Reconfigurable Intelligent Surface Aided Rate-Splitting Multiple Access for Multi-User Downlink Transmission

This paper examines the resource allocation problem in a wireless communication system utilizing rate-splitting multiple access (RSMA) assisted by the reconfigurable intelligent surface (RIS). We aim to jointly optimize user rate allocation, base station (BS) beamforming vectors, and RIS phase shift matrix to maximize the sum-rate of all users while satisfying quality of service requirements, power constraints, and constraints related to RIS reflecting elements. Firstly, closed-form expressions for the optimal user rate allocation scheme are derived, and the weighted minimum mean square error (WMMSE) method is employed to design BS beamforming vectors and nearly optimal solutions for ideal RIS phase shifts. Secondly, an alternative solution is proposed for the near-optimal design of non-ideal RIS phase shifts, utilizing successive convex approximation (SCA) and penalty methods. Ultimately, the simulation results confirm that the proposed algorithm surpasses benchmark algorithms in its rate performance. Additionally, compared to RIS-assisted non-orthogonal multiple access (NOMA) and RIS-assisted space division multiple access (SDMA), RIS-assisted RSMA can achieve rate performance improvements of 24.4% and 10.2%, respectively.