Throughput analysis in wideband MIMO broadcast channels with partial feedback

It has been recently shown that opportunistic transmit beamforming using partial channel state information (CSI) achieves the same throughput scaling obtained from dirty paper coding for a broadcast channel with fixed number of transmit antennas and many receivers [1]. In this paper, we study the generalization of this scheme to wideband broadcast channels. By using orthogonal frequency division multiplexing, an L-tap wideband channel can be decomposed to N parallel narrowband channels, where N is larger than L. Neighboring subchannels are therefore highly correlated, and it is intuitive to say that each group of neighboring subchannels (forming a cluster) can be characterized by one channel quality. We show in this paper that users need only feedback the best signal-to-noise-plus-interference ratio at the center of each cluster. Our results indicate that for cluster size of order Nu/L root log Kappa, where K is the number of users, this feedback scheme maintains the same throughput scaling as when full CSI is known. Simulation results show that larger cluster sizes (Nu/2L) can also be implemented for a small throughput hit.