Spatial and Time Variability of Fog Attenuations for Optical Wireless Links in the Troposphere

Line of sight optical wireless or free space optics (FSO) plays an increasingly important role in our communication infrastructure as they offer gigabit per second data rates and low system complexity. The biggest disadvantage of terrestrial FSO links is that low clouds, dry snow and especially dense fog conditions entail heavy losses to the optical beam transmitted in the terrestrial free-space. We address here the effect of fog on optical wireless signals theoretically and experimentally and analyse measured fog attenuations in terms of its temporal and spatial variability. Attenuation data presented and discussed here is collected in Graz, Austria, in the winter months of year 2004-05 and year 2005-06. It is shown that continental fog attenuations vary spatially and temporally in case of terrestrial FSO link and has very distinct seasonal and diurnal dependence during the fog life cycle of continental fog.