Iterative Multiuser Detection for Asynchronous Multi-Sequence Signaling Based CDMA Systems

In this paper, an iterative multiuser detector (MUD) is addressed for asynchronous direct sequence spread spectrum communications systems employing multi-sequence signaling over a quasi-static frequency-selective fading channel. A bit-interleaved coded modulation (BICM) scheme, facilitating the "turbo principle" receiver through iterative processing, is introduced to increase the time diversity. With a multi-sequence signaling set mapper constructed over expanded spreading sequences, each of which is regarded as a signal plane with uniformly distributed signal points assumed, user cross-correlations could be nevertheless increased under certain situations. A soft interference cancellation scheme, derived by the minimum mean-squared error (MMSE) sense on a basis of per sequence plane per user, is employed to compensate for the increased multiple access interference (MAI) at high system traffic load. Simulation results demonstrate system performance in comparison with both conventional standard coded and bandwidth-efficient coded DS-CDMA systems in terms of bit error rate under the same system traffic load, channel condition and given power and spectral efficiency.