On the atmosphere as an optical communication channel

A reasonable model for the atmosphere as an optical communication channel is presented and then reduced, with some approximation, to an equal-strength diversity system. By using the realistic assumption of a very large variance for the channel's random phase, the operations that the maximum-likelihood receiver must perform are determined. This resulting receiver is also a min-max receiver. Functional bounds to the error probability that results with orthogonal waveforms are determined. The variation of these bounds with the energy-to-noise ratio per diversity path and with the amount of atmospheric turbulence is discussed, and numerical results are presented for the low-rate (union-bound) performance. These results demonstrate that the error probability depends strongly on the amount of turbulence. However, if the amount of diversity can be controlled, the minimum energy-to-noise ratio per information bit for which the error probability can be made to vanish is independent of the turbulence. This minimum is just that which would exist in the absence of turbulence.