Beamforming Optimization in RIS-Aided Mimo Systems Under Multiple-Reflection Effects

We consider a controllable multi-user wireless communications system based on reconfigurable intelligent surface (RIS) and investigate the problem of optimizing active and passive beamforming jointly, with the presence of mutual coupling effects. Due to the sub-wavelength structure, mutual coupling between RIS elements is unavoidable. We investigate the effect of mutual coupling among RIS elements resulting in multiple reflection effects which are ignored in conventional (approximative) RIS models. We propose a novel method to optimize the RIS phase shifters considering the effect of multiple reflections using a more physically-consistent model (exact RIS model). Numerical results show that the inherent multiple reflection effects along with the optimized phase shifters and the active precoder can improve the performance of RIS-aided wireless communications systems.