Integrated passive reconfigurable intelligent surface and active relay assisted NOMA Systems

This paper investigates an integrated passive reconfigurable intelligent surface (RIS) and active relay assisted non-orthogonal multiple access (NOMA) system. To unleash the potential of distant user based on the downlink NOMA protocol, we identify a two-stage transmission strategy. In both stages, RISs provide coverage for paired users by actively modifying the channel response, and in the second stage, we introduce a dedicated active relay to realize the communication between the base station (BS) and distant user. Our goal is to maximize the system sum rate by jointly optimizing the power allocation at the BS and the passive beamforming at the RIS. To tackle the formulated non-convex problem, we propose an alternating penalty-based based algorithm. In particular, for the RIS phase-shift reconfiguration, a difference-of-convex (DC) approach is utilized to accurately detect the feasibility of the rank-one constraint. Numerical results demonstrate that: i) the integrated transmission scheme outperforms other baseline schemes; ii) the proposed scheme is capable of significantly enhancing the performance of distant NOMA users.