多径信道下辅助矢量接收机性能分析

研究了在直接序列码分多址(DS-CDMA)系统中,多径环境下一种基于RAKE结构的辅助矢量接收机。该接收机在收集多径信号的同时抑制系统中的多址干扰,从收集的多径信号中估计出信道响应矢量,并利用估计出的信道响应矢量对多径信号进行合并,最后进行判决。仿真结果表明,辅助矢量接收机能够有效地去除DS-CDMA系统中的多址干扰,同时具备良好的抗多径衰落性能,误码性能明显优于RAKE接收机。     

An equicorrelation-based multiuser communication scheme for DS-CDMA systems

An equicorrelation-based multiuser communication (ECBMC) scheme for direct-sequence code-division multiple-access (DS-CDMA) systems is presented. The ECBMC receiver has low computational complexity that is comparable to that of the conventional detector. By using the equality of cross correlations, the ECBMC scheme can completely eliminate multiple-access interference (MAI) in a synchronous single-path DS-CDMA network. The system performance is independent of the number of active users. The scheme is extended to include the effects of multipath fading. It is able to suppress a major portion of the MAI. This proposed ECBMC scheme is quite attractive for an MAI-dominant environment.     

一种新的盲空时多用户检测器

多用户检测是DS-CDMA系统中的一项关键技术,而阵列天线也是一项减轻多用户干扰的方法.许多现存的多用户检测器需要知道很多系统参数,并且其自适应实现需要发送训练序列.在多径衰落信道下,这些参数是很难获得的.本文提出了一种新的基于Kalman滤波的盲空时多用户检测器(BSTKAL),这种多用户检测器不需要发送训练序列.研究结果表明,检测器具有较强的抑制多址干扰和克服"远-近"效应的能力,并且能快速收敛.     

Postdetection optimal diversity combiner for DPSK differentialdetection

Postdetection diversity reception weights and combines all the detector outputs before symbol decision to combat the effects of multipath fading. A theoretical analysis of a postdetection optimal diversity combiner that can minimize the symbol error probability for differential phase shift keying (DPSK) differential detection in the presence of multiplicative Rayleigh fading, and co-channel interference (CCI) is presented. The effect of unequal average powers among diversity branches is taken into account. It is shown that the postdetection maximal-ratio combiner (MRC) described previously by the author is not optimal unless all branches have the same average power. It is also found that the combiner optimized for the effect of CCI (fading induced random FM noise) should weight each branch detector output in inverse proportion to the average CCI power (desired signal power). Assuming two-branch diversity, calculated BER (bit-error-rate) performance of π/4-shift QDPSK due to AWGN, CCI, and random FM is presented. In addition, the BER due to multipath channel delay spread (which is not treated in the theoretical analysis) is also computed to find the optimal combiner     

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。     

On the performance of multicarrier RAKE systems

In this paper, we integrate the multicarrier signaling technique and the RAKE receiver to design a DS-CDMA system with the capability to increase the data rate, mitigate the effect of correlation among the various subcarriers, and suppress partial-band interference. In the proposed system, a data sequence is serial-to-parallel converted, and multicarrier DS-CDMA is used on each of the parallel data streams. The receiver provides a RAKE for each subcarrier, and the outputs of the RAKEs are combined by a maximal-ratio combiner. We employ a correlated subcarrier and frequency-selective fading channel model to derive the average probability of error of the system and compare the results with those of both a single carrier RAKE system and a multicarrier DS-CDMA system in a frequency selective Rayleigh fading channel with an exponential multipath intensity profile, when multiple access interference and partial-band interference are present     

Space-Time Coded Beamforming for DS CDMA Downlink

1　Introduction　 In 3G mobile system, Code Division Multiple Access(CDMA) is a promising multiple access techniques. It iswell know that BeamFormin ( BF), Space Time Coding(STC) and Multiuser Detection(MUD) techniques in CDMAsystem can effectively resist Multi Path Interference (MPI),MAI and near far effects. So the system capacity will be im proved greatly by employing these techniques[1,2,8]. In Ref.[3] introduces Water Filling(WF) theory into STC to assignth…     

Error Rate Analysis of SIMO-CDMA with Complementary Codes Under Multipath Fading Channels

Broadband communication systems of the current generation are likely to offer higher bit rates for delivering high-speed multimedia services to the end users. The achievable capacity and data rate of wireless communication systems are limited to fading channels varying with time, leading to multiple access interference (MAI) and multipath interference (MPI). In this paper, we investigate downlink single-input multiple output transmission for complementary coded code-division multiple access (CC-CDMA) systems working in channels with multipath fading. Here, parallel interference cancellation is employed for analysis of CC-CDMA with different frequency domain equalization schemes to eliminate MPI and MAI over multipath fading channels. Error rate analysis for CC-CDMA employing receiver diversity is assessed using simulations under varying channel parameters. Further, we compare different equalization schemes to show the superiority of regularized zero forcing in reducing the error rate of CC-CDMA systems.     

Prerake joint with STTCM for TDD DS-CDMA system

A novel synthetical transmit diversity, namely Prerake joint with Space-Time Trellis Coded Modulation (STTCM), for TDD DS-CDMA system is proposed in this paper. Space-Time Code (STC) is applied under the multipath fading circumstance. Prerake is used in stead of Rake to overcome multipath fading for the simplicity of the receiver configuration. Through computer simulation, the performance of our proposed scheme in multipath fading can be better than that of just STC is used, the taps of the Prerake should be chosen to be equal to the multipath number in the matter of minimum mean square error. A new constellation (8APK) is presented to construct a kind of STC that achieves better performance than that of 8PSK.     

Effects of Nakagami-Fading Parameters and Power Control Error on Performance of DS-CDMA Cellular Systems with Adaptive Beamforming

It is well known that power control error (PCE) is a critical issue in CDMA cellular systems. In this paper, the bit error rate (BER) of a direct sequence-code division multiple access (DS-CDMA) receiver with imperfect power control, adaptive beamforming, and voice activity is derived in frequency-selective Nakagami fading channels. We discuss the effects of PCE, Nakagami-m fading parameter, and channel’s multipath intensity profile as average signal strength and rate of average power decay and their effects on the BER performance of DS-CDMA cellular systems. In this paper, the RAKE receiver consists of three stages. In the first stage, with conjugate gradient adaptive beamforming algorithm, the desired users’ signal in an arbitrary path is passed and the inter-path interference is canceled in other paths in each RAKE finger. Also in this stage, the multiple access interference (MAI) from other users is reduced. Thus, the matched filter (MF) can be used for the MAI reduction in each RAKE finger in the second stage. In the third stage, the output signals from the MFs are combined according to the conventional maximal ratio combining principle and then are fed into the decision circuit for the desired user. How the Nakagami fading parameters, power control imperfections, or the number of resolvable paths affect the reverse link capacity of the system is discussed in detail. Analytical and simulation results are also given for systems with different processing gains and number of BSs in the cell-selection process with various Nakagami fading parameters.     

Cramer-Rao bounds for the estimation of multipath parameters and mobiles' positions in asynchronous DS-CDMA systems

Commercial applications for the location of subscribers of wireless services continue to expand. Consequently, finding the Cramer-Rao lower bound (CRLB), which serves as an optimality criterion for the location estimation problem, is of interest. In this paper, we derive the deterministic CRLBs for the estimation of the specular multipath parameters and the positions of the mobiles in an asynchronous direct sequence code division multiple access (DS-CDMA) system operating over specular multipath fading channels. We assume a multilateral radio location system where the location estimates are obtained from some or all of the estimated signal parameters at different clusters of antennas of arbitrary geometry. Extension for unilateral and composite radio location techniques is also discussed. As an application example, we use numerical simulations to investigate the effects of specular multipath and multiple access interference (MAI) on the positioning accuracy for different radio location techniques.     

Antenna diversity combining and finite-tap decision feedbackequalization for high-speed data transmission

The next-generation wireless communication systems are expected to support high-speed data transmission. Associated with high transmission rates, however, is the problem of multipath intersymbol interference (ISI) due to frequency-selective fading. Decision feedback equalization (DFE) and antenna diversity combining are two practical techniques for combating multipath ISI. Through simulations we investigate the performance of diversity combining, together with DFE, under various numbers of antenna branches and equalization taps, in a quasistationary frequency-selective fading environment with additive white Gaussian noise (AWGN) and cochannel interference (CCI). We consider joint optimization combining and power selection diversity combining. We simulate the combiner, using quaternary phase shift keying (QPSK) modulation with up to four antenna branches. Our results show that using antenna diversity and DFE with joint optimization combining provides performance improvement with lower computational complexity, as compared to that of using either DFE or diversity combining alone for combating ISI     

Effect of bit-to-bit dependence on packet error rate using asynchronous DS-CDMA for mobile packet radio networks

The probability of packet success for asynchronous direct sequence code division multiple access (DS-CDMA) over the multipath fading channel is investigated under different fading conditions and for different cases. We studied the bit-to-bit dependence caused by the multipath fading and by the multiple access interference (MAI) originating from the chip and phase offsets between the desired and interfering signals. We investigated the effect of using the Gaussian versus the improved Gaussian process on the computation of probability of packet success as well as the effect of the bit-to-bit dependence within a packet. The improved Gaussian approximation provides more accurate values for the probability of bit error for any number of simultaneous users, while it also allows us to incorporate the effects of bit-to-bit dependence into the calculations of the probability of packet success.This paper was presented in part at the 17th Biennial Symposium on Communiations, Kingston, June 1994. This work was supported by a strategic grant (STR-0100720) from the Natural Sciences and Engineering Research Council (NSERC) of Canada.     

Performance of generalized multicarrier DS-CDMA over Nakagami-mfading channels

A class of generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) schemes is defined and its performance is considered over multipath Nakagami-m fading channels. The spacing between two adjacent subcarriers of the generalized MC DS-CDMA is a variable, allowing us to gain insight into the effects of the spacing on the bit error rate (BER) performance of MC DS-CDMA systems. This generalized MC DS-CDMA scheme includes the subclasses of multitone DS-CDMA and orthogonal MC DS-CDMA as special cases. We present a unified analytical framework for determining the exact average BER of the generalized MC DS-CDMA system over generalized multipath Nakagami-m (1960) fading channels. The optimum spacing of the MC DS-CDMA system required for achieving the minimum BER is investigated and the BER performance of the system having optimum spacing is evaluated. The resultant BER is compared with that of both multitone DS-CDMA and orthogonal MC DS-CDMA     

A Theoretical Evaluation of Parallel Interference Cancellation in M-Ary Orthogonal Modulated Asynchronous DS-CDMA System Over Multipath Rayleigh Fading Channels

In this paper, we tackle the problem of theoretical evaluation for the multistage parallel interference cancellation (PIC) scheme in a direct-sequence code division multiple access (DS-CDMA) system with orthogonal modulation and long scrambling codes. The studied system operates on the reverse link in a time varying multipath Rayleigh fading channel. By applying the Central Limit Theorem and some other approximations to multiple access interference (MAI) and intersymbol interference (ISI), as well as assuming identically distributed chips from a single interferer, the bit error rate (BER) performance of the PIC scheme at any stage can be recursively computed from the signal-to-noise ratio, number of users, the number of path per user, processing gain of the CDMA system, and the average received power of each path. For completeness, the BER expression is derived for chip synchronous and chip asynchronous systems over both equal and unequal power multipath channels. The proposed analysis is validated by the Monte Carlo simulations and proved to be reasonably accurate, and it gives insight into the performance and capacity one can expect from PIC-based receivers under different situations. For instance, the analytical results can be used to examine the convergence property, multipath diversity gains, and near-far resistance of the PIC scheme.     

Performance of asynchronous DS-CDMA using a maximal ratio combiningdiversity over a Rician fading channel

The final closed-form expression for bit error probability (BEP) is presented for a DS-CDMA system using a maximal ratio combining (MRC) diversity over a Rician fading channel. The accuracy of the BEP estimate evaluated by this expression is verified by comparison with a semi-analytic simulation result. The effect that diversity order has on the BEP is also considered for typical multipath delay profiles with different Rician ratios     

Multicarrier DS/SFH-CDMA systems

Multicarrier direct-sequence/slow-frequency-hopping (MC DS/SFH) code-division multiple-access (CDMA) systems are proposed, in which multiple carriers are modulated by the same DS waveform and hopped in frequency according to a random hopping pattern. The receiver dehops the received signal with the same pattern, provides RAKE receivers for each carrier, and combines the outputs with a maximal ratio combiner (MRC). The performance of the proposed system is investigated over a frequency-selective Rayleigh-fading channel and compared to that of the MC DS-CDMA systems. It is shown that for the same diversity order, the MC DS/SFH-CDMA systems are superior in reducing multiple-access interference (MAI) while preserving the good capability of narrowband interference suppression, when the system parameters are selected properly.     

Adaptive chip-rate equalization of downlink multirate wideband CDMA

We consider a downlink direct sequence-code division multiple access (DS-CDMA) system in which multirate user signals are transmitted via synchronous orthogonal short codes overlaid with a common scrambling sequence. The transmitted signal is subjected to significant time- and frequency-selective multipath fading. In response to this scenario, a novel two-mode receiver is proposed that accomplishes chip-rate adaptive equalization aided by filtering and/or cancellation of multiaccess interference (MAI). In the acquisition mode, a code-multiplexed pilot is used to adapt the equalizer from cold start or loss-of-lock. The use of MAI filtering results in a third-order least mean squares (LMS) algorithm, which has significant advantages over standard (i.e., first-order) LMS in nonstationary environments. In the tracking mode, decision-direction facilitates MAI-cancellation in the equalizer update, which enhances performance. The receiver monitors pilot decision quality as a means of switching between the two modes. The performance of the adaptive receiver is studied through analysis and simulation.     

Decentralized multiuser detection for time-varying multipath channels

Multiple-access interference (MAI) and time-varying multipath effects are the two most significant factors limiting the performance of code-division multiple-access (CDMA) systems. While multipath effects are exploited in existing CDMA systems to combat fading, they are often considered a nuisance to MAI suppression. We propose an integrated framework based on canonical multipath-Doppler coordinates that exploits channel dispersion effects for MAI suppression. The canonical coordinates are defined by a fixed basis derived from a fundamental characterization of the propagation effects. The basis corresponds to uniformly spaced multipath delays and Doppler shifts of the signaling waveform that capture the essential degrees of freedom in the received signal and eliminate the need for estimating arbitrary delays and Doppler shifts. The framework builds on the notion of active coordinates that carry the desired signal energy, facilitate maximal exploitation of channel diversity, and provide minimum-complexity MAI suppression. Progressively powerful multiuser detectors are obtained by incorporating additional inactive coordinates carrying only MAI. Signal space partitioning in terms of active/inactive coordinates provides a direct handle on controlling receiver complexity to achieve a desired level of performance. System performance is analyzed for two characteristic time scales relative to the coherence time of the channel. Adaptive receiver structures are identified that are naturally amenable to blind implementations requiring knowledge of only the spreading code of the desired user.     

异步DS-CDMA盲多用户检测

该文研究了频率选择性瑞利衰落信道中的异步DS-CDMA系统盲多用户检测。提出一种期望信号矢量估计方法,该方法同时利用了多径传播和接收机同步失调的特性,以利于把盲线性滤波优化技术应用于稳健的干扰抑制。为了抑制多址干扰(MAI),提出一种基于投影的辅助矢量(PAV)算法。该算法计算复杂度低,特别适用于短数据采样时滤波矢量的快速优化,仿真结果表明在低输入信号干扰噪声比(SINR)时能提供有效的干扰抑制,在高输入SINR时具有稳健的性能。