Max-Min Energy-Efficiency Fair Optimization in STAR-RIS Assisted System

Abstract: In this paper, we investigate simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS), which enables to communicate with users both sides by transmitting signals to users forward and reflecting signals to users backward simultaneously. We consider a communicatio system with a STAR-RIS, a base station (BS) and many users to maximize the minimum user energy efficiency (EE) by jointly optimizing the active beamforming, transmission and reflection coefficients with BS power consumption limited. To solve this optimization problem efficiently, we divide it into two sub-problems to optimize the transmitting beamforming matrix and phase shifts of STAR-RIS seperately. With the two subproblems fixed, an Alternating Optimization (AO) method is proposed to solve the maximize minmum user EE fair optimization problem. Numerical results can demonstrate that STAR-RIS behaves better than traditional reflecting-only RIS, and the algorithm we desigend can maximize the minum EE problem efficienty to ensure user fairness.