Energy Efficiency Optimization for Distributed RIS-Assisted MISO System Based on Statistical CSI

This paper optimizes the energy efficiency of distributed RIS-assisted communication system. Considering no channel estimation capability at Reconfigurable Intelligent Surface (RIS), an optimization scheme based on Statistical Channel State Information (SCSI) is given first. The traditional SCSI scheme cannot utilize the channel estimation capability of the Base Station (BS), so a scheme based on Two-Timescale (TTS) is further proposed. For the optimization, the triangular inequality and the successive convex approximation (SCA) method are employed in this paper to solve for the RIS elements phase shift. The singular value decomposition (SVD) and maximum ratio transmission (MRT) principle are adopted to solve the BS transmit beamforming in two schemes, respectively. By using partial derivatives, a closed-form equation for the optimal transmit power can be obtained. The system energy efficiency is maximized by optimizing the RIS element phase shift, BS transmit beam-forming and BS transmit power, jointly. The simulation results indicate that after the joint optimization, the energy efficiency has been significantly enhanced. The performance improvement is 10.3% (SCSI) / 10.8% (TTS) after optimization.