## 3 Numerical Simulation  
In this section, we illustrate the numerical simulation of outage probability and throughput of the system when facing a strong turbulence condition, based on the principles and modeling proposed on the previous section. In addition, we study the effect of various system parameters on the performance. The RF link receiving side SNR \(\mathrm{P}_{\mathrm{RF}}\), transmission rate \(\mathrm{R}\), and relative symbol rate \(\mathrm{\Psi}\) were all measured in subsequent experiments or adopted as actual values. Table 1 lists the parameters we used in the simulation. Under the strong turbulence condition, the refractive index structure parameter \(C_{n}^{2}\) is in the order of \(10^{-13}\)[24]. By substituting \(C_{n}^{2}\), simulation transmission distance \(\mathrm{L}\), wavelength \(\lambda\) into \(\sigma_{R}^{2}=1.23C_{n}^{2}(2\pi/\lambda)^{7/6}L^{11/6}\), specific values for the Rytov variance \(\sigma_{R}^{2}\) can be calculated, as well as the scintillation parameters \(\alpha\) and \(\beta\). All the simulations are under the conditions that the receiving side SNR of the RF link is fixed as 41.2, which is better than the best of FSO link, and it is a value obtained through experimental measurements. The relative symbol rate of the RF link over the FSO link is 0.25, and the modulation format is QPSK, which means the transmission rate \(\mathrm{R}=2\). The RF link receiving side SNR \(P_{\mathrm{RF}}\), transmission rate \(\mathrm{R}\), and relative symbol rate \(\mathrm{\Psi}\) were all measured in subsequent experiments or adopted as actual values. It should be noted that, in the previous theoretical analysis such as in Equation (9), \(M_{FSO}\) and \(M_{RF}\) represent the number of transmissions for the \(FSO\) and RF links, respectively. If it equals 1, it means one transmission; if it equals 2, it indicates one transmission followed by one retransmission, and so on. The theoretical analysis considered retransmissions separately for the \(RF\) and \(FSO\) links to more clearly analyze the impact of various parameters on system performance. However, in the subsequent simulations and experiments, we propose two retransmission strategies: one involves retransmitting only the RF link, and the other involves retransmitting both the RF and FSO links together. In these two retransmission strategies, either only the RF link's retransmissions are considered (in this case, \(M=M_{RF}\)), or the number of retransmissions for both links is the same (in this case, \(M=M_{RF}=M_{FSO}\)). Therefore, in representing the number of transmissions, we can omit the subscripts and simply use \(M\) to denote it. Figure 6 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 5%. It is observed from Figure 6 that although the symbol rate and the transmission rate of the RF link is a quarter to the FSO link, when \(\mathrm{M}=2\) and the RF link conduct a HARQ round, the outage probability still descends significantly compared to the single FSO link. The system performance of the RF link retransmission twice is similar to that of RF and FSO link retransmission once together. In terms of throughput performance, when the SNR on the receiving end of the FSO link is small, the FSO link is required to participate in retransmission to achieve a higher throughput due to the large rate ratio to the RF link. However, when SNR increases, the RF-only retransmission strategy can achieve higher throughput due to the fact that when the RF link is conducting a HARQ round, the FSO link can transmit the next data frame. The throughput of the single RF