Free-space optical communication through atmospheric turbulence channels

TLDR

Atmospheric turbulence in free‑space optical links causes intensity and phase fluctuations that degrade link performance. The study proposes techniques to mitigate turbulence‑induced intensity fluctuations (signal fading) in free‑space optical communication. The authors analyze several mitigation strategies—including symbol‑by‑symbol maximum‑likelihood detection with marginal fading statistics, maximum‑likelihood sequence detection using joint temporal statistics, and spatial diversity with multiple receivers employing ML detection—to improve performance when the receiver aperture is smaller than the fading correlation length and the observation interval is shorter than the correlation time.

Abstract

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers.