Joint iterative multiuser detection and channel estimation for differentially coded asynchronous CDMA systems

A joint channel estimation and multiuser detection scheme is proposed for the uplink of differentially coded asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems. A channel estimation is developed by exploiting the orthogonality between signal and noise subspaces and the soft estimate of coded bits of each user in conjunction with the output of the multiuser detector. The blind subspace channel estimation yields channel estimation with discrete valued phase ambiguities. By exploiting the robust combination of recursive systematic convolutional (RSC) decoder with a differential decoder for each user, an algorithm to resolve the phase error and simultaneously improving signal detection is proposed. Based on the proposed channel estimator and phase corrector, an iterative receiver is introduced for joint channel estimation, soft interference cancellation, linear minimum mean-square error (MMSE) filtering and iterative channel decoding. By exchanging soft information between different stages, the receiver performance is improved via iteration. Simulation results show that very close to single user performance in additive white Gaussian noise is possible.