Energy- Efficiency Optimization in Dual Reconfigurable Intelligent Surfaces Wireless Communication System

Reconfigurable Intelligent Surface (RIS) has become an emerging technology in wireless communication system. In this paper, we consider a RISs-assisted system where two RISs are deployed to cooperatively assist communications between a multi-antenna base station (BS) and multiple single-antenna cell-edge users. We optimize the transmitting precoding of the multi-antenna BS and the angular phase transfer matrix of RIS together to maximize energy efficiency (EE) of the RISs-assisted system. In order to solve the challenging EE optimization problem effectively, we turn the non-convex maximization problem into two equivalent subproblems based on mean-square error (MSE) method, and finally use iterative algorithm for optimization. We respectively use Lagrangian method and semidefinite relaxation techniques to optimize transmitting beamforming matrix and RIS phase shifts matrix. Numerical results demonstrate the significant performance improvement achieved by deploying RIS.