  Unmanned aerial vehicles (UAVs) have attracted significant interest recently
in assisting wireless communication due to their high maneuverability, flexible
deployment, and low cost. This paper considers a multi-UAV enabled wireless
communication system, where multiple UAV-mounted aerial base stations (BSs) are
employed to serve a group of users on the ground. To achieve fair performance
among users, we maximize the minimum throughput over all ground users in the
downlink communication by optimizing the multiuser communication scheduling and
association jointly with the UAVs' trajectory and power control. The formulated
problem is a mixed integer non-convex optimization problem that is challenging
to solve. As such, we propose an efficient iterative algorithm for solving it
by applying the block coordinate descent and successive convex optimization
techniques. Specifically, the user scheduling and association, UAV trajectory,
and transmit power are alternately optimized in each iteration. In particular,
for the non-convex UAV trajectory and transmit power optimization problems, two
approximate convex optimization problems are solved, respectively. We further
show that the proposed algorithm is guaranteed to converge to at least a
locally optimal solution. To speed up the algorithm convergence and achieve
good throughput, a low-complexity and systematic initialization scheme is also
proposed for the UAV trajectory design based on the simple circular trajectory
and the circle packing scheme. Extensive simulation results are provided to
demonstrate the significant throughput gains of the proposed design as compared
to other benchmark schemes.
