Low-altitude network is a key enabler for extending coverage and recovering connectivity in 6G systems, especially when terrestrial infrastructure is unavailable. This paper studies a uncrewed aerial vehicle (UAV)-mounted rotatable intelligent reflecting surface (IRS) as a low-altitude reflector between a blocked base station (BS) and a ground terminal (GT). Unlike the conventional isotropic-element assumption, each IRS element is modeled with a hemispherical directive radiation pattern, whose boresight can be adjusted via element rotations. We formulate a new optimization problem that jointly designs IRS phase shifts, per-element rotation vectors, and UAV placement to maximize the received signal-to-noise ratio (SNR). Leveraging the problem structure, we derive closed-form solutions for phase alignment and element rotations, showing that the optimal boresight points are along the internal angular bisector between the BS-IRS and GT-IRS directions. With these closed forms, the design reduces to a placement optimization problem over a box-constrained airspace; we solve it using an efficient projected gradient algorithm with majorization-minimization update and a global Lipschitz constant. Numerical results demonstrate substantial SNR gains from directive elements and reveal a fundamental trade-off between directional gain and path loss, yielding useful insights into low-altitude deployment of UAV-mounted IRSs.
Shanghai Jiao Tong University（上海交通大学）；University of Macau（澳门大学）；Nanjing University of Science and Technology（南京理工大学）；National University of Singapore（新加坡国立大学）
Energy-Efficient Trajectory Design of a Multi-IRS Assisted Portable Access Point
Research, Technology and Innovation Network (RTIN), Alba, The American College of Greece（美国希腊学院阿尔巴研究、技术与创新网络（RTIN））；Aalborg University（奥尔堡大学）；Lyra Electronics（莱拉电子公司）；The University of Manchester（曼彻斯特大学）；University College London（伦敦大学学院）
