  In this letter, we investigate an unmanned aerial vehicle (UAV) communication
system, where an intelligent reflecting surface (IRS) is deployed to assist in
the transmission from a ground node (GN) to the UAV in the presence of a
jammer. We aim to maximize the average rate of the UAV communication by jointly
optimizing the GN's transmit power, the IRS's passive beamforming and the UAV's
trajectory. However, the formulated problem is difficult to solve due to the
non-convex objective function and the coupled optimization variables. Thus, to
tackle it, we propose an alternating optimization (AO) based algorithm by
exploiting the successive convex approximation (SCA) and semidefinite
relaxation (SDR) techniques. Simulation results show that the proposed
algorithm can significantly improve the average rate compared with the
benchmark algorithms. Moreover, it also shows that when the jamming power is
large and the number of IRS elements is relatively small, deploying the IRS
near the jammer outperforms deploying it near the GN, and vice versa.
