  An unmanned aerial vehicle (UAV)-aided secure communication system is
conceived and investigated, where the UAV transmits legitimate information to a
ground user in the presence of an eavesdropper (Eve). To guarantee the
security, the UAV employs a power splitting approach, where its transmit power
can be divided into two parts for transmitting confidential messages and
artificial noise (AN), respectively. We aim to maximize the average secrecy
rate by jointly optimizing the UAV's trajectory, the transmit power levels and
the corresponding power splitting ratios allocated to different time slots
during the whole flight time, subject to both the maximum UAV speed constraint,
the total mobility energy constraint, the total transmit power constraint, and
other related constraints. To efficiently tackle this non-convex optimization
problem, we propose an iterative algorithm by blending the benefits of the
block coordinate descent (BCD) method, the concave-convex procedure (CCCP) and
the alternating direction method of multipliers (ADMM). Specially, we show that
the proposed algorithm exhibits very low computational complexity and each of
its updating steps can be formulated in a nearly closed form. Our simulation
results validate the efficiency of the proposed algorithm.
