An adaptive multiuser detector for DS‐CDMA systems in multipath fading channels

In this paper, we investigate the performance of an adaptive multistage detection scheme for direct‐sequence code‐division multiple‐access (DS‐CDMA) systems. The first stage consists of an adaptive multiuser detector which is based on the linear constrained minimum variance (LCMV) criterion. The interference cancellation (IC) occurs in the second stage. The performance of the iterative receiver over both flat and frequency‐selective fading channels is investigated and compared to the single‐user bound. In all cases, and under heavy system loads with near‐far problems, the iterative receiver is shown to offer substantial performance improvement and large gain in user‐capacity relative to the standard LCMV. In flat‐fading channels, our results show that the performance of the iterative detector is very close to the single‐user bound. For the frequency‐selective channel, this performance is noted to be in the order of 1 dB far from the single‐user bound. Copyright © 2007 John Wiley & Sons, Ltd.     

空时多用户检测器的级联结构

探讨了一种级联结构的空时多用户检测器。它可认为是改进的(或优化的)2D—RAKE接收机或改进的CMOE盲多用户检测器。其结构类似ST2D—RAKE接收机，但能充分利用所需用户的扩频码信息。用优化的方法生成解扩序列，从而大大改善了性能。     

A new chip‐level linear SOR‐SIC multiuser detector for long‐code systems

In this work, we introduce a new chip‐level linear modified‐SIC multi‐user structure that is asymptotically equivalent to successive over‐relaxation (SOR) iteration, which is known to outperform the conventional Gauss–Seidel iteration by an order of magnitude in terms of convergence speed. The main advantage of this scheme is that it uses directly the spreading codes and not the cross‐correlation coefficients and thus reduces significantly the overall computational complexity. This is critical for the design of low‐complexity multiuser detectors for long‐code CDMA systems such as IS95 and UMTS. We use a matrix algebraic approach to show the equivalence of the proposed scheme to linear matrix filtering. This allows obtaining an analytical expression for both the bit‐error rate (BER) and the asymptotic multiuser efficiency (AME). Moreover, we study the convergence behavior of the proposed scheme and prove that it converges if the relaxation factor is within the interval ]0, 2[. Simulation results are in excellent agreement with theory. Copyright © 2008 John Wiley & Sons, Ltd.     

异步CDMA系统中的CM多用户检测器

本文将恒模（CM）算法应用于异步CDMA系统的多用户信号检测。该CM检测器只要求了解目标用户的信息，运算较简单，且由于采用了自适应方法，检测器无需估计信号的幅度。分析及仿真结果均表明，CM检测器的误码性能和抗-近效应能力均优于传统检测器，且对于用户数目的变化有很好的适应性。     

基于Social Cognition粒子群算法多用户检测

最优多用户检测方法具有最优性能,但复杂度高,利用优化算法求解可以降低实现复杂度。粒子群算法是一种简单有效的新型群智能优化算法,研究了一种Socialcognition模型简化粒子群算法,并应用于大用户量CDMA多用户检测问题,主要考虑降低算法复杂度,提高算法的实现效率。分析及仿真表明该方法在系统用户数量较大时具有较好性能。     

PN sequence blind estimation in multiuser DS‐CDMA systems with multipath channels based on successive subspace scheme

In this paper, we proposed a new method based on expanding subspace algorithm and finite alphabet characteristics, for blind estimation of the users' spreading sequences in the multiuser direct sequence code division multiple access system in the presence of the multipath channels. In the proposed scheme, we show that the estimation of the users' overall channels in the direct sequence code division multiple access system is equivalent to the impulse response estimation of the multi‐input multi‐output finite impulse response channels. Our proposed approach is based on the successive estimation of the columns of the equivalent multi‐input multi‐output finite impulse response channels from the lowest degree columns to the highest degree ones. Accordingly, each user's overall channel that is the convolution of the original multipath channel and the spreading sequence is estimated. Then we extract PN sequences from the overall channel using finite alphabet characteristics of the spreading sequence chips for each user. According to simulation results, our proposed scheme outperforms the conventional methods in that it does not require symbol synchronization and does not have channel constraints (for example, AWGN and single user system) in the multipath channels.     

Turbo编码DS/CDMA系统中的迭代多用户接收器

本文提出了一种用于Turbo编码DS/CDMA系统的迭代多用户接收器。该接收器由一个软输入/软输出（SISO）的多用户检测器和一组单用户SISO信道译码器组成。每次迭代过程中，多用户检测器和信道译码器都输出信息作为下一代迭代的先验信息，仿真结果表明，这种接收器的比特误码性能接近Turbo编码系统的单用户限。     

Performance evaluation of multirate DS‐CDMA communication systems with adaptive antennas

Adaptive antennas, with spatial processing, used in code division multiple access (CDMA) systems can enhance range, reliability, and capacity. Moreover, the adaptive beamforming technologies can remove unwanted noise and interference from the received signal. In this paper, we propose the adaptive direct sequence (DS) CDMA antenna receivers and compare their performances in three existing multirate methodologies—variable processing gain (VPG), variable chip rate, and multicode systems for multiuser communications in multipath fading channels. Simulation results show that the processing powers of both narrowband and wideband adaptive antennas are not enough. We propose a powerful processing technique named Wiener Code Filter (WCF) to retrieve the shortcomings. Mean square error (MSE) analyses are also given in this paper. Some simulation examples are given to compare the performances of the proposed multirate structures. Copyright © 2010 John Wiley & Sons, Ltd.     

Asynchronous orthogonal decision‐feedback multiuser detector (AODFD) and its alternative decoding strategies

This paper proposes a new CDMA multiuser detector, asynchronous orthogonal decision‐feedback detector (AODFD), and its alternative decoding schemes. The proposed AODFD does not require an infinitely long whiten filter in its feed‐forward stage, however, which is necessary in the traditional ADDFD detector reported in the literature. The updating algorithm of the AODFD detector is also much simplified if compared with that of ADDFD that requires computational intensive z‐transformed matrix inversion and spectral factorization. Results show that, despite its low complexity, the AODFD performs very well under multiple access interference. The proposed two new decoding strategies can also be chosen to cater for different operating scenarios. Copyright © 2001 John Wiley & Sons, Ltd.     

基于ε逼近算法的CDMA盲自适应多用户检测

把码分多址通信系统的盲多用户信号检测问题转化为一个带约束的二次规划问题，利用二次规划问题的ε逼近算法给出了一种新的CDMA盲自适应多用户检测方法，仿真结果表明，该算法不但具有较强的多址干扰抑制性能，而且具有很快的收敛速度。对于时变信道特别适用。     

异步DS-CDMA系统中的最优时空线性多用户检测器

该文给出了一种适用于异步DS-CDMA系统的通用线性时空二维多用户检测方法,它将所有线性多用户检测器归结到一个统一的数学模型之下。基于此模型,该文将时空线性多用户检测问题转化为一个权矢量的优化问题,并在线性约束最小方差(LCMV)准则下给出了权矢量的优化解。对现有的几种典型线性多用户检测器的分析表明,这些方法都可以看作是该文提出的通用时空线性多用户检测方法的特例,且只能在某种限定条件下得到局部最优解,因此其抑制多址干扰和噪声的能力都低于文中的最优时空线性多用户检测器。最后,给出了数值仿真结果。     

Blind adaptive multiuser detector for nonlinearly modulated satellite mobile CDMA systems

This paper presents a novel blind adaptive noncoherent decorrelative multiuser detector for nonlinearly modulated satellite mobile Code Division Multiple Access （CDMA） systems. By using the known signature waveforms of the counterpart earth station in the blind adaptive multiuser detector, the system performance has been improved obviously. The computation results about the convergence properties of the new detector and the previous detectors demonstrate that the proposed multiuser detector has better performance than previous multiuser detectors for nonlinearly modulated CDMA systems.     

The LSCM_SUB algorithm for blind multiuser detection in multipath CDMA channels

1 Introduction Multiuser detection (MUD) is applied mainly in CDMA to cancel the multiaccess interference. The single-user matched filter or RAKE receiver treats interference from other users as noise, but MUD takes advantage of other user’s interference…     

MC‐CDMA MIMO systems with quasi‐orthogonal space–time block codes: Channel estimation and multiuser detection

This paper investigates blind channel estimation and multiuser detection for quasi‐synchronous multi‐carrier code‐division multiple‐access (MC‐CDMA) multiple‐input multiple‐output (MIMO) systems with quasi‐orthogonal space–time block codes (QO‐STBC). Subspace‐based blind channel estimation is proposed by considering a QO‐STBC scheme that involves four transmit antennas and multiple receive antennas. Based on the first‐order perturbation theory, the mean square error of the channel estimation is derived. With the estimated channel coefficients, we employ minimum output energy and eigenspace receivers for symbol detection. Using the QO‐STBC coding property, the weight analyses are performed to reduce the computational complexity of the system. In addition, the forward–backward averaging technique is presented to enhance the performance of multiuser detection. Numerical simulations are given to demonstrate the superiority of the proposed channel estimation methods and symbol detection techniques. Copyright © 2011 John Wiley & Sons, Ltd.     

光码分多址多用户干扰抑制系统性能研究

针对光码分多址多用户干扰问题,主要研究两种减小多用户干扰的方法(引入两个光硬限幅器法和多比特传输法)中哪种方法对多用户干扰的抑制效果更强.推导了上述方法的误码率性能,并进行了仿真.仿真结果表明,引入两个光硬限幅器的方法对多用户干扰的抑制效果比多比特传输的方法要好,当接收功率＞-52 dBm时,前者比后者的误码率可改善5个数量级.     

Adaptive blind receivers for MC‐CDMA communication systems

In this paper, we propose three adaptive blind algorithms for multiuser multicarrier code division multiple‐access systems in multipath fading channels. The proposed adaptive blind receivers are based on the property of the discreteness of the input data symbol and are updated in every symbol interval. We also use the concept that the variance of the output signal approaches to the variance of the desired signal to get the cost function. The three proposed receiver structures are the traditional finite impulse response (FIR) structure, the despreading (DES) filter structure and the generalized sidelobe canceller (GSC) structure. The advantage of the FIR filter is that the length of the filter weights does not have to be the same length as the spreading code. For the DES filter, the combination of the adaptive weight and the despreading code has the simplest structure than the other two proposed receiver structures. The constrained GSC filter is superior to the other two proposed receiver structures in the environments dominated by multiple‐access interference. By this constraint, the blind GSC filter can guarantee to converge to the desired solution. Simulation results are given to show the effectiveness and comparison of the proposed adaptive blind receivers. Copyright © 2012 John Wiley & Sons, Ltd.     

多径CDMA信道下的一种改进的盲空时多用户检测

该文在多径CDMA信道下基于Rosen梯度投影实现的最小均方盲空时多用户检测方法的基础上提出了一种改进的方法,该方法与原有的方法相比,运算量有所降低、对系统的要求降低,但通过仿真结果比较表明,该方法的检测性能与原有的方法大致相当,因而更加具有竞争力。     

A low‐complexity adaptive receiver for DS‐CDMA systems in unknown code delay environment

In direct‐sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple‐access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay‐locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low‐complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least‐mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean‐square error, of the proposed receiver. Copyright © 2010 John Wiley & Sons, Ltd.     

CDMA系统中联合功率控制、波束赋形与多用户检测

应用功率控制、波束赋形和多用户检测可以有效地消除和减少CDMA系统中的干扰。本文结合波束赋形和多用户检测的特性，提出一种应用波束赋形增益和多用户相关矩阵的解相关算法。基于算法的复杂性，提出一种区分聚集用户进行多用户检测的的算法。由于波束赋形能够消减主瓣波束之外的多用户干扰，对聚集移动台采用解相关检测达到减少运算量和消除多用户干扰的目的。对算法进行了仿真分析，证明了本算法的优越性。     

Differential space–time modulation for DS‐CDMA systems

Differential space–time modulation (DSTM) schemes were recently proposed to fully exploit the transmit and receive antenna diversities without the need for channel state information. DSTM is attractive in fast flat fading channels since accurate channel estimation is difficult to achieve. In this paper, we propose a new modulation scheme to improve the performance of DS‐CDMA systems in fast time‐dispersive fading channels. This scheme is referred to as the differential space–time modulation for DS‐CDMA (DST‐CDMA) systems. The new modulation and demodulation schemes are especially studied for the fast fading down‐link transmission in DS‐CDMA systems employing multiple transmit antennas and one receive antenna. We present three demodulation schemes, referred to as the differential space–time Rake (DSTR) receiver, differential space–time deterministic (DSTD) receiver, and differential space–time deterministic de‐prefix (DSTDD) receiver, respectively. The DSTD receiver exploits the known information of the spreading sequences and their delayed paths deterministically besides the Rake‐type combination; consequently, it can outperform the DSTR receiver, which employs the Rake‐type combination only, especially for moderate‐to‐high SNR. The DSTDD receiver avoids the effect of intersymbol interference and hence can offer better performance than the DSTD receiver. Copyright © 2001 John Wiley & Sons, Ltd.