Efficient transmission and resource allocation methods for multi-user MIMO downlink

The consideration of multiple-input multiple-output (MIMO) techniques for the cellular downlink of future wireless systems is motivated by the demand for high transmission rates to multiple users over limited frequency spectrum. Although the optimal approach for MIMO broadcast channels is dirty paper coding (DPC), it incurs very high complexity that limits its practicality. This thesis focuses on enhancing the feasibility of deploying multi-user MIMO techniques in practical downlink systems. In line with this, emphasis is placed on improving the performance of transmit zero-forcing beamforming (TZFBF), which has lower complexity but is sub-optimal.;However, optimal RAS/SMS incurs very large system overhead because the channel matrices of all potential users must be fed back to the base station. Another challenge is posed by the flexibility for spatial mode allocation at multi-antenna terminals to meet individual transmission rate requirements. A streamlined process that encompasses efficient selection with feedback reduction and systematic resource allocation with rate loss minimization, is developed for the sum rate maximization of TZFBF systems. In addition, bounds are developed for estimation of the ergodic TZFBF sum rates where RAS/SMS, user selection, signal-to-noise ratio and number of transmit elements are taken into account.;To narrow the sum rate performance gap with DPC systems, it is shown that receive antenna selection (RAS) is necessary for maximizing the achievable sum rate for TZFBF systems. This is true for TZFBF systems with multi-antenna terminals even when all receive antennas are equipped with RF chains and RAS reduces the upper bound on the broadcast sum capacity, and when the orthogonalized channels use optimal processing. Similarly, spatial mode selection (SMS) is necessary when receive-weight matrices are used for spatial mode allocation. Significantly, RAS/SMS helps to reduce the performance gap even for small user pool sizes. Optimal user selection for sum rate maximization is subsumed within an optimal RAS/SMS process for multi-antenna terminals and both selection processes become identical for single-antenna terminals. For a system with M transmit antennas, RAS/SMS increases the probability of scheduling M spatial modes compared to the case with sole reliance on user selection, especially when the potential user pool is small.