Free-Space Optical Communication Through
Atmospheric Turbulence Channels
Abstract—In free-space optical communication links, atmospheric
turbulence causes fluctuations in both the intensity and
the phase of the received light signal, impairing link performance.
In this paper, we describe several communication techniques to
mitigate turbulence-induced intensity fluctuations, i.e., signal
fading. These techniques are applicable in the regime in which the
receiver aperture is smaller than the correlation length of fading
and the observation interval is shorter than the correlation time
of fading. We assume that the receiver has no knowledge of the
instantaneous fading state. When the receiver knows only the
marginal statistics of the fading, a symbol-by-symbol ML detector
can be used to improve detection performance. If the receiver
has knowledge of the joint temporal statistics of the fading, maximum-
likelihood sequence detection (MLSD) can be employed,
yielding a further performance improvement, but at the cost of
very high complexity. Spatial diversity reception with multiple
receivers can also be used to overcome turbulence-induced fading.
We describe the use of ML detection in spatial diversity reception
to reduce the diversity gain penalty caused by correlation between
the fading at different receivers. In a companion paper, we
describe two reduced-complexity implementations of the MLSD,
which make use of a single-step Markov chain model for the
fading correlation in conjunction with per-survivor processing.
Index Terms—Atmospheric turbulence, free-space optical
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