Multiuser MIMO MAC with statistical CSI and MMSE receiver: Feedback strategies and transmitter optimization

We consider a multi-input multi-output (MIMO) multiple access channel (MAC) where the base station has perfect channel state information (CSI), while the mobiles have only partial CSI. The statistical CSI at the mobiles can be obtained either by implicit (during downlink transmission) or explicit (via a low rate control channel) feedback. The base applies a linear multiuser MMSE receiver and thus the optimization criteria is the average sum MSE. We optimize the average sum MSE with implicit and explicit mean- or covariance-feedback under individual power constraints. The optimal transmit directions of a user corresponds to the eigenvectors of his channel mean or correlation matrix. The optimal power allocation is characterized in terms of its beamforming range. Finally, the gain due to covariance feedback and all theoretical results are illustrated by numerical simulation.