We present an overview of an air-to-ground laser communications demonstration performed at MIT Lincoln
Laboratory. Error-free communication at 2.5 Gb/s was demonstrated along a 25-km slant path between a 1-in
transmit aperture on an aircraft at 12 kft altitude and ground terminal with 4 separate 1-cm receivers. Power
fluctuations from turbulence-induced scintillation are mitigated in the spatial domain by use of the multiple
ground receivers and in the time domain by the use of forward error correction and interleaving. The optical
terminals are monitored by multiple high-rate sensors which allow us to quantify total system performance.
KEYWORDS: Receivers, Scintillation, Turbulence, Free space optical communications, Transceivers, Information operations, Sensors, Transmitters, Free space optics, Signal detection
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.
MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully
demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary
goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667
Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was
accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data
from both the field and laboratory experiments were used to assess link performance as a function of system parameters
such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction.
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.
We present a design methodology for free-space laser communications systems. The first phase includes a
characterization through numerical simulations of the channel to evaluate the range of extinction and scintillation. The
second phase is the selection of fade mitigation schemes, which would incorporate pointing, acquisition, tracking, and
communication system parameters specifically tailored to the channel. Ideally, the process would include sufficient
flexibility to adapt to a wide range of channel conditions. We provide an example of the successful application of this
design approach to a recent set of field experiments.
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.
The free-space communications community has only recently recognized the complexity of atmospheric channel
interactions, which are highly dependent on the turbulence profile, beam propagation geometry, and transceiver design.
The search for models that accurately describe link performance and overall availability is currently an active field of
research. This paper describes a method for defining link availability based on statistical channel models, which can be
derived from measured signal fluctuations during periods of stable atmospheric conditions. Measurements made during
an extended communication link experiment conducted during the summer of 2008 indicate that the intervals of channel
stability, which impact the length of link outages, can vary in duration from a few minutes to several hours.
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.
As part of a free-space optical communications experiment over a 5km horizontal path, an extensive database of
tilt-stabilized receiver data was collected for Cn2
n conditions ranging from benign to very strong. This paper focuses
on the scintillation measurements made during those tests. Ensemble probability distributions are compiled from
these results, and are subsequently compared with standard channel models such as the log-normal and gammagamma
distributions. Statistical representations of temporal behavior are also developed from this database.
Accurate statistical models of atmospheric channel effects have proved to be invaluable in the development of
high-performance free-space transceivers.
The path-integrated turbulence strength is usually thought of as a parameter that varies slowly with time. In a
recent free-space communications experiment the Cn2
n value over a 5-km horizontal path was monitored almost
continuously for a period of nearly a month. In addition to well defined and repeatable diurnal fluctuations,
strong short-term fluctuations were observed in which the turbulence strength changed by an order of magnitude
within a period of minutes. These rapid changes were independently measured by a commercial scintillometer
and the high-rate output from the communications transceiver. The characteristics and probable causes for these
dynamic atmospheric events and their impact on the design of
free-space communication systems are discussed
in this article.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.