# A Survey on Intelligent-Reflecting-Surface-Assisted UAV Communications
## 1 Introduction
To provide a reliable power supply and stable control of the UAV-mounted IRSs, UAVs can be connected to other mobile platforms, such as ground BSs or vehicles with stable power supplies. The IRS on the UAV provides 360-degree panoramic reflective links toward the ground to the users and, thus, can support communications between the BS and users on the ground. IRS-assisted UAV communications can generally increase the data rates of nearby users and expand network coverage, thereby reducing the the number of cellular BSs and enabling energy-efficient communications systems. More specifically, the benefits and the main design considerations of IRS-assisted UAV communications can be summarized as follows:  
* In the case of link interrupt or performance degradation due to blockage of the LoS, it is possible to establish another LoS link bypassing the IRS. Therefore, the UAV-IRS systems can extend the coverage and improve the communication performance of users. * When the IRS is mounted on a UAV, its position can be dynamically changed according to the movement of the UAV. Compared with the ground IRS on a fixed surface, the mobility of UAVs provides a degree of freedom in the IRS placement. * When an IRS is mounted on a UAV, both the transmitter and receiver can establish an LoS link, thereby achieving full-angle reflection and extending the wireless coverage. * The IRS-assisted UAV communications provides improved security, as well as coverage expansion. In particular, the UAV can create a hot zone by generating artificial noise to protect legitimate users from eavesdroppers and external jamming attackers [41]. * IRS-assisted communications can operate in the mmWave or THz band, thereby enabling extremely wideband services. mmWave and THz links are very vulnerable  
Figure 1: A terrestrial communications system with UAV-mounted IRSs. to path blocking and loss [42]. However, the detrimental effect can be effectively compensated for by the reconfigurability of the IRS and the mobility of the UAV. * Optimization of the IRS-assisted UAV communications system enables energy efficiency management [43], for which various factors can be considered. The main factors for energy efficiency management include phase control of the IRS, the number of planar array components, and the trajectory and altitude of the UAV. Both the UAV and IRS described above are accepted as key enablers for next-generation communications systems, and they can be effectively combined to realize various services with innovative spatial configurations and further improve the communication performance [43; 44]. At the same time, there are still many issues arising from the integration and constraints to be overcome, and various studies are being performed. Technical surveys on the IRS and UAV can be found in [45; 46], respectively. In this article, we aim to present a comprehensive survey of IRS-assisted UAV communications systems, including research trends, important issues, technical challenges, and promising applications in future wireless networks. The remainder of this article is organized as follows. Section 2 describes various scenarios of IRS-assisted UAV communications. In Section 3, we provide an overview of relevant studies from the perspective of several performance criteria. Section 4 discusses the optimization methodologies used for developing IRS-assisted UAV communications systems. Section 5 highlights some technical challenges to be investigated in the future. Finally, conclusions are drawn in Section 6.