# Performance of Adaptive Bit-Interleaved Polar Coded Modulation in FSOC System
## 3 Performance Simulation Results and Discussion of A-Bipcm under Different Turbulence Conditions
However, due to limitations in generating turbulence conditions in our laboratory, we can only simulate weak turbulence conditions using the atmospheric turbulence simulation channel. Figure 2: FSOC system BER performance curve under different turbulence conditions: (**a**) BER performance curve under weak turbulence conditions, (**b**) BER performance curve under medium turbulence conditions, (**c**) BER performance curve under strong turbulence conditions. Figure 3 presents a schematic block diagram of the experimental system used to verify the performance of free-space wireless laser communication employing IM/DD. At the transmitting end of the system, a C-band continuous wave laser with a wavelength of 1550.116 nm and an output power of 10 dBm is utilized. Following polarization controller (PC) adjustment, the output power registers at 7.38 dBm. The polar code encoding modulation signal is generated offline using MATLAB and then loaded into an arbitrary waveform generator (AWG). The AWG has a 3 dB analog bandwidth of 25 GHz and a sampling rate of 32 GSa/s. It is responsible for generating the transmitted RF signal and performs 8 times interpolation, achieving an oversampling rate of 4 GS/s. In the experiment, a microwave amplifier (MA) is employed to amplify the amplitude of the output radio frequency signal from 237 mV to 4.5 V, which is necessary to drive a 10 GHz Mach-Zehnder modulator (MZM). The bias voltage of the MZM is set to 4 V. The optical signal output from the PC is then input into the MZM, and the amplified signal source from the microwave amplifier is added to it. Subsequently, the modulation output of the MZM is directed to the fiber collimator (FC). At the receiving end of the system, the FC is used to couple the received beam into an optical fiber. To control the optical power, a variable optical attenuator (VOA) is employed. The VOA enables real-time adjustment of the optical power of the received signal through attenuation. An avalanche photodiode detector (APD) is utilized for receiving the optical signals. The power input voltage of the detector is set to 12 V, and it has a saturated optical power of \(-\)16 dBm and the lowest detectable optical power of \(-\)27 dBm. For signal capture and analysis, a real-time digital storage oscilloscope (DSO) with a sampling rate of 20 GSa/s and a bandwidth of 4 GHz is utilized. The DSO captures the signal during the offline process, allowing for subsequent signal recovery and analysis. Offline digital signal processing (DSP) is performed using MATLAB to recover the transmitted signal. During the offline processing, the start and end positions of the transmission sequence are determined based on the flag bits added to the transmission sequence. The signal is then sampled, and appropriate thresholds are set to identify continuous sampling points as high and low levels of the binary sequence. The binary sequence is deinterleaved and sent to the decoder for decoding operations. The original bit sequence is restored using the Successive Cancellation List (SCL) decoding method combined with CRC. Figure 4 illustrates the experimental system diagram. Figure 5 depicts the random fluctuation of the time-domain optical signal data over a certain period. In Figure 6, the three conditions represented are as follows: (a) Under certain conditions, where the temperature of the upper and lower plates of the atmospheric turbulence simulation channel is 80 \({}^{\circ}\)C and the atmospheric coherence length is r\({}_{0}\) = 1.839 at a specific value. In this case, the optical signal experiences minimal jitter in the channel, and the energy of the signal is relatively concentrated and stable. (b) Under different conditions, with the temperature of the upper and lower plates set to 120 \({}^{\circ}\)C and the corresponding atmospheric coherence length r\({}_{0}\) = 1.384, a slightly stronger fluctuation in the time-domain waveform of the optical signal and a slightly wider dispersion of its energy is indicated.