A multiuser receiver for trellis-coded DS/CDMA systems inasynchronous channels

We propose and analyze a multiuser receiver using a decorrelating filter and Viterbi decoders for trellis-coded direct-sequence code-division multiple-access (DS/CDMA) systems with biorthogonal signal constellation in asynchronous channels. The biorthogonality is implemented by user signature waveforms and the decorrelating filter. The performance of the proposed system is investigated with emphasis on the asymptotic cases: it is shown that the proposed system provides us with some coding gain and near-far resistance. It is confirmed that we can enhance the performance of the proposed system by using base-station antenna arrays     

Design and analysis of concatenated coded DS/CDMA systems inasynchronous channels

We propose and analyze concatenated coding schemes for direct-sequence code-division multiple-access (DS/CDMA) systems in asynchronous channels. In the concatenated coding, bandwidth-efficient 2 ^2L-2-state L/(L+1)-rate 2-MTCM with biorthogonal signal constellation is used for the inner code, and (2^L-1,[(2^L -1)/L/2]) RS code is used for the outer code. It is shown that we can get considerable performance gain over the uncoded system without sacrificing the data transmission rate. The proposed system can be used as a coding scheme for reliable and high-speed integrated information services of mobile communication systems     

Joint decorrelating multiuser detection and channel estimation inasynchronous CDMA mobile communications channels

The asymptotic efficiencies of two decorrelators, path-by-path and channel-matched decorrelators, are analyzed in fading multipath propagation environments, and based upon the analytical results, a new joint multiuser detection and channel estimation scheme is proposed for asynchronous code division multiple access (CDMA) mobile communications channels. In the path-by-path decorrelator, each of the received signals corresponding to one of the multiple propagation paths is regarded as an independent interference source. On the contrary, in the channel-matched decorrelator, each composite signal transmitted from an identical user is regarded as a response of the multipath channel to the corresponding user's spreading sequence. The asymptotic efficiency of the path-by-path decorrelator is shown to drop rapidly as the number of simultaneous users increases. It is shown that the asymptotic efficiency can be made independent of the number of the propagation paths by the channel-matched decorrelator at the expense of requiring knowledge about the fading complex envelopes of all the propagation paths. The proposed joint multiuser detection and channel estimation scheme uses both path-by-path and channel-matched decorrelators. The path-by-path decorrelator is used for providing the channel estimator with the (noisy) channel information path-by-path, and decisions are made on the output of the channel-matched decorrelator. The decision results are fed back to the channel estimator, and used as the reference signals. The received complex envelope of each of the propagation paths is estimated in the channel estimator. Results of a series of exhaustive computer simulations are presented in order to demonstrate the overall performance of the proposed scheme, both in non-fading and fading multipath propagation environments     

Blind adaptive multiuser detection for DS/CDMA over multipathfading channels

Using a hard null scheme, multipath fading and multiple access interference suppression can be realised for a multiple constrained minimum variance (MCMV) detector at the same time. A modified version of the MCMV detector is also presented, which utilises the eigenstructure of the correlation matrix to enhance the performance of the MCMV detector. Numerical results demonstrate the effectiveness of the proposed detectors     

Memory search multiuser detector in DS/CDMA systems

The optimal multiuser detector (MUD) in DS/CDMA systems requires the solution of a NP-hard combinatorial optimisation problem (COP). A near optimal MUD based on solving the COP with memory search method is proposed. With almost the same computational complexity as the parallel interference cancellation (PIC) MUD, it produces a better performance gain than PIC-MUD     

An effective multiuser receiver for DS/CDMA systems

Multiuser detection (MUD) is a key technique for combating multiple access interference (MAI) in CDMA systems. We propose a simple and effective multiuser receiver, which we refer to as the objective function based multiuser detector (OFMUD). This is a successive multistage joint detector based on the statistical optimization of an objective function. Two objective functions are considered in this paper. The first one is based on a maximum likelihood solution, and the second one is based on the least squared error solution. A key feature of the proposed detector is that among all the K user bits, we update the estimate of one and only one bit (the one that is most likely to be wrong) in every stage. Analytical and numerical results show that significant performance improvement can be achieved compared with the conventional DS/CDMA receiver. At the same time, the near-far problem is alleviated. In addition, it is shown that the proposed receiver can outperform the popular multiuser parallel interference cancellation receiver in most cases with a smaller computational cost     

MMSE/PIC multiuser detection for DS/CDMA systems with inter- andintra-cell interference

This paper analyzes combinations of the linear minimum mean square error (MMSE) detector and a nonlinear parallel interference canceller (PIC) for multiuser code-division multiple-access (CDMA) detection. Both the case where all users' codes are known at the receiver and the case where only some codes are known are considered. An upper bound and an approximate formula for the error probability are derived and verified through simulation. It is shown that the combined MMSE/PIC detector can have a considerable performance gain over the MMSE. It is also shown that while the MMSE detector is sensitive to large code cross-correlation values, the combined MMSE/PIC detector is robust to code cross correlations. Finally, use of the MMSE/PIC detector for cellular systems with both inter- and intra-cell interference is considered     

Space-time blind multiuser detection for multirate DS/CDMA signals

Multiuser detection for multirate direct-sequence code-division multiple access (DS/CDMA) has been an active area of research. For example, nonblind low-rate (LR) and high-rate (HR) decorrelators have been proposed and analyzed in the literature for synchronous dual-rate systems with single receive antenna. Inspired by the subspace-based space-time (ST) blind linear detectors for synchronous single-rate systems, this paper extends the existing results and proposes the subspace-based ST-LR and ST-HR blind linear detectors, i.e., blind decorrelators and blind minimum mean-squared error (mmse) detectors, for synchronous dual-rate DS/CDMA. It is shown that: 1) ST-LR blind linear detectors can support no less users than ST-HR blind linear detectors as long as the desired spatial signature is identifiable (assuming that all the other system parameters are the same) and 2) the bit-error rate performance of ST-LR blind decorrelator is not inferior to that of its HR counterpart. The above conclusions are generalized to synchronous multirate systems. The extension to asynchronous systems is also described. Finally, the two-stage ST dual-rate blind detectors, which combine the adaptive purely temporal dual-rate blind mmse detectors with the nonadaptive beamformer, are presented.     

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。     

Partially blind adaptive MMSE interference rejection inasynchronous DS/CDMA networks over frequency-selective fading channels

In this work, the problem of joint suppression of multiple-access and narrow-band interference (NBI) for an asynchronous direct-sequence code-division multiple-access (CDMA) system operating on a frequency-selective fading channel is addressed. The receiver structure we consider can be deemed as a two-stage one: the first stage consists of a bank of minimum mean-square-error (MMSE) filters, each keyed to a given replica of the useful signal, and aimed at suppressing the overall interference; the second stage, assuming knowledge of the fading channel coefficients realizations, combines the MMSE filters outputs according to a maximal-ratio combining rule. Due to the presence of the NBI, the resulting structure is in general time-varying, and becomes periodically time-varying if the NBI bit-rate has a rational ratio to that of the CDMA system. Moreover, enlarging the observation window beyond the signaling interval and oversampling the signal space may yield a noticeable performance improvement. For the relevant case that the said ratio is rational, a new cyclic blind recursive least squares (RLS)-based algorithm is introduced, capable of tracking the periodically time-varying receiver structure, and allowing adaptive interference cancellation with a moderate complexity increase. We also come up with a closed-form expression for the conditional bit-error rate (BER), which is useful both to evaluate semi-analytical methods to assess the unconditional BER and to derive bounds on the system near-far resistance. The results indicate that the receiver achieves very satisfactory performance in comparison to previously known structures. Computer simulations also demonstrate that the cyclic blind RLS algorithm exhibits quite fast convergence dynamics     

An efficient blind multiuser detection for improper DS/CDMA signals

This paper proposes a new linear blind multiuser detection based on widely linear (WL) signal processing. The received signal and its complex conjugate are separately filtered and the results are linearly combined. The WL maximum/minimum (max/min) mean-output-energy (MOE) receiver is derived by applying the modified cost function. It is shown that a performance gain can be attained for the improper direct-sequence code division multiple access (DS/CDMA) signal, due to the additional information contained in pseudocovariance matrix of observations. The adaptive implementation with acceptable complexity is also developed. Computer-simulation results show that a significant performance gain is obtained over the other classical methods.     

Space-time multiuser detection in multipath CDMA channels

The problem of multiuser detection in multipath CDMA channels with receiver antenna array is considered. The optimal space-time multiuser receiver structure is first derived, followed by linear space-time multiuser detection methods based on iterative interference cancellation. Blind adaptive space-time multiuser detection techniques are then proposed, which require prior knowledge of only the spreading waveform and the timing of the desired user's signal. Single-user-based space-time processing methods are also considered and are compared with the multiuser approach. It is seen that the proposed multiuser space-time processing techniques offer substantial performance gains over the single-user-based methods, especially in a near-far situation     

I/Q multiplexed code assignment for fast cell search inasynchronous DS/CDMA cellular systems

For flexible system deployment, we consider asynchronous cell site operation which requires much longer cell search time than in synchronous operation. A new code assignment scheme is proposed to minimize the cell search time in which a group code is assigned to the in-phase pilot channel and a cell-specific code assigned to the quadrature pilot channel. Simulation results show that cell search can be accomplished in less than about 200 ms at 90% of the locations when the pilot code of length 1024 and eight group codes (64 cell-specific codes/group) are used     

Per-survivor processing sequence detection for DS/CDMA systems withpilot and traffic channels

A per-survivor processing (PSP) sequence detection receiver is proposed for DS/CDMA systems with pilot and traffic channels. The proposed receiver jointly estimates channel parameters and data symbols from pilot and traffic channels, and is derived by modifying the receiver of Choi (see IEEE Proc. Commun., vol.146, no.5, p.1-7, 1999). As a result, the new receiver can outperform the receiver of Choi in exchange for additional computation. Computer simulation results demonstrate the advantage of the proposed receiver over existing ones     

一种频选衰落信道下的Turbo多用户检测算法

联合MAP多用户检测与信道解码的迭代多用户检测(MUD)技术可显著提高宽带移动CDMA系统的容量和性能.在多径时变衰落的编码信道下,提出一种迭代实现干扰抑制、符号估计、信道解码的Turbo多用户检测算法.在每次迭代中,MUD自适应地实现干扰抑制并输出符号估计的软信息,软输入软输出的信道解码器使用LOG MAP方法实现信道解码并反馈符号估计的软信息作为下一次TurboMUD迭代的先验信息.仿真结果证实了该算法在频选衰落信道下经两次迭代就能逼近单用户编码CDMA系统的接收性能.     

Adaptive detection of DS/CDMA signals in fading channels

This paper examines the behavior of the minimum mean-squared error (MMSE) receiver in frequency-nonselective-fading channels. It is noted that the MMSE receiver will often lose phase lock on the desired signal when the desired signal dips into a deep fade. A modification to the MMSE receiver is presented which is demonstrated to function quite nicely in flat-fading channels. Analytical results for the modified MMSE receiver are presented and found to agree very well with simulation results. These analytical results are then compared to the theoretical performance of the conventional (i.e., correlator) receiver in terms of both bit-error rate (BER) and capacity. As expected, the modified MMSE receiver was found to offer a substantial improvement in both BER and capacity. Finally, a simple empirically derived formula is given which will give a good approximation to the BER of the modified MMSE receiver in a Rayleigh-fading environment. This formula can also be used to determine the number of users a given system can support. It is noted that as E_b/N₀ grows, it is quite feasible to approach 100% channel utilization with the MMSE receiver, whereas a conventional receiver is typically limited to a utilization of 10%-20%     

Blind multiuser detection over highly dispersive CDMA channels

This paper addresses blind multiuser detection in a direct-sequence code-division multiple-access (DS-CDMA) network in presence of both multiple-access interference and intersymbol interference. In particular, it considers a DS-CDMA system where K out of N users are transmitting; the N admissible spreading codes are known, and so is the code of the user to be demodulated. The number of interferers, the signatures of a certain number, possibly all, of the interferers, and the channel impulse response of each active user are unknown. The spreading codes of the unknown interferers are determined via a procedure that exploits the knowledge of the set of admissible transmitted codes and of the known active codes. The procedure applies to both single and multiple receiving antennas. The performance assessment of a blind decorrelating detector, implemented by resorting to the proposed identification procedure, shows that it outperforms a plain subspace-based blind decorrelator for small sizes of the estimation sample.     

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     

频率复用DS/CDMA系统中自适应盲多用户检测的一种方法

该文给出一种工作于强窄带干扰(NBI)多径衰落信道中DS/CDMA系统盲多用户检测接收机模型。它具有RAKE接收机的基本结构,在每一条并行支路中利用MMSE准则跟踪各路径信号。因此具有抗多径衰落,同时抑制NBI和多址接入干扰(MAI)的能力。模拟结果证明,它的性能优于未考虑NBI抑制的多用户检测接收机。     

Robust multiuser detection for multicarrier CDMA systems

Multiuser detection (MUD) for code-division multiple-access (CDMA) systems usually relies on some a priori channel estimates, which are obtained either blindly or by using training sequences, and the covariance matrix of the received signal, usually replaced by the sample covariance matrix. However, such prior estimates are often affected by errors that are typically ignored in subsequent detection. In this paper, we present robust channel estimation and MUD techniques for multicarrier (MC) CDMA by explicitly taking into account such estimation errors. The proposed techniques are obtained by optimizing the worst case performance over two bounded uncertainty sets pertaining to the two types of estimation errors. We show that although the estimation errors associated with the prior channel estimate and the sample covariance matrix are generally not bounded, it is beneficial to optimize the worst case performance over properly chosen bounded uncertainty sets determined by a parameter called bounding probability. At a slightly higher computational complexity, our proposed robust detectors are shown to yield improved performance over the standard detectors that ignore the prior estimation errors.