# Performance of Adaptive Bit-Interleaved Polar Coded Modulation in FSOC System  
[PERSON]  
1 School of Electronics and Information Engineering, Changchun University of Science and Technology, Changchun 130022, China;[EMAIL_ADDRESS] (Q.J. ); [EMAIL_ADDRESS] (S.L)  
[PERSON]  
2 National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun 130022, China 23 Institute of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun 130022, China 34 School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China; [EMAIL_ADDRESS] 4  
[PERSON]  
4 School of Optoelectronics Engineering, Changchun University of Science and Technology, Changchun 130022, China; [EMAIL_ADDRESS]. order to further improve the performance of the system, bit-interleaved coded modulation (BICM) is introduced into FSOC as a communication mode. Initially proposed by [PERSON], BICM is a scheme that combines error correction coding, interleaving, and modulation in a serial concatenation manner [3]. BICM offers lower computational complexity and improved system reliability in the channel [4]. Theoretical analysis has proven that polar codes can achieve the Shannon limit with linear encoding and decoding complexity, presenting new possibilities in the field of channel coding [5]. In 2016, [PERSON] proposed a polar coding scheme integrated with BICM for reliable communication over multiple channels. Although this scheme demonstrated the same error exponent as Arikan's polar codes and achieved multi-channel capacity, it lacks experimental validation [6]. In 2018, [PERSON] introduced error correction techniques to integrated polar codes, presenting a high-performance scheme known as bit-interleaved polar coded modulation with iterative decoding (BIPCM-ID). BIPCM-ID further improved performance by increasing the number of iterations and achieving higher diversity between transmitted bits. However, its application in fading channels remains unexplored [7]. In 2020, [PERSON] et al. established a comprehensive framework analyzing the theoretical performance of polar codes in block fading channels. Through the analysis of the polar spectrum, they revealed the explicit relationship between diversity order and codeword weight, yet this theoretical framework has not been applied to BIPCM systems [8]. In 2022, [PERSON] et al. proposed an interleaving method that achieves full diversity at a rate of 0.5 in the presence of two fading blocks. This indicates the potential of BICM techniques to optimize communication links in fading channels [9]. Also in 2022, [PERSON]'s team introduced the multi-aperture transmission and BICM techniques into FSOC, aiming to suppress atmospheric turbulence. By leveraging a random interleaver, they improved the minimum Euclidean distance between codewords, enhancing the link's resistance to fading. This research suggests that further exploration of advanced BICM techniques could strengthen the suppression capability against atmospheric turbulence and improve the environmental conditions of space links [10]. Traditional polar coding methods have limitations in effectively solving the problem of a long string of bit errors. We introduce a scheme of adaptive bit-interleaved polar coded modulation (A-BIPCM) to address the issue of a long string of bit errors caused by turbulence in FSOC systems. The A-BIPCM scheme employs the analysis of pairwise error probability (PEP) and polar spectrum (PS) to evaluate the minimum logarithmic upper bound weight (MLUW) approach, we use MLUW to construct the adaptive interleaving criterion. Simulations were conducted to evaluate the bit error rate (BER) performance of both conventional polar codes and the A-BIPCM method across various levels of turbulent channel intensities. The A-BIPCM scheme achieves a 0.96 dB improvement in weak turbulence, a 1.66 dB improvement in medium turbulence, and a 1.35 dB improvement in strong turbulence.