Linear Filter Design for Multi-User MIMO-Relay Downlink Systems with User Selection

This paper addresses the filter design and user selection issues for the downlink of a multi-user MIMO-relay system. To eliminate interuser interference, the filter design is based on the singular value decomposition (SVD) of the first hop link and block-diagonalization (BD) for the second hop link. Then the problem is converted to the power allocation problem at the relay station (RS). It can be determined in a closed- form by the water-filling policy. In a practical system with a large number of users, the RS need to select a subset of best users to serve. The exhaustive search for the optimal userset is, however, computationally prohibitive. Therefore, we propose a low-complexity algorithm that is based on the MIMO-relay channel capacity. Simulation results show that the proposed linear filter design scheme achieves significant system performance improvement compared with the equal power allocation scheme and the user selection scheme allows for a reasonable tradeoff between the complexity and performance.