AN ANALYTICAL INVESTIGATION OF THE ATMOSPHERIC LIMITATIONS ON THE CAPACITY OF AN OPTICAL CHANNEL

Abstract

A model optical communication system involving propagation through the 'clear', but turbulent atmosphere is chosen. The atmospheric turbulence which manifests in the form of various fluctuations, i.e. amplitude, phase and angle of arrival fluctuation are of interest In the present analysis. These variations are caused by random variation of the refractive index changes, which in turn is the result of turbulent mixing. These effects appear to severely limit the performance of a communication link through the turbulent atmosphere. As a result)the statis-tical description of the turbulent atmosphere becomes necessary and hence a quantitative discussion is given. In particular, this thesis deals with. a generalised analytical treatment of Signal-to-Noise ratio, considering the random beam scanning and intensity scintilliation, at the detector of the signal beam. The (S/N) expression is evaluated, considering the random beam scanning (r.m. s radial fluctuation) and intensity scintilliation, for the chosen system, where local oscillator spot and signal spot can assume unequal diameters. The expressions for (S/N) ratio and channel capacity, computed for various diameter ratios, indicate a definite degradation with increased wavefront distortion as measured by wave structure constant A. A similar -iv-trend is obtained with increased random beam scanning, as measured by normalised r.m. s radial fluctuation. As the design trade offs become better understood, one can always expect that the atmospheric turbulence will not seriously limit the reliability of a properly designed optical coeunication system