# A Survey on Intelligent-Reflecting-Surface-Assisted UAV Communications
## 2 Scenarios of IRS-Assisted UAV Communications  
Research on IRS-assisted UAV communications systems is being conducted from various perspectives. References [10; 16; 18; 31; 40; 47; 48; 49; 50; 51; 52; 53; 54; 55] described the implementation issues of IRS technology under various scenarios that integrate IRSs and UAVs to realize IRS-assisted UAV communications. In this section, we classify these scenarios into five representative ones according to the installation point of IRSs and the the role of UAVs. IRSs can be installed on the outer wall of a building on the ground to improve the performance of terrestrial and/or aerial communications. If no LoS link is secured in UAV communications, it is possible to build a bypassing LoS using the IRS. The IRS can also be mounted on a UAV, which allows the IRS to be intelligently and dynamically adjusted according to the movement of the UAV. This will also increase the probability of establishing an LoS link between the transmitter and receiver, thereby expanding wireless coverage. UAVs can perform various functions in communications systems. The UAV may serve as a moving BS or relay for supporting terrestrial and aerial communications. In multi-UAV environments, the UAV can also be a user that is served by a terrestrial or aerial BS.  
Figure 2 illustrates the five representative scenarios of IRS-assisted UAV communications. The scenarios are first classified according to the installation location of the IRS, and then, each class is divided into specific scenarios according to the role of the UAV. Scenarios (a)-(c) correspond to the case where IRSs are installed on the outer walls of buildings on the ground, whereas Scenarios (d)-(e) correspond to the case where IRSs are mounted on UAVs. In Scenario (a), UAVs function as aerial BSs, which will be effective when terrestrial BSs are not available (e.g., disaster areas, remote areas, and shadowed areas). The IRS installed on the wall of a building can establish a reflective LoS link between the aerial BS and users on the ground. The control of the reflection at the IRS can be carried out with a very low power consumption through phase adjustment of the signals incident to the IRS [56]. In Scenarios (b) and (d), UAVs act as moving relays for supporting communications between a BS and users (ground users or other UAVs). Both the UAV with the IRS and the UAV without the IRS can serve as a relay in these scenarios, and the IRS either attached to the wall of a building or mounted on a UAV can be used to facilitate the relaying functionality. In particular, relaying with the UAV-mounted IRS does not incur any additional noise, and thus, it is energy efficient. Furthermore, due to the nature of reflective surfaces, the connected LoS links that pass through one or more reflectors are allowed to share the same frequency band, thereby improving the spectral efficiency [48].  
In Scenarios (c) and (e), UAVs without IRSs are users served by a BS, and IRSs support communications between the BS and users. The IRS either attached to the wall of a building or mounted on a UAV helps communications between the BS and users. The UAV equipped with the IRS will be used to enhance various communication performance, such as signal amplification and coverage extension. The network configurations required for various scenarios are different, and various studies are in progress to optimize the system for each scenario. Furthermore, multiple scenarios can be combined together according to various applications of IRS-assisted UAV communications.