Opportunistic beamforming with limited feedback

Transmit-side channel state information (CSI) can improve the quality of communication over fading channels. One way this information can be exploited is via (opportunistic) multi-user diversity. Also, in a multi-antenna scenario, CSI allows the transmitter to propagate along the eigendirections of the channel, thus achieving beamforming gain or array gain. In the face of these possibilities, the question we raise is this: if we can have only limited (reliable) feedback for channel state information, what form should this information take, and how should it be used. In a previous work, we have shown that most of the gain of a single-antenna multi-user diversity system can be captured by feedback of only one bit. In this paper, we extend that system to a multi-antenna transmitter. We propose and analyze a compound strategy that uses one bit for multi-user diversity and any further information for beamforming. We show that this method not only enjoys good throughput, but also has desirable fairness properties. Simulations confirm our assertions.