This paper proposes an adaptive bit-interleaved polar coded modulation (A-BIPCM) method based on minimum logarithmic upper bound weight (MLUW). It is designed to reduce the fading effects and long string of bit error interference caused by atmospheric turbulence in free-space optical communications (FSOC). To assess the effectiveness of this method across turbulent channels of varying intensities, we conducted an evaluation of the bit error rate (BER) performance of polar codes in turbulent channels. The results demonstrate significant performance improvements provided by the A-BIPCM method compared to conventional polar code encoding and decoding. Specifically, under weak, moderate, and strong turbulence conditions, the A-BIPCM method achieves performance gains of 0.96 dB, 1.66 dB, and 1.35 dB, respectively. Additionally, an experimental verification platform for FSOC employing intensity modulation direct detection (IM/DD) with an atmospheric turbulence simulation channel, is established in this work. When the optical power of the detector is −16 dBm, the traditional polar code encoding and decoding performance at BER = 2.36 × 10⁻⁵, whereas the A-BIPCM scheme exhibits a significantly higher performance at BER = 2.11 × 10⁻⁶. The BER has been improved by representing an order of magnitude.

The multi-hop amplify-and-forward free-space optical communication (FSOC) system is studied in random fog using the I-function, considering Gamma-Gamma atmospheric turbulence and Beckmann pointing error. Outage probability, average bit error rate and average ergodic channel capacity are obtained. Channel-state-information assisted relay performs better than fixed-gain relay under high transmitted power. Increasing the hop number significantly improves the performance. More hops are needed in medium fog than in light fog to achieve the same performance. In addition, on a single-hop link, the influence of fog channel on system performance is dominant, while atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have little effect on the system performance. However, on a multi-hop link, atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have serious effects on system performance. Compared with correcting the normalized boresight error, compensating the normalized jitter standard deviation greatly improves the multi-hop FSOC system performance. Furthermore, optimizing beam width can further improves the performance. To ensure good communication, the system should select a low-order modulation scheme.