Multiuser space-time block coded MIMO with downlink precoding

Space-time block codes (STBC) are powerful techniques to achieve full spatial diversity in Rayleigh fading channels with low decoding complexity. When channel state information is available to the transmitter, space-time block codes can be improved through the use of a transmit precoder chosen based in part on the current channel state. We investigate the downlink transmission of an Alamouti space-time block coded multiuser wireless system in which complete channel knowledge is available to the base transceiver station. A novel unitary precoder is proposed that, with enough antennas, can effectively precancel co-channel interference at each mobile user thus enabling simple single user space-time block decoding. It is shown that excess transmit antennas, beyond the minimum required for precancellation, can he used to further improve the diversity performance through careful selection of the precoding matrix. Though the derivation assumes perfect channel state information, the impact of estimation error is investigated by deriving a lower bound on the signal to interference plus noise ratio. Monte Carlo simulations show that our proposed downlink precoder can effectively cancel the interference between mobiles, while still providing good diversity performance.