Opportunistic beamforming vs. space time coding in a queued downlink

We investigate the different usage of multiple transmit antennas in a SDMA/TDMA single-cell downlink system under random packet arrivals, correlated block-fading channels and non-perfect channel state information at the transmitter due to a feedback delay. We derive the arrival rate stability region and the adaptive scheduling policy that stabilizes any arrival rate point inside the region without knowing explicitly the arrival statistics. Then, we apply these results to the case of "opportunistic" beamforming and spacetime coding. The ability of accurately predicting the channel SNR dominates the performance of opportunistic beamforming. Hence, we propose to exploit synchronous pseudorandom beamforming matrices known a priori to the receivers in order to improve the channel state information quality. Under this scheme, it appears that for given feedback delay the relative merit of opportunistic beamforming versus space-time coding strongly depends on the channel Doppler bandwidth.