Downlink Limited Feedback Transmission Schemes for Asymmetric MIMO Channels

In multiple-input multiple-output (MIMO) broadcast channels, multiuser diversity is exploited by scheduling data transmission to users with best channel conditions. To find the best set of users, the base station requires knowledge of user channels, which for the case of non-reciprocal uplink and downlink channels may lead to a very heavy feedback overhead. In particular, well known transmission schemes for asymmetric MIMO downlink channels with more transmit than per user receive antennas, such as beamforming, require availability of full channel state information (CSI) at the transmitter. In this paper, two multiuser MIMO downlink techniques are presented, which only require partial CSI at the base station. The proposed schemes represent a combination of zero-forcing (ZF) receiver processing with MIMO point-to-point eigenmode transmission and transmit antenna selection. The results show that the scheme which is able to suppress inter-stream interference while maintaining maximum spatial multiplexing and high multiuser diversity gain, achieves higher system throughput.