Alan Harris

Free-space optical wavelength diversity scheme for fog mitigation in a ground-to-unmanned-aerial-vehicle communications link

Atmospheric weather conditions adversely affect the performance of free-space optical communications systems. Fog present in the atmosphere has the largest impact on free-space optical systems. We use simulation techniques to investigate the performance of a slant-path, wavelength-diversified free-space optical link between a ground station and an unmanned aerial vehicle in the presence of radiation fog. The free-space optical link is configured to operate using the simultaneous transmission of three wavelengths, 0.85, 1.55, and 10 µm, connecting a ground station to an unmanned aerial vehicle operating at either a 4- or 8-km altitude. The link is further analyzed by combining the multiple carrier wavelengths into either an equal-gain diversity scheme or a selective diversity scheme. The simulation results are obtained using ALTM and PcModWin simulation software from Ontar Corporation. The results obtained demonstrate that the use of either an equal-gain diversity scheme or a selective diversity scheme enables the deployment of a ground-to-air free-space optical communications link. Furthermore, a selective diversity scheme demonstrates a received power approximately three times greater than an equal-gain diversity scheme.

Alignment and tracking of a free-space optical communications link to a UAV

This paper investigates the ability of a mechanical gimbal to perform alignment and tracking of a free-space optical communications link between a ground station and an unmanned aerial vehicle. The repeatability and accuracy of a mechanical gimbal are experimentally analyzed in order to generate probability distribution functions for the gimbals performance. A simulation portion investigates the amount of divergence present in the laser beam of the communications link in the presence of atmospheric turbulence along with the scintillation index of the laser beam and the probability of the signal fading below a required threshold. The data from the experimental and simulation portions are further analyzed to verify the ability of a gimbal to provide acquisition and tracking functions for a ground-to-air link along with the expected geometric loss associated with such a communications link.

Atmospheric turbulence effects on a wavelength diversified ground-to-UAV FSO link

The use of free-space optical (FSO) communications links are envisioned as a viable option for providing a temporary high-bandwidth communications link between a ground station and an unmanned aerial vehicle (UAV). The presence of atmospheric turbulence causes three different phenomena to occur, namely beam wander, scintillation and beam spread, each of which is a wavelength dependent phenomenon. In this paper, simulation tools are used to investigate the effects of atmospheric turbulence on a wavelength diversified ground-to-UAV FSO communications link. This paper compares the effects of atmospheric turbulence on three different wavelengths, 1.55 μm, 0.85 μm and 10 μm. Each of these wavelengths has different advantages depending on prevalent weather conditions and atmospheric turbulence conditions. Based on the effects of atmospheric turbulence on each wavelength, a wavelength diversity scheme is proposed in order to optimize the performance of the FSO link. The largest problem associated with establishing a ground-to-UAV FSO link is alignment and tracking of the FSO link. For this reason, the wavelength diversity scheme is further analyzed as a method to optimize link acquisition and tracking of the ground-to-UAV link by exploiting various characteristics of each wavelength.

A novel wavelength hopping passive optical network (WH-PON) for provision of enhanced physical security

A novel secure wavelength hopping passive optical network (WH-PON) is presented in which physical layer security is introduced to the access network. The WH-PON design uses a pair of matched tunable lasers in the optical line terminal to create a time division multiplexed signal in which each data frame is transmitted at a unique wavelength. The transmission results for a 32-channel WH-PON operating at a data rate of 2.5 Gb/s are presented in this paper. The inherent security of the WH-PON design is verified through an attempted cross-channel eavesdropping attempt at an optical network unit. The results presented verify that the WH-PON provides secure broadband service in the access network.

An FPGA Implementation of Dirty Paper Precoder

Dirty paper code (DPC) can be used in a number of communication network applications; broadcast channels, multiuser interference channels and ISI channels to name a few. We study various implementation bottlenecks and issues with implementing a DPC pre-coder based on nested trellis technique. The aim is to achieve a practical hardware realization of the precoder for wireless LAN/DSL applications. We describe the architectural development process and realization of the precoder on a Xilinx Virtex 2V8000 FPGA. To the best of our knowledge this is the first reported DPC pre-coder hardware implementation.

Comparison of active beam steering elements and analysis of platform vibrations for various long-range FSO links

An important consideration when deploying free-space optical (FSO) communication links over ultra long distances is the ability to actively steer the laser beam. FSO links are currently being researched as an attractive option for deep-space communication links or as a link to provide broadband communications to aircraft in-flight. In order to establish ultra long FSO links or to actively track FSO links between moving platforms, an active tracking system based on hybrid technology is essential. These hybrid systems are usually a combination of a mechanical gimbal and some array of active optical components. The presence of active optical components in a beam steering device is necessary to provide a high bandwidth while offsetting vibrations present on the mounting platform. This study compares three active beam steering elements that can be used in FSO transmitters and receivers. Performance characteristics of MEMS-based micro mirror arrays, acousto-optic modulators and steerable mirrors are analyzed and compared. A comparison of aperture size, range of motion, resolution and scanning speed is performed. Simulations in order to show the effects of vibration on various different length FSO links are run. A simulation in order to verify the ability of a fast steering mirror to offset vibration effects in a ground-to-UAV link is performed.

Analysis of beam steering tolerances and divergence for various long range FSO communication links

Through the use of recent technological developments, it is now feasible to establish free-space optical (FSO) communication links over ultra-long distances. Recent research has shown that FSO systems could be deployed to establish high-rate data links to deep space. This study analyzes beam steering tolerances, beam divergence, and geometric loss for different distance ranges of interest for FSO communication links based on a mechanical gimbaled beam steering mechanism. The tolerance, divergence and geometric loss calculations are performed to evaluate the feasibility of establishing FSO links between the Earth and satellites, the Earth and aircraft, aircraft and satellites, the Earth and moon, the Earth and Mars, and the Earth and the edge of the solar system. The analysis and calculations performed take into consideration the availability of new technology such as low noise photon-counting detectors and fiber lasers and amplifiers. The beam steering tolerance and divergence calculations provide beneficial information for determining the extent to which future FSO systems could be deployed for both commercial, military and space exploration applications. Recommendations on the suitability of an FSO communication link for various applications are then made based on the beam steering tolerance and divergence calculations.

Security Enhancements in Novel Passive Optical Networks

Passive optical networks (PONs) solve the bandwidth bottleneck issue as they extend optical networks to homes and businesses. Security concerns in time division multiplexed PONs that are traditionally addressed through filtering and encryption techniques are replaced by an enhanced physical security technique that is presented herein. This security enhancement is accomplished through the inclusion of tunable lasers in the optical line terminal of a PON architecture. Simulation tools verify the viability of the physical security scheme. Presented results verify that implementation of this enhanced security technique can be successfully employed in a PON.

Minimization of Acquisition Time in a Wavelength Diversified FSO Link between Mobile Platforms

Free-space optical (FSO) communication links are envisioned as a viable option for the provision of temporary high-bandwidth communication links between moving platforms, such as a ground station and a mobile aerial platform such as an unmanned aerial vehicle. One of the limitations of FSO links is the transmission of laser beams through various weather phenomena. One technique to attempt to overcome the effects of weather, such as fog, is to implement a wavelength diversity scheme between the FSO transmitter and receiver. This paper investigates the minimization of link acquisition times using a wavelength diversity scheme between mobile FSO platforms. The wavelength diversity scheme consists of three different wavelengths, 1.55 μm, 0.85 μm and 10 μm. Each wavelength has different advantages and disadvantages for transmission depending of prevalent weather conditions and atmospheric turbulence conditions. A model of a ground-to-air FSO link is developed in order to predict the beam profile in the receiver plane. A simulation analysis of the transmission properties of the wavelength diversity schemes will be presented. Based on the transmission properties, a method for minimizing link acquisition times through the exploitation of various properties of each wavelength is presented and analyzed.

Quantifying Surface Roughness of Weathered Rock - Examples from Granite and Limestone

It is well established within the geotechnical community that weathering affects rock in a variety of ways. Weathering not only degrades engineering properties but also changes the surface appearance. Weathering classifications are partially based on the qualitative assessment of the surface appearance of rock. This study focuses on quantifying surface roughness of weathered rock. Surface textures of both granite and limestone specimens were collected using a commercially available laboratory laser scanning system. Point clouds were analyzed using two different techniques to assess surface roughness. Granite surfaces were assessed using triangulated point clouds and surface normal vectors. Limestone surfaces were assessed using statistical methods to describe deviations from a cylindrical shape. The most highly weathered granite had the largest range of surface normal vector orientation. Less weathered granites had smaller ranges of surface normal vector orientation. Results from weathered limestones were more ambiguous. The least weathered specimens had very small deviations from a perfect cylinder whereas the most weathered specimens had the greatest deviation from a perfect cylinder. However, no clear distinction could be made between deviations from a perfect cylinder for intermediate stages of weathering for limestone specimens. Close range laser scanning was able to capture surface textures from both granite and limestone specimens, however, relationships between weathering grade and surface texture were only statistically significant for granite specimens.