Multiple Scattering in Rain and Fog on Free-Space Optical Links

The multiple scattering effects of light propagation in rain and fog are investigated by means of numerical Monte Carlo photon propagation simulations. Rain and fog consisting of water droplets are described by realistic drop size distributions with parameters related to physical parameters of hydrometeors such as rain intensity, liquid water content, effective droplet radius of fog and atmospheric visibility. Simulations show that optical attenuation due to rain is about two times lower than predicted by single scattering approach. Fog attenuation is also reduced for the lowest visibilities. An impulse response of optical channel in rain and fog is obtained and the explicit models for the delay spread dependence are provided. Under realistic conditions, the RMS delay spread due to rain on 1 km long free space optics (FSO) path is limited below 10 ps. Moderate and dense fog can cause the delay spread about 50 ps or more. Frequency characteristics of the FSO channel impaired by rain and fog are given.