# A Survey on Intelligent-Reflecting-Surface-Assisted UAV Communications
## 1 Introduction  
Commercialization of Fifth-Generation (5G) mobile communications has led to a tremendous increase in the number of users and Internet of Things (IoT) devices, which require next-generation communication technologies. Furthermore, Sixth-Generation (6G) communications are emerging to be commercialized by 2030. Various studies have been conducted on its vision and research and development issues. According to these studies, future 6G communications is expected to target more advanced network performance than 5G, with the advent of challenging applications, such as extended reality, industrial automation, and tactile Internet. Non-terrestrial networks (NTNs) are being considered as one of the key elements of revitalizing 5G communications, expanding related industries, and developing future 6G networks [1]. NTNs can provide wireless services in a cost-effective manner in areas where 5G terrestrial networks are not available (such as isolated/remote areas, aboard aircraft, or ships) and vulnerable areas (out-of-town or rural areas) because of the extensive service coverage and reduced physical attacks on space/air platforms. They also provide service continuity to IoT devices or passengers on mobile platforms (such as aircraft, ships, high-speed trains, and buses) and enable reliable service for future communications for railways, sea, and aviation. In addition, NTNs can extend the availability of 5G networks by providing efficient multicast and broadcast resources for data delivery to network edges (or user terminals). These benefits can be obtained by a standalone NTN or an integrated terrestrial and non-terrestrial network. Furthermore, the benefits are expected to have a significant impact on various domains, including transportation, public safety, media and entertainment, e-health, energy, agriculture, finance, and automobiles. Unmanned aerial vehicles (UAVs),a part of aerial communication platforms, are emerging as important entities in future wireless networks. UAV-enabled communication networks are expected to improve the spectral efficiency, extend the coverage, and increase the flexibility of wireless networks [2]. In addition, UAVs can be effectively used for data dissemination to diverse IoT devices and for emergency communications in disaster scenarios [3]. From the perspective of wireless communications, the UAV has unique characteristics, such as high degrees of freedom in positioning and trajectories in three-dimensional (3D) space, low deployment and maintenance costs, and the ability to establish clear line-of-sight (LoS) links with other nodes. In particular, the mobility of UAVs will provide new opportunities for adaptive communications in aerial networks. UAVs can flexibly adjust their placement/trace over time based on the location and local wireless environments. If an LoS link can be built by the careful selection of the hovering position or trajectory of the UAV, communication quality can be improved [4]. Fixed UAVs can also find the best location for coverage extension. If the mobility of UAVs can be fully utilized, it is easier to meet the quality of service (QoS) requirements of wireless networks by optimizing UAV trajectories dynamically, depending on user location and surroundings [5]. Therefore, many works have been conducted to improve the performance of wireless networks using UAVs. The role of UAVs in such networks can be classified into mobile relays [4; 6], aerial base stations [5; 7], and information distributors and collectors [2; 8]. On the other hand, a UAV-UAV link is more likely to have a better channel condition than a UAV-ground link, due to higher LoS probability [6]. Owing to the high flexibility and low deployment cost, UAV-enabled communications provide promising solutions to a variety of problems [3; 9]. For example, the world's leading industrial companies, such as Facebook, Google, and Qualcomm, are investing in developing services that can provide high-speed Internet in the air through UAVs [2].