# Research on Polarization Coding Cooperative Communication Scheme for FSO System
## 5 Simulation Results and Analysis  
Through system simulation, this section will focus on the bit-error rate and outage probability of the FSO cooperative communication system under polarization coding. Under the proposed system scheme, the bit-error rate and outage probability of the system are simulated under the same communication distance, different turbulence intensity, and different communication distance under the same refractive index structure constant. The basic parameter settings of the simulation are shown in Table 1. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter Type** & **Value** \\ \hline Polarization code rate & 0.5 \\ Code length & 512, 1024, 2048, 4096 \\ Decoding algorithm & FastSCL \\ Relay cooperation strategy & Decode-and-Forward \\ Modulation mode & BPSK \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic parameter settings of polarization coding cooperative FSO communication system simulation. Under polarization coding, the BER performance of the FSO cooperative communication system is simulated for turbulent channels with different intensities. The polarization code length is set to 1024 and the code rate is 0.5. The decoding method uses fast serial offset list decoding (FastSCL) with CRC check. The cooperative communication mode uses the decoding and forwarding (DF) mode at the relay node and sets the bit-error rate threshold to \(10^{-5}\). The simulation results are shown in Figure 3. The solid line indicates that the cooperative communication mode is used, and the dotted line indicates that there is no cooperation (that is, there is a direct link between the source node and the destination node). The simulation results show that when the turbulence intensity is 0.3 and the system bit-error rate is \(10^{-3}\), the performance of the FSO cooperative communication system is about 0.5 dB higher than that of the direct link. Once the signal-to-noise ratio is greater than 13 dB, it shows better gain performance. When the turbulence intensity is 0.7 and the system bit-error rate is \(10^{-3}\), the performance of the FSO cooperative communication system is about 1 dB higher than that of the direct link, and the higher the signal-to-noise ratio is, the more obvious the gain is. When the turbulence intensity is 1.0 and the system bit-error rate is \(10^{-2}\), the performance of the FSO cooperative communication system is about 1.5 dB higher than that of the direct link,  
can be seen that for the FSO communication system with a longer communication distance and stronger atmospheric turbulence intensity, the use of a cooperative communication mode can effectively improve the performance of the communication system. The impact of polarization codes of systems with different code lengths under the same atmospheric turbulence intensity on the bit-error rate of the FSO cooperative communication system is shown in Figure 5. It can be seen from the figure that the shorter the polarization code length of the system, the worse the performance of the FSO cooperative communication system. When the signal-to-noise ratio of the system is lower than 8 dB, the polarization code length has little effect on the bit-error rate of the system. After 8 dB, the system polarization codes with a code length of 1024 and 2048 have similar improvements in the system bit-error rate performance. When the system bit-error rate is \(10^{-4}\), the system polarization code with a code length of 1024 is about 0.7 dB higher than the system polarization code with a code length of 512, and the system polarization code with a code length of 4096 is about 0.5 dB higher. In the FSO cooperative communication system, decoding will be carried out at both the relay node \(R\) and destination node \(D\). Multiple decoding will increase the bit-error rate of the system. Therefore, the system polarization code length is too long, which reduces the improvement of system bit-error rate performance. Figure 4: BER of cooperative FSO communication system and direct link under different communication distances.