Effects of the atmospheric channel on free-space laser communications

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In this paper we provide an overview of how the presence of atmospheric turbulence and scattering/absorbing media in the atmospheric channel degrade high-data-rate free-space laser communication performance. The impact of the atmospheric channel on overall link budget performance is discussed. Fog, rain, dust, snow, smoke, molecular absorption, and aerosol particulate matter all attenuate the signal-carrying laser beam, and to a certain extent can be compensated for by increasing the signal gain or by appropriate selection of the optical wavelength. In contrast, random fluctuations in the atmospheres refractive index severely degrade the wave-front quality of a signal-carrying laser beam, causing intensity fading and random signal losses at the receiver. This results in increased system bit error rates, especially along horizontal propagation paths. Atmospheric turbulence-induced signal losses increase as the distance between the transmitter and receiver is increased, and there is no wavelength window where these effects can be avoided, although longer wavelengths are less affected. With atmospheric turbulence, increasing the signal gain will not necessarily improve laser beam quality. For many cases of practical interest, the limiting factor in robust free-space optical communication link performance can be the presence of clear-air atmospheric turbulence in the optical channel. Various proposed probability density functions of laser intensity fluctuations through atmospheric turbulence will be discussed as they relate to laser communications performance and reliability under different weather conditions. Results from numerical simulations are presented for analyzing communications performance for various scenarios: Downlink, Uplink and Terrestrial (Horizontal) link.