Energy-Efficient UAV Relaying in Dual Communication Mode

In scenarios with intensive communication demands, heavy ground user (GU) traffic places significant pressure on ground base stations (BSs). Unmanned aerial vehicle (UAV)-assisted cellular communication offers a promising opportunity to enhance regional capacity and quality of service (QoS) by relaying GU data to neighboring underutilized BSs. In this paper, we propose an energy-efficient UAV-assisted relay communication system operating in a dual communication mode (cellular mode and UAV relay mode). Under UAV energy budget constraints, we jointly optimize users’ connection scheduling, bandwidth allocation, and UAV trajectory to maximize the long-term throughput of all GUs. The resulting mixed-integer nonlinear non-convex problem is difficult to solve. To address this, we decompose the proposed problem into two subproblems. Specifically, the path discretization technique, successive convex approximation (SCA) method, and difference-of-convex (DC) method are employed to handle trajectory design and variable service durations. The user connection scheduling and bandwidth resource allocation problems are solved via a penalty-DC framework. Simulation results demonstrate that the proposed algorithm achieves higher mean regional communication capacity than the benchmark schemes, while also revealing critical trade-offs between UAV trajectory, onboard energy limits, and user QoS requirements.