  This study investigates the integration of an intelligent reflecting surface
(IRS) into an unmanned aerial vehicle (UAV) platform to utilize the advantages
of these leading technologies for sixth-generation communications, e.g.,
improved spectral and energy efficiency, extended network coverage, and
flexible deployment. In particular, we investigate a downlink IRS-UAV system,
wherein single-antenna ground users (UEs) are served by a multi-antenna base
station (BS). To assist the communication between UEs and the BS, an IRS
mounted on a UAV is deployed, in which the direct links are obstructed owing to
the complex urban channel characteristics. The beamforming at the BS, phase
shift at the IRS, and the 3D placement of the UAV are jointly optimized to
maximize the sum rate. Because the optimization variables, particularly the
beamforming and IRS phase shift, are highly coupled with each other, the
optimization problem is naturally non-convex. To effectively solve the
formulated problem, we propose an iterative algorithm that employs block
coordinate descent and inner approximation methods. Numerical results
demonstrate the effectiveness of our proposed approach for a UAV-mounted IRS
system on the sum rate performance over the state-of-the-art technology using
the terrestrial counterpart.
