Correlated Channel Model for terrestrial Free Space Optics and project specifications evaluation of LT code in OOK modulation

Free Space Optics is a wireless line-of-sight communication system able to offer good bandwidth performance as well as straightforward installation and relocation. So that it represents a modern technology that allow to realize large bandwidth communication particularly when it is not possible to employ physical cable or radio technologies. Examples of FSO applications are: wireless backaul, fiber backup, temporary links, metro network extension and last mile access[1]. FSO is based on optical signals propagation in air and that is why it suffers of impairments which depend on atmospheric conditions. In detail, FSO suffers of scattering (i. e. Rayleigh and Mie) losses, absorption and scintillation. The former two impairments depend on a attenuation coefficient that is larger under hard atmospheric conditions, the latter is a random phenomena mainly noticeable also under clear sky. Scintillation phenomena depends on turbulence which is described by a local variations of the mean index of refraction and finally converted in a fluctuation of the signal optical irradiance. In this paper we describe for the first time a correlated channel model that predicts the temporal behavior of optical signal irradiance. We give also details on LT coding for OOK modulation.