Performance of an OFDM-SDMA based system in a time-varying multi-path channel

Significant investigation has been conducted towards combining OFDM which mitigates intersymbol interference and SDMA which improves the bandwidth efficiency. OFDM-SDMA systems need accurate channel estimation, because it is from this information that the separation of co-channel users is performed. Usually, the channel is assumed stationary because the frame duration is chosen smaller than the coherence time. However this choice limits the system efficiency. This paper discusses the performance of an OFDM-SDMA based system under a time-varying multi-path channel. The system under study uses the basic combining algorithm based on the linear minimum mean square error (MMSE) criterion for user separation and a pilot based scheme independent from channel statistics for channel estimation. We investigate in particular the effect of the pilot subcarrier arrangement on the BER performance and deduce an optimal configuration in a channel with both delay and Doppler spread