EM-Based Multiuser Detection in Fast Fading Multipath Environments

We address the problem of multiuser detection in fast fading multipath environments for DS-CDMA systems. In fast fading scenarios, temporal variations of the channel cause significant performance degradation even with the RAKE receiver. We use a previously introduced Time-Frequency (TF) RAKE receiver based on a canonical formulation of the channel and signals to simultaneously combat fading and multipath effects. This receiver uses the Doppler spread caused by rapid time-varying channel as another means of diversity. In dealing with multiaccess interference and as an attempt to avoid the prohibitive computational complexity of the optimum Maximum-Likelihood (ML) detector, we use the Expectation Maximization (EM) algorithm to derive an approximate ML detector. The new detector turns out to have an iterative structure very similar to the well-known multistage detector with some extra parameters. At the two extreme values of these parameters, the EM detector reduces to either oneshot TF RAKE or generalized multistage detector. For the intermediate values of the parameters, it combines the two estimates to obtain a better decision for the bits of the users. Because of using the EM algorithm, this detector has better convergence properties than the multistage detector; the bit estimates always converge, and if an appropriate initial vector is used, they converge to the global maximizer of the likelihood function. As a result, the new detector provides significantly improved performance while maintaining the low complexity of the multistage detector. Our simulation results confirm the expected performance improvements compared to the base case of the TF RAKE as well as the multistage detector used with the TF RAKE.