STAR-RIS Aided NOMA Communication System With Statistical CSI

Abstract

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have emerged a s a promising technology to reconFigure the radio propagation environment in the full space. However, a major challenge for reaping the beamforming gain of STAR-RISs is to deal with the large overhead that is required to estimate the instantaneous channel state information (CSI). To overcome this difficulty, we propose an efficient two-timescale (TTS) transmission protocol to maximize the average achievable sum-rate for a STAR-RIS aided non-orthogonal multiple access (NOMA) communication system. Specifically, the long-term STAR-RIS transmission and reflection coefficients are optimized based on the statistical CSI only, while the short-term power allocation at the base station (BS) is designed based on the estimated effective fading channels of all users. We further propose efficient algorithms to solve the respective long-term and short-term optimization problems. Simulation results are provided to validate the effectiveness of our design. Particularly, the channel estimation overhead of our proposed TTS protocol is substantially reduced as compared to the existing transmission schemes based on instantaneous CSI.