CSI Robust Joint User Selection and Precoder Design in MIMO Downlink Systems

In multiuser multiple-input multiple-output (MU-MIMO) systems, the selection of a subset of users to achieve the maximum sum rate is critical when resources are limited. In addition, designing suitable precoder and decoder matrices at the base station (BS) and at the user side requires unattainable perfect channel state information. In this letter, the downlink of a cellular system with multiple antennas at user equipments (UEs) is considered. A framework is derived for joint user selection and precoder/decoder design in the presence of imperfect channel estimation with neither instantaneous nor statistical error matrices. A robust method is proposed that incorporates a bounded worst-case channel in the design of precoders, decoders, and user selection. The optimization problem is solved iteratively by decomposition into multiple convex sub-problems which are solved successively. Finally, it is illustrated that for the case of a set of predesigned precoders, the interference pattern remains fixed for each user, implying that the problem of user selection could be transformed to that of beam selection.