Sum Rate Maximization for RIS-Aided NOMA With Direct Links

Reconfigurable intelligent surface (RIS) is an electromagnetic surface, and has abundant low-power reflecting elements which can dynamically tune the wireless propagation environment by changing their phase shifts. Non-orthogonal multiple access (NOMA) technology would be capable of greatly improving the spectral efficiency of communication via differentiating users through power. Via integrating RIS into NOMA to improve wireless system performance, we take an uplink RIS-aided NOMA network with direct links into account, where multiple users communicate with the access point under the assistance of a RIS with multiple elements. We aim to obtain the maximum sum rate by optimizing the phase matrix of RIS subject to users' transmitted power. Two algorithms are put forward to conquer the formulated intricate non-convex puzzle. More exactly, the semidefinite programming is proposed to relax the function while the majorization-minimization is used to derive the closed-form phase shifts. Finally, presented simulation results demonstrate the high performance of proposed schemes compared with no RIS and show the benefits of the existence of direct links.