Hysteretic, Scintillation-Driven Mode-Order Selection for Turbulent OAM Free-Space Optical Links

Orbital-angular-momentum (OAM) multiplexing raises FSO link capacity, but higher mode orders demand larger apertures and suffer more inter-modal crosstalk, so the usable order varies with turbulence and therefore with time. This work treats mode order as the output of a real-time controller driven only by the receiver's own intensity statistics — no wavefront sensor, no spatial light modulator, no separate scintillometer. The controller is a dead-band regulator with deliberately asymmetric time constants (immediate retreat, delayed advance), after the pattern of retinal light adaptation and thermoregulation. On a simulated 24 h diurnal turbulence cycle with a dust event, and with the controller fed a 30%-error estimate of r0, the proposed law holds outage at the link's own floor of 6.26% while a single-threshold selector doubles it to 12.44%, with 6.2x fewer mode switches. The cost is stated explicitly: 80% of oracle goodput rather than 93%. Limitation stated up front: the crosstalk threshold is a model parameter that has not been measured on hardware. All results are conditional on it and all results are simulation. Licensing: manuscript and figures under CC BY-NC-ND 4.0; source code under PolyForm Noncommercial 1.0.0. Commercial rights retained by the author.