Defense Notices

Power line communication (PLC) has received steady interest over recent decades because of its economic use of existing power lines, and is one of the communication technologies envisaged for Smart Grid infrastructure. However, power lines are not designed for data communication, and this brings unique challenges for data communication over power lines. In particular for broadband (BB) PLC, the channel exhibits linear periodically time varying (LPTV) behavior synchronous to the AC mains cycle due to time varying impedances, impulsive noise due to switching events in the power line network is present in addition to background noise. In this work, we focus on two major aspects of an orthogonal frequency division multiplexing (OFDM) system for BB PLC LPTV channels; bit and power allocation, and channel estimation (CE). 

For the problem of optimal bit and power allocation, we present that the application of a power constraint that is averaged over many microslots can be exploited for further performance improvements through bit loading. Due to the matroid structure of the optimization problem, greedy-type algorithms are proven to be optimal for the new LPTV-aware bit and power loading. Next, two mechanisms are utilized to reduce the complexity of the optimal LPTV-aware bit loading and peak microslot power levels: employing representative values from microslot transfer functions, and power clipping. 

Next, we introduce a robust CE scheme with low overhead that addresses the drawbacks of block-type pilot arrangement and decision directed CE schemes such as large estimation overhead, and difficulty in channel tracking in the case of sudden changes in the channel, respectively. A transform domain (TD) analysis approach is developed to determine the cause of changes in the channel estimates. The result of TD analysis is then exploited in the proposed scheme to mitigate the effects of LPTV channel and impulsive noise. 

Our results indicate that the proposed reduced complexity LPTV-aware bit loading with power clipping algorithm performs close to the optimal scheme, and the proposed CE scheme based on TD analysis has low estimation overhead and is robust to changes in the channel and noise, making them good alternatives for BB PLC LPTV channels.

Many of the innovative algorithms that permeate the field of array processing are based on a very simple signal model of an array. This simple, although powerful, model is at times a pale reflection of the complexities inherent in the physical world, and this model mismatch opens the door to the performance degradation of any solution for which the model underpins. This dissertation seeks to explore the impact of model mismatch upon common array processing algorithms. Model mismatch is examined in two ways: First, by developing a blind array calibration routine that estimates model mismatch and incorporates that knowledge into the RISR direction of arrival estimation algorithm. Second, by examining model mismatch between a transmitting and receiving antenna array, and assessing the impact of this mismatch on prolific direction of arrival estimation algorithms. In both of these studies it is shown that engineers have traded algorithm performance of model simplicity, and that if we are willing to deal with the added complexity we can recapture that lost performance.

A method of transmitting information clandestinely over a variety of network protocols is designed and discussed. A demonstrative implementation is created that utilizes the ubiquitous Hypertext Transfer Protocol (HTTP) and the world wide web. Key contributions include the use of access ordering to convey information, and the modulation of transaction level timing to emulate user behavior.

Multiple-input, multiple-output (MIMO) systems have received considerable attention over the last decade due to their ability to provide high throughputs and mitigate multipath fading effects. There are some limitations to get the most from MIMO, such as mutual coupling between 
antenna elements in an array. Mutual coupling and therefore inter element spacing have important effect on the channel capacity of MIMO communication system, its error rate and ambiguity of MIMO radar system. There are huge numbers of researches that focus on reducing the mutual coupling in antenna arrays and improve MIMO performance. Antenna design affects the performance of Multiple-Input–Multiple-output (MIMO) systems. Two aspects of antenna role in MIMO performance have been investigated in this thesis. Employing suitable antenna can have significant impact on performance of MIMO system. In addition to antenna design another antenna related issue that helps to optimize the system performance is to reduce mutual coupling between antenna elements in an array.Effect of antenna configuration in array on mutual coupling has been studied in this research. Main purpose is to find the array configuration which provides minimum mutual coupling between elements. U-slot patch antenna which because of its features like wide bandwidth ,multi band resonance and ease to achieve different polarizations has attracted lots of researchers has been used in this study.

The desire to hide communications has existed since antiquity. This includes hiding the existence of the transmission and the location of the sender. Wireless networks offer an opportunity for hiding a transmission by placing a signal in the radio frequency (RF) occupied by a target network which also has the added benefit of lowering its probability of detection. 

This research hides a signal within the RF environment of a packet based wireless (infrastructure) network. Specifically, in this research the interfering (covert) signal is placed in the guard band of the target network’s orthogonal frequency division multiplexed (OFDM) signal. We show that the existence of adaptive protocols allow the target network to adjust to the existence of the covert signal. In other words, the wireless network views the covert network as a minor change in the RF environment; this work shows that the covert signal can be indistinguishable from other wireless impairments such as fading. 

The impact of the covert signal on the target system performance is discovered through analysis and simulation; the analysis and simulation begin at the physical layer where the interaction between the target and covert systems occurs. After that, analysis is performed on the impact of the covert link on the target system at data-link layer. Finally, we analyze the performance of the target system at the transmission control protocol (TCP) layer which characterizes the end-to-end performance. The results of this research demonstrate the potential of this new method for hiding the transmission of information. The results of this research could encourage the creation of new protocols to protect these networks from exploitation of this manner.

Measurement of wind velocity is of essential to enhance the wind energy utilization which is very important considering the fact that it one of most important renewable source of energy and LIDAR (Light Detection and Ranging) has become a very popular technology for such measurements. In this study, a pulsed Doppler Lidar operating at 1.5µm is demonstrated with coherent detection technique for measurement of velocity of spinning disc which is a hard target used in this project. This Lidar uses the principle of Doppler shift to measure the velocity and an Acousto-optic modulator is used for frequency shifting in the transmitter to produce an intermediate frequency. A data acquisition board (DAQ) was used to generate the pulses and also to process the data once it was collected by the receiver using mat lab. A graphical user interface was used to interface the DAQ with the system and changing parameters like PRF, pulse width, record directory etc. could be changed directly from the computer. A thorough study of literature has been done and same has been presented. The architecture of the Lidar, velocity results, future work and an analysis of SNR’s dependence on range and pulse energy under predefined atmospheric conditions will be discussed.

Ever since the invention of near infrared optical spectroscopy almost three decades ago, research has still been going actively to improve the accuracy of biological tissue oxygenation measurements and make it commercially available in clinics for medical diagnosis and imaging. Hemoglobin concentration in blood especially near brain can be found by determining the absorption and scattering coefficient of chromosphere at a particular wavelength. But there are many challenges to overcome when infrared light penetrates deeper into our skull which acts as a high scattering medium. This improvement has taken a new shape with the application of diffusion theory to separate the absorbed light and scattered light in tissues. With this motivation, in this project we designed a dual-frequency (120MHz and 125MHz) based two wavelengths LED transmitting system to transmit optical power through fibers and penetrate NIR light into a scattering medium which is then received by the detection and demodulation circuit for data acquisition. Different methods available to measure absorption and reduced scattering coefficient for non-homogenous medium will be discussed.

Understanding network behavior under challenges is essential to constructing a resilient and survivable network. Due to the mobility and wireless channel properties, it is more difficult to model and analyze mobile wireless networks under various challenges. We provide a comprehensive model to analyze malicious attacks against mobile ad hoc networks. We analyze comprehensive graph-theoretical properties and network performance of the dynamic networks under attacks against the critical nodes using both synthetic and real-world mobility traces. Our study provides insights into the design and construction of resilient and survivable mobile wireless networks.

Online Social Networks (OSNs) are integrated into business, entertainment, politics, and education; they are integrated into nearly every facet of our everyday lives. They have played essential roles in milestones for humanity, such as the social revolutions in certain countries, to more day-to-day activities, such as streaming entertaining or educational materials. Not surprisingly, social networks are the subject of study, not only for computer scientists, but also for economists, sociologists, political scientists, and psychologists, among others. In this dissertation, we build a model that is used to classify content on the OSNs of Reddit, 4chan, Flickr, and YouTube according the types of lifespan their content have and the popularity tiers that the content reaches. The proposed model is evaluated using 10-fold cross-validation, using data mining techniques of Sequential Minimal Optimization (SMO), which is a support vector machine algorithm, Decision Table, Naïve Bayes, and Random Forest. The run times and accuracies are compared across OSNs, models, and data mining algorithms. 
Our experiments compared the runtimes and accuracy of SMO, Naïve Bayes, Decision Table, and Random Forest to classify the lifespan of content on Reddit, 4chan, and Flickr as well as classify the popularity tier of content on Reddit, 4chan, Flickr, and YouTube. The experimental results indicate that SMO is capable of outperforming the other algorithms in runtime across all OSNs. Decision Table has the longest observed runtimes, failing to complete analysis before system crashes in some cases. The statistical analysis indicates, with 95% confidence, there is no statistically significant difference in accuracy between the algorithms across all OSNs. Reddit content was shown, with 95% confidence, to be the OSN least likely to be misclassified. All other OSNs, were shown to have no statistically significant difference in terms of their content being more or less likely to be misclassified when compared pairwise with each other.

To enhance the capabilities of autonomous flight systems for Unmanned Aircraft Systems (UASs), a multichannel Frequency-Modulated Continuous Wave (FMCW) collision-avoidance radar with a center frequency of 1.445 GHz is designed. The radar is intended to provide situational awareness for a 40% Yak-54 model aircraft by providing in real time range, radial velocity and angle-of-arrival (AoA) information on surrounding targets with an update rate of 10 Hz. A target’s range and Doppler is determined by employing a two-dimensional (2-D) Fast Fourier Transform (FFT) on the received signal which maps the target to a specific range-Doppler bin. Tests have shown that the proto-type radar is capable of providing range detection up to 430 m with an accuracy of 0.6 m for a target with a 1-m2 radar cross section (RCS). The radar is designed to provide a Doppler resolution of 10 Hz. An array of receiving antennas is used to determine a target’s elevation and azimuth angles by exploiting the received signal’s phase difference at each individual antenna. The AoA measurement error due to thermal noise was found to be less than 3° for a signal-to-noise ratio (SNR) of 18 dB.