Hybrid Reconfigurable Intelligent Surfaces and Unmanned Aerial Vehicles Assisted Cooperative NOMA

While existing studies have investigated the integration of reconfigurable intelligent surfaces (RIS) and unmanned aerial vehicle (UAV) systems to improve wireless communication, only single-user scenarios or orthogonal multiple access (OMA) have been considered, which suffers from spectral inefficiency. To serve multiple users efficiently, this paper proposes a novel hybrid RIS and UAV-assisted cooperative non-orthogonal multiple access (NOMA) scheme. Specifically, a hybrid RIS comprising both active and passive elements mounted on a UAV, which acts as an aerial relay to assist the downlink NOMA communication. The sum rate maximisation problem is investigated by optimising the three-dimensional (3D) placement of the UAV and the NOMA power allocation while guaranteeing the users’ quality of service requirements. The formulated non-convex problem is solved iteratively using alternating optimisation with the interior-point method applied to each subproblem. The simulation results demonstrate that the proposed NOMA scheme with a hybrid RIS-UAV achieves a better sum rate performance than the NOMA schemes with conventional fully passive RIS-UAV, full-duplex UAV relay, and baseline OMA schemes, highlighting its potential for future multiuser wireless networks.