Transmit Power Minimization With QoS Guarantees in STAR-RIS-Aided RSMA Networks

Rate-splitting multiple access (RSMA) and simultaneously transmitting and reflecting (STAR) reconfigurable intelligent surface (RIS) have emerged as promising technologies for multiple access and relaying in sixth-generation (6G) networks. This paper investigates a STAR-RIS-aided RSMA system that minimizes transmit power while satisfying users’ quality of service (QoS) requirements under spatially correlated channels. The formulated non-convex optimization problem jointly optimizes the precoding vector, common message ratio, phase shift matrix, and transmission and reflection ratio. To solve this, the problem is decomposed into three subproblems: (i) precoding and common message ratio control at the base station (BS), (ii) phase shift matrix design, (iii) transmission/reflection ratio control at the STAR-RIS. These subproblems are transformed into convex forms via penalty functions and linear approximations. A stationary solution is obtained using interior-point initialization, successive convex approximation, and alternating optimization algorithms. Through extensive simulations under various channel environments, including spatially correlated scenarios, the proposed method achieves much lower transmit power while meeting QoS requirements, outperforming conventional (STAR-)RIS-aided RSMA, non-orthogonal multiple access, and space-division multiple access. The performance gain becomes more significant when the number of STAR-RIS elements or BS antennas is limited.