Soft-Output Multiuser Estimation for Asynchronous CDMA Channels

In this paper, we consider several multiuser demodulation schemes for a direct-sequence code-division multiple access (DS-CDMA) system that provide soft-outputs to enhance single-user soft-decision decoders. First, we derive an optimum soft-output multiuser estimation (OSOME) algorithm for asynchronous CDMA channels with white Gaussian noise (WGN). The algorithmgen-erates a posteriori probabilities (APP) for coded bits given the received signal. This forward-recursive algorithm requires complexity and storage exponential in the number of users K and linear in the decision lag L. Second, we outline a reduced-computation suboptimal version of this algorithm (SSOME) which compares favorably in complexity and storage with multiuser soft-output Viterbi algorithm (SOVA). Furthermore, we note that the Bayesian conditional decision feedback estimation (BCDFE) algorithm 3] provides symbol APP estimates that can be considerably improved upon by a slight modiication. We derive the modiied BCDFE algorithm for whitened multiuser channels. Finally, we present simulation results of a 4-user, convolutionally coded, asynchronous CDMA system. The results indicate that the SSOME algorithm with L = K?1 achieves a 0:4?0:5 dB gain over SOVA with much greater lag (and complexity) and keeps within 0:25 dB of the OSOME algorithm. Moreover, the modiied BCDFE algorithm provides a good performance/complexity tradeoo.