A Survey of Multiple Reconfigurable Intelligent Surface Aided Systems

Reconfigurable intelligent surface (RIS) can establish reflection links between base stations and users, and provide a high passive beamforming gain, thereby significantly improving the system’s achievable rate. Due to the outstanding performance of the RIS, we survey the achievable rate optimization for the multi-RIS system, including single-hop and multi-hop RIS. Specifically, we first introduce the principle and application scenarios of the RIS. Meanwhile, we review the recent works of the achievable rate optimization for the traditional single-hop RIS system. We then extend the single-hop RIS system to the multi-hop RIS system to enhance the coverage of dense urban area. Finally, we use the semidefinite relaxation and alternating optimization methods to obtain the upper bound of the achievable rate for the multi-hop RIS system. In addition, we identify several future research directions for the multi-hop RIS system.