## 3 Numerical Simulation
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Symbol** & **Value** \\ \hline Refractive index structure parameter & \(C_{n}^{2}\) & \(10^{-13}\mathrm{m}^{-2/3}\) \\ Simulation transmission distance & \(\mathrm{L}\) & 2800 m \\ Wavelength & \(\lambda\) & 1550 nm \\ Scintillation parameter & \(\alpha\) & 10.31 \\ Scintillation parameter & \(\beta\) & 37.38 \\ Rytov variance & \(\sigma_{R}^{2}\) & 13.15 \\ RF link receiving side SNR & \(P_{RF}\) & 41.2 dB \\ Transmission rate & \(\mathrm{R}\) & 2 \(\mathrm{p}\mathrm{b}\mathrm{s}/\mathrm{Hz}\) \\ Relative symbol rate & \(\mathrm{\Psi}\) & 0.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. link HARQ strategy outperformed the RF/FSO hybrid HARQ strategy in the large SNR condition, as is shown in Figure 6b. Figure 7 illustrates the outage probability and throughput versus the receiving side SNR of the FSO link at different HARQ rounds and HARQ strategies when the FEC overhead equals 15%. It is observed that the performance of the system when the RF link retransmitted twice was much better than the system performance of the RF/FSO hybrid retransmitting once together compared to the aforementioned OH = 5%. And the outage probability became zero when RF retransmitted three times. The larger the OH, the greater the gain brought by RF retransmission. However, in terms of the throughput, because of the larger proportion of OH occupying the transmitted data frame, the total net amount of information became less and the total throughput decreased compared to the OH = 5% condition. Figure 6: The outage probability and throughput versus the receiving side SNR of the FSO link at different HARQ rounds and HARQ strategies when the FEC overhead equals 5%: (**a**) outage probability; (**b**) throughput. Figure 7: The outage probability and throughput versus the receiving side SNR of the FSO link at different HARQ rounds and HARQ strategies when the FEC overhead equals 15%: (**a**) outage probability; (**b**) throughput. Figure 8(a) depicts the outage probability versus the receiving side SNR of the FSO link under different FEC OH with RF-only retransmission. Figure 8(b) depicts the throughput versus the receiving side SNR of the FSO link under the same circumstances. The overhead of FEC affects the total amount of net information transferred. When the length of the data frame to be transmitted is fixed, the change in the net information transmitted due to a different FEC overhead can be equivalent to a change in the modulation order and thus to a change in the transmission rate R. From Figure 8(a), it is observed that the higher the FEC OH cost, the lower the transmission rate, the lower the interrupt probability, and therefore system performance is improved. Thus, it can be concluded that when the total number of bits transmitted is constant, the gain of FEC to the system transmission is brought by the coding gain on the one hand and the decrease in equivalent modulation order on the other. As for the throughput, it is shown in Figure 8(b) that the higher the FEC OH cost, the lower the throughput. This was due to the reduction of net information transmitted when the proportion of OH was higher.