Joint Dual-Hop Beamforming and Trajectory Design for UAV Relay Networks

The reconfigurable deployment capability offers unmanned aerial vehicles (UAVs) substantial potential when utilized as relay nodes for cellular coverage enhancement. However, achieving optimal performance requires the co-design of UAV trajectory planning and dual-hop communication strategies. In this paper, we explore a UAV-Assisted downlink relay network, where the base station transmits signals to the UAV in the first hop, and the UAV decodes and forwards signals to mobile users in the second hop. We formulate an energy consumption model that characterizes both UAV propulsion energy and communication energy, with the latter determined by the beamforming vectors at each hop. To minimize total energy consumption, we propose an optimization framework that jointly considers the UAV's flight trajectory and dual-hop beamforming vectors. The non-convex problem is addressed using a secretary bird optimization algorithm, which enables efficient convergence in high-dimensional solution spaces. Simulation results demonstrate that compared to flight planning based on the center point, the proposed algorithm reduces total energy consumption by at least 20%.