Trace balancing for multiuser MIMO downlink transmission

We consider the problem of balancing individual capacities in a multi-user MIMO downlink channel. If perfect channel state information is available at the base station, the rates of all users can be jointly controlled by adjusting the transmit covariance matrices. We propose a new transmission strategy based on trace balancing, which is optimal in the low SNR regime. Otherwise, it provides an approximation. An iterative algorithm is proposed, which rapidly converges to the global optimum of the trace balancing problem.