MIMO capacity with interference

System capacity is considered for a group of interfering users employing single user detection and multiple transmit and receive antennas for at Rayleigh fading channels with independent fading coeecients for each path. The focus is on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. It is shown that the optimum signaling is sometimes diierent from cases where the users do not interfere with each other. In particular, the optimum signaling will sometimes put all power into a single transmitting antenna, rather than divide power equally between independent streams from the diierent antennas. If the interference is either suuciently weak or suuciently strong, we show that either the optimum interference-free approach, which puts equal power into each antenna, or the approach that puts all power into a single antenna is optimum and we show how to nd the regions where each approach is best.