Adaptive Probabilistic Constellation Shaping based on Enumerative Sphere Shaping for FSO Channel with Turbulence and Pointing Errors

TL;DR

This paper introduces an adaptive probabilistic constellation shaping system using enumerated spherical shaping for free-space optical channels, enabling continuous rate control and improved reliability under turbulence and pointing errors.

Contribution

It proposes a novel adaptive PCS system with enumerated spherical shaping that offers finer control and higher spectral efficiency than existing methods for FSO channels.

Findings

Achieves quasi-continuous spectral efficiency control with fine granularity.

Provides higher spectral utilization compared to CCDM-based systems.

Demonstrates 99.999% reliability under severe turbulence and pointing errors.

Abstract

Free-space optical (FSO) transmission enables fast, secure, and efficient next-generation communications with abundant spectrum resources. However, atmospheric turbulence, pointing errors, path loss, and atmospheric loss induce random attenuation, challenging link reliability. Adaptive rate control technology enhances spectrum utilization and reliability. We propose an adaptive probabilistic constellation shaping (A-PCS) coherent system utilizing enumerated spherical shaping (ESS) for distribution matching. With PCS-64QAM, the system achieves continuous rate control from conventional QPSK-equivalent to 64QAM spectral efficiency, providing quasi-continuous control with granularities of approximately $0.05$~bits/4D for spectral efficiency and $0.1$~dB for the post-FEC SNR threshold, and a maximum control depth of $12.5$~dB. Leveraging ESS for efficient sequence utilization, it offers higher spectral utilization and finer control granularity than constant composition DM (CCDM)-based A-PCS systems. We further model and analyze the FSO channel, presenting calculations and comparisons of outage probability and ergodic capacity under varying turbulence intensities and pointing errors. Results demonstrate 99.999~\% reliability at maximum $\sigma_\mathrm{R}^2 = 1.39$ and $\sigma_\mathrm{s} = 0.5~\mathrm{m}$, meeting requirements under severe turbulence and large pointing errors.