Sum-Rate Maximization Scheme for Multi-RIS-Assisted NOMA Uplink Systems

Abstract

Reconfigurable intelligent surface (RIS) and non-orthogonal multiple access (NOMA) are both highly promising technologies for future communication. Compared with traditional single-RIS-assisted NOMA systems, this paper considered multi-RIS-assisted NOMA uplink communication systems and proposed a sum-rate maximization scheme. At present, most research on RIS-assisted NOMA systems has not considered the joint optimization of users’ power, multi-RIS deployment, and multi-RIS phase shifts. Firstly, this paper proposed a sum-rate problem with multiple variates, which are involved in users’ power, multi-RIS deployments, and multi-RIS phase shifts. This problem is usually very complex and non-convex, which makes it very difficult to obtain an optimal solution. Then, the original problem was decomposed into three sub-problems through several derivations, which are relatively simple and easy to solve. Finally, the optimal multi-RIS deployment locations were obtained by a simulated annealing particle-swarm optimization algorithm, and a suboptimal solution based on positive semidefinite relaxation was adopted to solve the joint optimization problem of users’ power and multi-RIS phase shifts, respectively. The research results indicate that the sum-rate for the considered systems with the multi-RIS optimization algorithm can be improved by about 1 bps/Hz, compared with that of non-optimization, and under the same total number of RIS reflection units as a single-RIS scheme, the performance of the proposed scheme in this paper is superior to the single-RIS scheme, which proves the effectiveness of the proposed algorithm.