# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 5 Conclusions  
To enhance rapid deployment of post-disaster communication networks, extend coverage, and address the challenges of future 6G communications and information warfare, this study introduces an innovative hybrid multi-hop FSO/THz link system for UAVs. The FSO and THz links are modeled following the gamma-gamma and respective distributions, providing a robust framework for analyzing system performance under varying conditions. This research incorporates the impact of atmospheric turbulence, pointing errors, and AOA fluctuations on both FSO and THz links, which are critical factors in real-world applications. Utilizing the Meijer-G function and the complementary error function (erfc), we have derived closed-form expressions for the outage probability, BER, and average channel capacity of FSO multi-hop links, THz multi-hop links, and multi-hop links employing both hard- and soft-switching strategies. These mathematical formulations allow for a comprehensive evaluation of the system's performance and facilitate a detailed analysis of the strategic differences in link management. Our findings underscore the effectiveness of both hard- and soft-switching strategies in enhancing the performance of the UAV-based hybrid multi-hop FSO/THz link system. Notably, soft switching provides a marginal gain over hard switching by reducing the necessity for frequent transitions between links, thereby optimizing system efficiency and reducing potential disruptions in communication continuity. The integration of IRS significantly mitigates the adverse effects of atmospheric conditions and alignment errors. Importantly, system performance escalates with the increase in the number of IRS reflective elements, demonstrating the scalability and adaptability of this approach. Employing UAVs as relays transcends the limitations of traditional relay systems and notably enhances system performance, particularly when IRS technology is integrated onto the UAVs. This configuration not only stabilizes the communication links but also extends their operational range and reliability. The strategic use of THz links as a backup for wireless optical communications further bolsters the system robustness, ensuring stable connectivity even under challenging conditions. Numerical analysis confirms the superiority of the proposed hybrid link system over conventional systems, highlighting its potential in revolutionizing UAV communication frameworks. This research not only provides valuable theoretical insights for the optimal design of UAV communication systems but also pioneers a novel approach for advancing next-generation communications. Looking forward, these insights lay a solid foundation for future experimental validations and practical deployments, promising to significantly  
Figure 10: Variation curve of channel capacity at different numbers of links.  
impact the development of resilient and efficient communication infrastructures in both civilian and military contexts.  
Conceptualization, methodology, Y.W. and H.L.; software, validation, writing--original draft preparation, Y.W.; formal analysis, investigation, data curation, R.L., R.W. and Z.W. ([PERSON]); writing--review and editing, visualization, J.Y., Z.W. ([PERSON]) and J.L.; supervision, project administration, funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Nos. 62175070, 62305116 and 92365109), the Guangdong Basic and Applied Basic Research Foundation (Nos. 2022A15151 10752, 2023A1515011385), and the Science and Technology Program of Guangzhou (Nos. 202201010340 and 2019050001).  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflicts of interest.