## 5 Simulation Results and Analysis  
In order to verify the bit error rate and average channel capacity performance of the FSO communication system shown in Figure 1, the atmospheric channels under different turbulence intensities were simulated, and the system simulations with different priority parameters and different channel asymmetric parameters were carried out. The simulation parameters are shown in Table 1:  
Aiming at the impact of atmospheric turbulence with different intensity on the performance of the FSO communication system, that is, in this system, the stronger the turbulence \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter Type** & **Value** \\ \hline Refractive index structure constant/(m\({}^{-2/3}\)) & \(1.2\times 10^{-14}\), \(2.8\times 10^{-14}\), \(7.2\times 10^{-14}\) \\ Laser wavelength (m) & \(1.55\times 10^{-6}\) \\ Hierarchical modulation mode & 4/16-QAM \\ Network code type & Physical layer network code \\ Communication distance of direct link & 5000 m \\ Communication distance of relay link & 3535 m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameter settings. intensity, the worse the BER performance at the user node and the total average channel capacity of the system. The source information was layer-modulated at the source node, and the received information was layer-modulated and network-encoded at the relay node to overcome the interference of atmospheric turbulence on the performance of the FSO communication system. In this study, because the transmission distance of each communication link was inconsistent, the intensity of the atmospheric turbulence channel is reflected by the atmospheric refractive index structure constant \(C_{n}^{2}\), the source information is modulated by 4/16-QAM, and the received information is encoded by the physical-layer network at the relay node. By deducing the expressions of BER and the average channel capacity of the FSO communication system, the BER performance and average channel capacity performance of the system under different turbulence intensities, different constellation priority parameters, and different channel asymmetry parameters were analyzed. In order to obtain the change of BER performance of the cooperative FSO communication system, with constellation priority parameters under different atmospheric turbulence intensity and different channel average signal-to-noise ratio, the system simulation, as shown in Figure 4, was carried out. In Figure 4, the atmospheric turbulence intensity is represented by the atmospheric refractive index structure constant \(C_{n}^{2}\). We know that the bit error rate of the cooperative FSO communication system decreases with the increase of the constellation priority parameter. Under the same signal-to-noise ratio, the atmospheric refractive index structure constant \(C_{n}^{2}\)C increased. The stronger the atmospheric turbulence intensity, the greater the impact on the BER performance of the cooperative FSO communication system, so the BER performance of the system became worse and worse. Under the same atmospheric turbulence intensity, such as the atmospheric refractive index structure constant \(C_{n}^{2}=1.2\times 10^{-14}\), the average signal-to-noise ratio of the channel increased from \(SNR=5 dB\) to \(SNR=15 dB\), and the BER performance of the system became better and better, and could drop below \(10^{-8}\). The higher the signal-to-noise ratio, the more obvious the BER performance of the system. The atmospheric refractive index structure constants \(C_{n}^{2}=2.8\times 10^{-14}\) and \(C_{n}^{2}=7.2\times 10^{-14}\) also had the same improvement effect. This shows that the layered modulation, combined with the physical-layer network coding scheme, has a better improvement effect on the high SNR channel. Figure 4: Comparison of system BER performance under different constellation priority parameters. Figure 5 shows the change of BER performance of the cooperative FSO communication system with channel asymmetry parameters.