Joint multipath-Doppler diversity in mobile wireless communications

We introduce a new approach for achieving diversity in spread-spectrum communications over fast-fading multipath channels. The RAKE receiver used in existing systems suffers from significant performance degradation due to the rapid channel variations encountered under fast fading. We show that the Doppler spread induced by temporal channel variations in fact provides another means for diversity that can be further exploited to combat fading. We develop the concept of Doppler diversity and propose a framework that exploits joint multipath-Doppler diversity in an optimal fashion. Performance analysis shows that even the relatively small Doppler spreads encountered in practice can be leveraged into significant diversity gains via our approach. The framework is applicable in several mobile wireless multiple access systems and can provide substantial performance improvement over existing systems     

Time-selective signaling and reception for communication overmultipath fading channels

The mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code-division multiple-access systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel, which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the intersymbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the intersymbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudorandom codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel, thereby facilitating the use of powerful existing coding techniques for Gaussian channels     

Suboptimal multiuser detector for frequency-selective Rayleighfading synchronous CDMA channels

Proposes a suboptimal low-complexity multiuser receiver for synchronous CDMA frequency-selective Rayleigh fading channels. In contrast to the conventional RAKE receiver, which suffers from near-far effects due to channel fading, the proposed multiuser receiver is shown to alleviate the near-far problem while preserving multipath diversity gain. This is demonstrated by comparing the symbol error probability and asymptotic multiuser efficiency of the proposed multiuser detector and RAKE receiver     

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.     

Pilot-based estimation of time-varying multipath channels forcoherent CDMA receivers

Reliable coherent wireless communication requires accurate estimation of the time-varying multipath channel. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) systems: (i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission and (ii) impact of channel estimation errors on coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of filters is derived. A key channel characteristic controlling system performance is the normalized coherence time, which is approximately the number of symbols over which the channel remains strongly correlated. It is shown that the estimator performance is characterized by an effective signal-to-noise ratio (SNR)-the product of the pilot SNR and the normalized coherence time. A simple uniform averaging estimator is also proposed that is easy to implement and delivers near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits joint multipath-Doppler diversity. It is shown that the overall receiver performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite effects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can significantly mitigate the error probability floors that plague conventional CDMA receivers under fast fading due to errors in channel estimation     

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.     

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.     

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     

An iterative maximum SINR receiver for multicarrier CDMA systems over a multipath fading channel with frequency offset

A robust iterative multicarrier code-division multiple-access (MC-CDMA) receiver with adaptive multiple-access interference (MAI) suppression is proposed for a pilot symbols assisted system over a multipath fading channel with frequency offset. The design of the receiver involves a two-stage procedure. First, an adaptive filter based on the generalized sidelobe canceller (GSC) technique is constructed at each finger to perform despreading and suppression of MAI. Second, pilot symbols assisted frequency offset estimation, channel estimation and a RAKE combining give the estimate of signal symbols. In order to enhance the convergence behavior of the GSC adaptive filters, a decisions-aided scheme is proposed, in which the signal waveform is first reconstructed and then subtracted from the input data of the adaptive filters. With signal subtraction, the proposed MC-CDMA receiver can achieve nearly the performance of the ideal maximum signal-to-interference-plus noise ratio receiver assuming perfect channel and frequency offset information. Finally, a low-complexity partially adaptive (PA) realization of the GSC adaptive filters is presented as an alternative to the conventional multiuser detectors. The new PA receiver is shown to be robust to multiuser channel estimation errors and offer nearly the same performance of the fully adaptive receiver.     

Small delay multipath diversity in spread spectrum communication systems

A new double-filter receiver for spread spectrum wireless systems is presented. The receiver exploits the inherent diversity due to the small delay difference between the multipaths which may not be exploited by the conventional RAKE receiver. The bit error rate performances of both the coherent and the noncoherent detectors are analyzed and compared with the ideal RAKE receiver and the matched filter bound. We also examine the best and the worst cases of a three-path fading channel for the proposed receiver. Optimum pulse shapes under various channel conditions are designed. Implementation issues in code-division multiple access systems are discussed. The numerical results show that the proposed receiver achieves significant gains for a given spreading factor under a complexity constraint.     

数字移动通信中的抗多径衰落技术

移动信道的主要特征是由移动和多径传播所产生的多径衰落现象。在高速移动通信系统中，必须采用抗多径衰落技术，才能实现有效传输。本文首先概述了移动信道的数学模型及其典型形式，在此基础上，分别讨论了时分多址（ＴＤＭＡ）和码分多址（ＣＤＭＡ）系统的抗多径衰落方法──自适应信道均衡技术及多径分集接收技术，并简述了近年来较为活跃的多用户检测技术和自适应天线分集技术。     

Combined multiuser detection and diversity reception for wirelessCDMA systems

Code division multiple-access (CDMA) techniques using interference cancellation are being explored for the capacity increase in third-generation universal mobile telecommunications systems. However, multipath fading is a major constraint on the performance of wireless CDMA systems, with multipath propagation worsening the effects of multiple-access interference, and fading on propagation paths leading to the near far problem. Multiuser detection, exploiting the knowledge of other users to cancel multiple-access interference, has the capability of eliminating the near far problem and providing a significant capacity increase in CDMA systems. On the other hand, diversity techniques effectively combat the fading effects of the channel. This paper investigates multiuser receivers that combine explicit antenna diversity, RAKE multipath diversity, and multipath decorrelating detection. Both coherent reception with maximal-ratio combining and differentially coherent reception with equal-gain combining are analyzed. The results demonstrate a significant increase in up-link capacity over the conventional RAKE receiver, at the expense of complexity. In the case of limited receiver complexity, where the number of correlators is less than the number of resolvable paths at the RAKE front-end, antenna diversity is shown to be effective in reducing residual multiple-access interference     

一种新的基于特征向量合并的超宽带系统分集方案

针对超宽带系统在室内环境中面临严重的多径衰落问题,通常在接收端需使用RAKE接收机来收集多径能量改善性能。该文提出了一种新的分集方案,在发送端信号进行预处理,在接收端使用RAKE合并收集多径能量,同时给出了基于信道矩阵特征值估计的最佳合并权重和时延参数估计算法。理论分析和仿真结果都表明,该算法得到的输出信噪比总是大于传统的RAKE接收机输出信噪比。     

Performance of DS-CDMA receivers with MRC in nakagami-m fading channels with arbitrary fading parameters

The receivers that combine spatial antenna diversity with temporal multipath diversity are known as two-dimensional (2-D) RAKE receivers. In this paper, we consider the outage probability and the bit error rate performance of a coherent binary phase shift keying 2-D RAKE receiver in the context of an asynchronous direct sequence (DS)-code division multiple access (CDMA) system operating in a Nakagami-m fading channel with real and arbitrary fading parameters. The closed-form expressions derived for the two wireless performance measures are easily evaluated numerically and enable the link designer to examine the effects of system parameters, such as the number of receive antennas, RAKE fingers per antenna, and asynchronous CDMA users in the cell, as well as channel conditions, such as the amount of fading in the combined paths and the multipath intensity profile of the channel on the link performance. In addition, the diversity loss due to correlated fading among the spatially separated RAKE fingers is quantified.     

MMSE RAKE Reception for Asynchronous DS/CDMA Overlay Systems and Frequency-Selective Fading Channels

In this work the authors introduce and analyze a detection scheme for simultaneous suppression ofmultiaccess and digital narrowband interference (NBI) for an asynchronousDS/CDMA system operatingover a frequency-selective fading channel. The proposed detector may possibly consider a processing interval greater than the bit-interval, andamounts to a RAKE structure, wherein each branch is designed under an MMSE optimization strategy, followedby a propercombining rule. It is shown that narrowband interference rejection entails ingeneral a time-varyingdetection structure. As to the performance assessment, we give closed-form formulas for the systemBit Error Rate (BER),Near-Far Resistance and Signal-to-Interference Ratio (SIR).Numerical results show that the proposed receiver proves effective in suppressing interference, and that itlargely outperforms conventional multiuser detectors which do not account forthe presence of theNBI, at the price of little complexity increase.Finally, we provide a blind adaptive implementation of the proposedreceiver, based on the exponentially weighted Recursive-Least-Squares (RLS)algorithm, andrequiring knowledge of the relevant parameters of the desired user only, andof the NBI signaling time.     

Performance of antenna diversity multiuser receivers in CDMA channels with imperfect fading estimation

The performance of antenna diversity coherent and differentially coherent linear multiuser receivers is analyzed in frequency-nonselective Rayleigh fading CDMA channels with memory. The estimates of the complex fading processes are utilized for maximal-ratio combining and carrier recovery of the coherent multiuser receiver. To analyze the impact of channel estimation errors on the receiver performance, error probability is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimation as well as the fading-induced error probability floor. The impact of fading dynamics on the differentially coherent decorrelating receiver with equal-gain combining is quantified. While performance of multiuser receivers at lower SNR is determined by both the fading dynamics and the number of active CDMA users, performance at higher SNR is given by an error probability floor which is due to fading only and has the same value as in a single-user case. The comparison of the two receiver structures indicates that the coherent decorrelating receiver with diversity reception may be preferable to the differentially coherent one in nonselective fading CDMA channels with memory.     

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.     

WCDMA系统中导频符号辅助的空时二维RAKE接收机

针对WCDMA上行链路的时分导频和复扩频方式的特点 ,提出了一种适用于WCDMA系统的线性自适应空时二维RAKE接收机 .利用导频符号进行信道估计 ,并作为空间波束成型器的参考信号 .理论分析和在多用户时变频率选择性衰落信道下的仿真研究表明 ,对导频信号的充分利用可以降低空时二维接收机的实现复杂度 ,所提的接收机能明显提高接收机的输出信干噪比 ,降低误码率     

Error probability performance for W-CDMA systems with multiple transmit and receive antennas in correlated Nakagami fading channels

The bit error rate (BER) performance of a two-dimensional (2-D) RAKE receiver, in combination with transmit diversity on the downlink of a wide-band CDMA (W-CDMA) system, is presented. The analyses assume correlated fading between receive antenna array elements, and an arbitrary number of independent but nonidentical resolvable multipaths combined by the RAKE receiver in the general Nakagami-m (1960) fading channel framework. The impact of the array configuration (e.g., the number of transmit antennas and receive antennas, the antenna element separation) and the operating environment parameters (such as the fading severity, angular spread and path delay profile) on the overall space-path diversity gain can be directly evaluated. In addition, the exact pairwise error probability of a convolutional coded system is obtained, and the coding gain of a space-path diversity receiver is quantified.     

频率选择性衰落信道下DS-CDMA系统发射分集技术

本文研究多径频率选择性衰落信道下,DS-CDMA系统开环发射分集技术.提出了一种将线性多用户检测与发射分集相结合的新方法,该方法能在克服信道衰落的同时抑制多址干扰和码间串扰.针对两种不同的扩频码分配方式,本文分别给出了盲和半盲的信道估计和检测算法.仿真结果表明,本文所提方法的性能优于基于空时分组码的RAKE接收机,特别是存在远近效应时性能改善更大.