# Performance Analysis of UAV-IRS Relay Multi-Hop FSO/THz Link
## 4 Numerical Results
This indicates that the use of IRS can largely improve the performance degradation due to the increase in the number of links. Figure 5: BER variation curves for different turbulence intensity, pointing error, and mode effects. Figure 6: BER variation curves under the effect of different numbers of reflective surface elements. In Figure 8, for the number of links N = 10, where the parameters are \(\gamma^{FSO}_{th,l}=3\) dB, \(\gamma^{FSO}_{th,u}=7\) dB, and \(\gamma^{FSO}_{th}=\gamma^{THz}_{th}=4\) dB and the number of reflective surface elements W = 4, this study investigates the impact of varying operational modes, pointing errors, and turbulence strengths on the system's channel capacity. Analysis of the data reveals that optimal channel capacity is achieved under conditions of moderate turbulence and minimal pointing errors, within the same operational mode. This finding suggests that the system's channel capacity is significantly higher under conditions of reduced turbulence and pointing errors. Additionally, the performance of channel capacity varies across different modes; specifically, the soft-switching scheme outperforms the hard-switching scheme, indicating that employing soft-switching and hard-switching strategies can enhance the system's channel capacity. Furthermore, the channel capacity of a standalone THz link is observed to be inferior to that in the other three modes. Because FSOs employ the visible or near-infrared wavelengths, which have higher frequencies than THz bands, and so, theoretically give a broader bandwidth, this discrepancy can be linked to [PERSON]'s theorem. This leads to a reduced terahertz channel capacity at an equivalent SNR. As SNR increases, the channel capacity of the FSO link converges with those observed during hard and soft switching. This convergence is explained by the system's increased reliance on the FSO link for communication at higher SNRs, reducing the likelihood of switching to the THz link. Consequently, with rising SNR, the channel capacity of the FSO link approaches that observed during both switching modes. According to the results in Figure 9, a soft-switching scheme is considered under the influence of different turbulence intensities, pointing errors, and the number of reflective surface elements. Where the parameters are \(\gamma^{FSO}_{th,l}=3\) dB, \(\gamma^{FSO}_{th,u}=7\) dB, and \(\gamma^{FSO}_{th}=\gamma^{THz}_{th}=4\) dB, with the number of links set to N = 10. The analysis of the figure reveals that, holding the degree of turbulence and pointing errors constant, the system's channel capacity progressively increases with the addition of IRS. This finding underscores the significant role that increasing the number of IRS elements plays in enhancing the system's channel capacity. Notably, under conditions of strong turbulence and pronounced pointing errors, the system's performance with sixteen IRS elements surpasses that under mild turbulence and slight pointing errors with only eight IRS elements. Thus, it is evident that increasing the IRS count not only boosts the system's channel capacity but also mitigates the adverse effects of turbulence and pointing errors on system performance. Consequently, selecting the optimal number of IRS elements based on the specific requirements of the communication scenario can optimize the IRS's utilization efficiency. Figure 7: BER variation curves for different numbers of links. In the context of strong turbulence and significant pointing errors, where the parameters are \(\gamma_{th,J}^{FSO}=3\,\mathrm{dB}\), \(\gamma_{th,\mu}^{FSO}=7\,\mathrm{dB}\), and \(\gamma_{th}^{FSO}=\gamma_{th}^{THz}=4\,\mathrm{dB}\), we examine the system's channel capacity with varying numbers of links (N = 5, 10, 15, 20). This analysis explores the capacity implications of employing IRS versus traditional configurations without IRS, across different link counts. Figure 10 illustrates that the system's channel capacity decreases as the number of links increases.