  The method of photon distribution function (PDF) is used to study
fluctuations of light beams propagating through a turbulent atmosphere. Our
analysis concerns the regime of saturated fluctuations. The focus is on the
phenomena of beam wandering and the effect of partial coherence on photon
density fluctuations. It is shown that the size of the quasiclassical part of
wandering decreases with the propagation distance, while the quantum part
increases. We explain this qualitative difference by beam fragmentation, which
is accompanied by a loss of correlation between individual parts. The effect of
the phase diffuser on the fourth moment (FM) of the irradiance is taken into
account. The obtained explicit expression for the FM indicates the possibility
of a significant reduction of noise in the communication channel. The diffuser
changes the shot noise from delta-correlated (in the spatial domain) to
smoothly distributed. The theory developed here is used to estimate the
influence of the phase diffuser on light fluctuations.
