For some free-space laser communications (lasercom) links impacted by atmospheric-induced scintillation, the principle of reciprocity provides knowledge of a remote terminal's received intensity fluctuations based on measurements of the local terminal's received intensity fluctuations. We evaluate a reciprocity-enhanced technique using optical switching between multiple spatially-diverse transmit apertures to mitigate against atmospheric-induced scintillation. Experimental bit-error rate measurements are presented to quantify the expected performance of this approach in comparison to standard static diversity approaches. We investigate system constraints of this reciprocity-enhanced transmitter diversity approach, and identify candidate optical switches and digital logic for implementation.
This paper reports on a two-color acquisition beam with continuously variable divergence for free-space laser communication links. This approach is useful for terminals using wavelength-separated beacon and communi- cation signals amplified by a common high-power optical amplifier (HPOA). The acquisition beam features a controllable power-split ratio between the two waveforms that varies during the acquisition sequence. With this scheme, the area of the acquisition beam can be expanded by a factor of 100, while maintaining power levels of the two colors within a specified range on an acquisition sensor in a partner terminal. An optical transmitter produces beacon and communication waveforms from distinct master oscillators, and balances these signal levels during a multi-step transition sequence to achieve the desired relative power levels at each wavelength. An HPOA then boosts this transmitter signal to power levels necessary to overcome link losses. The HPOA and transmitter adjust power levels during the sequence to maintain beacon and communication irradiances within specified ranges. A divergence setting assembly (DSA) simultaneously adjusts the beam width from a 10x broadened beacon to a narrow, diffraction-limited beam. We demonstrate control of the on-axis beacon power to within a 10-dB dynamic range while the beacon area varies by a factor of approximately 20 dB. This paper describes the hardware and software control of the various units used to perform acquisition.
KEYWORDS: Navigation systems, Laser communications, Telecommunications, Data communications, Satellites, Clocks, Phase measurement, Receivers, Space operations, Global Positioning System
The Lunar Laser Communication Demonstration (LLCD) flown on the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite achieved record uplink and downlink communication data rates between a satellite orbiting the Moon and an Earth-based ground terminal. In addition, the high-speed signals of the communication system were used to accurately measure the round-trip time-of-flight (TOF) of signals sent to the Moon and back to the Earth. The measured TOF data, sampled at a 20-kS/s rate, and converted to distance, was processed to show a Gaussian white noise floor typically less than 1 cm RMS. This resulted in a precision for relative distance measurements more than two orders-of-magnitude finer than the RF-based navigation and ranging systems used during the LADEE mission. This paper presents an overview of the LLCD TOF system, a summary of the on-orbit measurements, and an analysis of the accuracy of the measured data for the mission.
Free-space optical communication provides rapidly deployable, dynamic communication links that are capable
of very high data rates compared with those of radio-frequency systems. As such, free-space optical
communication is ideal for mobile platforms, for platforms that require the additional security afforded
by the narrow divergence of a laser beam, and for systems that must be deployed in a relatively short
time frame. In clear-weather conditions the data rate and utility of free-space optical communication links
are primarily limited by fading caused by micro-scale atmospheric temperature variations that create
parts-per-million refractive-index fluctuations known as atmospheric turbulence. Typical communication
techniques to overcome turbulence-induced fading, such as interleavers with sophisticated codes, lose viability
as the data rate is driven higher or the delay requirement is driven lower. This paper, along with
its companion [J. H. Shapiro and A. Puryear, “Reciprocity-Enhanced Optical Communication through Atmospheric
Turbulence–Part I: Reciprocity Proofs and Far-Field Power Transfer”], present communication
systems and techniques that exploit atmospheric reciprocity to overcome turbulence which are viable for
high data rate and low delay requirement systems. Part I proves that reciprocity is exhibited under rather
general conditions, and derives the optimal power-transfer phase compensation for far-field operation. The
Part II paper presents capacity-achieving architectures that exploit reciprocity to overcome the complexity
and delay issues that limit state-of-the art free-space optical communications. Further, this paper uses
theoretical turbulence models to determine the performance—delay, throughput, and complexity—of the
proposed architectures.
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.
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.
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.
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.
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.
KEYWORDS: Scintillation, Optical simulations, Modulation, Data modeling, Transmitters, Receivers, Atmospheric modeling, Free space optics, Numerical simulations, Near field
In this paper we discuss several models for the probability density function (pdf) of the irradiance of a Gaussian-beam
wave from ground to space. We consider cases of tracked beams and untracked beams, both of which involve a certain
amount of beam wander. The various pdf models that we introduce are all compared with simulation data over a broad
range of beam diameters. We find that certain well-known models fit the simulation data in one of the regimes defined
by the ratio of beam radius W0 to Fried's parameter r0 (W0/r0 <<1, W0/r0 ~ 1, W0/r0 >> 1), but not generally in the other regimes. This is true for tracked beams as well as untracked beams. Two new pdf models, developed here as a
modulation of either the gamma-gamma pdf or the gamma pdf, are shown to provide excellent fits to the simulation
data over all three regimes defined above.
First-order weak-fluctuation Rytov theory predicts that the longitudinal (on-axis) component of the scintillation index of an uplink collimated beam will become significantly smaller as the size of the transmitter aperture increases up to around 100 cm. However, the results of recent computer simulations are at odds with this behavior, and we believe that this discrepancy is due to the fact that the conventional Rytov theory does not correctly account for the effects of beam wander on the scintillation index. We present a theoretical structure that accurately describes far-field irradiance fluctuations caused by uncorrected beam wander. This new theory is validated by demonstrating excellent agreement between the predicted scintillation index and computer code results for both tracked and untracked beams. For many applications of practical interest, such as free-space optical communications, a good understanding of the time-average Strehl ratio is also essential simulation results for this parameter are presented and shown to be in good agreement with the theory.
Recent studies of ground-to-space beam propagation have revealed significant departures from Rytov theory when the beam diameter w0 is on the order of the atmospheric coherence width r0. It has been shown that such departures from Rytov theory are primarily a consequence of beam wander and other low-order aberrations. In this paper we discuss modeling of the probability density function (PDF) for uplink beams. In particular, we show how the PDF transitions from lognormal statistics when w0/r0 << 1 to the negative exponential distribution when w0/r0 >> 1. The most interesting regime is the transition region near w0/r0 = 1, where the statistical behavior of tracked and untracked beams differs significantly.
First-order weak-fluctuation Rytov theory predicts that the longitudinal (on-axis) scintillation component of an untracked focused beam projected along a horizontal path will become significantly smaller as the size of the transmitter aperture increases. At the same time, the radial component near the diffractive edge of the beam is predicted to increase without bound. The results of recent computer simulations are at odds with this behavior, and we believe that this discrepancy is due to the fact that Rytov theory does not correctly account for the effects of beam wander. We present a theoretical structure that accurately describes far-field irradiance fluctuations caused by uncorrected tilt jitter. This new theory is validated by demonstrating excellent agreement between the predicted scintillation index and computer code results for both tracked and untracked beams. For many applications of practical interest, such as free-space optical communications, a good understanding of the time-average Strehl ratio is also essential; simulation results for this parameter are presented and shown to be in good agreement with theory.
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