Multi-Functional RIS for Sum-Rate Maximization in Rate-Splitting Multiple Access Networks

Wen Wang, Wanli Ni, Hui Tian, Zhaohui Yang, Changsheng You, Dusit Niyato

Abstract: In this paper, we propose a multi-functional reconfigurable intelligent surface (MF-RIS) assisted rate-splitting multiple access (RSMA) scheme to improve the spectral efficiency in the downlink communications. With the ability of simultaneous signal reflection, transmission/refraction, and amplification, the proposed MF-RIS enables full-dimensional communications with enhanced signal power, thereby improving the signal reception of RSMA users in whole space. Under the constraints on the power budget, MF-RIS coefficients, and minimum rate requirement, a non-convex optimization problem is formulated to maximize the sum-rate by jointly designing the rate allocation, transmit beamforming, and MF-RIS configuration. Then, a low-complexity alternating optimization algorithm is proposed to solve this challenging problem. Simulation results show that the proposed MR-RIS-aided RSMA scheme can achieve up to 33.6% and 51.9% higher spectral efficiency than the simultaneous transmitting and reflecting RIS-aided RSMA scheme and the MF-RIS-aided non-orthogonal multiple access scheme, respectively.