UAV-Enabled Broadcast Channel: Trajectory Design and Capacity Characterization

Unmanned aerial vehicles (UAVs) have recently gained growing popularity in wireless communications owing to their many advantages such as swift and cost- effective deployment, line-of-sight (LoS) aerial-to- ground link, and controllable mobility in three- dimensional (3D) space. In this paper, we consider a UAV-enabled two-user broadcast channel (BC), where a UAV flying at a constant altitude is deployed to send independent information to two users at different fixed locations on the ground. We aim to characterize the capacity region of this new type of BC over a given UAV flight duration, by jointly optimizing the UAV's trajectory and transmit power/rate allocations over time, subject to the UAV's maximum speed and maximum transmit power constraints. Although the considered problem is shown to be non-convex and difficult to solve in general, we obtain the optimal solution by exploiting its particular structure and applying tools from convex optimization. In particular, we show that the optimal UAV trajectory should follow a general hover- fly-hover (HFH) structure, i.e., the UAV successively hovers at a pair of initial and final locations above the line segment of the two users each with a certain amount of time and flies unidirectionally between them at the maximum speed, and superposition coding (SC) with interference cancellation at the receiver of the nearer user is generally needed. Finally, simulation results are provided to verify our analysis, which also reveal useful guidelines to the practical design of UAV trajectory and communication jointly.