Secure turbulence-resistant coherent free-space optical communications via chaotic region-optimized probabilistic constellation shaping.

We propose a chaotic region-optimized probabilistic constellation shaping (CRPCS) scheme to enhance the security and the resistance to turbulence for free-space optical (FSO) communications. For this approach, a four-dimensional hyperchaotic system generates a pseudorandom sequence to rotate and encrypt the constellation. Constellation distribution of short pseudorandom sequences behaves as the law of a non-uniform character. Grouping long pseudorandom sequences and counting the characteristics of constellation distribution can realize probabilistic constellation shaping with low and fixed redundant information. We demonstrate a 56 Gbyte/s coherent FSO communication system based on log-normal and Gamma-Gamma turbulence models with a key space of 10. The results show that the optical receiver sensitivity is improved by 0.3-1.1 dB, and the transmission distance is also improved by 3.2%-7.0% in different shaping cases.