# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 4 Numerical Results
\begin{table}
\begin{tabular}{l l l} \hline \hline  & **Parameter** & **Value** \\ \hline \multirow{7}{*}{FSO Link} & Angle of arrival & \(\psi_{AOA}=4\) mrad \\  & Wavelength & \(\lambda_{F}=1550\) nm \\  & Link length & \(L_{\mathrm{FSO},i}=250\) m \\  & Visibility & \(V=20\) km \\  & Moderate turbulence & \(C_{n}^{2}=1\times 10^{-13}\) m\({}^{-2/3}\) \\  & Strong turbulence & \(C_{n}^{2}=1\times 10^{-12}\) m\({}^{-2/3}\) \\  & Small pointing errors & \(\sigma_{(i-1),j,u}=0.25\) m, \(\sigma_{ij,\mu}=0.25\) m, \(\sigma_{(i-1),\mathrm{angle}}=0.4\) m \\  & Large pointing errors & \(\sigma_{(i-1),j,u}=0.5\) m, \(\sigma_{ij,\mu}=0.5\) m, \(\sigma_{(i-1),\mathrm{angle}}=0.8\) m \\ \hline \multirow{7}{*}{THz Link} & Frequency & \(f_{T}=100\) GHz \\  & Link length & \(d_{T}=200\) m \\  & Antenna gain & \(G_{T}^{T}=G_{T}^{T}=55\) dBi \\  & Standard atmospheric conditions & \(p=101,325\) Pa, \(T=25\)\({}^{\circ}\)C, \(\phi=50\%\) \\  & Moderate turbulence & \(\pi_{T}=17,\,\mu_{T}=1.7\) \\  & Strong turbulence & \(\pi_{T}=1.7,\,\mu_{T}=1.5\) \\  & Small pointing errors & \(\sigma_{(i-1),j,u}=0.25\) m, \(\sigma_{ij,\mu}=0.25\) m, \(\sigma_{(i-1),\mathrm{angle}}=0.4\) m \\  & Large pointing errors & \(\sigma_{(i-1),j,u}=0.5\) m, \(\sigma_{ij,\mu}=0.5\) m, \(\sigma_{(i-1),\mathrm{angle}}=0.8\) m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameter settings for FSO, THz, and RF links without special instructions. Figure 2: Variation curves of outage probability for different modes, pointing errors, and atmospheric turbulence. performance as the number of links increases. This decline is attributed to the heightened probability of an error occurring in one of the relays when employing a serial IRS-UAV as a relay, consequently elevating the system's outage probability. Moreover, the system utilizing the IRS scheme outperforms its counterpart without the IRS, notably exhibiting several orders of magnitude lower outage probability when IRS numbers = 4 and the number of links N = 10, compared to a non-IRS setup with N = 4 links. This indicates that IRS deployment can significantly mitigate the adverse effects of increasing link numbers on system performance. Thus, by strategically setting the IRS numbers and link count, it is feasible to extend communication coverage while striving for optimal outage probability performance. Based on the findings presented in Figure 4, this study examines the performance of the system's outage probability when employing a soft-switching scheme under various conditions, including different levels of turbulence intensity, pointing errors, and the number of reflecting surface elements. Where the parameters are \(\gamma_{th,l}^{FSO}=3\,\mathrm{dB}\), \(\gamma_{th,u}^{FSO}=7\,\mathrm{dB}\), and \(\gamma_{th}^{FSO}=\gamma_{th}^{THz}=4\,\mathrm{dB}\), with the number of links set to N = 4. As depicted in Figure 4, under consistent turbulence intensity and pointing error conditions, the system's outage probability performance notably enhances as the number of IRS elements increases. This improvement highlights the significant role of increasing IRS elements in bolstering the system's resilience, by facilitating additional transmission pathways and signal amplification, thereby mitigating the adverse impacts of turbulence and pointing inaccuracies. Remarkably, at a constant SNR, the system's performance under severe turbulence and pronounced pointing errors, with an IRS count of 16, parallels that observed under milder turbulence and pointing conditions with just 4 IRS elements. This parallelism underscores the potential of escalating IRS counts to counteract the detrimental effects of turbulence and pointing errors effectively. Consequently, scaling up the IRS array can markedly elevate system performance, allowing for tailored IRS configurations based on the severity of turbulence and pointing errors encountered in diverse communication contexts. This strategic adjustment is crucial for optimizing system architecture and enhancing overall communication efficacy.