As a light beam radiates from a source and propagates through the atmosphere,
it passes through regions of air with varying density. The optical phenomenon
of refraction causes the entire beam or parts of the beam to change its course slightly
as the density of medium (the air) changes. As the beam propagates through more of
the atmosphere, the variation accumulates such that the beam arrives at the receiving
end with uneven power density distributed along its cross-section. Different part of the
cross-sectional area of the beam end up with different optical power densities. Due to
turbulence, uneven distribution of the power density also changes constantly over time,
resulting in fluctuation of the received signal. Such fluctuation happens typically in the
order of a few milliseconds.
The net effect of scintillation on an FSO system is in the fluctuation of the received
signal. However, if a link has sufficient link margin to accommodate the fluctuation,
scintillation would not adversely affect link performance. For a typical FSO system
in a typical deployment, signal fluctuation due to scintillation is anywhere from
1 to 3 dB.
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