  This paper investigates a novel unmanned aerial vehicles (UAVs) secure
communication system with the assistance of reconfigurable intelligent surfaces
(RISs), where an UAV and a ground user communicate with each other, while an
eavesdropper tends to wiretap their information. Due to the limited capacity of
UAVs, a RIS is applied to further improve the quality of the secure
communications. The time division multiple access (TDMA) protocol is applied
for the communications between the UAV and the ground user, namely, the
downlink (DL) and the uplink (UL) communications. In particular, the channel
state information (CSI) of the eavesdropping channels is assumed to be
imperfect. We aim to maximize the average worst-case secrecy rate by the robust
joint design of the UAV's trajectory, RIS's passive beamforming, and transmit
power of the legitimate transmitters. It is challenging to solve the Joint
UL/DL optimization problem due to its non-convexity. To this end, we develop an
efficient algorithm based on the alternating optimization (AO) technique.
Specifically, the formulated problem is divided into three sub-problems, and
the successive convex approximation (SCA), S-Procedure, and semidefinite
relaxation (SDR) are applied to tackle these non-convex sub-problems. Numerical
results demonstrate that the proposed algorithm can considerably improve the
average secrecy rate compared with the benchmark algorithms, and also confirm
the robustness of the proposed algorithm.
