Fade statistics associated with a space/ground laser communication link at large zenith angles

Laser satellite communication systems are subject to signal fading below a prescribed threshold value owing primarily to optical scintillations associated with the received signal. At large zenith angles between the transmitter and receiver the intensity fluctuations can be much stronger than at small zenith angles, easily exceeding the limitations imposed by weak fluctuation theory. Under such strong conditions the intensity fluctuations cannot be properly modeled by the longitudinal distribution. In this paper we use recently developed expressions for the scintillation index associated with an uplink or downlink path at large zenith angles and calculate the probability of signal fade as a function of threshold below the mean signal level. The analysis presented here is based on both the conventional lognormal model and the gamma-gamma distribution that has recently been proposed for the intensity fluctuations over all conditions of atmospheric turbulence. The gamma-gamma distribution, based on a model that treat intensity fluctuations as a modulation of small-scale scintillations by large-scale scintillations, has two parameters that are naturally linked to the large-scale and small-scale scintillations of the new scintillation model.