Characterization and Mitigation of Atmospheric Turbulence for Coherent Free-Space Optical Communications Systems

Free-space optical (FSO) communication systems employing coherent modulation offer high spectral efficiency and improved receiver sensitivity compared to intensity-modulation systems. However, coherent detection is inherently more sensitive to atmospheric turbulence, as degradation of a beam's spatial coherence severely impairs detection efficiency. In this work, we present a detailed characterization of the impact of atmospheric turbulence on coherent FSO links through transmission experiments performed over an 824 m free-space link. By simultaneously measuring turbulence strength and single-mode fibre (SMF) coupled power at the receiver, we evaluate both the statistical and temporal fading behavior of the coherent FSO channel. Building on these measurements, we demonstrate turbulence-resilient coherent communication using 10 Gbit/s 16-QAM over the same link using the pilot-assisted transmission scheme. The performance of the pilot-assisted scheme is compared with that of a fibre-coupled coherent system to assess its effectiveness as a turbulence mitigation strategy. The presented results provide insight into turbulence-impaired coherent FSO channels and inform the design of physical-layer mitigation and signal processing techniques for robust coherent optical wireless communication.