On performance of RIS-aided ground-to-air and air-to-ground communications in multi-user NOMA systems

This paper examines the performance of a wireless system where two reconfigurable intelligent surfaces (RISs) and an unmanned aerial vehicle (UAV) are combined to aid multiple ground users adopting nonorthogonal multiple access (NOMA) scheme. Specifically, the UAV acts as an aerial relay station, the first and the second RISs are respectively used to aid ground-to-air (G2A) and air-to-ground (A2G) communications. Besides the reflected link from the RISs, the direct link between a ground base station (S) and the UAV, the UAV and ground users are exploited to increase the received power at UAV and ground users. We successfully derive the theoretical formulas of outage probability (OP) and ergodic capacity (EC) of RIS-aided G2A/A2G-NOMA system over Nakagami-m fading channel. The derived formulas are validated by Monte-Carlo simulations, and the impacts of system parameters on the OP and EC are fully evaluated to achieve useful insights into the system behaviors. Numerical results clarify that by using two RISs where every RIS is equipped with 100 reflecting elements (REs), the OP of RIS-aided G2A/A2G-NOMA system is greatly lower than the OP of the G2A/A2G-NOMA system without RISs, in which there are only S-UAV and UAV-ground users direct links. In the case of EC, the benefit of using RISs focuses on a low transmit power regime. It is because the EC of the RIS-aided G2A/A2G-NOMA system is only higher than the EC of the system without RISs in the low transmit power region. For the high transmit power regions, the ECs with and without RISs are similar.