Defense Notices

Graph is an extremely useful representation of a wide variety of practical systems in data analysis. Recently, with the fast accumulation of stream data from various type of networks, significant research interests have arisen on spectral clustering for network streams (or evolving networks). Compared with the general spectral clustering problem, the data analysis of this new type of problems may have additional requirements, such as short processing time, scalability in distributed computing environments, and temporal variation tracking. 

However, to design a spectral clustering method to satisfy these requirements certainly presents non-trivial efforts. There are three major challenges for the new algorithm design. The first challenge is online clustering computation. Most of the existing spectral methods on evolving networks are off-line methods, using standard eigensystem solvers such as the Lanczos method. It needs to re-compute solutions from scratch at each time point. The second challenge is the parallelization of algorithms. To parallelize such algorithms is non-trivial since standard eigen solvers are iterative algorithms and the number of iterations cannot be predetermined. The third challenge is the very limited existing work. In addition, there exists multiple limitations in the existing method, such as computational inefficiency on large similarity changes, the lack of sound theoretical basis, and the lack of effective way to handle accumulated approximate errors and large data variations over time. 

In this thesis, we proposed a new online spectral graph clustering approach with a family of three novel spectrum approximation algorithms. Our algorithms incrementally update the eigenpairs in an online manner to improve the computational performance. Our approaches outperformed the existing method in computational efficiency and scalability while retaining competitive or even better clustering accuracy. We derived our spectrum approximation techniques GEPT and EEPT through formal theoretical analysis. The well-established matrix perturbation theory forms a solid theoretic foundation for our online clustering method. In addition, we discussed our preliminary work on approximate graph mining with evolutionary process, non-stationary Bayesian Network structure learning from non-stationary time series data, and Bayesian Network structure learning with text priors imposed by non-parametric hierarchical topic modeling.

Multiple-Input Multiple-Output (MIMO) systems with large-scale transmit antenna arrays, often called massive MIMO, is a very promising direction for 5G due to its ability to increase capacity and enhance both spectrum and energy efficiency. To get the benefit of massive MIMO system, accurate downlink channel state information at the transmitter (CSIT) is essential for downlink beamforming and resource allocation. Conventional approaches to obtain CSIT for FDD massive MIMO systems require downlink training and CSI feedback. However, such training will cause a large overhead for massive MIMO systems because of the large dimensionality of the channel matrix. In this research proposal, we propose an efficient downlink beamforming method to address the challenging of downlink training overhead. First, we design an efficient downlink beamforming method based on partial CSI. By exploiting the relationship between uplink (UL) DoAs and downlink (DL) DoDs, we derive an expression for estimated downlink DoDs, which will be used for downlink beamforming. Second, we derive an efficient downlink beamforming method based on downlink CSIT estimated at the BS. By exploiting the sparsity structure of downlink channel matrix, we develop an algorithm that select the best features from the measurement matrix to obtain efficient CSIT acquisition that can reduce the downlink training overhead compared with the conventional LS/MMSE estimators. In both cases, we compare the performance of our proposed beamforming method with traditional method in terms of downlink achievable rate and simulation results show that our proposed method outperform the traditional beamforming method.

Nowadays, wireless communication are becoming so tightly integrated in our daily lives, especially with the global spread of laptops, tablets and smartphones. This has paved the way to dramatically increasing wireless network dimensions in terms of subscribers and amount of flowing data. With the rapidly growing data traffic, interference has become a major limitation in wireless networks. To deal with this issue and in order to increase the spectral efficiency of wireless networks, various interference mitigation techniques have been suggested among which interference alignment (IA) has been shown to significantly improve network performance. However, how to practically use IA to mitigate inter-cell interference in a downlink multi-cell multi-user MIMO networks still remains an open problem. Besides, more recently, the attention of researchers has been drawn to a new technique for improving the spectral efficiency, namely, massive/full dimension multiple-input multiple-output. Although massive MIMO/FD-MIMO brings a large diversity gain to the network, its practical implementation poses a research challenge. Moreover, new techniques that can mitigate interference impact in such systems remain unexplored. To address these challenges, this proposed research targets to 1) develop an IA technique for downlink multi-cell multi-user MIMO networks; 2) mathematically characterize the performance of IA with limited feedback; and 3) evaluate the performance analysis of IA technique (with/without limited feedback) in massive MIMO/FD-MIMO networks. Preliminary results show that IA with limited feedback significantly increase the spectral efficiency of downlink multi-cell multi-user MIMO networks.

Cyber-physical systems (CPS) are expected to revolutionize the world through a myriad of applications in health-care, disaster event applications, environmental management, vehicular networks, industrial automation, and so on. The continuous explosive increase in wireless data traffic, driven by the global rise of smartphones, tablets, video streaming, and online social networking applications along with the anticipated wide massive sensors deployments, will create a set of challenges to network providers, especially that future fifth generation (5G) cellular networks will help facilitate the enabling of CPS communications over current network infrastructure. 
In this dissertation, we first provide an overview of CPS taxonomy along with its challenges from energy efficiency, security, and reliability. Then we present different tractable analytical solutions through different 5G technologies, such as device-to-device (D2D) communications, cell shrinking and offloading, in order to enable CPS traffic over cellular networks. These technologies also provide CPS with several benefits such as ubiquitous coverage, global connectivity, reliability and security. By tuning specific network parameters, the proposed solutions allow the achievement of balance and fairness in spectral efficiency and minimum achievable throughout among cellular users and CPS devices. To conclude, we present a CPS mobile-health application as a case study where security of the medical health cyber-physical space is discussed in details. 

Future wireless networks will face a compound challenge of supporting large traffic volumes, providing ultra-reliable and low latency connections to ultra-dense mobile devices. To meet this challenge, various new technologies have been introduced among which mutual-information accumulation (MIA), an advanced physical (PHY) layer coding technique, has been shown to significantly improve the network performance. Since the PHY layer is the fundamental layer, MIA could potentially impact various network layers of a wireless network. Accordingly, the understanding of improving network design based on MIA is far from being fully developed. The purpose of this dissertation is to study the fundamental performance improvement of MIA over wireless networks and to apply these fundamental results to guide the design of practical systems, such as cognitive radio networks and massive machine type communication networks. 
This dissertation includes three parts. The first part of this dissertation presents the fundamental analysis of the performance of MIA over wireless networks. To begin with, we first analyze the asymptotic performance of MIA in an infinite 2-dimensional(2-D) grid network. Then, we investigate the optimal routing in cognitive radio networks with MIA and derive the closed-form cooperative gain obtained by applying MIA in cognitive radio networks. Finally, we characterize the performance of MIA in random networks using tools from stochastic geometry. 
The second and third part of this dissertation focuses on the application of MIA in cognitive radio networks and massive machine type communication networks. An optimization problem is formulated to identify the cooperative routing and optimal resources allocation to minimize the transmission delay in underlay cognitive radio networks with MIA. Efficient centralized as well as distributed algorithms are developed to solve this cross-layer optimization problem using the fundamental properties obtained in the first part of this dissertation. A new cooperative retransmission strategy is developed for massive MTC networks with MIA. Theoretical analysis of the new developed retransmission strategy is conducted using the same methodology developed in the fundamental part of this dissertation. Monte Carlo simulation results and numerical results are presented to verify our analysis as well as to show the performance improvement of our developed strategy. 

Advanced switching algorithms and modulation methods for power electronics converters controlled with model predictive control (MPC) strategies have been proposed in this work. The methods under study retain the advantage of conventional MPC methods in programing the nonlinear effects of the converter into the design calculations to improve the overall dynamic and steady state performance of the system and builds upon that by offering new modulation technique for MPC to minimize the voltage and current ripples through using a fixed switching frequency. The proposed method is easy to implement and provides flexibility to prioritize different objectives of the system against each other using the objective weighting factor. To demonstrate the effectiveness of the proposed method, it has been used to overcome the stability problems caused by a constant power load (CPL) in a multi converter system as a case study. 
In addition, to further evaluate the merits of the proposed method, it has been used to control modular multilevel converters (MMCs) in voltage source converter-high voltage DC (VSC-HVDC) systems. The proposed method considers the nonlinear properties of the MMC into the design calculations while minimizing the line total harmonic distortion (THD), circulating current ripple and steady-state error by generating modulated switching signals with a fixed switching frequency. In this work, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to a MMC system is detailed. Experimental results from the systems under study illustrate the effectiveness of proposed strategies. 

Through silicon via (TSV) based three-dimensional (3D) integrated circuit (IC) aims to stack and interconnect dies or wafers vertically. This forefront technology offers a promising near-term solution for further miniaturization and the performance improvement of electronic systems and follows a more than Moore strategy. 
Along with the need for low-cost and high-yield process technology, the successful application of TSV technology requires further optimization of the TSV electrical modeling and design. In the millimeter wave (mmW) frequency range, the root mean square (rms) height of the TSV sidewall roughness is comparable to the skin depth and hence becomes a critical factor for TSV modeling and analysis. The impact of TSV sidewall roughness on electrical performance, such as the loss and impedance alteration in the mmW frequency range, is examined and analyzed following the second order small perturbation method. Then, an accurate and efficient electrical model for TSVs has been proposed considering the TSV sidewall roughness effect, the skin effect, and the metal oxide semiconductor (MOS) effect. 
However, the emerging application of 3D integration involves an advanced bio-inspired computing system which is currently experiencing an explosion of interest. In neuromorphic computing, the high density membrane capacitor plays a key role in the synaptic signaling process, especially in the spike firing analog implementation of neurons. We proposed a novel 3D neuromorphic design architecture in which the redundant and dummy TSVs are reconfigured as membrane capacitors. This modification has been achieved by taking advantage of the metal insulator semiconductor (MIS) structure along the sidewall, strategically engineering the fixed oxide charges in depletion region surrounding the TSVs, and the addition of oxide layer around the bump without changing any process technology. Without increasing the circuit area, this reconfiguration of TSVs can result in substantial power consumption reduction and a significant boost to chip performance and efficiency. Also, depending on the availability of the TSVs, we proposed a novel CAD framework for TSV assignments based on the force-directed optimization and linear perturbation. 

Ice basal reflectivity is much needed for the determination of ice basal conditions and for the accurate modeling of ice sheet to estimate the future global mean sea level rise. Reflectivity values can be determined from the received radio echo sounding data if the power loss caused by different components along the two-way transmission of EM wave are accurately compensated. 

For the large volume of received radio echo sounding data collected over Byrd glacier in 2011-2012 with multichannel radar, the spherical spreading loss caused due to two-way propagation, power reduction due to roughness and relative englacial attenuation are compensated to estimate the relative reflectivity values of the Byrd glacier. 
In order to estimate the scattered incoherent power component due to roughness, the distributions of echo amplitudes returned from air-firn interface and from ice – bed interface are modeled to estimate RMS height variations. The englacial attenuation rate of wave for two-way propagation along the ice depth is modeled using the observed data. The estimated air-firn interface roughness parameters are relatively cross verified using the Neal’s method and with the correlations from the Landsat image mosaic of Antarctica. Estimated relative basal reflectivity values are validated using the cross-over analysis and abruptness index measurements. From the Byrd relative reflectivity map, the corresponding echograms at the locations of potential subglacial water systems are checked for the observable lake features. 
The obtained results are checked for correlations with previously predicted lake locations and subglacial flow paths. While the results doesn’t exactly match with the previously identified locations with elevation changes, high relative reflectivity values are observed close to those locations, aligning exactly or close to previously predicted flow paths providing a new window into the hydrological network of the glacial. Relative reflectivity values are clustered to indicate the different potential basal conditions beneath the Byrd glacier