I. Introduction
Unmanned aerial vehicle (UAV) has attracted an increasing attention for driving the exploration and implementation of the future communication networks because it provides the controllable-mobility, low-cost and low-latency wireless communication [1], [2]. Recently, UAVs have been deployed in many practical scenarios, such as wireless communication base station [3], [4], wireless power transfer [5], mobile edge computing [6] and smart grids [7]. However, it is still challengeable to apply the UAV to satisfy the system requirements, such as the higher communication rate, higher energy efficiency, and lower latency.