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We investigate the impact of multiple transmit antennas on the performance of 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 delay in the feedback link. 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" beam-forming and space-time coding. The ability of accurately predicting the channel SNR dominates the performance of opportunistic beamforming. Hence, we propose to exploit synchronous pseudo-random beamforming matrices known a priori by 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 and space-time coding (transmit diversity) strongly depends on the channel Doppler bandwidth.