NURSE-MAC: A Service-Differentiated MAC Protocol Based on RIS-Enhanced NOMA for High Spectral and Temporal Efficiency in UAV-Assisted Emergency Communication

Unmanned Aerial Vehicles (UAVs) can be rapidly deployed as aerial base stations to provide temporary wireless coverage in post-disaster areas, while reconfigurable intelligent surfaces (RISs) can further improve the propagation environment. However, designing an efficient multiple access control (MAC) mechanism for RIS-enhanced UAV-assisted emergency communications remains challenging, especially when heterogeneous traffic requires differentiated access and service support. This paper proposes NURSE-MAC, a service-differentiated MAC protocol based on RIS-enhanced non-orthogonal multiple access (NOMA), to improve spectral and temporal efficiency in UAV-assisted emergency communication networks. NURSE-MAC adopts a hybrid superframe structure consisting of a contention period (CP) and a schedule period (SP) to support different types of traffic. For general traffic, an initial contention window (CW)-adjustable power back-off (PB) strategy is designed during the CP to improve spectral efficiency while balancing throughput and fairness. For critical traffic with minimum rate requirements, a reservation-based mechanism is introduced so that user equipment (UE) can register during the CP and receive scheduled transmission opportunities during the SP. Under the quasi-static assumption at the superframe time scale during the UAV hovering period, an alternating iterative optimization strategy is employed during the SP to jointly optimize RIS phase shifts and transmit power for scheduled transmissions. Simulation results show that NURSE-MAC achieves higher spectral efficiency than orthogonal multiple access (OMA) and better temporal efficiency than representative NOMA-based and RIS-assisted MAC schemes. The results also demonstrate that the proposed hybrid design provides an effective way to support minimum rate constrained critical traffic while maintaining efficient contention-based access for general traffic.