Introduction
Recently, unmanned aerial vehicle (UAV) assisted air–ground integrated networks (AGINs) have played an important role in disaster rescue, emergency support, and network expansion, and have drawn wide attention from academia and industry [1], [2]. Due to the advantages of low cost, high mobility, and flexible deployment, UAV hovering in the air is easier to establish the wireless communication link than the ground fixed communication facilities [3], [4]. UAVs possess the ability to establish superior line of sight (LoS) links by air superiority. Hence, they can sufficiently approach ground terminals to transmit and receive data [5]. In the AGIN, there are mainly two types of applications of UAV: 1) UAV base station (BS), which can quickly recover the ground communications by forming an air-to-ground cellular network in the area where the ground BSs are paralyzed [6]; and 2) UAV relay, which can help the ground users suffering severe channel fading to achieve high-speed and reliable communications via better aerial LoS transmissions [7].