Joint multi-UAV deployments for air–ground integrated networks

Unmanned aerial vehicles (UAV) can perform high-speed and reliable transmissions with the properties in air superiority, high agility, and fast deployment. They have shown advantages in flexibility and reliability when the ground communication facilities cannot provide satisfactory services. In this article, a multi-UAV-based air–ground integrated network (AGIN) model is established with a novel joint optimal multi-UAV deployment framework, where UAVs serve as aerial base stations (BSs) or relays. To improve the communication performance of the AGIN, the following deployment schemes are proposed: 1) a static multi-UAV BS deployment to maximize the number of covered users by user clustering; 2) a mobile multi-UAV BS deployment to maximize the transmission performance of users, which is achieved by jointly optimizing UAV trajectories and user scheduling; 3) an optimized UAV multihop relay deployment to minimize the number of UAV relays and the communication outage probability. Moreover, UAV formation relay is deployed as a virtual multiantenna array to improve the relaying performance by beamforming and orthogonal space-time block coding, and some further open researches and challenges are also discussed.