in the pairwise error event probability for 16 QAM     

Asymptotic normality of linear multiuser receiver outputs

This paper proves large-system asymptotic normality of the output of a family of linear multiuser receivers that can be arbitrarily well approximated by polynomial receivers. This family of receivers encompasses the single-user matched filter, the decorrelator, the minimum mean square error (MMSE) receiver, the parallel interference cancelers, and many other linear receivers of interest. Both with and without the assumption of perfect power control, we show that the output decision statistic for each user converges to a Gaussian random variable in distribution as the number of users and the spreading factor both tend to infinity with their ratio fixed. Analysis reveals that the distribution conditioned on almost all spreading sequences converges to the same distribution, which is also the unconditional distribution. This normality principle allows the system performance, e.g., the multiuser efficiency, to be completely determined by the output signal-to-interference ratio (SIR) for large linear systems.     

On adaptive minimum probability of error linear filter receiversfor DS-CDMA channels

Receiver architectures in the form of a linear filter front-end followed by a hard-limiting decision maker are considered for DS-CDMA communication systems. Based on stochastic approximation concepts a recursive algorithm is developed for the adaptive optimization of the linear filter front-end in the minimum BER sense. The recursive form is decision driven and distribution free. For additive white Gaussian noise (AWGN) channels, theoretical analysis of the BER surface of linear filter receivers identifies the subset of the linear filter space where the optimal receiver lies and offers a formal proof of guaranteed global optimization with probability one for the two-user case. To the extent that the output of a linear DS-CDMA filter can be approximated by a Gaussian random variable, a minimum-mean-square-error optimized linear filter approximates the minimum BER solution. Numerical and simulation results indicate that for realistic AWGN DS-CDMA systems with reasonably low signature cross-correlations the linear minimum BER filter and the MMSE filter exhibit approximately the same performance. The linear minimum BER receiver is superior, however, when either the signature cross-correlation is high or the background noise is non-Gaussian     

Theoretical reliability of MMSE linear diversity combining inRayleigh-fading additive interference channels

We derive an exact closed-form solution for the reliability of an ideal M-branch MMSE (minimum mean-squared error) diversity combiner operating in a Rayleigh-fading channel with N interferers, each having some specified average power. The reliability is defined as the probability, taken over fading of the desired and interfering signals, that the combiner's output signal-to-interference ratio (SINR) is greater than some specified threshold. This kind of metric is important in evaluating the potential capacity improvements of using diversity combining and adaptive array processing in interference-limited wireless systems. Our result is remarkably simple, fast, straightforward to compute, and numerically stable. We show a set of special cases, which relate to standard results and reveal valuable insights into how this type of array processing operates in interference-limited environments. We also present a set of numerical examples, which show that our calculated reliabilities agree with estimates from Monte Carlo simulation     

A multiuser receiver for trellis-coded DS/CDMA systems inasynchronous channels

We propose and analyze a multiuser receiver using a decorrelating filter and Viterbi decoders for trellis-coded direct-sequence code-division multiple-access (DS/CDMA) systems with biorthogonal signal constellation in asynchronous channels. The biorthogonality is implemented by user signature waveforms and the decorrelating filter. The performance of the proposed system is investigated with emphasis on the asymptotic cases: it is shown that the proposed system provides us with some coding gain and near-far resistance. It is confirmed that we can enhance the performance of the proposed system by using base-station antenna arrays     

Note on a new computational data smoothing procedure suggested by minimum mean square error estimation

Computational procedures for obtaining minimum mean square error estimates of parameters are developed for the case in which the observation functions, describing the dependence of observed data on the unknown parameters, can be accurately approximated by expansions through quadratic terms in the unknown parameters. Analytical closed-form solutions are obtained for the minimum mean square error estimates of real valued functions of the unknown parameters. It is pointed out that even in cases where the statistical assumptions are not satisfied the resulting computational procedures may be applicable. In such cases adequate, although nonoptimum, estimates may still be obtained and the computational procedure may be more convenient than iterative least-square methods.     

Multiple-antenna-aided OFDM employing genetic-algorithm-assisted minimum bit error rate multiuser detection

The family of minimum bit error rate (MBER) multiuser detectors (MUD) is capable of outperforming the classic minimum mean-squared error (MMSE) MUD in terms of the achievable bit-error rate (BER) owing to directly minimizing the BER cost function. In this paper, we will invoke genetic algorithms (GAs) for finding the optimum weight vectors of the MBER MUD in the context of multiple-antenna-aided multiuser orthogonal frequency division multiplexing (OFDM) . We will also show that the MBER MUD is capable of supporting more users than the number of receiver antennas available, while outperforming the MMSE MUD.     

Asymptotic minimum bit error rate property of maximum-likelihood multiuser detection

In multiuser detection, the maximum-likelihood (ML) joint detection method is optimal in the sense that it maximizes the probability of correctly detecting all user symbols. However, it is not known whether the ML detector achieves the minimum bit error rate (BER) for each individual user although the ML detector often exhibits superior BER performance in practice. We prove that under high signal-to-noise ratios, the ML detector approaches the minimum BER.     

多径条件下最小方差空时多用户检测的性能分析

本文研究和分析了在一般多径信道条件下，CDMA阵列接收机对于多址干扰和符号间串扰的联合抑制。文中推广了多用户检测的正则表示，利用最小最大原理实现了广义正则开式的最小方差空时多用户检测器：进一步分析并改进了空时信道的估计方法；最后定量分析了系统的干扰抑制能力。     

Lower bounds on the mean square estimation error

A general class of lower bounds on the mean square error (mse) in random parameter estimation is formulated. These bounds are generated using functions of the parameter and the data that are orthogonal to the data. A particular choice in the class yields a new lower bound which is superior to both the Cramer-Rao and Bobrovsky-Zakai lower bounds.     

Change of error criterion in mean square optimisation

System error is defined as the output of a linear time-invariant filter H(s) whose input is the system input minus the system output. The effect of changes of H(s) on the resulting optimum mean-square error system is studied.     

A least mean square error synthesis with pattern constraints atmultiple frequencies

The pattern synthesis techinque described allows the power pattern to be optimized in a very useful sense over the desired operating band of the antenna array. This is an improvement over previous techniques involving constraints on such artificial parameters as source norm and superdirectivity ratio, which do not, in fact, ensure a useful pattern bandwidth. The procedure described provides minimum mean square error over an arbitrary number of discrete frequencies     

Performance analysis of an improved MMSE multiuser receiver formismatched delay channels

Motivated by the fact that time delays in a practical direct-sequence code-division multiple-access (DS-CDMA) system can never be perfectly estimated, an improved minimum-mean squared-error (MMSE)-based receiver is proposed and analyzed. Via the simple assumption of a probability distribution for the delay estimation errors, the proposed receiver can achieve a performance superior to that of the conventional MMSE (CMMSE) receiver. The performances of this improved receiver and the CMMSE receiver are compared in terms of the mean squared error (MSE), probability of error, and asymptotic multiuser efficiency (AME). As the original definition of AME does not consider mismatched channels, the behavior of three single-user receivers bearing imperfect delay estimation is also investigated. These single-user receivers are employed to define a more appropriate AME. Finally, an efficient update mechanism to accommodate dynamic channel statistics, and thus practical implementation, is proposed     

On the probability of error in linear multiuser detection

The problem of determining the bit-error probability in linear multiuser detection analytically is considered. A number of upper and lower bounds are developed, some of which are very tight, and issues of asymptotic behavior, optimality, and Gaussian approximation are explored. Several special cases, including those of equi-energy and equi-correlated signals, are treated in some detail.