# A Survey on Intelligent-Reflecting-Surface-Assisted UAV Communications
## 1 Introduction
Furthermore, UAVs can be integrated with various technologies in wireless networks [10]. Recently, advanced technologies, such as non-orthogonal multiple access (NOMA), millimeter-wave (mmWave), and terahertz (THz), have been applied to UAV-enabled communications under various scenarios. The basic idea of the NOMA technique is to allocate non-orthogonal resources among the users at the cost of increased receiver complexity, which is required for separating the non-orthogonal signals [11; 12; 13]. We can support massive devices more over the UAV by the UAV-enabled communications scenarios adopting NOMA. More devices can be served by adopting NOMA in UAV-enabled communications systems [14]. Both the mmWave and THz bands provide a wide bandwidth to support a high data rate in UAV-enabled communications [15]. However, UAV systems typically have strict size, weight, and power constraints [6]. These constraints affect the flight time, durability, and communication performance. In particular, UAVs consume propulsion energy to support mobility, which is typically several times higher than the communication energy, in addition to transceiver power consumption. A UAV at high altitude typically forms LoS links to ground nodes; however, UAV-ground channels are occasionally blocked by trees and skyscrapers in urban areas, which may degrade the communication performance. Due to size and power limitations, UAVs have also faced limitations in adopting advanced technologies that can support high data rates and an ever-growing number of devices [9]. Those problems can be alleviated if wireless channels become more reliable and less power-consuming. This necessitates a new communication paradigm, that is controllable or reconfigurable wireless environments. Mobile communications systems are usually designed and optimized in the transmitter and receiver, which constitutes the termination nodes of a communication link. The primary goal of the system design is to compensate for the wireless channel between the transmitter and receiver, on the premise that artificial control of the wireless channel is impossible. If we are able to control the wireless channel, however, we can expect a significant performance improvement in the communications system. Recently, numerous studies have been conducted to optimize the wireless channel, as well as the transmitter and receiver. Wireless environments created using this approach are called smart radio environments (SREs) or intelligent radio environments to emphasize the fundamental differences from historical designs and optimization criteria [16; 17; 18; 19; 20; 21; 22; 23]. Intelligent reflecting surfaces (IRSs) [24], also known as reconfigurable metasurfaces [22], smart large intelligent surfaces [23], large intelligent surfaces [25; 26; 27], or reconfigurable intelligent surfaces [28], are accepted as a promising technology that realizes SREs. In this study, the term \"IRS\" will be used as a representative of these terminologies. The IRS has recently attracted attention as a core technology for next-generation wireless communications [29; 30], as it can control the wireless propagation environment. The IRS can be viewed as a continuous surface of reflective elements with successively induce phase shifts and individual reflection coefficients. The reflective elements can be controlled using mechanical operations, special materials (e.g., graphene), and electronic devices (e.g., positive-intrinsic-negative diodes) [31]. The capability of the IRS that reflects and controls incident signals enables shaping the wireless propagation environment. This contrasts with the traditional approach that mitigates the harmful effects of the wireless channel at the transmitter and receiver sides instead of changing it. The IRS uses passive elements and serves as an artificial two-dimensional metasurface. It does not require analog-digital converters and power amplifiers.