Combined space-time chip equalization and parallel interference cancellation for DS-CDMA downlink with spatial multiplexing

In the downlink of traditional single input single output (SISO) DS-CDMA systems, multipath propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Chip equalization can restore the orthogonality and suppress the MUI. In the context of multiple input multiple output (MIMO) DS-CDMA systems, spatial multiplexing emits independent data streams from the M_T transmit antennas which allows one to significantly enhance the spectral efficiency. However, combined with DS-CDMA, spatial multiplexing also leads to an M_T-fold increase of the MUI. Linear chip equalization only is not able to cope with the induced MUI. In this paper, we therefore propose a new combined linear and non-linear multi-user receiver for the spatially multiplexed DS-CDMA (SM DS-CDMA) downlink. The proposed receiver consists of an initial linear chip equalization stage and possibly multiple non-linear parallel interference cancellation (PIC) stages with RAKE combining. With a single PIC/RAKE stage, it achieves a 5.1 dB gain given a BER of 10^-3, compared to the linear MMSE receiver only. A second PIC/RAKE stage brings an additional 0.9 dB gain at the expense of increased complexity.