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

In this paper, we investigate simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS), which enables to communicate with users in both sides by transmitting signals to users forward and reflecting signals to users backward simultaneously. We consider a communication system with a STAR-RIS, a base station (BS) and many users to maximize the minimum user energy efficiency (EE) by jointly optimizing active beamforming, transmission and reflection coefficients with power constraint. Considering the fractional programming form of the formulated problem, we adopt Dinkelbach algorithm to transform the problem into the form of convex function subtraction, and then divide it into two sub-problems optimizing the beamforming vectors at BS and coefficient matrices at STAR-RIS separately. With the two subproblems fixed, an Alternating Optimization (AO) algorithm is proposed to solve the maximize minimum user EE fair optimization problem. Numerical results demonstrate that STAR-RIS can enhance energy and spectrum efficiency of the STAR-RIS communication system, and the proposed algorithm maximizes the minimum EE effectively to ensure user fairness.