  This letter proposes a novel UAV-enabled mobile jamming scheme to improve the
secrecy rate of ground wiretap channel. Specifically, a UAV is employed to
transmit jamming signals to combat against eavesdropping. Such a mobile jamming
scheme is particularly appealing since the UAV-enabled jammer can fly close to
the eavesdropper and opportunistically jam it by leveraging the UAV's mobility.
We aim to maximize the average secrecy rate by jointly optimizing the UAV's
trajectory and jamming power over a given flight period. To make the problem
more tractable, we drive a closed-form lower bound for the achievable secrecy
rate, based on which the UAV's trajectory and transmit power are optimized
alternately by an efficient iterative algorithm applying the block coordinate
descent and successive convex optimization techniques. Simulation results
demonstrate that the proposed joint design can significantly enhance the
secrecy rate of the considered wiretap system as compared to benchmark schemes.
