Doubly opportunistic beamforming for downlink multiuser MIMO

We develop a novel opportunistic beamforming scheme referred to as doubly opportunistic beamforming (DOBF), which is doubly selective in both users and beams based on a new type of channel state information (CSI) feedback to improve the limited feedback downlink system performance. DOBF benefits from very flexible scheduling at the transmitter, compared to the conventional opportunistic beamforming. We provide new findings as to how much the inaccurate signal to interference and noise ratio (SINR) estimation due to lack of scheduling information at the receivers affects the performance of zero-forcing beamforming (ZFBF) with limited feedback. On the contrary, DOBF does not suffer from such a SINR inaccuracy problem. Rather surprisingly, numerical results show that our scheme can noticeably outperform even ZFBF under a comparable amount of CSI feedback load when spatially correlated fading and performance degradation due to limited scheduling information are taken into account.