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The outsourcing and offshoring competitive landscape and its uncertainties

Sultan, Ziad R. (Ziad Raymond)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 42 leaves; 1818079 bytes; 1819731 bytes; application/pdf; application/pdf
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The outsourcing and offshoring competitive landscape is rapidly and constantly evolving, presenting new challenges and opportunities for providers and customers alike. Outsourcing providers are pressured to increase the value delivered to clients. American providers are rushing offshore in an attempt to achieve savings through lower labor cost, while Indian providers are attempting to establish a stronger presence onshore, to capture a greater share of the high-value services market. Meanwhile, the importance of business processes is being emphasized at every level. These market forces add to the difficulty of reaching a coherent understanding of outsourcing as a whole. Market research and consulting reports address the different aspects of outsourcing in a very haphazard manner, and inexperienced customers are having difficulties formulating realistic expectations. Cost savings vary significantly from case to case, and many other factors need to be taken into account, such as the risk of intellectual property loss and hidden costs due to loss of flexibility, both of which can be hard to quantify. Benchmarks are being used extensively in the industry, and associated with penalties. While the use of benchmarks can be a powerful tool...

Auto-configuration of Savants in a complex, variable network

Yu, Joseph Hon
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 64 p.; 2569339 bytes; 2571914 bytes; application/pdf; application/pdf
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In this thesis, present a system design that enables Savants to automatically configure both their network settings and their required application programs when connected to an intelligent data management and application system. Savants are intelligent routers in a large network used to manage the data and events related to communications with electronic identification tags [10]. The ubiquitous nature of the identification tags and the access points that communicate with them requires an information and management system that is equally ubiquitous and able to deal with huge volumes of data. The Savant systems were designed to be such a ubiquitous information and management system. Deploying any ubiquitous system is difficult, and automation is required to streamline its deployment and improve system management, reliability, and performance. My solution to this auto-configuration problem uses NETCONF as a standard language and protocol for configuration communication among Savants. It also uses the Content-Addressable Network (CAN) as a discovery service to help Savants locate configuration information, since a new Savant may not have information about the network structure. With these tools, new Savants can configure themselves automatically with the help of other Savants.; (cont.) Specifically...

Feedback loop design and experimental testing for integrated optics with micro-mechanical tuning

Waller, Laura A. (Laura Ann)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 78 p.; 3374959 bytes; 3378130 bytes; application/pdf; application/pdf
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I designed a capacitive sensor with feedback control for precision tuning of a MEMS controlled wavelength-selective switch. The implementation is based upon a customized feedback loop with a PID controller. The positional stability of the bridge can be controlled to within 0.5 [Angstroms], and the tuner has a time constant of 1 gs for the desired 5V actuation voltages. I created a realistic noise model for the capacitive sensor circuit and its controller, and added this to the noise models already developed for the device. Using these models, the parameters of the system can easily be changed to model device performance under varying conditions and device iterations. I also developed an equipment test set-up for accurately measuring the optical properties, both spectral and temporal, of these devices. The apparatus can be modified slightly to be used in testing of other integrated optoelectronic devices. The procedure for aligning lensed fibers to integrated waveguides and optimizing light throughput is described, and some test device values are presented.; by Laura A. Waller.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.; Page 78 blank. Vita.; Includes bibliographical references (p. 69).

Electron Bernstein wave current drive modeling in toroidal plasma confinement

Decker, Joan, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 340 p.; 14541425 bytes; 14556444 bytes; application/pdf; application/pdf
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The steady-state confinement of tokamak plasmas in a fusion reactor requires non-inductively driven toroidal currents. Radio frequency waves in the electron cyclotron (EC) range of frequencies can drive localized currents and are thus particularly attractive for control of the current profile. In the high-[beta] regimes of spherical tokamaks (ST) such as NSTX and MAST, heating and current drive (CD) by conventional electron cyclotron waves is not possible. However, electron Bernstein waves (EBW) have been proposed as an alternative for CD in these overdense devices. Given the important role predicted for CD by EBWs in high-[beta] STs, a detailed study of EBWCD must be undertaken. In this thesis a systematic analysis of EBWCD is provided. In particular, the characteristics of EBWs, the physics of resonant wave-particle interaction, and the CD mechanisms are investigated in detail. The CD efficiency and the current deposition profile are calculated using the numerical code DKE, which solve the drift-kinetic equation. Two scenarios for EBWCD are identified. The first scenario consists of approaching a harmonic of the EC resonance from a lower B-field region and drives current in the plasma core using the Fisch-Boozer mechanism.; (cont.) The other scenario consists of approaching a harmonic of the EC resonance from a higher B-field region and drives current off-axis on the outboard side using the Ohkawa mechanism. Both schemes drive current in the toroidal direction opposite to the parallel wave vector. The EBWCI) efficiency is found to be higher than ECCD efficiency because the EBW power is deposited in the tail of the electron distribution function. The results of this thesis confirm the important role of EBWs for driving currents in high-[beta] plasmas. The analytical and numerical tools developed as part of this thesis can be used to design...

Ultrahigh resolution optical coherence tomography for the detection of early stage neoplastic pathologies

Hsiung, Pei-Lin, 1975-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 121 p.; 15727032 bytes; 15732142 bytes; application/pdf; application/pdf
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Identification of changes associated with early stage disease remains a critical objective of clinical detection and treatment. Effective screening and detection is important for improving outcome because advanced disease, such as metastatic cancer, can be difficult to impossible to cure. Many existing diagnostic modalities, including x-ray imaging, magnetic resonance imaging, ultrasound, and endoscopy do not have sufficient resolution to detect changes in architectural morphology associated with early neoplasia and other pathologies. Diagnostic modalities capable of identifying pre-malignant tissue at an early stage could therefore significantly improve treatment outcome. Optical coherence tomography (OCT) is an emerging biomedical imaging technique that can potentially be used as an in vivo tool for identifying early stage neoplastic pathologies. Recent advances in solid-state laser and nonlinear fiber technology have enabled the development of ultrahigh resolution and spectroscopic OCT techniques which promise to improve tissue differentiation and image contrast. Previous ex vivo, benchtop ultrahigh resolution OCT imaging studies suggest that differentiation of architectural morphology associated with pathology is feasible. This thesis covers the development and investigation of ultrahigh resolution OCT for studies of early neoplastic pathologies.; (cont.) A section of this thesis will focus on development and evaluation of a novel turn-key broadband source for OCT. Feasibility studies were performed using ultrahigh resolution OCT for imaging human tissues ex vivo in the clinical pathology laboratory setting. Imaging results will be presented examining a variety of normal and neoplastic lesions in preliminary studies of the thyroid gland...

Investigating the role of simulation fidelity in laparascopic surgical training; Design of an experimental platform for measuring training effectiveness of virtual reality surgical simulations

Kim, Hyun K. (Hyun Kyu), 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 55, [4] leaves; 6689687 bytes; 6692074 bytes; application/pdf; application/pdf
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Minimally invasive surgery (MIS), with its aptitude for quick recovery and minimal scarring, has revolutionized surgery over the past few years. As a result, the development of a VR-based surgical trainer for MIS has been a popular area of research. However, there still remains a fundamental question of how realistic the simulation has to be for effective training. On the one hand, learning surgical practices with an unrealistic model may lead to negative training transfer. However, because of the learning abilities and perceptual limitations of the sensory, motor, and cognitive system of the human user, perfect simulation is unnecessary. Furthermore, given the large variations in human anatomy and physiology, there is no single perfect model. The question is how simple a simulation can we get away with, while at the same time preserving a level of fidelity between the virtual and real organ behavior that leads to positive training transfer. A dual station experimental platform was set up for this study. The two stations consisted of a real environment testing station and a virtual environment training station. The fidelity of the simulation could easily be adjusted in the virtual training station so that subjects could be treated with different modes of training. With the dual station setup the real environment performance of a subject before and after VE training could be measured.; (cont.) First round of experiments on the setup were conducted to investigate the effect of haptic fidelity and the effect of part task training on surgical training. Haptic fidelity was adjusted by modeling a material of non-linear stiffness to different degrees of accuracy. Subjects were initially tested on the real station performing a bimanual pushing and cutting task. They were then trained on the virtual station...

Distributed computing with imperfect randomness

Vaikuntanathan, Vinod
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 43 p.; 1925681 bytes; 1925680 bytes; application/pdf; application/pdf
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Randomness is a critical resource in many computational scenarios, enabling solutions where deterministic ones are elusive or even provably impossible. However, the randomized solutions to these tasks assume access to a pure source of unbiased, independent coins. Physical sources of randomness, on the other hand, are rarely unbiased and independent although they do seem to exhibit somewhat imperfect randomness. This gap in modeling questions the relevance of current randomized solutions to computational tasks. Indeed, there has been substantial investigation of this issue in complexity theory in the context of the applications to efficient algorithms and cryptography. This work seeks to determine whether imperfect randomness, modeled appropriately, is "good enough" for distributed algorithms. Namely, can we do with imperfect randomness all that we can do with perfect randomness, and with comparable efficiency ? We answer this question in the affirmative, for the problem of Byzantine agreement. We construct protocols for Byzantine agreement in a variety of scenarios (synchronous or asynchronous networks, with or without private channels), in which the players have imperfect randomness. Our solutions are essentially as efficient as the best known randomized Byzantine agreement protocols...

Image statistics and the perception of surface reflectance

Sharan, Lavanya
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 223 p.; 9617077 bytes; 9631583 bytes; application/pdf; application/pdf
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Humans are surprisingly good at judging the reflectance of complex surfaces even when the surfaces are viewed in isolation, contrary to the Gelb effect. We argue that textural cues are important for this task. Traditional machine vision systems, on the other hand, are incapable of recognizing reflectance properties. Estimating the reflectance of a complex surface under unknown illumination from a single image is a hard problem. Recent work in reflectance recognition has shown that certain statistics measured o an image of a surface are diagnostic of reflectance. We consider opaque surfaces with medium scale structure and spatially homogeneous reflectance properties. For such surfaces, we find that statistics of intensity histograms and histograms of filtered outputs are indicative of the diffuse surface reflectance. We compare the performance of a learning algorithm that employs these image statistics to human performance in two psychophysical experiments. In the first experiment, observers classify images of complex surfaces according to the perceived reflectance. We find that the learning algorithm rivals human performance at the classification task. In the second experiment, we manipulate the statistics of images and ask observers to provide reflectance ratings. In this case...

On attack correlation and the benefits of sharing IDS data

Katti, Sachin (Katti Rajsekhar)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 49 p.; 2429738 bytes; 2430326 bytes; application/pdf; application/pdf
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This thesis presents the first wide-scale study of correlated attacks, i.e., attacks mounted by the same source IP against different networks. Using a large dataset from 1700 intrusion detection systems (IDSs), this thesis shows that correlated attacks are prevalent in the current Internet; 20% of all offending sources mount correlated attacks and they account for more than 40% of all the IDS alerts in our logs. Correlated attacks appear at different networks within a few minutes of each other, indicating the difficulty of warding off these attacks by occasional offline exchange of lists of malicious IP addresses. Furthermore, correlated attacks are highly targeted. The 1700 DSs can be divided into small groups with 4-6 members that do not change with time; IDSs in the same group experience a large number of correlated attacks, while IDSs in different groups see almost no correlated attacks These results have important implications on collaborative intrusion detection of common attackers. They show that collaborating IDSs need to exchange alert information in realtime. Further, exchanging alerts among the few fixed IDSs in the same correlation group achieves almost the same benefits as collaborating with all IDSs, while dramatically reducing the overhead.; by Sachin Katti.; Thesis (S.M.)--Massachusetts Institute of Technology...

Interferometric-spatial-phase imaging for sub-nanometer three-dimensional positioning

Moon, Euclid E. (Euclid Eberle), 1965-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 206 p.; 55784965 bytes; 55784278 bytes; application/pdf; application/pdf
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Current alignment technology is incapable of satisfying the needs of imminent generations of lithography. This dissertation delineates a novel method of alignment and three-dimensional position metrology that is compatible with many forms of proximity lithography. The method is called Interferometric-Spatial-Phase Imaging (ISPI), and is based on encoding three-dimensional position information in the spatial phase and frequency of interference fringes, viewed with specialized oblique-incidence, dark-field optical microscopes. Alignment detectivity is <0.5 nm, and detection range is >500 gm. Unlike amplitude-based interferometers, this spatial-phase-encoding interferometer achieves high alignment detectivity without sensitivity to variations in wavelength, gap and other factors, such as resist layers and changes in the index of refraction in the beampath. Several novel gap detection methods are introduced, with gap detectivity <1 nm, measured over gaps between <1 am and >500 jgm.; (cont.) Gap is confirmed with exposure of patterns in resist, taking advantage of near-field interference in a novel Chirped Talbot Effect. Alignment and pattern overlay are confirmed in experiments combining x-ray exposures with continuous ISPI position feedback. Dynamic overlay of patterns in resist is demonstrated to be 2.7 nm...

Effect of varying gate-drain distance on the RF power performance of pseudomorphic high electron mobility transistors

Wong, Melinda F
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 137 p.; 5738316 bytes; 5745373 bytes; application/pdf; application/pdf
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AIGaAs/lnGaAs Pseudomorphic High Electron Mobility Transistors (PHEMTs) are widely used in satellite communications, military and commercial radar, cellular telephones, and other RF power applications. One key figure of merit in these applications is RF power output. Increasing the gate-to-drain length (LRD) of the PHEMT leads to an increase in its breakdown voltage. This should theoretically allow the selection of a higher drain operating voltage and consequently result in higher output power at microwave frequencies. However, experimentally, a decrease in output power and peak power-added efficiency is generally observed with increasing LRD In order to understand this, we have studied in detail the RF power performance of industrial PHEMTs with different values of LRD. We have found that there is an optimum value of LRD beyond which the maximum RF power output that the device can deliver drops. In addition, we have found that the output power of long LRD devices declines significantly with increasing frequency. We explain the difference in RF power behavior of the different devices through the evolution of load lines with frequency, LRD, and operating voltage. We have found that the presence of oscillations in the NDR region limit the maximum allowable operating voltage of long LRD devices through catastrophic burnout. The maximum voltage of short LRD devices is limited by electrical degradation. Pulsed I-V measurements have revealed that long LRD devices increasingly suffer from surface state activity that limit the maximum drain current under RF operation. A delay time analysis has shown an increasing extension of the depletion region toward the drain with increasing LRD that limits the frequency response of long LRD devices.; by Melinda F. Wong.; Thesis (S.M.)--Massachusetts Institute of Technology...

Development of three-dimensional passive components for power electronics; Development of 3D passive components for power electronics

Cantillon-Murphy, Pádraig J
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 88 leaves; 3793176 bytes; 3796792 bytes; application/pdf; application/pdf
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As component and power densities have increased, printed circuit boards (PCBs) have taken on additional functionality including heatsinking and forming constituent parts of electrical components. PCBs are not well suited to these tasks. A novel fabrication method is proposed to develop an enhanced circuit board fabrication approach which overcomes this problem. This method uses a photoresistive scaffold and subsequent metallization to realize the proposed structures. These structures are suitable as heatsinks, inductor windings, busbars and EMI shields among other applications.; by Pádraig J. Cantillon-Murphy.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.; Includes bibliographical references (leaves 85-88).

Learning the dynamics of deformable objects and recursive boundary estimation using curve evolution techniques

Sun, Walter
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 176 p.; 13312623 bytes; 17797080 bytes; application/pdf; application/pdf
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The primary objective of this thesis is to develop robust algorithms for the incorporation of statistical information in the problem of estimating object boundaries in image data. We propose two primary algorithms, one which jointly estimates the underlying field and boundary in a static image and another which performs image segmentation across a temporal sequence. Some motivating applications come from the earth sciences and medical imaging. In particular, we examine the problems of oceanic front and sea surface temperature estimation in oceanography, soil boundary and moisture estimation in hydrology, and left ventricle boundary estimation across a cardiac cycle in medical imaging. To accomplish joint estimation in a static image, we introduce a variational technique that incorporates the spatial statistics of the underlying field to segment the boundary and estimate the field on either side of the boundary. For image segmentation across a sequence of frames, we propose a method for learning the dynamics of a deformable boundary that uses these learned dynamics to recursively estimate the boundary in each frame over time. In the recursive estimation algorithm, we extend the traditional particle filtering approach by applying sample-based methods to a complex shape space.; (cont.) We find a low-dimensional representation for this shape-shape to make the learning of the dynamics tractable and then incorporate curve evolution into the state estimates to recursively estimate the boundaries. Experimental results are obtained on cardiac magnetic resonance images...

SIP : a smart digital image processing library; Smart Image Processing : a smart digital image processing library

Zhou, Mengyao
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 214 p.; 13749716 bytes; 13758748 bytes; application/pdf; application/pdf
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The Smart Image Processing (SIP) library was developed to provide automated real-time digital image processing functions on camera phones with integer microprocessors. Many of the functions are not available on commercial camera phones and some are not found even in desktop image processing software. Five patents are pending for key functions in this library. These functions create realistic water reflections in varying weather conditions, perform localized magnification and pinching, transform photographs to perspective view, provide fast, high-quality spin radial blur, and provide a fast integer implementation for arbitrary rotation. Details on all pending patents are given in five chapters of this thesis. Other operations performed by the library include adding fog and shadow, creating a neon image, and creating a translucent comer fold. All library functions have been successfully implemented on an integer microprocessor for real-time performance in an existing camera phone system. The library also provides solutions to a number of long-standing problems in image processing, including direct application of transforms in subsampled YCbCr and YCrCb images.; by Mengyao Zhou.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

Fault-tolerance and load management in a distributed stream processing system

Balazinska, Magdalena
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 199 p.; 6212416 bytes; 7581442 bytes; application/pdf; application/pdf
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Advances in monitoring technology (e.g., sensors) and an increased demand for online information processing have given rise to a new class of applications that require continuous, low-latency processing of large-volume data streams. These "stream processing applications" arise in many areas such as sensor-based environment monitoring, financial services, network monitoring, and military applications. Because traditional database management systems are ill-suited for high-volume, low-latency stream processing, new systems, called stream processing engines (SPEs), have been developed. Furthermore, because stream processing applications are inherently distributed, and because distribution can improve performance and scalability, researchers have also proposed and developed distributed SPEs. In this dissertation, we address two challenges faced by a distributed SPE: (1) faulttolerant operation in the face of node failures, network failures, and network partitions, and (2) federated load management. For fault-tolerance, we present a replication-based scheme, called Delay, Process, and Correct (DPC), that masks most node and network failures.; (cont.) When network partitions occur, DPC addresses the traditional availability-consistency trade-off by maintaining...

Études in non-interactive zero-knowledge

Shelat, Abhi
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 77 p.; 3801962 bytes; 3805072 bytes; application/pdf; application/pdf
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N a ZERO-KNOWLEDGE PROOF [GMR85], Prover interactively convinces Verifier that theorem 7r is true in such a way that (a) a corrupt Prover cannot convince Verifier of a false theorem and (b) a corrupt Verifier cannot "learn" anything other than the fact that r is true. In a NON-INTERACTIVE ZERO-KNOWLEDGE PROOF [BFM88], the Prover must do the above by sending only a single message to Verifier! To make this possible, Prover and Verifier are not tabula rasa, but rather born with some setup information. Much in the fashion of a musical TUDE, in this thesis, we explore several variations on the setup assumptions for non-interactive zero-knowledge in order to enjoy a richer understanding. Our labor brings forth * various unconditional characterizations of computational and statistical NIZK proofs, * new constructions that have practical applications to non-malleable encryption and CCAz encryption, * new constructions which form the building blocks of "fair" versions of interactive zero-knowledge and collusion-free multi-party computation protocols, * and conceptual contributions which underlie the recent works on how cryptography can be used to achieve equilibrium in game theory.; by Abhi Shelat.; Thesis (Ph. D.)--Massachusetts Institute of Technology...

A quantitative design and analysis of magnetic nanoparticle heating systems

Khushrushahi, Shahriar Rohinton
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 89 leaves; 4096919 bytes; 4298707 bytes; application/pdf; application/pdf
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Magnetic particles under the influence of an alternating magnetic field act as localized heating sources due to various loss mechanisms. This effect has been extensively investigated in hypothermia studies over the past decades and has recently been applied at the molecular level to control the dehybridization of DNA molecules. As a result, it has the potential of controlling and studying biological systems. To ensure that the nanoparticles are the only source of heat requires a very efficient system that minimizes heat transfer from sources other than the magnetic field. A quantitative analysis of the requirements and the design of such a system was investigated and tested experimentally. Although the results were affected by transmission line effects, the theory supporting the approach is sound and explains the crucial parameters that are necessary for optimizing localized ferromagnetic nanoparticle heating.; by Shahriar Rohinton Khushrushahi.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2006.; Includes bibliographical references (leaves 81-89).

Acoustic signal estimation using multiple blind observations

Lee, Joonsung
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 111 p.; 3670858 bytes; 3951085 bytes; application/pdf; application/pdf
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This thesis proposes two algorithms for recovering an acoustic signal from multiple blind measurements made by sensors (microphones) over an acoustic channel. Unlike other algorithms that use a posteriori probabilistic models to fuse the data in this problem, the proposed algorithms use results obtained in the context of data communication theory. This constitutes a new approach to this sensor fusion problem. The proposed algorithms determine inverse channel filters with a predestined support (number of taps). The Coordinated Recovery of Signals From Sensors (CROSS) algorithm is an indirect method, which uses an estimate of the acoustic channel. Using the estimated channel coefficients from a Least-Squares (LS) channel estimation method, we propose an initialization process (zero-forcing estimate) and an iteration process (MMSE estimate) to produce optimal inverse filters accounting for the room characteristics, additive noise and errors in the estimation of the parameters of the room characteristics.; (cont.) Using a measured room channel, we analyze the performance of the algorithm through simulations and compare its performance with the theoretical performance. Also, in this thesis, the notion of channel diversity is generalized and the Averaging Row Space Intersection (ARSI) algorithm is proposed. The ARSI algorithm is a direct method...

Dynamic sensor tasking in heterogeneous, mobile sensor networks

Jones, Peter B. (Peter B.), S.M., Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 101 leaves; 5289913 bytes; 5289646 bytes; application/pdf; application/pdf
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Modern sensor environments often attempt to combine several sensors into a single sensor network. The nodes of this network are generally heterogeneous and may vary with respect to sensor complexity, sensor operational modes, power costs and other salient features. Optimization in this environment requires considering all possible sensor modalities and combinations. Additionally, in many cases there may be a time critical objective, requiring sensor plans to be developed and refined in real-time. This research will examine and expand on previous work in multi-sensor dynamic scheduling, focusing on the issue of near optimal sensor-scheduling for real-time detection in highly heterogeneous networks. First, the issue of minimum time inference is formulated as a constrained optimization problem. The principles of dynamic programming are applied to the problem. A network model is adopted in which a single "leader" node makes a sensor measurement. After the measurement is made, the leader node chooses a successor (or chooses to retain network leadership). This model leads to an index rule for leader/action selection under which the leader is the sensor node with maximum expected rate of information acquisition. In effect, the sensor and modality with the maximum ratio of expected entropic decrease to measurement time is shown to be an optimal choice for leader.; (cont.) The model is then generalized to include networks with simultaneously active sensors. In this case the corresponding optimization problem becomes prohibitively difficult to solve...

Radiofrequency detector coil performance maps for parallel MRI applications; Radio frequency detector coil performance maps for parallel magnetic resonance imaging applications

Lattanzi, Riccardo
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 70 p.
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Parallel MRI techniques allow acceleration of MR imaging beyond traditional speed limits. In parallel MRI, arrays of radiofrequency (RF) detector coil arrays are used to perform some degree of spatial encoding which complements traditional encoding using magnetic field gradients. As the acceleration factor increases, coil design becomes critical to the overall image quality. The quality of a design is commonly judged on how it compares with other coil configurations. A procedure to evaluate the absolute performance of RF coil arrays is proposed. Electromagnetic calculations to compute the ultimate intrinsic signal-to-noise ratio (SNR) available for any physically realizable coil array are shown, and coil performance maps are generated based on the ratio of experimentally measured SNR to this ultimate intrinsic SNR.; by Riccardo Lattanzi.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.; Includes bibliographical references (p. 67-70).