Performance of different user selection algorithms for transmit power minimization

In multi-user communication from one base station (BS) to multiple users, the problem of minimization of transmit power to achieve some target guaranteed performance (rates) at users has been well investigated in literature. Similarly various user selection algorithms have been proposed and analyzed when BS has to transmit to a subset of the users in the system, mostly for the objective of the sum rate maximization. We study the joint problem of the minimization of the transmit power at the BS with user selection. The general analytical results for the average transmit power required to meet guaranteed performance at the users´ side are difficult to obtain even without user selection due to joint optimization required over beamforming vectors and power allocation scalars. Nevertheless, we characterize analytically the average transmit power required to meet guaranteed performance with various users selection algorithms, namely semi-orthogonal user selection (SUS), norm-based user selection (NUS) and angle-based user selection (AUS), in the limiting case when only 2 users are selected for simultaneous transmission. The SUS performs better than other presented selection algorithms due to its user selection mechanism.