Multiuser downlink beamforming with limited feedback

Downlink beamforming is well studied in wireless communications over the last decade, including some optimization issues. In this paper, we study the multiuser downlink beamforming when the channel state information (CSI) is provided to the transmitter with limited feedback. With limited feedback, the CSI becomes imperfect. Joint beamforming and power allocation are addressed with limited feedback. We also show that the multiuser beamforming becomes interference-limited even though the number of active users is less than the number of transmit antennas due to limited feedback. From this, we can see that there is capacity loss and the performance is limited by the residual interference due to imperfect CSI.