Exploiting Multiuser Diversity in MIMO Broadcast Channels with Limited Feedback

We consider a multiple antenna broadcast channel in which a base station equipped with M transmit antennas communicates with K ≥ M single-antenna receivers. Each receiver has perfect channel state information (CSI), whereas the transmitter has partial channel knowledge obtained via a limited rate feedback channel. Built upon multiuser interference bounds, we propose scalar feedback metrics that incorporate information on the channel gain, the channel direction, and the quantization error, with the goal to provide an estimate of the received signal-to-noise plus interference ratio (SINR) at the transmitter. These metrics, combined with efficient user selection algorithms and zero-forcing beamforming on the quantized channel are shown to achieve a significant fraction of the capacity of the full CSIT case by exploiting multiuser diversity. A multi-mode scheme that allows us to switch from multiuser to single-user transmission is also proposed as a means to compensate for the capacity ceiling effect of quantization error and achieve linear sum-rate growth in the interference-limited region. The asymptotic sum-rate performance for large K, as well as in the high and low power regimes, is analyzed and numerical results demonstrate the performance and the advantages of the proposed metrics in different system configurations.