## 4 Simulation and Discussion
When the scintillation index is fixed, the average BER of the hybrid system is significantly lower than that of the FSO system; as the scintillation index increases, the intensity of the scintillation ascends, and the BER of the two systems increases accordingly. In addition, when \(m^{2}<0.3\), the variation range of BER caused by the scintillation index is less than that when \(m^{2}>0.3\). For example, in the RF/FSO systems, when the SNR is 28 dB, the difference of the BER is between \(m^{2}=0.1\) and 0.3 is \(5.5\times 10^{-5}\), and between \(m^{2}=0.5\) and 0.3 is \(2.25\times 10^{-4}\). It can be seen that the impact of increasing scintillation on BER performance may become more intense. [PERSON] theory is only aimed at the weak scintillation region, and the feature of the strong scintillation area needs to be studied in further research. Figure 8 illustrates the effect of the modulation technology on the average BER of the system and here we set the scintillation index as 0.3. Figure 8a shows the average BER of the hybrid RF/FSO system under three modulation modes. When \(L>8\), the average BER  
Figure 6: Outage probability against average signal-to-noise ratio (SNR) per hop for various scintillation intensities and different pointing error parameters. The threshold of the hybrid system is \(\gamma_{th}=10\) dB. Figure 7: Average BER of 16-PPM–BPSK–SIM RF/FSO system and the FSO system against the average SNR under various scintillation indices and the pointing error parameter \(\xi=1\). of the hybrid modulation is lower than that of BPSK, and the average BER decrease as \(L\) increases. The average BER performance of the PPM modulation in the hybrid system is still the best among these three modulation methods. However, considering other factors, such as the power efficiency of the \(L\)-PPM, the hybrid modulation is also worth studying. Figure 8b compares the BER performance of the hybrid RF/FSO system (S-R-D) and pure FSO system (S-D) for \(L\)-PPM-BPSK-SIM modulation. The average BER of the hybrid system is significantly reduced, and when the average SNR reaches 20 dB, the average BER of the hybrid system at \(L=8\) is even lower than that of the FSO system at \(L=64\). Combining Figure 8a,b, considering that an excessively large value of \(L\) may reduce the communication rate, setting \(L\) at 16 will be a decent option. Figure 9 reveals the average BER performance of the hybrid RF/FSO system for optical wave propagation through coronal turbulence under various parameters. As shown in Figure 9a,b, the increasement of the turbulent outer scale and spectral index will lead to the reducement the average BER of the system. Moreover, when the values of \(l_{o}\) and \(p\) are larger, their influence on the BER will attenuate, because the amplitude fluctuation variance is smaller. However, it is worth noting that compared with an FSO system in some other research studies [41], when the turbulence outer scale and spectral index increase, the BER of the system will change significantly. Consequently, hybrid systems and hybrid modulation will enhance the stability of the BER performance. Considering the effect of the wavelength on the scintillation index in Figure 3, the average BER versus wavelength is further studied in Figure 9c. As expected, the reduction of the wavelength improves the BER performance, so that the communication achieves better BER even with a low SNR. Therefore, decreasing the wavelength or increasing the carrier frequency are alternative methods to eliminate the coronal turbulence. As shown in Figure 9d, the relative solar wind density fluctuation factor also has an impact on the BER. As \(\eta\) increases, the average BER of the system increases, correspondingly. Usually, we set \(\eta\) within 0.2 because excessively large values of \(\eta\) may contribute to an abnormal increase of the radio Sun. Here, to investigate the effect of \(\eta\) on the BER more intuitively, we set the maximum value of \(\eta\) at 0.4.