Probabilistic Amplitude Shaping Performance over FSO Atmospheric Turbulence Channels

Free-space optics (FSO) shows great promise for ultra-high-capacity wireless communication. Still, it faces limitations due to power fluctuations from atmospheric turbulence, weather, and pointing errors. Probabilistic Amplitude Shaping (PAS) addresses these challenges, optimizing the achievable rate in Log-normal and Gamma-Gamma turbulence channels. This study uses a Modified Multi-Repeat Mapping (MMRM) scheme optimized via the Water Cycle Algorithm (WCA) to adjust probability mass functions (PMFs) and reduce FSO impairments. Results demonstrate that the proposed non-uniformly shaped QAM significantly outperforms conventional uniform QAM under similar turbulence conditions, yielding SNR gains of {4.3, 2.8} dB for PAS-16-QAM at L={10,30}L={10,30}, {3.5, 2.5} dB for PAS-32-QAM at L={50,70}L={50,70}, and {4.1, 2.6} dB for PAS-64-QAM at 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> SER compared to uniform QAM.