Turbo multiuser detection 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 wide-band code-division multiple-access (CDMA) systems. We develop an iterative multiuser receiver for turbo-coded CDMA systems with aperiodic spreading sequences operating in multipath fading, 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 (i.e., the outputs of the maximum ratio combiners for all the users), and performs soft-interference cancellation and instantaneous minimum mean-square error (MMSE) filtering. The soft multiuser detector has a complexity of O(K²) per bit per iteration, 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 the proposed multiuser receiver offers more than 0.5-dB gain over the single-user RAKE receiver at a bit-error rate (BER) of 10^-3 and even outperforms the single-user RAKE receiver operating in a single-user environment.