Multiuser Detection in MIMO DS-CDMA Systems Over Slow-Fading Channels

Transmit diversity designed for fast-fading channels is examined in a multiuser direct-sequence code division multiple access (DS-CDMA) system over slowly-fading channels. The underlying space-time system employs two transmit antennas and M receive antennas at the user side and base-station receiver, respectively. The receiver employs a decorrelator multi-user detector. In our analysis, we derive a closed form expression for the bit error probability. These theoretical results, when compared to simulations, are shown to be very accurate. Both simulations and analytical results demonstrate that, regardless of the system load, the full diversity order of NM is always maintained and only a signal-to-noise ratio (SNR) loss is incurred when using a decorrelator detector at the receiver side. This SNR degradation is shown to be a function only of the number of users and independent of the number of transmit and/or receive antennas. Using our theoretical results, we show that the loss in SNR from the single-user bound can be well approximated by 10log(C/2) where C is a parameter of the interference level