Multiuser-MIMO downlink TX-RX design based on SVD channel diagonalization and multiuser diversity

We address the problem of transmitter and receiver design in Multi-User Multi-Input Multi-Output (MU-MIMO) communications over fading wireless channels. We assume perfect channel state information at the transmitter (CSIT). Attractive algorithmic solutions have been proposed in the past for the problem of minimizing total transmit power through joint beamforming and power control for MU-MISO systems with SINR constraints. Extensions of this approach to the MIMO case are not immediate, due to dependency on the receiver used for the multiple streams of a given user. By introducing a limited number of zero-forcing constraints, only between streams of the same user, the SINRs for all streams become decoupled, and the MU-MISO solutions can be straightforwardly adapted to the MU-MIMO problem. A multi-user diversity aspect can be introduced also, since the MU-MISO solution allows for straightforward identification of the most power hungry streams. When optimizing the number of streams in this way for maximum sum rate, the solution thus obtained is not far away from the Sato bound. Simulations are presented to also show the increased performance of the proposed approach over some alternative proposals.