Effects of multi user MIMO scheduling freedom on cellular downlink system throughput

The paper studies different degrees of channel quality-based scheduling freedom for the downlink of a DS-CDMA system where the base station (BS) and each mobile station (MS) have 2 antennas. The system combines opportunistic user time slot scheduling, opportunistic spatial multiplexing, and adaptive modulation and coding (AMC) with limited and unlimited allowable constellation sizes. System performance is investigated in a time dispersive propagation environment, with varying degrees of antenna correlation. Three transmission schemes with increasing degrees of resource allocation freedom are identified. The full freedom scheme allows for time slots and transmit antennas to be independently allocated to different users and is compared to two schemes of more limited flexibility in antenna allocation. For a system with little multi user diversity gain, i.e., fewer than 5 users, the option of single antenna selection transmit diversity is favorable, while dual antenna transmission with independent antenna-to-user allocation becomes increasingly important when a higher degree of multi user diversity can he exploited.