A two-stage CMA-based receiver for blind joint equalization and multiuser detection in high data-rate DS-CDMA systems

The paper deals with the problem of blind mitigation of intersymbol interference (ISI) as well as multiple-access interference (MAI) in asynchronous high data-rate direct-sequence code-division multiple-access systems. A blind adaptive multiuser receiver based on the constant-modulus algorithm (CMA) is proposed, which demodulates each desired user by exploiting only the knowledge of its spreading code, without requiring estimation of the users's channels and timings. In order to overcome the CMA interference capture problem, which arises in a multiuser scenario, a two-stage adaptive receiver is adopted: In the first stage, partial MAI and ISI suppression is blindly achieved by exploiting the desired user signature structure properties; in the second stage, the residual MAI and the ISI are removed by using the CMA, and the information symbols of the desired user are reliably recovered. Theoretical analysis and simulation results show that the first stage is an effective blind adaptive strategy which allows the CMA detector in the second stage to lock on the desired-user symbol, at a particular delay. The proposed blind receiver achieves a significant performance gain in comparison with existing blind methods.     

Group-blind multi-user detection under multi-path channel

This article proposes an improved generalized side-lobe canceller(GSC)receiver structure based on a new group constraint,which is suitable to eliminate multiple-access interference(MAI),inter-symbol interference(ISI)in direct-sequence code-division multiple access(DS/CDMA)downlink multi-user system.The new group constraint is deduced to construct the group-blind adaptive multi-user detector.The analysis indicates that the improved GSC under the new constraint not only retains the property of suppressing ISI and canceling MAI,but also can extract useful information from the succeeding bit for the desired user.The simulations show that the improved GSC filter can cancel the MAI efficiently whether the wireless channel length is short or long,and achieve better performance than other compared filters.     

Symbol-by-symbol based adaptive interference canceller forasynchronous DS/CDMA systems in multipath fading channels

The authors propose a new interference cancellation scheme called the symbol-by-symbol based adaptive interference canceller (SAIC), which adaptively estimates and removes both multiple access interference (MAI) and intersymbol interference (ISI) at the output of the Rake receiver. The SAIC is considerably simpler to implement than existing techniques; computer simulation results demonstrate that it can perform much better than conventional Rake receivers     

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.     

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.     

STBC MIMO CDMA系统CMVDR接收机研究

提出了STBC MIMO CDMA系统中一种新的基于多用户ML估计的带制约的最小方差无失真响应(CMVDR)接收机，能有效抑制多址干扰(MAD和符号间干扰(ISI)。给出了多用户、多径的STBC MIMO CDMA系统信道模型及ML参数估计，并给出了分部加权和整体加权两种CMVDR接收机，本接收机也适用于SIMO CDMA系统。进行了系统模拟，并对分部加权和整体加权两种CMVDR接收机的性能以及自适应CMMSE接收机的性能进行了比较分析。     

An efficient receiver scheme for downlink ZP-CDMA

A signal transmitted through a wireless channel may be severely distorted by intersymbol interference (ISI) and multiple access interference (MAI). In this paper, we propose an efficient CDMA receiver based on frequency domain equalization (FDE) with a regularized zero forcing (RZF) equalizer and parallel interference cancellation with a unit clipper decision function (CPIC) to combat both the ISI and the MAI. We call this receiver the FDE-RZF-CPIC receiver. This receiver is suitable for downlink zero padding CDMA cellular systems. The effects of the decision function, the channel estimation, the number of cancelled users, and the user loading on the performance of the proposed receiver are discussed in the paper. The bit error rate (BER) of the data received by the proposed receiver is evaluated by computer simulations. The experimental results show that the proposed receiver provides a good performance, even with a large number of interfering users. At a BER of 10^?3, the performance gain of the proposed receiver is about 2 dB over the RAKE receiver with a clipper decision function and PIC in the half-loaded case (eight users) and is much larger in the full-loaded case (16 users).     

Reduced Complexity Adaptive MAP Receiver for Frequency-Flat,Time-Varying CDMA Channels

An adaptive maximum a posteriori (MAP) receiver withmultiple-access interference (MAI) mitigation is proposed for DS-CDMAsystems in frequency-flat, time-varying fading channels. The receiverconsists of a sequence matched filter (MF), an adaptive channelestimator, and a MAP demodulator. By oversampling the MF output, thereceiver can characterize the MAI symbol-by-symbol to suppress thestrong and time-varying MAI. Without any pilot insertion and apriori knowledge, the channel estimator can adapt to various fadingchannels using a general, model-based approach. For practicalimplementations, a reduction in complexity is introduced and is shownto have a negligible effect on both uncoded and decoded bit errorrates (BER's).     

基于U-D分解滤波的高性能盲自适应多用户检测算法

提出利用U-D分解渐消记忆滤波算法估计CDMA系统多用户接收器的最优判决向量，构造出一种收敛速度快、跟踪性能好、数值稳定性好的高性能盲自适应多用户检测算法。仿真实验表明，本文提出的方法具有很强的抗多址干扰能力和较高的数值鲁棒性。     

IDMA水声多用户下行通信链路迭代接收算法

水声通信网络的快速发展对多用户水声物理层通信技术提出了迫切的需求。本文主要针对水声多用户下行通信链路中存在严重码间干扰和多址干扰的问题,提出了一种交织多址（IDMA）水声多用户下行通信链路迭代接收算法,能够根据译码器输出的软信息对估计的信道进行迭代更新,同时根据信道估计结果重建各用户对当前用户的多址干扰（MAI）,并使用干扰抵消器予以抵消,通过在均衡器、译码器和干扰抵消器之间的迭代逐次提高当前用户信息的输出信噪比,最终实现正确解调。其中,信道估计算法采用LMMSE初始化的分簇OMP稀疏信道估计算法,多普勒估计采用时频二维搜索结合自相关相位估计的方法,计算机仿真结果表明该接收机具有与已知信道时接近或相当的性能。进行了多用户水声通信海上试验,在4.5km、6km和7km三个距离实现了对2个用户、3个用户和4个用户的同时通信,每个用户的通信净速率约为500bps,证明了本文所提出的方法能够有效应用在多用户水声通信中,为水声通信网络的发展提供了一种重要的物理层通信技术手段。      

A Decision Aided CDMA Receiver with Partially Adaptive Decorrelating Multi-User Interference Cancellation

A CDMA receiver with enhanced multiple access interference (MAI) suppressionis proposed for a pilot symbols assisted system over multipath channels. Thedesign of the receiver involves the following procedure. First, blind adaptivecorrelators are constructed at different fingers based on the scheme ofgeneralized sidelobe canceller (GSC) to collect the multipath signals andsuppress MAI. A low-complexity partially adaptive (PA) realization of the GSCcorrelators is proposed which incorporates multi-user information for reducedrank processing. By a judiciously designed decorrelating procedure, a new GSCstructure is obtained in which the MAI are decorrelated and suppressedindividually. The next step is then a simple coherent combining of thecorrelator outputs with pilot aided channel estimation. Finally, furtherperformance enhancement is achieved by an iterative scheme in which the signalis reconstructed and subtracted from the GSC correlators input data, leadingto faster convergence of the receiver. The proposed low-complexity PA CDMAmulti-user receiver is shown to be robust to multipath fading and channelerrors, and achieve nearly the same performance of the ideal maximum SINR andMMSE receivers by using a small number of pilot symbols.     

一种空时盲自适应RAKE接收机

多址干扰是直扩码分多址系统中的一个主要问题.多用户检测技术和阵列天线已成为解决这一问题的两种主要方法,将这两种方法结合起来的空时处理能大大提高系统的容量和性能.本文提出了一种空时盲自适应RAKE接收机.研究结果表明,该接收机具有较强的抑制多址干扰和克服"远-近"效应的能力,且能快速收敛.     

一种频率选择性衰落信道下的盲自适应去相关Rake接收机

多用户检测是DS－CDMA系统中的一项关键技术,而Rake接收是解决多径效应的一种有效方法,本文将基于Kalman滤波的多用户检测器与Rake接收相结合,提出了一种频率选择性衰落信道下的盲自适应去相关Rake接收机,研究结果表明,这种接收机具有较强的抑制多址干扰和克服“远－近”效应能力,并且能快速收敛。     

Iterative (turbo) soft interference cancellation and decoding forcoded CDMA

The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive soft-output decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached     

Zero forcing and minimum mean-square-error equalization formultiuser detection in code-division multiple-access channels

In code-division multiple-access (CDMA) systems transmitting over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. The conventional suboptimum receiver consisting of a bank of matched filters is often inefficient because interference is treated as noise. The optimum multiuser detector is too complex to be implemented at present. Four suboptimum detection techniques based on zero forcing (ZF) and minimum mean-square-error (MMSE) equalization with and without decision feedback (DF) are presented and compared. They combat both ISI and MAI. The computational complexity of all four equalizers is essentially the same. All four equalizers are independent of the size of the data symbol alphabet. It is shown that the performance of the MMSE equalizers is better than that of the corresponding ZF equalizers. Furthermore, the performance of the equalizers with DF is better than that of the corresponding equalizers without DF. The impairing effect of error propagation on the equalizers with DF is reduced by channel sorting     

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     

综合业务的CDMA系统的业务容量分析

个人通信与移动计算的发展促进了将综合业务引入移动通信中。ＣＤＭＡ系统是新一代传输综合业务的系统。本文采用一种业务接入控制算法，分析了综合业务的ＣＤＭＡ系统的业务容量。显见，在计算综合业务的ＣＤＭＡ系统容量时，所需的Ｅｂ／Ｉ０、速率Ｒ及激活因子大的业务对系统容量的影响大。同时在上行信道中，非理想功率控制对Ｅｂ／Ｉ０、Ｒ及激活因子大的业务的容量影响也较大。为此，可以对不同的业务置以不同的功率调整步长来减小这种影响。     

Reduced-rank interference suppression for DS-CDMA based on interpolated FIR filters

Reduced-rank receivers based on interpolated finite impulse response (FIR) filters for direct sequence code division multiple access (DS-CDMA) systems are proposed and a novel scheme where the interpolator is rendered time-varying is introduced. The interpolated minimum mean squared error (MMSE) and constrained minimum variance (CMV) solutions are derived for both receiver and interpolator to mitigate multiple access interference (MAI) and intersymbol interference (ISI) in a downlink scenario.     

Interference Cancellation with Space Diversity for Downlink MC-CDMA Systems

Modern wireless communications require an efficient spectrum usage and high channel capacity and throughput. Multiple-input and multiple-output (MIMO), Linear equalizers, multi-user detection and multicarrier code-division multiple access (MC-CDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate multiple access interference (MAI), eliminate Inter symbol interference (ISI) and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Multiple-output (SIMO) downlink MC-CDMA systems. It employs an interference cancellation scheme to suppress the interference caused by the multipath fading channel. Also, the proposed scheme is developed for MIMO MC-CDMA system. The performance analysis of Downlink MIMO MC-CDMA systems with V-BLAST over frequency selective fading channel is investigated under various number of transmit and receive antennas. The simulation results show proposed SIMO equalization with parallel interference cancellation scheme is effective in reducing the ISI and the MAI. It improves the performance significantly and the simulation results show that MIMO MC-CDMA with V-BLAST multi-user detection provides high data rate and the BER significant improvement.     

Soft-Limiting Receiver Structures for Time-Hopping UWB in Multiple-Access Interference

A novel receiver structure based on soft-limiting is proposed for detecting a time-hopping ultrawide-bandwidth (UWB) signal in the presence of multiple-access interference (MAI). The proposed structure contains a nonlinear limiter for suppressing the MAI. Simulation results show that the proposed receiver achieves better bit-error-rate performance than the conventional matched receiver when operating in MAI. When operating in an MAI-plus-Gaussian-noise environment, the receiver structure outperforms the conventional matched-filter receiver for moderate to large values of signal-to-noise ratio (SNR). A receiver structure with adaptive limiting threshold is further proposed to ensure that the performance of the soft-limiting receiver always meets or surpasses the performance of the conventional UWB receiver for all values of SNR. Application of the soft-limiting structure in the fingers of a rake receiver is shown to enhance the signal-to-noise-plus-interference ratio.