# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 4 Numerical Results
Figure 3: Variation curve of outage probability for different numbers of links in the scheme with soft switching. The analysis presented in Figure 5 investigates the scenario where the number of links (N) is eight and the number of reflective surface elements (W) is four, where the parameters are \(\gamma_{th,l}^{FSO}=3\,\mathrm{dB}\), \(\gamma_{th,u}^{FSO}=7\,\mathrm{dB}\), and \(\gamma_{th}^{FSO}=\gamma_{th}^{THz}=4\,\mathrm{dB}\). The impact of various modes, pointing errors, and turbulence strengths on the BER is analyzed. Figure 5 reveals that the system's BER performance is superior under conditions of moderate turbulence and minimal pointing errors compared to scenarios involving strong turbulence and significant pointing errors within the same mode. This observation underscores the detrimental effects of turbulence and pointing errors on system efficiency. Nonetheless, with an increase in the SNR, the BER performance under both moderate and strong turbulence and pointing errors converges, particularly when employing FSO multi-hop links and THz multi-hop links. This convergence suggests that the adverse impacts of turbulence and pointing errors can be markedly mitigated through the use of IRS. Furthermore, Figure 5 also highlights the performance disparities across different modes, with the soft-switching scheme yielding the best BER performance, followed by the hard-switching scheme. This indicates that the adoption of soft- and hard-switching schemes can significantly enhance the system's BER performance, thereby improving the quality of communication transmission. The findings presented in Figure 6 explore the impact of varying turbulence intensities, pointing errors, and the quantity of reflective surface elements on the system's BER within a soft-switching scheme context. Here, \"IRS-Number\" refers to the count of reflective surface elements. The parameters are \(\gamma_{th,l}^{FSO}=3\,\mathrm{dB}\), \(\gamma_{th,u}^{FSO}=7\,\mathrm{dB}\), and \(\gamma_{th}^{FSO}=\gamma_{th}^{THz}=4\,\mathrm{dB}\), with the number of links set to N = 4. The figure demonstrates that the system's BER performance progressively enhances with a higher count of IRS elements, maintaining constant turbulence strength and pointing error levels. This improvement suggests that increasing the IRS element count can markedly boost system performance, particularly under conditions of strong turbulence and significant pointing errors. Notably, a configuration with 16 IRS elements outperforms one with 8 IRS elements in scenarios of both moderate turbulence and minimal pointing errors. Consequently, an increase in IRS elements can effectively mitigate the adverse impacts of turbulence and pointing errors on system performance. In environments where turbulence and pointing errors are prevalent, adjusting the number of reflective surface elements upwards can optimize system performance. Elevating the IRS count serves to diminish the detrimental effects of turbulence and pointing errors, thereby enhancing the system's BER performance. Figure 4: Variation curves of the outage probability for different numbers of reflective surface elements. In Figure 7, under the action of strong turbulence and strong pointing errors, we set the number of links to be N = 4, 6, 8, and 10, and analyzed the effect on the average BER of the system with or without the use of an IRS and with different numbers of links. From the figure, we can observe that as the number of links increases, the performance of the system becomes progressively worse, this is because when serial relaying is used, the increase in the number of relays increases the probability of error in a particular relay, which leads to a decrease in the performance of the system. In addition, we can observe that the performance of the system with the IRS scheme is better than the performance of the system without the IRS scheme; especially, when IRS number = 4 and the number of links N = 10, the BER performance is orders of magnitude lower than that without IRS and the number of links N = 4.