Robust Beamforming Design for Energy Efficiency and Spectral Efficiency Tradeoff in Multi-STAR-RIS-Aided C-HRSMA

This paper investigates a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted cognitive hierarchical rate-splitting multiple access (C-HRSMA) system to enhance the system performance under imperfect channel state information (ICSI). By exploiting the natural user grouping afforded by the STAR-RIS and its distinct channel manipulation capabilities for the transmission and reflection users, we effectively mitigate inter-group common stream interference within C-HRSMA, consequently facilitating the achievement of higher spectral efficiency. Subsequently, the design is formulated as a non-convex optimization problem that incorporates the phase-shift matrix of STAR-RIS, the beamforming vector of the base station, and the common rate allocation vector. To address this non-convex problem, an alternating optimization (AO) technique is employed to decouple the primary problem and solve the subproblems using S-procedure and successive convex approximation (SCA). The simulation results validate that the proposed algorithm exhibits superior SE and EE performance against benchmark algorithms.