# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 4 Numerical Results  
In this section, we assess the performance of the UAV-IRS hybrid multi-hop FSO/THz link system utilizing the mathematical expressions formulated in Section 3. To streamline the analysis, as both FSO and THz links are relayed to subsequent nodes via identical IRS-UAV repeaters, the link pointing error parameters for FSO and THz remain consistent throughout the analysis. Table 2 shows typical system data used, including performance parameters of FSO and the THz link if not otherwise specified. New conditions are specified if they are introduced during the qualitative analysis. In addition, the numerical simulation plots below, whenever case 1 and case 2 are involved, are for the following conditions: Case 1 is moderately turbulent with small pointing errors; case 2 is strongly turbulent with large pointing errors. The outage probability of the UAV-based IRS hybrid multi-hop PSO/THz link system is shown in Figure 2. Figure 2 illustrates the case where the number of links N = 4, where \(\gamma_{th,l}^{FSO}=3\,\mathrm{dB}\), \(\gamma_{th,u}^{FSO}=7\,\mathrm{dB}\), \(\gamma_{th}^{FSO}=\gamma_{th}^{THz}=4\,\mathrm{dB}\), and the number of IRS reflective elements \(W=24\). The system's outage probability fluctuates with the average SNR under various turbulence conditions, pointing errors, and operational modes, as depicted in the figure. It is observed that, holding turbulence conditions and modes of operation constant, the system's outage probability consistently diminishes as the average SNR increases. This indicates that enhancing the link system's SNR can significantly improve the system's communication performance. At the same average SNR, for identical modes (FSO multi-hop link and THz multi-hop link), the outage probability performance closely resembles that observed under conditions of medium turbulence with weak pointing errors, as well as under strong turbulence with strong pointing errors. This observation indicates that IRS can effectively reduce the adverse effects of turbulence and pointing errors on system performance. Moreover, the outage probability when employing hard switching is superior to that observed in both FSO and THz links. Specifically, when the FSO link's communication quality is subpar and fails to meet the required performance standards,the system seamlessly transitions to the THz link to maintain communication reliability. Furthermore, under identical SNR conditions, the performance of the outage probability with soft switching surpasses that of hard switching. This suggests that soft switching not only extends the operational duration of the FSO link but also minimizes the negative consequences of frequent transitions associated with hard switching on the system's outage probability performance. In the simulation depicted in Figure 2, the system employing soft switching demonstrates superior performance regarding outage probability. Consequently, the subsequent phase involves utilizing soft switching to explore additional aspects of the system's performance. Figure 3 presents a sequence of experiments designed to assess the system's response to significant turbulence and the impact of pronounced pointing errors, where the parameters are \(\gamma_{FB,l}^{FSO}=3\,\mathrm{dB}\), \(\gamma_{th,u}^{FSO}=7\,\mathrm{dB}\), and \(\gamma_{th}^{FSO}=\gamma_{th}^{THz}=4\,\mathrm{dB}\), with the number of links set to N = 4, 6, 8, and 10. The system's performance is compared both with and without the implementation of the IRS scheme. The analysis reveals a decline in system