  Unmanned aerial vehicles (UAVs) are anticipated to be widely deployed in
future wireless communications, due to their advantages of high mobility and
easy deployment. However, the broadcast nature of air-to-ground line-of-sight
wireless chan- nels brings a new challenge to the information security of UAV-
ground communication. This paper tackles such a challenge in the physical layer
by exploiting the mobility of UAV via its trajectory design. We consider a
UAV-ground communication system with multiple potential eavesdroppers on the
ground, where the information on the locations of the eavesdroppers is
imperfect. We formulate an optimization problem which maximizes the average
worst-case secrecy rate of the system by jointly designing the robust
trajectory and transmit power of the UAV over a given flight duration. The
non-convexity of the optimization problem and the imperfect location
information of the eavesdroppers make the problem difficult to be solved
optimally. We propose an iterative suboptimal algorithm to solve this problem
efficiently by applying the block coordinate descent method, S-procedure, and
successive convex optimization method. Simulation results show that the
proposed algorithm can improve the average worst-case secrecy rate
significantly, as compared to two other benchmark algorithms without robust
design.
