Beamforming For Multiple User MIMO Downlink

Future development of wireless communications demands higher data rates. With limited spectral resource, it requires to improve the spectral efficiency. Spatial diversity or spatial multiplex can be got by Multiple-Input Multiple-Output (MIMO) technique. This paper introduces single-user MIMO system, multi-user MIMO system and related techniques, including Spatial Time Block Coding, Layered Spatial Time Coding and beamforming. The paper focus on beamforming in multi-user MIMO downlink.Transmit beamforming is a nonconvex problem in multi-user MIMO downlink. The key technique for joint design of transmit and receive beamformers is Uplink-Downlink Duality. By constructing virtual dual uplink of original downlink, transmit beamformers can be solved as optimal receivers. This paper exploits weighted sum antenna power optimization problem. It is shown that dual relations exist between weights of transmit antenna powers and powers of receive noises.Most of researches on multi-user downlink beamforming are based on sum power constraints. However, in realization, antenna powers are usually limited individually, especially in Cooperative MIMO systems and Distributed MIMO systems. Beamforming under per-antenna power constraints is under consideration in this paper. In the light of the dual relations, transmit antenna powers can be managed by powers of dual uplink noises. Under this scheme, three algorithms are proposed for joint design of transmit and receive beamforming with per-antenna power constraints in multi-user MIMO downlink. Algorithm A tries to minimizes total transmit power, Algorithms B balances power utilization ratios over antennas, and Algorithm C tries to maximizes SINR vectors under per-antenna power constraints. The algorithms show good performances in simulations.