Algorithmic decoding of dense OAM signal constellations for optical communications in turbulence.

We demonstrate an optical detection and decoding strategy to increase the information rate and spectral efficiency of free-space laser communication links affected by turbulence by means of dense orbital angular momentum (OAM) modulation. Using three candidate receiver architectures-based on a Shack-Hartmann sensor, a Mode Sorter, and a complex conjugate projection scheme as a base case-we demonstrate an algorithmic classification system based on the received OAM spectra produced by these architectures. This classification scheme allows low-error-rate data transmission in turbulence using 16-OAM, 32-OAM, and 64-OAM symbol constellations, with OAM states between -20 and 20. We evaluate and compare their performance under weak to strong atmospheric turbulence conditions using an accuracy metric and confusion matrices.