  Unmanned aerial vehicle (UAV) communication is anticipated to be widely
applied in the forthcoming fifth-generation (5G) wireless networks, due to its
many advantages such as low cost, high mobility, and on-demand deployment.
However, the broadcast and line-of-sight (LoS) nature of air-to-ground wireless
channels gives rise to a new challenge on how to realize secure UAV
communications with the destined nodes on the ground. This paper aims to tackle
this challenge by applying the physical layer security technique. We consider
both the downlink and uplink UAV communications with a ground node, namely
UAV-to-ground (U2G) and ground-to-UAV (G2U) communications, respectively,
subject to a potential eavesdropper on the ground. In contrast to the existing
literature on wireless physical layer security only with ground nodes at fixed
or quasi-static locations, we exploit the high mobility of the UAV to
proactively establish favorable and degraded channels for the legitimate and
eavesdropping links, respectively, via its trajectory design. We formulate new
problems to maximize the average secrecy rates of the U2G and G2U
transmissions, respectively, by jointly optimizing the UAV's trajectory and the
transmit power of the legitimate transmitter over a given flight period of the
UAV. Although the formulated problems are non-convex, we propose iterative
algorithms to solve them efficiently by applying the block coordinate descent
and successive convex optimization methods. Specifically, the transmit power
and UAV trajectory are each optimized with the other being fixed in an
alternating manner, until the algorithms converge. Simulation results show that
the proposed algorithms can improve the secrecy rates for both U2G and G2U
communications, as compared to other benchmark schemes without power control
and/or trajectory optimization.
