  This paper investigates an aerial reconfigurable intelligent surface
(RIS)-aided communication system under the probabilistic line-of-sight (LoS)
channel, where an unmanned aerial vehicle (UAV) equipped with an RIS is
deployed to assist two ground nodes in their information exchange. An
optimization problem with the objective of maximizing the minimum average
achievable rate is formulated to jointly design the communication scheduling,
the RIS's phase shift, and the three-dimensional (3D) UAV trajectory. To solve
such a non-convex problem, we propose an efficient iterative algorithm to
obtain its suboptimal solution. Simulation results show that our proposed
design significantly outperforms the existing schemes and provides new insights
into the elevation angle and distance trade-off for the UAV-borne RIS
communication system.
