Channel fade modeling for free-space optical links

A recent series of experimental measurements over atmospheric channels using an optical receiver with tilt-tip control revealed that the first order statistics were well described by a log-normal probability density function under some conditions. The second order statistics in the form of the covariance function for the natural logarithm of the normalized optical intensity were observed to be well described by a double exponential decay. This paper presents a model for the channel in terms of a pair of stochastic differential equations which can be numerically evolved in time to produce time records of faded normalized optical intensities. The first and second order statistics of the faded intensity can be matched to those actually observed in the real channel. These simulated records can then be used to design free-space optical receivers that employ packet retransmission and or forward error corrective coding architectures.