Energy Efficiency Optimization of STAR-RIS Empowered RSMA System with SWIPT

This paper first incorporates the concept of simultaneous wireless information and power transfer (SWIPT) into simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) empowered rate-splitting multiple access (RSMA) system to improve the energy efficiency (EE). On the basis of the power limit at the BS, a multi-user resource optimization problem of maximizing EE is studied by jointly optimizing the transmission-reflection coefficients (TRCs) at the STAR-RIS, the active beamforming vectors at BS, the power splitting (PS) ratios, and the common message rates. To solve the non-convex optimization problem, we decompose the optimization problem into three sub-problems: amplitude coefficient optimization, phase optimization, and common rate PS ratio optimization. Two optimization schemes are proposed: fractional programming and semidefinite relaxation (FP-SDR) based scheme and successive convex approximation and FP (SCA-FP) based scheme. By iteratively optimizing the three sub-problems using alternating optimization (AO), we can obtain the suboptimal solution to original optimization problem. Simulation results show that the STAR-RIS-RSMA scheme with SWIPT can achieve better EE performance than other multiple access technologies.