I. Introduction
Recently, unmanned aerial vehicle (UAV)-assisted communication has attracted increasing attention from both industry and academia due to its multi-fold advantages [2]–[5]. UAVs can be deployed flexibly for on-demand wireless communication systems due to their high manoeuvrability [6]–[8]. UAVs can also provide line-of-sight (LOS) links to ground users (GUs), which can improve the performance of communication systems [9]–[11]. In addition, the UAVs can act as the complement of terrestrial networks to serve the heavy traffic loads in hotspot areas, such as large-scale sport or musical events [12]–[14].