Sub-optimal iterative MMSE receivers for space-time trellis coded CDMA systems in multi-path fading channels

A code division multiple access (CDMA) scheme with multiple transmit and receive antennas is combined with space-time trellis codes over a frequency selective channel. Multi-user minimum mean square error (MMSE) detection, generalized to accommodate multiple antennas and multiple paths, is combined with the turbo principle in an iterative fashion, allowing interference regeneration and cancellation at the receiver. The resulting optimal iterative multi-user MMSE detector is able to remove completely multiple access interference, inter symbol interference and intra antenna interference and achieves single user performance, even in a fully loaded system. As a consequence of improved performance, the computational complexity becomes very high. We propose two suboptimal iterative multi-user MMSE receivers that allow a computational complexity reduction of three orders of magnitude compared to the optimal solution and, still, outperform the non-iterative optimal multi-user MMSE detector. The proposed sub-optimal iterative equalizers are analysed and supported by computer simulations.