# A Survey on Intelligent-Reflecting-Surface-Assisted UAV Communications
## 1 Introduction
From the perspective of electromagnetic theory, the IRS can control the amplitude and phase of the electromagnetic waves that collide with a planar array, thereby changing the direction and intensity of the reflective waves. Consequently, the IRS can perform anomalous reflection/diffraction, absorption, polarization manipulation, beam focusing, beam splitting, and multi-beam collimation. From the perspective of communication technology, space modulation, encoding, and multi-stream transmission can be realized by the IRS [16; 17; 18; 19; 20; 21; 22]. In particular, the IRS can generate additional signal paths with a desired direction, amplitude, and phase with low noise amplification. Accordingly, the IRS can convert non-LoS (NLoS) channels to LoS channels and alleviate strong interference channels. As a result, the use of the IRS is expected to support highly efficient (in terms of spectrum and energy), secure, reliable, and flexible designs for future wireless networks [27; 32]. The use of the IRS will improve the performance of existing wireless communications systems by enabling a greater degree of freedom in the control of wireless channels. More specifically, the IRS can be used to increase coverage, increase the data rate, suppress interference, facilitate beamforming, and step up physical-layer security (PLS) in response to randomly fluctuating wireless channels in real-time. A real-time-adjustable IRS can be used to mitigate and eliminate the multipath and Doppler effects caused by a mobile transmitter/receiver [33]. IRS-enabled systems provide a more energy-efficient alternative than conventional multi-antenna amplification and delivery relay systems [34]. Actually, the operation of the IRS is similar to that of a half-duplex relay, in that it is based on a manual beamforming at an intermediate point of the channel [35; 36; 37]. Unlike active relays based on signal regeneration and amplification, however, the IRS is more energy-efficient and cost-efficient, because it operates passively without requiring radio frequency chains comprising amplifiers, filters, mixers, tuners, and detectors. Therefore, the IRS can primarily change the phase of the received signals, although it can also adjust the amplitude of the received signal at the cost of higher complexity. Recently, there have been many studies on IRS technology. In [38], the authors investigated the use of IRSs for SREs, wherein key applications, advantages over existing technologies, hardware architectures, and signal models for IRS-enabled systems were presented. Reference [29] discussed the implementation issues of the IRS, highlighted future research direction of the IRS and its role in 6G communications, and classified research works related to the technology. In [22], the authors presented the potential needs and applications of machine learning (ML) to IRS tasks. The authors of [17] pointed out the main problems in the design and implementation of IRS-enabled communication systems. IRS-assisted UAV communications, which incorporate IRSs into UAV communications, is expected to overcome the aforementioned limitations and problems of UAV communications and further provide energy-efficient communications to IoT networks [39]. First of all, the easy installation of an IRS and a conformal geometry allow the IRS to be installed on the front of buildings. An IRS coated on a high-rise building is likely to establish an LoS link with a UAV and with ground users owing to its high altitude and shorter distance. Accordingly, the reflective LoS links created by the IRS help signals bypass propagation obstacles between the UAV and ground users. The IRS can also be installed on indoor walls or ceilings to serve users in blocked regions [40]. Another application can deploy an IRS on an aerial platform, i.e., UAVs, to help create bypassing LoS links between the ground base station (BS) and users. Figure 1 illustrates a terrestrial communications system with UAV-mounted IRSs, which is one of the application scenarios of IRS-enabled UAV communications.