RIS-Assisted MIMO System Design: A Low Complex MIMO Precoder Selection Method

By intelligently reconfiguring wireless propagation environment, reconfigurable intelligent surfaces (RISs) can enhance signal quality, suppress interference, and improve channel conditions, thereby serving as a powerful complement to multiple-input multiple-output (MIMO) systems. However, the joint optimization of RIS phase shifts and the selection of MIMO precoder vectors for next-generation communication systems remains largely unexplored. This paper proposes a low-complex singular value-based RIS optimization strategy, where the phase shifts are configured to maximize the dominant singular values of the cascaded channel matrix, and the corresponding singular vectors are utilized for precoding. The proposed precoder selection is independent of the number of transmission layers and does not require mutual information computation across subbands, thereby reducing time complexity. The simulation results show that the proposed precoder selection method generally outperforms the conventional approach and sometimes achieves similar performance. In addition, for multilayer transmission and two-dimensional planar antenna arrays, the proposed method provides a significant reduction in time complexity compared to the conventional selection approach.