Multi-User Multi-Input Multi-Output (MU-MIMO) Downlink Beamforming Systems with Limited Feedback

This paper considers zero-forcing beamforming (ZFBF) MU-MIMO systems in which one stream is transmitted per user. Assuming no knowledge of other users´ channel matrices, user terminals (UT) feedback the effective vector channel (not the full MIMO channel matrix) using channel vector quantization and a channel quality index (CQI) indicating its estimated receive signal-to-noise-ratio (SINR). Limited feedback causes residual multi-user interference at the UT, which otherwise would be removed completely by ZFBF. Consequently, neither the UT nor the base station has sufficient information to calculate SINR accurately making MU-MIMO design quite challenging. A blind CQI estimation at the UT is proposed which averages expected interference over all possible co-scheduled transmit beamformers, which is then corrected at the BS based on the actual transmit beamformers used. A joint method for receive beamformer design and channel codeword selection which optimizes received SINR is also presented. Link and system level results demonstrate performance gains compared to prior work.