## 7 Simulation Results  
In this section, MATLAB was used for the simulations to validate the analysis and assess the performance of the cooperative relay-assisted THz-VLC link in terms of BER and outage probability. All the parameters used for the simulation are listed in Table 1, they were selected based on credible literature sources. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Number of bits & \(1\times 10^{5}\) \\ \hline Distance [\(\mathrm{THz}\) link] & 30 m \\ \hline Frequency [\(\mathrm{THz}\) link] & 0.3 \(\mathrm{THz}\) \\ \hline Radius of the reception antenna & 0.1 m \\ \hline Transmission beam footprint radius & 0.6 m \\ \hline Jitter variance, \(\sigma_{r}\) & 0.05–0.2 m\({}^{2}\) \\ \hline Distance [VLC link] & 5 m \\ \hline Photodiode area & 0.05 \\ \hline Source power, \(P_{s}\) & 1 \(\mathrm{dB}\) \\ \hline Relay power, \(P_{r}\) & 1 \(\mathrm{dB}\) \\ \hline \(\theta\) & 53.13\({}^{0}\) \\ \hline \(\psi_{c}\) & 75\({}^{0}\) \\ \hline \(\psi\) & 53.13\({}^{0}\) \\ \hline Refractive index of a lens & 1.5 \\ \hline Distance [\(\mathrm{FSO}\) link] & 30 m \\ \hline \(\alpha\) [\(\mathrm{FSO}\) link] & 4.1 \\ \hline \(\beta\) [\(\mathrm{FSO}\) link] & 1.4 \\ \hline Distance [\(\mathrm{RF}\) link] & 30 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. Figure 8 illustrates the influence of pointing errors on the BER performance of the proposed THz-VLC scheme. An increase in BER corresponding to heightened pointing errors highlights the system's sensitivity to alignment inaccuracies. The system demonstrates optimal performance under ideal conditions without pointing errors, underscoring the critical need for precise alignment. Additionally, the close match between the simulated BER and theoretical BER in the absence of pointing errors validates the theoretical framework presented in Lemma 2. This correlation confirms the robustness and accuracy of the theoretical model in predicting system performance under ideal conditions. Theoretical and simulation models from the literature [5] confirm that lower jitter variance is necessary for a longer transmission distance. Considering this and the values provided in Table 1, for the remainder of the simulations in this study, a jitter variance of 0.05 m\({}^{2}\) is utilized. In Figure 9, the BER performance is compared across three different communication paradigms: RF-VLC, FSO-VLC, and THz-VLC. The first link from the source to the relay (using RF, THz, or FSO) employs BPSK modulation with a link distance of 30 m, while the subsequent VLC link uses OOK with a link distance of 5 m. This figure illustrates that RF-VLC and FSO-VLC outperform THz-VLC under lower SNR conditions. This is mainly due to the shorter wavelengths of THz signals compared to RF and optical signals, making them more susceptible to absorption by atmospheric gases and attenuation from obstacles, resulting in higher signal loss and degradation at lower SNR levels. However, as the SNR increases, THz-VLC exhibits superior performance, achieving a BER of \(10^{-5}\) at a significantly lower SNR compared to FSO-VLC and RF-VLC. The disadvantages of the THz link, such as higher absorption and attenuation, diminish with increasing SNR. Additionally, THz signals offer higher bandwidth and data rates compared to RF and optical signals, allowing THz-VLC to leverage this advantage more effectively with higher SNR, leading to improved BER performance. Figure 8: Comparison of BER performance for varying pointing errors in THz–VLC link. Furthermore, in Figure 9, the simulated BER for the THz-VLC link aligns closely with the theoretical BER, providing further validation for Lemma 1. An assessment was conducted by varying the distances for the second hop over VLC while maintaining a constant distance for the first hop (THz, RF, PSO). The results are illustrated in Figure 10.  
As the relay-destination distance increases, the BER correspondingly rises due to higher attenuation and other losses over longer distances.