# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 1 Introduction
Meanwhile, reference [16] provides a detailed analysis of the cumulative distribution function (CDF) and the probability density function (PDF) for a multi-hop UAV FSO communication system model, also under the influence of atmospheric turbulence, pointing errors, and AOA fluctuations. They derive explicit expressions for both the link outage probability and the end-to-end outage probability. Furthermore, they optimize the system's performance by adjusting parameters such as beamwidth, field of view, and the UAV's positioning. Reference [17] investigates a dual-hop hovering UAV FSO communication system that leverages decode-and-forward (DF) relaying, which takes into consideration numerous aspects that can affect the system's performance, such as atmospheric route loss, atmospheric turbulence, aiming mistakes, and AOA fluctuations. Although the studies mentioned above describe the progress made in using UAVs as mobile relays to overcome some of the limitations of conventional ground-based relays, they do not address the ability to achieve 360\({}^{\circ}\) panoramic coverage with full-angle reflections or ensure the attainment of the maximum signal-to-noise ratio (SNR) [18]. The emergence of intelligent reflecting surface (IRS) technology has laid a strong foundation for the development of advanced, intelligent, and controllable wireless communication systems that are expected to be part of future 6G networks [19]. IRS is composed of an array of electromagnetic material with numerous reflective elements, each capable of directionally reflecting incoming waves without necessitating an external power supply. Optimizing the number of reflective elements on an IRS, as well as adjusting its emission coefficients, makes it a cost-effective and easily deployable solution for enhanced communication systems. Reference [20] delved into the intricacies of an IRS-assisted hybrid RF/THz system, which utilizes a fixed-gain amplification forwarding protocol. They derived the CDF for the end-to-end channel gain of the hybrid RF/THz link, as well as the closed-form expressions for the PDF. Utilizing these statistical characteristics, they calculated the closed-form expressions for key performance metrics, including the outage probability, the average bit error rate (BER), and the average channel capacity. Additionally, they conducted asymptotic analyses to evaluate the system's performance in terms of outage probability and average BER under extreme operating conditions. Reference [21] explores IRS-assisted RF and wireless optical communication systems, as well as the performance of wireless optical communication systems using the gamma-gamma distribution and the generalized Magara distribution. Reference [22] proposed a theoretical framework for cascaded IRS composite turbulence and pointing errors, provided a PDF and CDF of cascaded composite turbulence and pointing errors, from which new closure formulas were derived, and investigated cascaded multi-IRS FSO and THz wireless systems. The insights gleaned from the aforementioned studies have spurred researchers to explore the integration of IRS with UAVs and IoT infrastructure to enhance overall system performance. Reference [23] studied a UAV-based IRS in a hybrid FSO/RF dual-hop communication system, with the FSO acting as a secondary link to the RF, which considered the effect of IRS reflective elements on phase shift errors and provided closed-form formulae for the average symbol BER and spectral efficiency of the system. Reference [24] explores the deployment of a legitimate UAV as a relay, alongside an IRS, to enhance the reach of free-space optical communication systems. Reference [25] develops a model for an IRS-assisted three-hop communication system, integrating RF and FSO for the initial two hops and underwater wireless optical communication (UWOC) for the final hop, formulating closed-form expressions for system performance metrics. Reference [26] investigates a hybrid backhaul network that incorporates FSO and THz technologies, augmented by reconfigurable IRS.