# Performance of Adaptive Bit-Interleaved Polar Coded Modulation in FSOC System
## 3 Performance Simulation Results and Discussion of A-Bipcm under Different Turbulence Conditions  
Based on the method mentioned above, this paper conducts simulation analysis on key parameters such as turbulence intensity, decoding width, and before and after adaptive interleaving to evaluate the bit error performance of A-BipCM under atmospheric turbulence. To achieve this goal, this article introduces CRC into the traditional SCL to improve decoding performance and uses CRC check to verify the accuracy of the decoding results, which has proven potential in short block length delay-constrained systems. In order to provide a comprehensive performance evaluation and take into account the application requirements in the actual system, during the simulation process, the following parameters were set: code length N = 2048, code rate R = 0.5, wavelength \(\lambda=1550.116\) nm, CRC check code length is 8, and the number of iterations is set to \(10^{6}\). In addition, simulation analysis was also conducted on different list sizes, which were, respectively, set to L = 1, L = 4, and L = 8. In this work, a comparison is made between the proposed A-BipCM scheme and the conventional polar code encoding and decoding. The comparison is based on simulations that analyze the change in BER with SNR under weak turbulence, medium turbulence, and strong turbulence conditions (as shown in Figure 2a-c). The Rytov variances under the three turbulence conditions are 0.2, 1.6, and 3.5, respectively. The A-BipCM scheme is discussed, particularly focusing on the differences observed under different code rates and the list size. To investigate the performance gap between polar codes and the Shannon limit in turbulent channels from an information theory perspective, the Shannon limit curve is also plotted for each turbulent condition, considering a code rate of 0.5. The simulation results indicate that at the BER = 3.8 \(\times\)\(10^{-3}\), code rate R\({}_{2}\) = 0.75, the proposed A-BipCM scheme achieves an improvement compared to conventional polar code encoding and decoding. Specifically, it achieves an improvement of 0.94 dB under weak turbulence conditions, a  
BIPCM scheme is increased from 1 to 4 and 8, additional improvements are observed. In weak turbulence, there is a 1.63 dB improvement and a 2.18 dB improvement. In medium turbulence, improvements of 2.37 dB and 3.22 dB are achieved, and in strong turbulence, improvements of 1.913 dB and 2.75 dB are achieved. The simulation results demonstrate that as the turbulence intensity increases, there is a decrease in bit error performance, reflecting the adverse impact of atmospheric turbulence on channel quality. Additionally, under the same turbulent conditions, as the SNR increases, the BER performance improves. Experimental Verification Results and Discussion of FSOC System Performance under Weak Turbulence Conditions  
To assess the actual performance of the A-BIPCM scheme in combating atmospheric turbulence, this work conducted an experimental verification study on the performance of the FSOC system based on the A-BIPCM scheme. The simulation channel had dimensions of 2 m \(\times\) 1 m \(\times\) 0.35 m and was designed to simulate a weak turbulence channel. In this experimental setup, the simulation channel utilized the air between hot and cold plates to create an environment that mimics real atmospheric channels. Convection was employed to replicate the turbulent characteristics of the atmosphere. The intensity of the simulated turbulence was adjusted by controlling the temperature difference between the hot and cold plates, which closely resembles the characteristics of actual channels affected by atmospheric turbulence. The atmospheric coherence length \(r_{0}=A_{turbulence}\Delta T^{\Delta}\), an important parameter in characterizing the turbulence effects, \(A_{turbulence}\) was measured and determined based on the fitted curve. The linear coefficient is the exponential coefficient of the temperature \(\Delta T\) difference between the heating plate and the cooling plate.