Capacity enhancement in the downlink using space-division opportunistic beamforming

With many users waiting to be served in a queue, opportunistic beamforming (OBF) permits true beamforming to be performed without the need of channel state information (CSI) being known at the transmitter so that multiuser diversity can still be exploited in a little scattering environment. In this regard, it is commonly felt that OBF does not provide any gain in independent Rayleigh fading channels because the channel fluctuation among the users is already at maximum. In this paper, however, OBF is proposed for support of multiple users in the spatial domain, dubbed space-division OBF and therefore the sum-throughput of the system can be enhanced even in fast independent Rayleigh fading channels. The interpretation is that asymptotically, there are some users whose channels are near-orthogonal and match with the antenna weights randomly generated at the transmitter to produce true beamforming. This paper also presents an efficient scheduling scheme to mediate access the channel using space-division OBF.