Capacity Analysis and Rate Maximization Design in RIS-Aided Uplink Multi-User MIMO

Reconfigurable intelligent surface (RIS) has recently drawn intensive attention due to its potential of simultaneously realizing high spectral and energy efficiency in a sustainable way. This paper focuses on the design of efficient transmission methods to maximize the uplink sum throughput in a RIS-aided multi-user multi-input multi-output (MU-MIMO) system. To provide an insightful basis, the channel capacity of RIS-aided MU-MIMO is theoretically analyzed. Then, the conventional transmission schemes based on orthogonal multiple access are presented as the baseline. From the information-theoretic perspective, we propose two novel schemes, i.e., $\mathit{\text{joint}}\mathit{\text{transmission}}$ based on the semidefinite relaxation of quadratic optimization problems and $\mathit{\text{opportunistic}}\mathit{\text{transmission}}$ relying on the best user selection. The superiority of the proposed schemes over the conventional ones in terms of achievable rates is justified through simulation results.