| Multiuser detection and turbo coding are two of the most powerful techniques for enhancing the performance of next-generation wideband code-division multiple-access (CDMA) systems. Multiuser detection for three types of uplink CDMA is addressed in this work.; First, we propose a geometric view of multiuser detection and present results on the problem of efficient computation of the optimal decision regions (Voronoi diagram) for synchronous uncoded DS/CDMA over known channels. Among other results, it is shown that Verdu's algebraic notion of "indecomposable error sequences" relates directly to the geometric construction of the Voronoi diagram for multiuser detection. Based on the presented geometric results, we introduce two multiuser detectors whose performance compares favorably to some popular existing detectors.; Next, we consider the problem of joint channel estimation and data detection in uplink asynchronous CDMA systems employing aperiodic spreading sequences in the presence of unknown multipath fading channels and additive white Gaussian noise, in which case the maximum likelihood sequence estimation is too complex to perform. Sequential blind multiuser receivers are proposed, based on the sequential expectation-maximization (EM) algorithm. With the prior knowledge of only the signature waveforms, the delays and the second-order statistics of the fading channel, the receivers sequentially estimate the channel using the sequential EM algorithm. Moreover, the snapshot estimates of each path are tracked by linear minimum-mean-squared-error (MMSE) filters.; Finally, we develop an iterative multiuser receiver for turbo-coded CDMA systems with aperiodic spreading sequences operating in known multipath fading, which exploits the power of both multiuser detection and turbo coding. The key innovation in the proposed receiver is a low-complexity soft multiuser detector which uses the same decision statistics as the conventional RAKE receiver (i.e., the outputs of the maximum ratio combiners for all the users), and performs soft-interference cancellation and instantaneous MMSE filtering. The soft multiuser detector has a complexity of O (K2) per bit per iteration, where K is the number of users.; In all three types of channel, simulation results demonstrate that the performance of the proposed techniques significantly outperforms those of some existing popular techniques. |
