  This paper investigates the problem of secure energy efficiency maximization
for a reconfigurable intelligent surface (RIS) assisted uplink wireless
communication system, where an unmanned aerial vehicle (UAV) equipped with an
RIS works as a mobile relay between the base station (BS) and a group of users.
We focus on maximizing the secure energy efficiency of the system via jointly
optimizing the UAV's trajectory, the RIS's phase shift, users' association and
transmit power. To tackle this problem, we divide the original problem into
three sub-problems, and propose an efficient iterative algorithm. In
particular, the successive convex approximation method (SCA) is applied to
solve the nonconvex UAV trajectory, the RIS's phase shift, and transmit power
optimization sub-problems. We further provide two schemes to simplify the
solution of phase and trajectory sub-problem. Simulation results demonstrate
that the proposed algorithm converges fast, and the proposed design can enhance
the secure energy efficiency by up to 38\% gains, as compared to the
traditional schemes without any RIS.

Signal Processing