This dissertation aims to reduce urban traffic congestion with street traffic signal control. The traffic signal controllers in the U.S. follow the National Electrical Manufacturing Association Standards (NEMA Standards). In a NEMA controller, the control parameters for a coordinated control are cycle, green splits, and offset. This dissertation proposed a virtual phase-link concept and developed a macroscopic model to describe the dynamics of a traffic network. The coordinated optimal splits control problem was solved using model predictive control. The outputs of the solution are the green splits that can be used in NEMA controllers. I compared the proposed method with a state-of-the-practice signal timing software under coordinated-actuated control settings. It was found that the proposed method significantly outperformed the benchmarking method. I compared the proposed NEMA-based virtual phase-link model and a Max Pressure controller model using Vissim. It was found that the virtual phase-link method outperformed two control strategies and performed close, but not as good as, the Max Pressure control strategy. The disadvantage of the virtual phase-link method stemmed from the waste of green time during a fixed control cycle length and the delay which comes from the slowing down of platoon during a road link to allow vehicles to switch lanes. Compared to the Max Pressure control strategy, the virtual phase-link method can be implemented by any traffic controller that follows the NEMA standards. The real-time requirement of the virtual phase-link method is not as strict as the Max Pressure control strategy. I introduced the offsets optimization into the virtual phase-link method. I modeled the traffic arrival pattern based on the optimization results from the virtual phase-link control method. I then derived a phase delay function based on the traffic arrival pattern. The phase delay function is a function of the offset between two consecutive intersections. This phase delay function was then used for offsets optimization along an arterial. I tested the offsets optimization method against a base case using microscopic simulations. It was found that the proposed offset optimization method can significantly reduce vehicle delays. / Doctor of Philosophy / The goal of this work is to reduce traffic congestion by providing optimized signal timing plans to controllers. Knowing that the controllers in the U.S. follow National Electrical Manufacturing Association (NEMA) Standards, I proposed a virtual phase-link concept and modeled the road traffic network under NEMA controllers’ control as a set of virtual phase-links. Each virtual phase-link corresponds to a NEMA phase at an intersection. I then proposed a NEMA-based virtual phase-link street traffic model. The control variables are the green time allocated to each phase. I compared the proposed NEMA-based virtual phase-link control method with a state-of-the-practice signal timing software using simulation experiments. It was found that the proposed control methods significantly outperformed the signal timing software. I implemented a state-of-the-art adaptive control strategy, Max Pressure control. I compared the proposed NEMA-based virtual phase-link control method with the Max Pressure control strategy. I found that the virtual phase-link control method performed close, but not as good as, the Max Pressure control strategy. The disadvantage of the virtual phase-link method stemmed from the waste of green time during a fixed control cycle length and the delay which comes from the slowing down of platoon during a road link to allow vehicles to switch lanes. The Max Pressure control needs non-conventional controllers which can potentially switch to any phase at any time. Compared to the Max Pressure control strategy, the virtual phase-link method can be implemented by any traffic controller that follows the NEMA standards. The real-time requirement of the virtual phase-link method is not as strict as the Max Pressure control strategy. I then augmented the virtual phase-link method with optimal offsets control. The offsets are the time differences of the coordinated phases comparing to a reference point in a control cycle. I derived a phase delay function and used that function to optimize the offsets by minimizing the associated delays. The simulation experiments showed that the proposed offsets optimization method could reduce the delay along the coordinated path significantly.

Traditionally medium access control protocols for ad hoc networks have been designed for nodes using omni directional antennas. Through the use of directional antennas, it is possible to obtain higher efficiency. In this thesis we investigate the impact of these antennas on aggregate throughput and end-to-end delay. The use of omni-directional antennas not only results in lower power efficiency, but also decreases network efficiency due to interference caused by the transmission of packets in undesired directions. This thesis explores the effect of using smart antennas and proposes a signaling mechanism for forming the extended links using the network layer. For the performance assessment of the wireless networks using directional antennas, baseline models of phased array antenna and channel have been developed using the discrete event simulator OPNET ModelerTM 8.0. Simulation scenarios have been created for single hop as well as multihop networks. From the results of the simulation we observe that although the nodes forming the extended link experience decrease in end-to-end delay, the data successfully transmitted using extended link is correlated to the spatial distribution of nodes. / Master of Science

In Earth-to-Space communications, well-known propagation effects such as path loss and atmospheric loss can lead to fluctuations in the strength of the communications link between a satellite and its ground station. Additionally, a less-often considered effect of shadowing due to the geometry of the satellite and its solar panels can also lead to link degradation. As a result of these anticipated channel impairments, NASA's communication links have been traditionally designed to handle the worst-case impact of these effects through high link margins and static, lower rate, modulation formats. This thesis first characterizes the propagation environment experienced by a software-defined radio on the NASA SCaN Testbed through a full link-budget analysis. Then, the following chapters propose, design, and model a link adaptation algorithm to provide an improved trade-off between data rate and link margin through varying the modulation format as the received signal-to-noise ratio fluctuates. / Master of Science

An airport node-link network model is an important input for most airport simulation models. Developing, maintaining and updating detailed airport surface node-link models require significant work. A model to convert airport geographic and geometric information into a node-link network is thus needed. In this thesis, an efficient model to automate the procedure of converting airport geographic and geometric information into a node-link network is proposed. The geographic and geometric information are obtained from the Aeronautical Information Exchange Model (AIXM). The node-link network is generated by converting the geographic and geometric information contained into the AIXM files using coincident geometry management and polygon representation development. Finally, using the airport node-link network generation model, a standalone computer application, called Taxiway Toolkit, is developed to improve the Airfield Delay Simulation Model (ADSIM+). / Master of Science

For a successful CubeSat mission, it is imperative to schedule events in a fashion that will generate maximum useful science data. Intuitive uplink commanding software is required for the Lower Atmosphere/Ionosphere Coupling Experiment (LAICE) CubeSat to ensure best results. The ground station up-link software is created with this aim in mind. This will make the operation center for the LAICE project efficient. This will also help in evaluating the effect of a particular schedule on LAICE instrument interface board (LIIB) before sending the commands to it. The interactive User Interface (UI) that makes the entire process intuitive guides the user to create an uplink schedule without any human error. The control software creates the command sequence taking in to account all the limitations and specification of the systems and instruments on LAICE. These data are backed up in an efficient format in Virginia Tech’s database for future processing. This web-based application ensures a smooth scheduling process without any errors. Assistive flight-ready software is provided on the flight computer on the LAICE CubeSat to upload the correct uplink sequence to the LIIB. / Master of Science

This thesis presents two methods for designing four-bar function generators by matching a plot of the desired function with curves and charts derived from the computer solution of the displacement equation relating the input and output angles of the four-bar linkage. A series of curves and charts is presented for various assumed ratios of link lengths. An accuracy check procedure is presented, and tabulated computer data are included so that accuracy checks can be made for the function generators designed by these methods. / M.S.

Collagen type I is an extracellular matrix (ECM) protein that affords tensile strength and biological scaffolding to numerous vertebrate and invertebrate tissues. This strength has been attributed to the triple-helical structure of the collagen type I molecules, their organization into fibrils, and the presence of inter-molecular, covalent, enzymatic crosslinks. There are several different types of these crosslinks; their composition is tissue-specific and dependent upon factors such as age and health. Furthermore, these enzymatic crosslinks tend to form specifically at amino/N- and carboxy/C-terminal crosslinking sites. The mechanical behavior of collagen type I has been investigated, via experiment and theory, at the level of the molecule, microfibril, fibril, and fiber. However, the influence of different enzymatic crosslinks and their location (e.g., N- vs. C-site) on the mechanics of collagen type I has not been investigated in the literature. We employed molecular dynamics to model the mechanical behavior of uncrosslinked and crosslinked ~23-nm-long molecular segments and ~65-nm-long microfibril units of collagen type I. We then used these molecular simulations to construct a model of a single collagen type I fibril by considering the ~65-nm-long microfibril units arranged in series and then in parallel. When a uniaxial deformation was applied along the long axis of the molecular models, N-crosslinks aligned rapidly at lower strains followed by C-crosslinks more gradually at higher strains, leading to a two-stage crosslink recruitment. Then when comparing the influence of different enzymatic crosslinks, significant differences were observed for the high-strain elastic moduli of our microfibril unit models, namely and in increasing order, uncrosslinked, immature crosslinked (HLKNL and deH-HLNL), mature HHL-crosslinked, and mature PYD-crosslinked. At the fibril level, our low- and high-strain elastic moduli were in good agreement with some literature data, but in over-estimation of several other literature reports. Future work will seek to address simplifications and limitations in our modeling approach. A model such as this, accounting for different enzymatic crosslink types, may allow for the prediction of the mechanics of collagen fibrils and collagenous tissues, in representation of healthy and diseased states. / Ph. D.

Communication systems have an important role to play in managing the safe and efficient operation of the National Airspace System (NAS). The users of the NAS, from the commercial transport aircraft to hobby airplanes can all benefit from the recent advances in digital communication technology especially as the capacity of the analog voice systems is surpassed by the growth in air traffic. One of the benefits of the new digital data links being developed is to allow delivery of real time weather information to the cockpit of aircrafts. Weather information is essential to flight operations and until recently there were only limited voice and text weather reports available to the pilot. With data links, graphical weather in the cockpit is possible and also highly desired by the aviation community. This thesis will develop a decision model and analysis tool for product developers of weather information systems that need to select appropriate data link technology for which to develop their weather information systems. A comparative analysis of the aeronautical communications systems is done with the view of satisfying end-user requirements for weather information and achieving market success. A study of the various data links is conducted to gain an understanding of their performance characteristics and implementation issues. A consumer survey was designed and valuable insight into the requirements and opinions of pilots was gained. The method of Quality Function Deployment (QFD) was employed to find the most important technical characteristics of the data links to satisfy the consumer requirements captured in the survey. The decision model consisting of the QFD and also further technical assessment was implemented in software to allow any scenario of product requirements and data link to be performed. / Master of Science

Approved for public release, distribution is unlimited / A recent improvement to the Seaweb underwater wireless network was the implementation of a Selective Automatic Repeat Request (SRQ) mechanism. SRQ is a protocol implemented in the Seaweb link layer as a measure for mitigating unreliability inherent in the telesonar physical layer. In January 2004, an experiment was performed in St. Andrew's Bay, Panama City, Florida. The goal was to transmit large data files through the network, in accordance with a Naval Special Warfare need for imagery file telemetry. For three point-to-point test geometries, SRQ was tested with a noisy and variable physical layer. Through the incorporation of SRQ, the unreliability was overcome. A link-budget model calibrated with the sound channel data collected from the experiment establishes the benefit of a "SRQ gain." / Ensign, United States Navy

This thesis presents the design, simulation and analysis of a new network based on the Automatic Packet Reporting System (APRS) concept to enhance communications of CubeSat. Created in the 1980s, APRS is arguably one of the world’s largest Radio Frequency (RF) networks maintained by a community of licensed amateur radio volunteers for a variety of terrestrial purposes. Using these established nodes we can create a virtualized ground station network for satellite downlinks. Based on literature reviews of CubeSat communications systems and their orbital elements, as well as direct surveys of the amateur radio community, comprehensive far-field antenna models were created in 4nec2 for both the ground and space sections of the CubeSat downlink. The ground station antenna models include: a directional AMSAT/OSCAR Yagi-Uda, an omnidirectional J-pole, ground plane, and a whip antenna. The CubeSat antenna models used include dipoles, monopoles, and turnstiles. These models were evaluated and then imported into the Analytical Graphics Inc (AGI) Systems Tool Kit (STK) to form a discrete event simulation based on nonhomogeneous WGS84 location data extracted from real world APRS Internet Gateway (I-Gate) packets. All objects within the simulation use realistic antenna models, node locations, propagation models and satellite orbital mechanics. This simulation can act as a suite of generally applicable example satellites models (ExampleSat) for analytical comparisons that include link budgets, access times, differing data rates, antenna designs, orbital altitudes and ground station locations. Furthermore, it represents a North American continental scale RF satellite communications downlink network model. Using the virtual ground station network alters the net probability distribution of the received signal from the spacecraft. Specifically, the thesis compares non-stationary time series analysis methods upon the net received signal. The virtualized ground station network enables the aggregated received signal to appear stationary. This in turn may allow for alternative signals analysis techniques to improve a CubeSat’s downlink performance. The simulation allows us to generate representative received signals from ExampleSat to the ground station nodes for exploratory analysis, utilizing less arbitrary assumptions. This simulation and the set of models enable a more rapid start to trade-off studies for link budget design. This set of ExampleSat downlink models can be used by new designers of small-satellite communications systems. / February 2017