  This paper studies a secrecy unmanned aerial vehicle (UAV) communication
system with coordinated multi-point (CoMP) reception, in which one UAV sends
confidential messages to a set of distributed ground nodes (GNs) that can
cooperate in signal detection, in the presence of several colluding suspicious
eavesdroppers. Different from prior works considering the two-dimensional (2D)
horizontal trajectory design in the non-CoMP scenario, this paper additionally
exploits the UAV's vertical trajectory (or altitude) control for further
improving the secrecy communication performance with CoMP. In particular, we
jointly optimize the three dimensional (3D) trajectory and transmit power
allocation of the UAV to maximize the average secrecy rate at GNs over a
particular flight period, subject to the UAV's maximum flight speed and maximum
transmit power constraints. To solve the non-convex optimization problem, we
propose an alternating-optimization-based approach, which optimizes the
transmit power allocation and trajectory design in an alternating manner, by
convex optimization and successive convex approximation (SCA), respectively.
Numerical results show that in the scenario with CoMP reception, our proposed
3D trajectory optimization significantly outperforms the conventional 2D
horizontal trajectory design, by exploiting the additional degree of freedom in
vertical trajectory.
