Optimal user pairing in downlink MU-MIMO with transmit precoding

In this paper consider a downlink multiuser-MIMO transmission system in which at most two users can transmit simultaneously on different spatial channels or beams. We show how to find the optimal user subsets for K users in a system where inter-user interference is mitigated via transmit precoding and sum capacity is maximized using a linear programming algorithm that defines the optimal user subsets under a particular fairness criterion.