Transmit Diversity vs. Opportunistic Beamforming in Data Packet Mobile Downlink Transmission

We compare space-time coding (transmit diversity) and random “opportunistic” beamforming in a SDMA/TDMA single-cell downlink system with random packet arrivals, correlated block-fading channels and non-perfect channel state information at the transmitter due to a feedback delay. Our comparison is based on system stability. The ability of accurately predicting the channel SNR dominates the performance of opportunistic beamforming, even under the optimistic assumption that the sequence of beamforming matrices is perfectly known a priori by the receivers. Our results show that the relative merit of opportunistic beamforming versus space-time coding strongly depends on the channel Doppler bandwidth. Therefore, previous naive conclusions on the fact that transmit diversity always hurts the system performance under multiuser-diversity scheduling should be taken with great care.