RIS-mounted UAV-assisted backscatter NOMA system for emergency communications in wireless networks

This paper investigates a Reconfigurable Intelligent Surface (RIS)-mounted Unmanned Aerial Vehicle (UAV)-assisted emergency wireless communication system enhanced with a Backscatter Device (BD) relay. The system addresses coverage challenges in scenarios where direct links between a Base Station (BS) and User Equipment (UE) are obstructed. We employ Non-Orthogonal Multiple Access (NOMA) with Successive Interference Cancellation (SIC) to serve near and far users simultaneously, while the RIS dynamically adjusts its phase shifts to reflect signals via the BD coherently. MATLAB simulations model realistic mobility scenarios with time-varying BS-RIS, RIS-BD, and BD-UE channels, including Additive White Gaussian Noise (AWGN), path loss, and fading. We analyse the effect of BD positioning on the effective cascaded channel, Signal-to-Noise Ratio (SNR), and achievable data rate. Results show that closer BD placement significantly enhances near-user performance with SIC, while far-user rates benefit from power allocation and constructive reflections. The proposed topology demonstrates robust connectivity, high spectral efficiency, and cost-effective deployment, highlighting the importance of BD positioning and RIS phase optimisation in multi-hop NOMA networks.