  Unmanned aerial vehicles (UAVs) in cellular networks have garnered
considerable interest. One of their applications is as flying base stations
(FBSs), which can increase coverage and quality of service (QoS). Because FBSs
are battery-powered, regulating their energy usage is a vital aspect of their
use; and therefore the appropriate placement and trajectories of FBSs
throughout their operation are critical to overcoming this challenge. In this
paper, we propose a method of solving a multi-FBS 3D trajectory problem that
considers FBS energy consumption, operation time, flight distance limits, and
inter-cell interference constraints. Our method is divided into two phases: FBS
placement and FBS trajectory. In taking this approach, we break the problem
into several snapshots. First, we find the minimum number of FBSs required and
their proper 3D positions in each snapshot. Then, between every two snapshots,
the trajectory phase is executed. The optimal path between the origin and
destination of each FBS is determined during the trajectory phase by utilizing
a proposed binary linear problem (BLP) model that considers FBS energy
consumption and flight distance constraints. Then, the shortest path for each
FBS is determined while taking obstacles and collision avoidance into
consideration. The number of FBSs needed may vary between snapshots, so we
present an FBS set management (FSM) technique to manage the set of FBSs and
their power. The results demonstrate that the proposed approach is applicable
to real-world situations and that the outcomes are consistent with
expectations.
