Joint Transmit Beamforming and Phase Shift Design for RIS and NOMA Assisted Wireless Caching Systems With Non-Linear Energy Harvesting

This paper studies a new reconfigurable intelligent surface (RIS) aided wireless caching system. The RIS is equipped with a smart controller, and the functions of content caching and energy harvesting (EH) are endowed to the controller. We design a two-phase transmission policy. In the first phase, the requested contents that are not cached are delivered by the base station (BS) to RIS, and the controller conducts EH from surrounding radio frequency signals. In the second phase, the controller distributes cached contents utilizing non-orthogonal multiple access (NOMA) transmission with harvested energy, while the RIS forwards fetched contents sent from the BS to users. The beamforming vectors of the BS, RIS phase shifts, and NOMA power allocation factors are jointly optimized to maximize the achievable sum rate. The problem is intractable owing to coupled variables. After using the methods of fractional programming and quadratic transformation, we divide it into three sub-problems. The first sub-problem is solved by introducing auxiliary variables; the second one is addressed using the Riemannian manifold optimization; and the third one is converted to be convex by transforming the quality of service constraint. The sub-optimal solutions are achieved by alternatively solving the sub-problems. Simulation results reveal that the problem converges quickly, and the proposed algorithm outperforms benchmarks in terms of the sum rate.