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     

A Turbo Multiuser Detector Based on Samples Reliability

A turbo multiuser detector based on the reliability of the samples is proposed. Reliability thresholds are used to classify a sample as reliable or not. The detector complexity decreases each time a sample is classified as reliable. The proposed detector can reach similar performance, when compared to the turbo multiuser detector, but with smaller complexity. Performance is evaluated in terms of bit error rate and complexity obtained by simulation, as a function of signal-to-noise ratio and the reliability threshold. We present the performance results for different processing gain. These results can be applied to code division multiple access and orthogonal frequency division multiple access systems.     

Performance analysis of linear multiuser detectors for randomlyspread CDMA using Gaussian approximation

The performance of a linear decorrelating detector (LDD) and a minimum mean square error (MMSE) detector is analyzed for random spreading waveforms. The performance of the LDD and MMSE detectors is expressed in terms of the so-called near-far resistance, defined by a reciprocal of a diagonal component of inverse matrix. For random code division multiple access, which employs random spreading waveforms, the near-far resistance can be regarded as a random variable. Many papers have dealt with the analysis of multiuser detectors for random spreading sequences. In most cases, however, these analyses derived only the expectations or bounds for the near-far resistance. In this paper, we directly derive the approximate probability density function (PDF) of the near-far resistance and corresponding bit error rate expression for random spreading sequences. It is based on Gaussian approximation of the cross correlation between any two randomly generated spreading codes. The resulting PDF turned out to be a reversed-and-scaled version of chi-square distribution. The approximate expressions, both the PDF and the corresponding bit error rate expression, were verified via Monte Carlo simulations. The results showed that the approximation is quite close to the simulation results when the number of users is less than half the processing gain     

Near optimum tree-search detection schemes for bit-synchronousmultiuser CDMA systems over Gaussian and two-path Rayleigh-fadingchannels

Simple reduced tree-search detection schemes of the breadth-first type are applied to suboptimal joint multiuser detection in bit-synchronous code-division multiple access (CDMA) systems over both Gaussian and two-path Rayleigh-fading channels. It is pointed out that in contrast to the case of the optimal multiuser detector, the choice of the receiver filter severely influences the performance of suboptimal multiuser detectors. Simulation results supported by analysis show that breadth-first tree-search algorithms using a decorrelating noise whitening receiver filter perform better than similar receivers, which solely use a matched filter (MF) for virtually all nonsingular spreading code sets studied. Most of the code sets are randomly generated. The M- and T-algorithm detectors based on decorrelating noise whitening filter (WF) outputs can achieve near optimum performance at a very low complexity compared to the optimal detector, although the proposed detectors are more complex than some known suboptimum detectors. Furthermore, the use of combining techniques is considered for a two-path Rayleigh-fading channel, and a semi-synchronous CDMA structure is proposed. It is shown that if maximum ratio combining (MRC) is employed, the decorrelating noise WF still exists. The corresponding suboptimal combining detector with a decorrelating noise WF outperforms a similar noncombining detector     

CDMA移动通信系统中的分组并行多用户信号检测器

该文提出了一种新的分组并行多用户信号检测器结构,推导了这种多用户信号检测器的检测算法,算法的计算复杂度与用户数呈线性关系,而性能优于解相关检测器,检测器的总延时小于判决反馈多用户信号检测器。数值及仿真结果表明,当接收到的干扰信号功率高于有用信号:3dB以上时,分组并行多用户信号检测器的误码性能趋于某一值,因而该检测器可以有效地克服CDMA移动通信中的远近效应。     

Linear space-time multiuser detection for multipath CDMA channels

We consider the problem of detecting synchronous code division multiple access (CDMA) signals in multipath channels that result in multiple access interference (MAI). It is well known that such challenging conditions may create severe near-far situations in which the standard techniques of combined power control and temporal single-user RAKE receivers provide poor performance. To address the shortcomings of the RAKE receiver, multiple antenna receivers combining space-time processing with multiuser detection have been proposed in the literature. Specifically, a space-time detector based on minimizing the mean-squared output between the data stream and the linear combiner output has shown great potential in achieving good near-far performance with much less complexity than the optimum space-time multiuser detector. Moreover, this space-time minimum mean-squared error (ST-MMSE) multiuser detector has the additional advantage of being well suited for adaptive implementation. We propose novel trained and blind adaptive algorithms based on stochastic gradient techniques, which are shown to approximate the ST-MMSE solution without requiring knowledge of the channel. We show that these linear space-time detectors can potentially provide significant capacity enhancements (up to one order of magnitude) over the conventional temporal single-user RAKE receiver     

Genetically modified multiuser detection for code division multipleaccess systems

The problem of multiple access interference (MAI) and intersymbol interference (ISI) suppression in code division multiple access (CDMA) systems is considered. By combining the theory of multiuser detection (MUD) and evolutionary computation, a hybrid genetic engine is proposed, suitable for the detection of CDMA signals in the presence of MAI and ISI. The proposed hybrid detector structure can be extended to most multiuser detectors and used as the base detector within the structure. Using random selection, mutation and crossover operators and a unique chromosome structure, the genetic algorithm evolves the base detector to a group of more efficient detectors in terms of bit-error rate performance. First, a new packet-level genetic MUD technique, using a conventional single user detector as the base detector, is proposed for asynchronous CDMA (ACDMA) with negligible ISI. Then the signal-subspace-based minimum mean square error detector is chosen as a base detector and wrapped inside the hybrid genetic engine to evolve to a better structure nearly to eliminate both ISI and MAI. The novelty of the proposed structure is the way the deterministic closed-form solution of the base detector is mapped to a genetic engine resulting in a group of more efficient and adaptive detectors     

Pilot to data channel power allocation for PCA-DS/CDMA withinterference canceler

The purpose of this letter is to examine channel power assignment for multiuser direct sequence/code division multiple access (DS/CDMA) uplinks. The channels under consideration are pilot channels and data channels. A multiuser detector is also included. The pilot channel estimates are used for multiple access interference (MAI) regeneration, as well as for data channel detection. Simulation results allowed us to pinpoint optimum pilot-to-data channel power ratios (PDRs) for the best bit error rate (BER) performance. In addition, the optimum PDRs of the receivers with the multiuser detector are high compared with those of conventional receivers without the multiuser detector. This study may prove helpful in the effort to optimize the channel power assignment for channel estimation and coherent detection of systems with the multiuser detector     

Performance comparison of MRC and EGC on a MC-CDMA system with synchronization errors over fading channels

This work derives the average bit error rate (BER) of the uplink and downlink multicarrier code division multiple access (MC-CDMA) systems using maximum ratio combining (MRC) and equal gain combining (EGC) with synchronization errors over fading channels. The derived equation can simultaneously incorporate the parameters of the fading channel and all of the synchronization errors, including frequency offset, carrier phase jitter, and timing jitter. Numerical results indicate that those two combining schemes on the uplink and downlink MC-CDMA systems are degraded by all of the normalized synchronization errors over 10⁻². The comparison outcomes between MRC and EGC reveal that the MRC generally outperforms EGC in the uplink MC-CDMA system. However, EGC achieves better performance when the number of users is small, the normalized synchronization errors are low and the signal to noise ratio (SNR) is high. In the downlink system, EGC mainly outperforms MRC when the SNR and the number of users are gradually increased and the normalized synchronization errors are low. Therefore, the selection of MRC or EGC depends on the SNR, the synchronization errors and the number of users in uplink and downlink MC-CDMA systems.     

Asynchronous multiuser CDMA detector made simpler: noveldecorrelator, combiner, canceller, combiner (DC3) structure

A code division multiple access (CDMA) detector that employs a combination of a decorrelator and an adaptive interference canceller was proposed by Siveski, Bar-Ness and Chen (1994) for the synchronous channel. For the asynchronous channel, the one-shot approach was suggested. It requires a (2K-1) dimensional decorrelator and a canceller (K is the number of users) and must be repeated once for every user if adequate performance for all users is sought. Instead, we propose in this paper a decorrelator, data combiner, canceller, data combiner arrangement. Using maximum signal-to-noise ratio (SNR) combining at both data combining stages, we show that the resulting decorrelator canceller performs with an asynchronous channel as does that for the synchronous case     

An MBER–BLAST algorithm for OFDM–SDMA communication using particle swarm optimization

A multiple antenna‐aided, minimum bit error rate–Bell Laboratories‐layered space–time (MBER–BLAST) multiuser detection algorithm is proposed for uplink orthogonal frequency division multiplexing–space division multiple access (OFDM–SDMA) communication to increase the capacity of the system. The proposed algorithm overcomes the limitations of the conventional detectors when the number of users exceed the number of receiver antennas. A particle swarm optimization (PSO) algorithm is employed for finding the optimum weight vectors for MBER detector. PSO is well suited for physically realizable, real‐time applications, where low complexity and fast convergence are of absolute importance, while an optimum maximum likelihood (ML) detection using an exhaustive search method is prohibitively complex. The proposed algorithm outperforms the MBER detector and is capable of achieving performance close to that attained by ML detector at a significantly lower complexity, especially under high user loads. Simulation results show that MBER–BLAST detector promises substantially improved performance compared with the existing systems and offers a good performance–complexity trade‐off. It supports a large number of users by exploiting the capacity advantages of multiple antenna systems in rich scattering environments. Copyright © 2010 John Wiley & Sons, Ltd.     

Genetic algorithm assisted joint multiuser symbol detection andfading channel estimation for synchronous CDMA systems

A novel multiuser code division multiple access (CDMA) receiver based on genetic algorithms is considered, which jointly estimates the transmitted symbols and fading channel coefficients of all the users. Using exhaustive search, the maximum likelihood (ML) receiver in synchronous CDMA systems has a computational complexity that is exponentially increasing with the number of users and, hence, is not a viable detection solution. Genetic algorithms (GAs) are well known for their robustness in solving complex optimization problems. Based on the ML rule, GAs are developed in order to jointly estimate the users' channel impulse response coefficients as well as the differentially encoded transmitted bit sequences on the basis of the statistics provided by a bank of matched filters at the receiver. Using computer simulations, we showed that the proposed receiver can achieve a near-optimum bit-error-rate (BER) performance upon assuming perfect channel estimation at a significantly lower computational complexity than that required by the ML optimum multiuser detector. Furthermore, channel estimation can be performed jointly with symbol detection without incurring any additional computational complexity and without requiring training symbols. Hence, our proposed joint channel estimator and symbol detector is capable of offering a higher throughput and a shorter detection delay than that of explicitly trained CDMA multiuser detectors     

基于混合量子进化的STBC-MIMO MC-CDMA系统多用户检测算法研究

针对STBC-MIMO MC-CDMA多用户检测方法存在计算复杂度高、工程实现困难的问题,提出利用一种混合量子进化算法（HQEA）解决STBC-MIMO MC-CDMA多用户检测问题,该算法不仅具有较低的计算复杂度,而且解决了量子进化算法容易陷入局部最优解、参数个数较多和通过查表来更新量子门的问题。仿真表明,该算法在STBC-MIMO MC-CDMA系统多用户检测问题上,具有比其他智能算法更强的跳出局部最优和寻找全局最优的能力。     

BER of MC-CDMA systems with MRC in correlated Nakagami-m fading

Based on the characteristic function of the sum of correlated gamma random variables and an alternative expression of Q-function, a simple bit error rate (BER) expression is derived for the multicarrier code division multiple access (MC-CDMA) systems with maximal ratio combining (MRC) in correlated Nakagami-m fading channels. The much simpler approximate average BER expression using exponential approximation of the Q-function is also derived.     

一种新的应用于多载波码分多址系统的干扰对消算法

本文提出了一种适用于多载波直扩码分多址系统(MC-DS-CDMA)上行信道的低复杂度部分并行干扰对消(LC-PPIC)算法.该算法充分利用MC-DS-CDMA系统的信道估计与子载波分集,在各个子载波上将用户分为可靠,普通和无效三类,对可靠用户进行干扰对消处理并消去后,再对普通用户进行相同操作,并放弃无效用户,最后将各个子载波上的结果通过最大比合并(MRC)输出.这样,在降低运算复杂度的同时,更好地抑制了多址干扰(MAI).通过仿真比较,可以看出LC-PPIC算法的误码率(BER)性能明显优于置于载波合并前后的两种部分并行干扰对消(PPIC)算法,特别是在系统负载较重和信噪比较高的情况下,这种优势十分显著.     

Hybrid SNR-Adaptive Multiuser Detectors for SDMA-OFDM Systems

Multiuser detection (MUD) and channel estimation techniques in space-division multiple-access aided orthogonal frequency-division multiplexing systems recently has received intensive interest in receiver design technologies. The maximum likelihood (ML) MUD that provides optimal performance has the cost of a dramatically increased computational complexity. The minimum mean-squared error (MMSE) MUD exhibits poor performance, although it achieves lower computational complexity. With almost the same complexity, an MMSE with successive interference cancellation (SIC) scheme achieves a better bit error rate performance than a linear MMSE multiuser detector. In this paper, hybrid ML-MMSE with SIC adaptive multiuser detection based on the joint channel estimation method is suggested for signal detection. The simulation results show that the proposed method achieves good performance close to the optimal ML performance at low SNR values and a low computational complexity at high SNR values.     

Adaptive Multiuser Decision Feedback Detector: A Novel Parallel Interference Canceller

Adaptive decision feedback detector based on the parallel interference cancellation approach (ADFD-PIC) is presented for code division multiple access transmission, which not only combats the intersymbol interference and multiple access interference but also suppresses other-cell interference. For frequency-selective smoothly time-varying channels, the proposed ADFD-PIC structure using the erasure algorithm based soft-slicer and channel estimator (EC-ADFD-PIC) offers performance improvement by mitigating the adverse effects of error propagation. The simulation results are presented to demonstrate the substantial improvement in bit error rate performance of the minimum mean square error EC-ADFD-PIC over other multiuser detection techniques.     

基于优化多带复小波的多载波CDMA及反向链路性能

在分析多载波码分多址的原理基础上,利用优化生成的多带复小波,提出了一种基于优化多带复小波的多载波CDMA系统反向链路模型,并采用基于多天线的空间分集合并(SDC)技术进一步完善所提系统.该系统可利用优化多带复小波的优良特性来避免通常多载波CDMA由于插入循环前缀所带来的频谱效率的下降,在此基础上我们研究了所提系统采用空间分集合并技术时在瑞利衰落信道下的反向链路性能;给出相应的误比特率分析.理论分析和仿真结果表明采用基于最大比合并的SDC技术可显著提高所提系统抗空间衰落和各种干扰的能力,而且基于SDC技术的所提系统要好于同样基于SDC技术的通常多载波CDMA系统和基于实小波包的多载波CDMA系统.     

Iterative multiuser detection, macrodiversity combining, anddecoding for the TDMA cellular uplink

A soft-input soft-output (SISO) multiuser detector (MUD) suitable for inclusion in iterative processing architectures is presented and applied to the detection of the coded time division multiple access (TDMA) cellular uplink. A SISO-MUD processor is located at each base station in the network, and adjacent base stations share information concerning the mobiles they serve. Because the MUD outputs are soft, they are suitable for postdetection macrodiversity combining. The combined signals are then passed to a SISO forward error correction (FEC) decoder, and the soft outputs are fed back to the multiuser detectors. Processing continues in an iterative fashion in accordance with the turbo principle. Simulation results are presented that indicate that use of such a scheme enables cellular systems to be overloaded with more than just one cochannel user per cell at the price of a minimal loss in signal-to-noise ratio (SNR). The proposed implementation assumes the availability of both perfect channel state information and a high capacity backhaul     

An asynchronous multiuser CDMA detector based on the Kalman filter

We introduce a multiuser receiver based on the Kalman filter, which can be used for joint symbol detection and channel estimation. The proposed algorithm has the advantage of working even when the spreading codes used have a period larger than one symbol interval (“long codes”), unlike adaptive equalizer-type detectors. Simulation results which demonstrate the performance advantage of the proposed receiver over the conventional detector, the minimum mean squared error (MMSE) detector and a recursive least squares (RLS) multiuser detector are presented. A thorough comparison of the MMSE detector and the proposed detector is attempted because the Kalman filter also solves the MMSE parameter estimation problem, and it is concluded that, because the state space model assumed by the Kalman filter fits the code division multiple access (CDMA) system exactly, a multiuser detector based on the Kalman filter must necessarily perform better than a nonrecursive, finite-length MMSE detector. The computational complexity of the detector and its use in channel estimation are also studied