An iterative multiuser detector for turbo-coded CDMA in multipath fading channels

Multiuser detection and turbo coding are two of the most powerful techniques for enhancing the performance of next-generation wideband CDMA systems. In this paper, we develop an iterative multiuser receiver structure for turbo-coded CDMA systems with aperiodic spreading sequences in a multipath fading environment, which exploits the power of both techniques. The key innovation in the proposed receiver is a low-complexity soft multiuser detector which uses the same decision statistic as the conventional RAKE receiver and performs soft-interference cancelation and instantaneous MMSE filtering. During iterations, extrinsic information is computed and exchanged by the soft multiuser detector and a bank of modified turbo decoders, to achieve successively refined estimate of the users' signal. The soft multiuser detector has a complexity of O(K-2) per bit per user, where K is the number of users. A single-user receiver which employs a soft RAKE detector followed by a turbo decoder is also considered. Simulation results demonstrate that under various channel impairment, such as multiple-access interference, multipath dispersion and time-variant fading, the proposed multiuser receiver offers significant performance gain over the single-user RAKE receiver. Moreover, it is seen that the turbo multiuser receiver in a multiuser environment even outperforms the single-user RAKE receiver in a single-user environment, because the RAKE receiver neglects intersymbol interference.