William E. Wilcox

Architecture overview and data summary of a 5.4 km free-space laser communication experiment

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.

Air-to-ground lasercom system demonstration

This article presents an overview of the air to ground lasercom demonstration performed under the Free-Space Optical Communications Airborne Link (FOCAL) program. Techniques used to mitigate fading, demonstrated in 2008 and reported earlier, are reviewed as are the basic equipment approach, also reported earlier. We overview the new results for tracking, fiber coupling, channel measurements and communications performance for the air-to-ground link. 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.

Comparison of Scintillation Measurements from a 5 km Communication Link to Standard Statistical Models

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.

Experimental comparison of tracking algorithms in the presence of aircraft boundary-layer distortions for emulated free-space laser communication links.

We report experiments comparing different focal plane array (FPA) tracking algorithms for emulated laser communication links between an aircraft and spacecraft. The links include look-angle-dependent phase disturbances caused by boundary-layer turbulence replicated by using a deformable mirror. Impairments from platform jitter, atmospheric scintillation, and propagation delay are also included. We study a hyperhemispherical dome geometry that provides a large field of regard but generates boundary-layer turbulence. Results from experiments comparing peak and centroid FPA tracking algorithms in various environments show that power delivered to the optical fiber varies with algorithm and look angle. An improvement in steady-state fiber-coupled power of up to 1.0 dB can be achieved through appropriate choice of algorithm. In a real system, this advantage could be realized by implementing a tracking processor that dynamically changes its tracking algorithm depending on look angle and other parameters correlated to boundary-layer turbulence.

Analysis of rapid Cn2 fluctuations observed during a 5-km communication link experiment

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.

A Conical Scan Free Space Optical Tracking System for Fading Channels

Free space optical communication systems require robust pointing and tracking to establish and maintain lineof- sight (LOS). Atmospheric scintillation can present a challenge to the LOS tracking systems located at each end of the link. This paper describes a pointing, acquisition, and tracking (PAT) approach for single-mode fiber coupling, which was successfully demonstrated over a 5.4 km lasercom link that was subject to severe turbulence conditions. One of the primary advantages of the scheme described is its compensation for thermo-mechanical drift, which simplifies optomechanical design and allows use of simple COTS hardware. An overview of the PAT system and performance data are presented.

Experimental emulation of air-to-space laser communication links

We demonstrate an experimental emulator for evaluating pointing, acquisition, and tracking performance of laser communication systems. Results show the ability to assess the performance impact from a wide range of impairments relevant to air-to-space links.

The use of statistical channel models, full-field propagation codes, and field data to predict link availability

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.