  In this paper, we investigate the unmanned aerial vehicle (UAV)-aided
simultaneous uplink and downlink transmission networks, where one UAV acting as
a disseminator is connected to multiple access points, and the other UAV acting
as a base station (BS) collects data from numerous sensor nodes. The goal of
this paper is to maximize the sum of the UAV-BS and UAV-AP system throughput by
jointly optimizing the 3D UAV trajectory, communication scheduling, and
UAV-AP/SN transmit power. We first consider a special case where the UAV-BS and
UAV-AP trajectories are pre-determined. Although the resulting problem is an
integer and non-convex optimization problem, a globally optimal solution is
obtained by applying the polyblock outer approximation (POA) method based on
the problem's hidden monotonic structure. Subsequently, for the general case
considering the 3D UAV trajectory optimization, an efficient iterative
algorithm is proposed to alternately optimize the divided sub-problem based on
the successive convex approximation (SCA) technique. Numerical results
demonstrate that the proposed design is able to achieve significant system
throughput gain over the benchmarks. In addition, the SCA-based method can
achieve nearly the same performance as the POA based method with much lower
computational complexity.
