Over a two-year period beginning in early 2008, MIT Lincoln Laboratory conducted two free-space optical
communication experiments designed to test the ability of spatial beam diversity, symbol encoding, and interleaving to
reduce the effects of turbulence-induced scintillation. The first of these exercises demonstrated a 2.7 Gb/s link over a
ground-level 5.4 km horizontal path. Signal detection was accomplished through the use of four spatially-separated 12
mm apertures that coupled the received light into pre-amplified single-mode fiber detectors. Similar equipment was
used in a second experiment performed in the fall of 2009, which demonstrated an error-free air-to-ground link at
propagation ranges up to 60 km. In both of these tests power levels at all fiber outputs were sampled at 1 msec intervals,
which enabled a high-rate characterization of the received signal fluctuations.
The database developed from these experiments encompasses a wide range of propagation geometries and turbulence
conditions. This information has subsequently been analyzed in an attempt to correlate estimates of the turbulence
profile with measurements of the scintillation index, characteristic fading time constant, scintillation patch size, and the
shape parameters of the statistical distributions of the received signals. Significant findings include observations of rapid
changes in the scintillation index driven by solar flux variations, consistent similarities in the values of the alpha and
beta shape parameters of the gamma-gamma distribution function, and strong evidence of channel reciprocity.
This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002.
Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by
the United States Government.