Near-Field Beam Sharing and Energy Harvesting in RIS-Assisted NOMA Networks

This article investigates an unconventional simultaneous wireless information and power transfer (SWIPT) system, wherein a base station (BS) and an energy station (ES) serve multiple zero-energy devices (ZEDs) present in the dead-zone region via reconfigurable intelligent surface (RIS) aided nonorthogonal multiple access (NOMA) systems. The multi-antenna BS and ES transmit information and energy signals to distant users, which are then reflected by RIS to near-field ZEDs which lacks a direct line-of-sight with the BS and ES. Specifically, we aim to develop a beam-sharing algorithm that jointly optimizes the RIS phase profile and beamforming vector of BS and ES to maximize the overall sum rate of the ZEDs while ensuring that the minimum energy harvesting criterion at the ZEDs and transmit power budget at BS and ES. Consequently, a non-convex resource optimization problem is formulated which is solved using an iterative approach, leveraging majorization-minimization and second-order cone programming transformation. Finally, numerical results are presented to demonstrate the performance of the proposed mixed near-far field RIS-aided NOMA SWIPT system under varying network parameters. The results confirm that the proposed solution attains better sum-rate performance when compared to a random beamforming design.