  Mobile relaying is emerged as a promising technique to assist wireless
communication, driven by the rapid development of unmanned aerial vehicles
(UAVs). In this paper, we study secure transmission in a four-node (source,
destination, mobile relay, and eavesdropper) system, wherein we focus on
maximizing the secrecy rate via jointly optimizing the relay trajectory and the
source/relay transmit power. Nevertheless, due to the coupling of the
trajectory designing and the power allocating, the secrecy rate maximization
(SRM) problem is intractable to solve. Accordingly, we propose an alternating
optimization (AO) approach, wherein the trajectory designing and the power
allocating are tackled in an alternating manner. Unfortunately, the trajectory
designing is a nonconvex problem, and thus is still hard to solve. To
circumvent the nonconvexity, we exploit sequential convex programming (SCP) to
derive an iterative algorithm, which is proven to converge to a
Karush-Kuhn-Tucker (KKT) point of the trajectory design problem. The simulation
results demonstrate the efficacy of the joint power and trajectory design in
improving the secrecy throughput.
