Adaptive minimum-BER linear multiuser detection for DS-CDMA signalsin multipath channels

The problem of constructing adaptive minimum bit error rate (MBER) linear multiuser detectors is considered for direct-sequence code division multiple access (DS-CDMA) signals transmitted through multipath channels. Based on the approach of kernel density estimation for approximating the bit error rate (BER) from training data, a least mean squares (LMS) style stochastic gradient adaptive algorithm is developed for training linear multiuser detectors. Computer simulation is used to study the convergence speed and steady-state BER misadjustment of this adaptive MBER linear multiuser detector, and the results show that it outperforms an existing LMS-style adaptive MBER algorithm presented by Yeh et al. (see Proc. Globecom, Sydney, Australia, p.3590-95, 1998)     

Adaptive interference suppression for DS-CDMA

We propose an adaptive scheme to reject multiuser interference using as little as 20-30 taps in direct-sequence code-division multiple-access systems employing arbitrary processing gains (31-400 considered here). We demonstrate that our receiver using this scheme does not require training signals, either initially when the desired user powers on in the presence of several strong interferers, or during transmission, as other users (possibly near-far interferers) gain access to the system. Moreover, knowledge of interfering users spreading codes, timing, or phase information is not required with our adaptive receiver.     

ADAPTIVE STEP－SIZE CONSTANT MODULUS ALGORITHM FOR BLIND MULTIUSER DETECTION IN DS－CDMA SYSTEMS

Blind Adaptive Step-size Constant Modulus Algorithm (AS-CMA) for multiuser detection in DS-CDMA systems is presented.It combines the CMA and the concept of variable step-size,uses a second LMS algorithm for the step size.It adjusts the step-size according to the minimum output-energy principle within a specified range,thus overcomes the problems of bad effect of fixed step-size LMS algorithm.Compared with Adaptive Step-size LMS (AS-LMS) algorithm,through simulations,this algorithm can adapt the changes of the environment,suppress multiple access interference in the dynamic environment and the stability of Signal to Interference Ratio (SIR) is superior to that of AS-LMS.     

Adaptive implementation of polynomial expansion detector formulti-user detection in DS-CDMA systems

The polynomial expansion (PE) multi-user detector was proposed to eliminate the computational complexity of matrix inversion involved in the decorrelating detector and the MMSE detector. Computation of the polynomial coefficients in a PE detector is, however, a complex task. A fast converging adaptive algorithm based on the Davidon-Fletcher-Powell (DFP) method is proposed for computing polynomial weights for a PE detector     

Multi-user detection for DS-CDMA communications

Direct-sequence code-division multiple access (DS-CDMA) is a popular wireless technology. In DS-CDMA communications, all of the users' signals overlap in time and frequency and cause mutual interference. The conventional DS-CDMA detector follows a single-user detection strategy in which each user is detected separately without regard for the other users. A better strategy is multi-user detection, where information about multiple users is used to improve detection of each individual user. This article describes a number of important multiuser DS-CDMA detectors that have been proposed     

Adaptive BLAST-type decision-feedback equalizers for DS-CDMA systems

In this paper, we propose two new adaptive equalization algorithms for direct sequence code division multiple access (DS-CDMA) systems operating over time-varying and frequency selective channels. The equalization schemes consist of a number of serially connected stages and detect users in an ordered manner, applying a decision feedback equalizer (DFE) at each stage. Both the equalizer filters and the order in which the users are extracted are updated in a recursive least squares (RLS) manner, efficiently realized through time- and order-update recursions. V-BLAST detection ordering is implemented, that is, the stronger signal is extracted first so that the weaker users can be more easily detected. The spreading codes are unavailable at the receiver of the first scheme, whereas the second algorithm employs the RAKE receiver concept, incorporating knowledge of the spreading sequences to offer performance improvement. The bit error rate (BER) performance of the equalizers is evaluated via simulations, in both mild and severe near-far environments. Their superiority over existing techniques is demonstrated.     

Subspace multiuser detection for multicarrier DS-CDMA

A subspace-based linear minimum mean-squared error (MMSE) multiuser detection scheme is proposed for a multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) system. Typically, a MC-DS-CDMA system employs a band-limited chip waveform. The band-limited nature of the chip waveform causes problem in applying standard subspace techniques because no non noise subspace can be formed. It is shown that channel and timing information needed for the construction of the linear MMSE detector can be identified by a multiple-signal-classification-like algorithm based on a finite-length truncation approximation of the chip waveform. In practice, since perturbed versions of the subspaces assumed in the finite-length truncation approximation are actually observed, and because of the band-limited property of the chip waveform, the accuracy of the channel estimation and, hence, the performance of the MMSE detector are degraded. This effect is investigated in this paper.     

An Adaptive MMOE-PIC Detector for Asynchronous DS-CDMA Communications

We propose a modified linear parallel interference cancelation (PIC) structure using the adaptive minimum mean output-energy (MMOE) algorithm for direct-sequence code-division multiple-access (DS-CDMA) systems. The complexity of the proposed receiver structure is shown to be linear in the number of users and hence, lower complexity than the centralized minimum mean-squared error (MMSE) multiuser detector. It is demonstrated that the proposed receiver structure can significantly reduce the long training period required by the standard adaptive MMOE receiver in near-far environments. Both numerical and theoretical results show that the proposed receiver performs close to the optimum MMSE receiver whereas the conventional adaptive MMOE detector suffers from high BER’s due to the imperfect filter coefficients. Also our results show a three fold increase in the number of users when the MMOE-PIC is used relative to the conventional MMOE receiver. Furthermore, the transient behavior of the proposed MMOE-PIC receiver due to abrupt changes in the interference level is examined. It is shown that the proposed adaptive receiver offers much faster self recovery, with less signal-to-interference ratio (SIR) degradation, than the standard MMOE in sever near-far scenarios.     

Adaptive blind retrieval techniques for multiuser DS-CDMA signals

In a multiuser direct sequence code division multiple access system, retrieval of the transmitted signals with a conventional receiver is difficult when there is chip asynchronism, multipath propagation and the associated near-far problem. It is shown that despite these problems, an adaptive linear receiver based on a mixed cross-correlation and constant modulus algorithm has the potential to retrieve all users simultaneously. Moreover, for the assumed transmission model, a necessary condition for global convergence of the proposed algorithm is also provided     

Adaptive selection/maximal-ratio combining based on error detection of multidimensional multicode DS-CDMA

A novel two-dimensional diversity combining is proposed for the uplink in a single cell which employs multidimensional multicode direct-sequence code-division multiple-access signaling and two receive antennas. First, the signaling is combined with precoding to obtain a constant envelope signal that is suitable for the uplink, and the resulting error detection is applied to the diversity combining. Based on the error detection, an adaptive selection-combining/maximal-ratio combining (SC/MRC) is performed, for which initial data detection is made by the SC to avoid the combining loss of the very noisy paths. When the initial SC is unreliable (indicated by the error detection), the MRC is attempted to fully exploit the space and multipath diversity. The adaptive SC/MRC is generalized to further increase the diversity gain over the MRC, offered by the error-detection capability. Through analysis and simulation results, it is shown that the adaptive SC/MRC and its generalized diversity receiver outperforms the other schemes in terms of the symbol-error rate, and also its bit-error rate can be lower than that of the variable spreading factor scheme using a single code.     

Double window multi-user detection for asynchronous DS-CDMA

One-shot (window) detection allows multi-user detection schemes to operate symbol by symbol in asynchronous systems and retain near-far resistance. A novel technique, double window detection, is presented, which significantly outperforms single window detection, especially for decorrelating receivers. Results for decorrelation and MMSE detection, using both windowing types are presented     

MC DS-CDMA 系统中的常模盲检测方法

提出了MC DS-CDMA（多载波直扩码分多址）系统中的SDCMA（最陡下降常模算法）和LSCMA（最小二乘常模算法）的盲检测方法，并把这2种CMA（常模算法）盲检测和基于子空间的MMSE（最小均方误差）检测方法相比较。仿真结果表明这2种CMA盲检测方法性能明显优于基于子空间的MMSE检测方法，而且运算复杂度也低于后者。在收敛速度上，LSCMA检测方法远快于SDCMA检测方法。     

Robust detection in DS-CDMA

Robust single-user detection is employed in a direct sequence code-division multiple-access (DS-CDMA) system in which the noise process contains impulsive components. The breakdown point is computed for a mixture noise model. The bit error probability expressions are derived under a Gaussian mixture. The performance is also evaluated in the presence of power imbalance and asynchronous reception. Noise, rather than interference, is shown to be the primary obstacle in achieving good performance for certain practical signal power and user load levels. It is concluded that DS-CDMA employing a robust correlator receiver performs better than the conventional matched filter in an impulsive noise environment     

Adaptive maximum SINR RAKE filtering for DS-CDMA multipath fadingchannels

The conventional signature-matched RAKE processor for multipath direct-sequence code division multiple access channels is viewed as a regular linear tap-weight filter of length equal to the sum of the system processing gain and the user channel memory. In this paper, performance improvements are sought in the context of adaptive filtering under maximum signal-to-interference-plus-noise-ratio criteria. The minimum-variance-distortionless-response RAKE (RAKE-MVDR) filter and the lower complexity scalar optimized auxiliary-vector RAKE (RAKE-AUX) filter are developed. Bit error rate (BER) comparisons with the conventional RAKE signature-matched filter are carried out for training sets of reasonably small size, perfectly known, and mismatched/estimated channel coefficients, and extreme near-far system configurations     

Adaptive interference cancellation for DS-CDMA systems using neuralnetwork techniques

The objective of this study is to apply and investigate a neural network-based decision feedback scheme for interference suppression in direct sequence code division multiple access (DS-CDMA) wireless networks. It is demonstrated that a decision feedback functional link equalizer (DFFLE) in combination with an eigenvector network can closely approximate a Bayesian receiver with significant advantages, such as improved bit-error ratio (BER) performance, adaptive operation, and single-user detection in a multiuser environment. It is assumed that the spreading codes of the interfering users will be unknown to the receiver. This detector configuration is appropriate for downlink communication between a base station and a mobile user in a digital wireless network. The BER performance in the presence of interfering users is evaluated. The improved performance of such a DFFLE receiver for CDMA is attributed to the nonlinear decision boundary it evaluates for the desired user. The receiver structure is also capable of rapid adaptation in a dynamic communications scenario for which there is entry/exit of users and imperfect power control. The convergence performance and error propagation of the DFFLE receiver are also considered and exhibit reasonable promise for third generation wireless DS-CDMA networks     

Fast Converging Adaptive MMSE Multiuser Detector for DS-CDMA System

A fast converging adaptive minimum-mean-squared-error (MMSE) multiuser detector is proposed for direct-sequence code-division multiple-access (DS-CDMA) systems with severe near-far problem where the convergence rate of adaptive MMSE detectors for distinct users can be very different. It is shown that by successively cancelling the interference signals of strong power users, the convergence rate of the proposed detectors for weak power users can be significantly increased, which helps to reduce the length of training sequence for tracking. It is also shown that the order of cancellation and several important parameters required for interference cancellation can be determined from the convergence behavior of the proposed detector. Numerical results are presented to show that the proposed detector offers improved performance in various DS-CDMA environments.Zhiwei Mao received the B.Sc. degrees from Beijing University of Posts and Telecommunications (BUPT), Beijing, China in 1996 and 1999, respectively. Since 2000, she had been a Research Assistant and graduate student in the Department of Electrical and Coumputer Engineering, University of Victoria, Victoria, BC, Canada. She received the Ph.D. degree in electrical engineering in 2003. Currently, she is an Assistant Professor at Lakehead University, Thunder Bay, Ontario, Canada.Her research interests include wireless communications, multiuser detection, digital communications and digital singal processing.Vijay K. Bhargava received the B.Sc., M.Sc., and Ph.D. degree from Queens University, Kingston, ON, Canada in 1970, 1972 and 1974 respectively.Currently, he is a Professor and Head of the Department of Electrical and Computer Engineering at the University of British Columbia, Vancouver, Canada. Previously he was with the Univeristy of Victoria (1984–2003) and with Concordia University in Montréal (1976–1984). He is a co-author of the book Digital Communications by Satellite (New York: Wiley, 1981), co-editor of Reed-Solomon Codes and Their Applications (New York: IEEE, 1994) and co-editor of Communications, Information and Network Security (Boston: Kluwer, 2002). His research interest are in wireless communications.Dr. Bhargava is a Fellow of the B.C. Advanced Systems Institute, Engineering Institute of Canada (EIC), the IEEE, the Canadian Academy of Engineering and the Royal Society of Canada. He is a recipient of the IEEE Centennial Medal (1984), IEEE Canadas McNaughton Gold Medal (1995), the IEEE Haraden Pratt Award (1999), the IEEE Third Millennium Medal (2000), IEEE Graduate Teaching Award (2002), and the Eadie Medal of the Royal Society of Canada (2004).Dr. Bhargava is very active in the IEEE and was nominated by the IEEE Board of Director for the Office of IEEE President-Elect. Currently he serves on the Board of Communications Society. He is an Editor for the IEEE Transactions on Wireless Communications. He is a Past President of the IEEE Information Theory Society.     

Multiuser detection for DS-CDMA systems using evolutionary programming

This article introduces a new multiuser detection scheme which uses evolutionary programming (EP) to detect the user bits based on the maximum-likelihood decision rule. The major advantage of the proposed detector is that it has a lower computational complexity compared to other popular evolutionary-algorithm-based detectors. The simulation results show that the EP has always converged to the optimum solution with a small number of generations. The simulated average computational time performance demonstrates that this approach achieves practical ML performance with polynomial complexity in the number of users.     

LMMSE detection for DS-CDMA systems in fading channels

The linear minimum mean-squared-error (LMMSE) criterion can be used to obtain near-far resistant receivers in direct-sequence code-division multiple-access systems. The standard version of the LMMSE receiver (postcombining LMMSE) minimizes the mean-squared error between the filter output and the true transmitted data sequence. Since the detector depends on the channel coefficients of all users, it cannot be implemented adaptively in fading channels due to severe tracking problems. A modified criterion for deriving LMMSE receivers (precombining LMMSE) in fading channels is presented. The precombining LMMSE receiver is independent of the users' complex channel coefficients, and it effectively converts the time-varying Rayleigh fading channel to an equivalent fixed additive white Gaussian noise channel from the point of view of updating the detector. The performance of the LMMSE receivers in fading channels is studied via computer simulations and numerical analysis. The results show that the postcombining LMMSE receiver has potentially larger capacity, but it cannot be used in fast fading channels. The precombining LMMSE receiver has slightly worse capacity than the postcombining LMMSE receiver, but remarkably larger capacity than the conventional RAKE receiver at the signal-to-noise ratios of practical interest     

一种用于抑制直扩通信中窄带干扰的非线性自适应方法

在分析线性自适应滤波抑制单频干扰、自回归过程窄带干扰和窄带BPSK调制信号干扰的性能的基础上,提出了一种非线性自适应波抑制方法.在对窄带干扰进行预测前,通过对扩谱信号的估计,使得窄带干扰预测的背景噪声由非高斯转变为高斯,并消除扩谱信号对干扰估计的影响,再通过NLMS和RLS自适应算法进行干扰抑制.理论分析和仿真试验结果表明,这种改进方法对于系统性能的改善效果,明显优于常规自适应滤波抑制方法.