Multiuser detection in multipath environments for variablespreading-factor CDMA systems

A way to allow variable data rates in CDMA is by varying the spreading factor in accordance with the data rate requested by the user. A detection scheme suitable for multirate transmission is required in addition to one that combats multiple access interference of other users and intersymbol interference from multipath. In this letter, an energy-add multiuser detection method is combined with a scheme called Cholesky-iterative detection to cope with these challenges. It allows the users to be estimated on a symbol-by-symbol basis, making receiver complexity independent of data package length. Simulation results correspond closely to a single-user lower bound. The results also show that the bit-error probability performances of the various users for the Cholesky-iterative detector are closely clustered, while those of a previously published scheme, decorrelation feedback detection, are more widely spread     

异步CDMA并行混沌遗传算法的多用户检测

基于传统遗传算法的多用户检测器易陷入局部最优解且收敛较慢,影响实时性。利用混沌优化算法的优势来弥补遗传算法的这一缺陷,使2种算法优势互补,同时采用具有“迁移策略”的并行搜索机制,提出了一种新的次优多用户检测方法——并行混沌遗传混合算法(PCGA),并在异步CDMA系统中对其性能进行了研究。仿真结果表明,提出的这种算法能有效克服传统遗传算法易陷入局部极小的问题,收敛速度更快,在抗干扰与克服“远一近”效应方面均优于传统遗传算法,有较明显的性能优势。     

Multiuser detection for packet-switched CDMA networks with retransmission diversity

In this paper, multiuser detection is developed with exploitation of retransmission diversity. A maximum likelihood detector (MLD) that has the same order of complexity as the MLD using only the data in one transmission is developed. It is shown that the retransmission diversity increases the minimum signal distance and, therefore, significantly improves the performance of MLD. The linear MMSE, decorrelating, and MF detectors are developed under two design approaches. In the first approach,we have considered the weighted sum of outputs of a number of linear detectors, where each is implemented for one transmission. The optimum set of weights that maximizes the signal-to-interference-and-noise ratio (SINR) is obtained. In the second approach, a linear detector jointly utilizes the statistics collected from all transmissions. When the SINR of a linear detector is the same in each transmission, the retransmission diversity is shown to increase the SINR by 10log/sub 10/J dB for a packet of J transmissions. The limiting bit error rate for these three linear detectors in large networks with random spreading sequences is obtained in closed form, which enables further network throughput evaluation.     

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.     

Multiuser receivers for CDMA systems in Rayleigh fading channels

Multiuser demodulation in relatively fast fading channels is analyzed. The optimal maximum likelihood sequence detection (MLSD) receiver is derived and a general suboptimal receiver to approximate the MLSD is proposed. The performance of the parallel interference cancellation (PIC) and decorrelating receivers is compared. The PIC receiver is demonstrated to achieve better performance in known channels than the decorrelating receiver, but it is observed to be more sensitive to the channel coefficient estimation errors than the decorrelator. At high channel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas the decorrelating receiver does not. The performance of data-aided (DA) and decision-directed (DD) multiuser channel estimation is also compared. DA channel estimation is shown to be more robust than DD channel estimation, which may suffer from BER saturation caused by hangups at high signal-to-noise ratios (SNRs). The impact of channel estimation filter impulse response on the BER is studied by comparing optimal and suboptimal channel estimation filters. The implementation complexity of the decorrelating and PIC receivers is compared in terms of required floating point operations and clock cycles in a practical communication scenario. It is observed that the PIC receiver is only moderately more complex to implement than the conventional matched filter bank receiver, whereas the decorrelating receiver is significantly more complex     

二维W-OCDMA系统中的多用户检测技术

抑制多用户干扰是无线光码分多址(OCDMA)通信系统中的一项关键技术。分析了多用户干扰产生的原因,提出采用光限幅器技术和多用户检测技术抑制多用户干扰。分析了采用素数码的无线OCDMA系统中光限幅器技术工作原理及采用二维地址码的无线OCDMA系统中多用户检测技术工作原理。分析结果表明：光限幅器技术仅能部分抑制多用户干扰,而多用户检测技术能够有效地消除多用户干扰,多用户检测技术是适用于无线OCDMA系统消除多用户干扰的有效技术。     

Multiuser detection and diversity reception for asynchronous CDMA mobile communication

In CDMA mobile communication systems, multiple access interference can be canceled by multiuser detection technique. The Degradation by channel fading can be reduced by diversity reception. This paper investigates a family of multiuser receivers that combined decor-relating detection, antenna diversity and RAKE multipath diversity. The performance of the multiuser receivers is analyzed. The results demonstrate a significant increase in the performance of the receivers by using multiuser detection and diversity reception.     

Multiuser detection of quasisynchronous CDMA signals using lineardecorrelators

A linear decorrelator detector is proposed for a quasisynchronous code-division multiple-access (QS-CDMA) cellular system. It is assumed that each of N users has a Global Positioning System (GPS) generated local clock and attempts to transmit synchronously with the other users in its cell. The base station thus receives N signature waveforms which are approximately synchronized, but with a small timing error due to local oscillator drift, Doppler shifts, and variable distances from the mobiles to the base. The residual timing offset is discretized, and a decorrelator is constructed which attempts to reject the multiuser interference vectors corresponding to the set of possible offsets. A noncoherent version of the decorrelator detector is presented, along with a maximum-likelihood scheme for synchronizing to the desired signal. Bounds and analytical results for the bit-error rate (BER) are obtained for various interferer conditions     

Multiuser detection of asynchronous CDMA with frequency offset

This article presents a near-far resistant detection scheme for asynchronous code-division multiple access with frequency offset. Based on a one-shot technique and Taylor expansion, a zeroth-order and a first-order one-shot linear decorrelating detector (LDD) are proposed. The zeroth-order LDD has simple architecture but suffers performance degradation for large frequency offset. The first-order one-shot LDD, with increased complexity, has very good near-far resistant property even for large frequency offset. Two versions of the first-order one-shot LDD are investigated according to different Taylor expansion approaches. The feasibility of the proposed detectors is demonstrated by computer simulations     

Multiuser detection in asynchronous CDMA frequency-selective fading channels

Multipath fading severely limits the performances of conventional code division multiple-access (CDMA) systems. Since every signal passes through an independent frequency-selective fading channel, even modest cross-correlations among signature sequences may induce severe near-far effects in a central multiuser receiver. This paper presents a systematic approach to the detection problem in CDMA frequency-selective fading channels and proposes a low complexity linear multiuser receiver, which eliminates fading induced near-far problem.We initially analyze an optimal multiuser detector, consisting of a bank of RAKE filters followed by a dynamic programming algorithm and evaluate its performance through error probability bounds. The concepts of error sequence decomposition and asymptotic multiuser efficiency, used to characterize the optimal receiver performance, are extended to multipath fading channels.The complexity of the optimal detector motivates the work on a near-far resistant, low complexity decorrelating multiuser detector, which exploits multipath diversity by using a multipath decorrelating filter followed by maximal-ratio combining. Analytic expressions for error probability and asymptotic multiuser efficiency of the suboptimal receiver are derived that include the effects of multipath fading, multiple-access interference and signature sequences correlation on the receiver's performance.The results indicate that multiuser detectors not only alleviate the near-far problem but approach single-user RAKE performance, while preserving the multipath diversity gain. In interference-limited scenarios multiuser receivers significantly outperform the RAKE receiver.This paper was presented in part at the Twenty-Sixth Annual Conference on Information Sciences and Systems, Princeton, NJ, March 1992 and MILCOM'92, San Diego, CA, October 1992. This work was performed while author was with the Department of Electrical and Computer Engineering, Northeastern University, Boston, USA.     

码分多址系统中的多用户检测

CDMA通信系统是一个自干扰受限系统，如何克服CDMA系统中的多址干扰问题已成为提高CDMA系统容量的关键。多用户检测是克服CDMA多址干扰最有潜力的关键技术之一。本文研究了CDMA移动通信系统中几种常见的多用户检测技术，分析了他们的原理，并对各种多用户检测的优缺点进行了比较。     

Multiuser detection with neural network and PIC in CDMA systems for AWGN and Rayleigh fading asynchronous channels

In this paper, multiuser detection in code division multiple access (CDMA) was performed by using neural network (NN) and parallel interference cancellation (PIC). Neural network is used as a front-end stage of one stage PIC circuit. PIC is a classical technique in multi user detection process and its bit error rate (BER) performance is not good in one stage for most of the applications. For improving its BER performance, generally multi stage PIC which has high computational complexity is used. In this study, we have got better BER performance than the three stage PIC receiver in AWGN channel, almost same BER performance with the three stages PIC receiver in the Rayleigh fading channel.     

On pilot-based multipath channel estimation for uplink CDMA systems: an overloaded case

The paper considers a coding scheme for multipath channel estimation in uplink code-division multiple-access systems where each user transmits an individual pilot signal (sequence) to estimate its multipath channel coefficients. Assuming a common radio channel model with a uniform power delay profile, we derive lower bounds on the maximum mean square error for two types of linear channel estimators: an inverse filter and a linear minimum mean square error (MMSE) estimator. In contrast to previous work, the main focus here is on overloaded systems where the total number of multipath channel coefficients of all users is greater than processing gain. We show that the inverse filter bound is attained if and only if each pilot sequence is a perfect root-of-unity sequence. Interestingly, the conventional matched filter achieves the same lower bound if pilot sequences form a complementary periodic sequence set. In case of the MMSE estimator, the lower bound is either met or not depending on some system parameters. We provide a necessary and sufficient condition for achieving the bound when pilot sequences are arbitrary vectors on the unit sphere. This paper gives insight into the performance limits of practical systems.     

CDMA系统中的多用户干扰抵消

本文介绍在多步检测结构中把干扰对消和利用天线阵的自适应MMSE检测器［1］结合起来,并介绍一种新的基于信号子空间估计的检测器。使用这种新的检测器可以使当子空间的维数减少时,干扰就从接收到的信号中抵消,并用正交投影子空间跟踪估计（OPAST）［2］来自适应的估计检测器和空间特征。与MMSE检测器相比,这种检测器在性能上更优。     

Multiuser detectors for large CDMA random access systems over Rayleigh fading channels

In this paper, large code division multiple access (CDMA) random access systems employing the decorrelator and minimum mean square error (MMSE) detectors are investigated over Rayleigh fading channels under the assumption that both the number of users and the spreading gain tend to infinity, but their ratio converges to a constant. The signal to interference ratio (SIR) is shown to converge almost surely to a constant and the bit‐error rate (BER) is expressed as a function of the traffic load, transmission probability, channel coefficient, and distribution of transmission power. Furthermore, the throughput, the spectrum efficiency, and the stability region are analyzed and simulated. For dominating systems, it is shown that the MMSE detector achieves much higher throughput and spectral efficiency than decorrelator detector. Besides, it is also disclosed that, when the signal to noise ratio (SNR) is larger than an optimum value, the spectrum efficiency increases as the ratio of bit energy to noise power spectrum density (E_b/N₀) increases; however, when SNR is smaller than the optimum value, the spectrum efficiency decreases as E_b/N₀ increases. For ordinary stable systems, it is demonstrated that their stability region gets narrower as the traffic load increases. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

CDMA多用户检测技术

首先分析了CDMA移动通信系统中采用多用户检测的必要性,然后介绍了各种多用户检测技术的工作原理,对各种多用户检测技术的特点进行了比较,最后提出了目前多用户检测技术的一些最新发展方向。     

CDMA多用户检测技术

首先分析了CDMA移动通信系统中采用多用户检测的必要性 ,然后介绍了各种多用户检测技术的工作原理 ,对各种多用户检测技术的特点进行了分析比较 ,最后提出了目前多用户检测技术的一些最新发展方向。     

Collaborative spreading for the downlink of overloaded CDMA

A new scheme called ‘Collaborative Spreading’ is proposed for the downlink of CDMA to allow the sharing of the same spreading sequence by more than one user. In particular, it addresses the problem of user overloading and maintain the use of the same set of available orthogonal sequences and simple receiver structure. In this scheme, a total of K users are divided into G groups each containing T users which are collaboratively coded to form uniquely decodable composite codewords. These codewords are spread using a single sequence to perform the CDMA function between the groups. At the receiver, a low complexity maximum likelihood (ML) joint detection and decoding is carried out over a small set of allowed composite codewords to recover the desired user's data. Theoretical and simulation performance analysis of the bit error rate (BER) and user capacity are presented in different channel conditions. It is shown that the proposed collaborative spreading is a simple and very effective means for extending the user capacity at the cost of a modest degradation in BER performance compared with non‐overloaded fully orthogonal CDMA. It can achieve higher signal‐to‐interference plus noise ratio (SINR) and higher overloading ratio compared with Orthogonal CDMA/Orthogonal CDMA schemes and other group orthogonal CDMA schemes. Copyright © 2009 John Wiley & Sons, Ltd.     

Robust multiuser detection for multicarrier CDMA systems

Multiuser detection (MUD) for code-division multiple-access (CDMA) systems usually relies on some a priori channel estimates, which are obtained either blindly or by using training sequences, and the covariance matrix of the received signal, usually replaced by the sample covariance matrix. However, such prior estimates are often affected by errors that are typically ignored in subsequent detection. In this paper, we present robust channel estimation and MUD techniques for multicarrier (MC) CDMA by explicitly taking into account such estimation errors. The proposed techniques are obtained by optimizing the worst case performance over two bounded uncertainty sets pertaining to the two types of estimation errors. We show that although the estimation errors associated with the prior channel estimate and the sample covariance matrix are generally not bounded, it is beneficial to optimize the worst case performance over properly chosen bounded uncertainty sets determined by a parameter called bounding probability. At a slightly higher computational complexity, our proposed robust detectors are shown to yield improved performance over the standard detectors that ignore the prior estimation errors.