  In this paper, we consider the application of intelligent reflecting surface
(IRS) in unmanned aerial vehicle (UAV)-based orthogonal frequency division
multiple access (OFDMA) communication systems, which exploits both the
significant beamforming gain brought by the IRS and the high mobility of UAV
for improving the system sum-rate. The joint design of UAV's trajectory, IRS
scheduling, and communication resource allocation for the proposed system is
formulated as a non-convex optimization problem to maximize the system sum-rate
while taking into account the heterogeneous quality-of-service (QoS)
requirement of each user. The existence of an IRS introduces both
frequency-selectivity and spatial-selectivity in the fading of the composite
channel from the UAV to ground users. To facilitate the design, we first derive
the expression of the composite channels and propose a parametric approximation
approach to establish an upper and a lower bound for the formulated problem. An
alternating optimization algorithm is devised to handle the lower bound
optimization problem and its performance is compared with the benchmark
performance achieved by solving the upper bound problem. Simulation results
unveil the small gap between the developed bounds and the promising sum-rate
gain achieved by the deployment of an IRS in UAV-based communication systems.
