Given the increased interest in aerial communications, free-space optics (FSO) has gained considerable attention for its ability to deliver wider bandwidth, license-free, and secure communication. FSO was initially developed for fixed platforms. As such, the current push to introduce mobility is one of the greatest challenges for FSO developers. This paper presents the first open-loop alignment/stability analysis of hovering multirotors proven to maintain an FSO link despite inherent instability. Communication distance, wavelength, and platform deviation are among the many parameters evaluated in our model. We characterize fiber-bundle transceivers, as an example of optical arrays and their applicability to aerial FSO communication. Our simulation is based on static lab measurements combined with theoretical analysis and optical geometrical intersection models. Analyses indicated that rotational deviation has a much higher impact on performance than translational deviation. Current commercial multirotor platforms proved adequate for future FSO communication, with 16% to 30% expected throughput when employing suitable optical arrays. We conducted a parametric sweep test to determine optimal and marginal receiver parameters based on platform characteristics and performance metrics. This analysis was conducted and reported as one example of the many applications that our developed mathematical models support.

Multielement FSO Transceivers Alignment for Inter-UAV Communications

: This paper provides an overview of the Laser Communications Relay Demonstration Project (LCRD), a joint project between NASA?s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory, California Institute of Technology (JPL), and the Massachusetts Institute of Technology Lincoln Laboratory (MIT/LL). LCRD will provide two years of continuous high data rate optical communications in an operational environment demonstrating how optical communications can meet NASA?s growing need for higher data rates, or for the same data rate provided by a comparable RF system, how it enables lower power, lower mass communications systems on user spacecraft. In addition, LCRD?s architecture will allow it to serve as a testbed in space for the development of additional symbol coding, link and network layer protocols, etc. This paper reviews the current concepts and designs for the flight and ground optical communications terminals, the critical technologies required, and the concept of operations. It reports preliminary conclusions from several trade studies conducted at GSFC, JPL, and MIT/LL. The flight optical communications terminals will be flown on a commercial communications satellite in geosynchronous orbit to be launched no earlier than December 2016, and will demonstrate a technology critical for NASA?s Next Generation Tracking and Data Relay Satellite.

https://arc.aiaa.org/doi/pdf/10.2514/6.2012-1261897

Overview of the Laser Communications Relay Demonstration Project

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

Free-space optical communications research and demonstrations at the U.S. Naval Research Laboratory

Deep (>;10 dB) long-duration (>;1 ms) scintillation fades, caused by propagation through refractive-index turbulence, are the principal impairment that must be overcome to realize Gbps-class laser communication over line-of-sight atmospheric paths in clear-weather conditions. Spatial diversity reception can ameliorate such fades, to a degree, but current systems typically rely on forward error-correction and interleaving to achieve reliable communication over the atmospheric channel. This paper, together with its companion [A. L. Puryear, J. H. Shapiro, and R. R. Parenti, “Reciprocity-enhanced optical communication through atmospheric turbulence - Part II: Communication architectures and performance,” to be submitted to J. Opt. Commun. Netw.], comprise a two-part study that introduces and analyzes an alternative approach, in which atmospheric reciprocity is exploited to eliminate the need for interleaving and minimize the amount of forward error-correction required. The present work (Part I) first describes the problem setting and then presents proofs for reciprocity principles - with and without phase compensation - that apply under rather general conditions. By specializing to the far-field regime, the optimum (power-transfer maximizing) phase compensation is identified. These results underlie the communication architectures and performance analysis that will be reported in the Part II paper.

Reciprocity-enhanced optical communication through atmospheric turbulence — Part I: Reciprocity proofs and far-field power transfer optimization

This article describes observations made during a recent series of single-mode lasercom experiments in which high-rate data transmission was demonstrated between a small aircraft and a ground station separated by distances up to 80 km. A significant result of the subsequent data analysis was the discovery of near-unity correlations between the signal fluctuations observed by power monitors at the two ends of the link. This evidence of reciprocity is presented, along with the description of a preliminary concept for utilizing this channel state information to improve link performance.

Experimental observations of channel reciprocity in single-mode free-space optical links.

We describe a flexible high-sensitivity laser communication transceiver design that can significantly benefit performance
and cost of NASA's satellite-based Laser Communications Relay Demonstration Optical communications using
differential phase shift keying, widely deployed for use in long-haul fiber-optic networks, is well known for its superior
sensitivity and link performance over on-off keying, while maintaining a relatively straightforward design However,
unlike fiber-optic links, free-space applications often require operation over a wide dynamic range of power due to
variations in link distance and channel conditions, which can include rapid kHz-class fading when operating through the
turbulent atmosphere Here we discuss the implementation of a robust, near-quantum-limited multi-rate DPSK
transceiver, co-located transmitter and receiver subsystems that can operate efficiently over the highly-variable free-space
channel Key performance features will be presented on the master oscillator power amplifier (MOPA) based TX,
including a wavelength-stabilized master laser, high-extinction-ratio burst-mode modulator, and 05 W single
polarization power amplifier, as well as low-noise optically preamplified DSPK receiver and built-in test capabilities

Multi-rate DPSK optical transceivers for free-space applications

The multi-rate DPSK format, which enables efficient free-space laser communications over a wide range of data rates, is
finding applications in NASA’s Laser Communications Relay Demonstration We discuss the design and testing of an
efficient and robust multi-rate DPSK modem, including aspects of the electrical, mechanical, thermal, and optical
design The modem includes an optically preamplified receiver, an 05-W average power transmitter, a LEON3 rad-hard
microcontroller that provides the command and telemetry interface and supervisory control, and a Xilinx Virtex-5 radhard
reprogrammable FPGA that both supports the high-speed data flow to and from the modem and controls the
modem’s analog and digital subsystems For additional flexibility, the transmitter and receiver can be configured to
support operation with multi-rate PPM waveforms

A multi-rate DPSK modem for free-space laser communications

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.

/pdf/architecture-overview-and-data-summary-of-a-5-4-km-free-r2mz2t3eot.pdf

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

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.

Air-to-ground lasercom system demonstration

We demonstrate phase modulation, pulse carving, and burst-mode windowing in a single Mach-Zehnder modulator, significantly reducing size, weight, power, and complexity of prior three-modulator designs with only 0.6-dB additional performance penalty.

R. J. Magliocco

Papers

Multi-rate DPSK optical transceivers for free-space applications

A multi-rate DPSK modem for free-space laser communications

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

Air-to-ground lasercom system demonstration

A consolidated multi-rate burst-mode DPSK transmitter using a single Mach-Zehnder modulator