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Atmospheric laser communications using direct-detection systems do suffer from severe degradation caused by scintillation. Because the atmospheric cut-off frequency can be as low as 100 Hz, temporal averaging is not applicable in high-speed communications. The simplest way of reducing fading is to increase the receiver size and to take advantage of aperture averaging. Spatial and temporal variations of the received intensity have to be investigated in order to predict the efficiency of aperture averaging. This paper reviews briefly the theory of spatial averaging that characterizes the direct-detected optical power. For comparison purposes, results of measurements are presented. These measurements consist of recorded pupil intensity patterns for a scenario with known turbulence profile. Statistics derived from measurement data are compared with theoretical second-order statistics.

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