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Massachusetts Institute of Technology Dept. of Electrical Engineering and Computer Science

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Works: 2,625 works in 2,632 publications in 1 language and 2,636 library holdings
Roles: Funder
Classifications: qc361, 537.62305
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Publications about Massachusetts Institute of Technology Publications about Massachusetts Institute of Technology
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Most widely held works by Massachusetts Institute of Technology
by ( Book )
1 edition published in in English and held by 10 libraries worldwide
by ( Book )
1 edition published in in English and held by 4 libraries worldwide
by ( Book )
1 edition published in in English and held by 4 libraries worldwide
by ( Book )
1 edition published in in English and held by 3 libraries worldwide
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
Mode filtering is most commonly implemented using the sampled mode shape or pseudoinverse algorithms. Buck et al [1] placed these techniques in the context of a broader maximum a posteriori (MAP) framework. However, the MAP algorithm requires that the signal and noise statistics be known a priori. Adaptive array processing algorithms are candidates for improving performance without the need for a priori signal and noise statistics. A variant of the physically constrained, maximum likelihood (PCML) algorithm [2] is developed for mode filtering that achieves the same performance as the MAP mode filter yet does not need a priori knowledge of the signal and noise statistics. The central innovation of this adaptive mode filter is that the received signal's sample covariance matrix, as estimated by the algorithm, is constrained to be that which can be physically realized given a modal propagation model and an appropriate noise model. The first simulation presented in this thesis models the acoustic pressure field as a complex Gaussian random vector and compares the performance of the pseudoinverse, reduced rank pseudoinverse, sampled mode shape, PCML minimum power distortionless response (MPDR), PCML-MAP, and MAP mode filters. The PCML-MAP filter performs as well as the MAP filter without the need for a priori data statistics. The PCML-MPDR filter performs nearly as well as the MAP filter as well, and avoids a sawtooth pattern that occurs with the reduced rank pseudoinverse filter. The second simulation presented models the underwater environment and broadband communication setup of the Shallow Water 2006 (SW06) experiment.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
Autonomous underwater vehicles (AUVs) are increasingly being considered for remotely supervised missions, primarily for routine subsea inspection tasks currently performed by tethered remotely operated vehicles (ROVs). This project is a step in the development of a high speed, networked wireless communication capability for AUVs using MIT Sea Grant's software defined Reconfigurable Modem (R-Modem) acoustic communications platform. We have demonstrated a test implementation of live streaming video with a digital camera connected to an R-Modem transceiver; and explored a range of tuning parameters for the video link.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
Optical Coherence Tomography (OCT) is an emerging optical biomedical imaging technology that enables cross-sectional imaging of scattering tissue with high sensitivity and micron-scale resolution. In conventional OCT, the reference arm path length in a Michelson interferometer is scanned in time to generate a profile of backscattering versus depth from the sample arm. In conventional OCT, a broadband, low coherence light source is used to achieve high axial resolution. However, clinical and research applications of conventional OCT have been limited by low imaging speeds. Recently, new Fourier domain OCT detection methods have enabled speeds of ~20,000-40,000 axial scans per second, which are ~50-100x faster than conventional OCT. These methods are called "Fourier domain" because they detect the interference spectrum and do not require mechanical scanning of the reference arm path length in time. In this thesis, two different technologies for Fourier domain OCT are investigated. The first technology, called spectral OCT, uses a broadband light source and a spectrometer to measure the interference spectrum. The second technology, called swept source OCT, uses a rapidly tunable narrowband laser to measure the interference spectrum over time. Applications of these new technologies for retinal imaging are illustrated, including three-dimensional retinal imaging in animal models, clinical imaging of retinal pathologies, quantification of photoreceptor morphology, and functional imaging of intrinsic stimulus-induced scattering changes in the retina. Finally, using a rapidly tunable laser, ultrahigh-speed swept source OCT imaging at 249,000 axial scans per second, roughly three orders of magnitude faster than conventional OCT, is demonstrated. This technology is applied for three-dimensional snapshots of the retina and optic nerve head and unprecedented visualization of retinal anatomy.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
The impracticality of the ambulatory electrocardiogram for long-term physiological monitoring has lead to the development of many new, compact sensors that have been designed with form factor and user comfort in mind. Nevertheless, there currently is no single sensor module that would be ideal to use for continuous, long-term monitoring. The sensors tend to either lack wireless capabilities, have a short battery life, or are financially unfeasible. After conducting a quick survey of recently developed sensors, we propose the design of iCalm: a novel, wrist-worn, low-power, low-cost, and wireless physiological sensor module. Its performance is compared against an FDA-approved platform through numerous experiments, including a few user studies. The iCalm skin conductance sensor greatly reduced noise due to motion and pressure artifacts; the iCalm heart rate sensor performed similar to the FDA-approved sensor. In addition, all of the participants in the experiments preferred the iCalm to the FDA-approved comparison sensors we tested. With iCalm, we hope to enable comfortable, long-term monitoring of the autonomic nervous system physiology and improve upon the current commercial sensors on the market.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
Web application users spend considerable time clicking on hyperlinks and buttons to complete frequent tasks. Individual application developers can optimize their interfaces to improve typical usage; however, no single task model will accurately reflect the needs of a wide audience of users. This thesis describes EasyLink, an automated optimization to the view of Web applications. EasyLink facilitates the common activities of individual users without explicit customization by each user. Using a record of the user's actions, EasyLink adapts the view of the page on later visits. The new view reduces unused elements by decreasing their contrast and emphasizes the most used elements by enlarging their size and ease of pointing. An evaluation of EasyLink on Gmail shows that it accurately models 64% of user behaviour, significantly reduces the time to complete simple tasks, and is preferred by users over the default view of Gmail.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
In relational query processing, one generally chooses between two classes of access paths when performing a predicate lookup for which no clustered index is available. One option is to use an unclustered index. Another is to perform a complete sequential scan of the table. Online analytical processing (OLAP) workloads often do not benefit from the availability of unclustered indices; the cost of random disk I/O becomes prohibitive for all but the most selective queries. Unfortunately, this means that data warehouses and other OLAP systems frequently perform sequential scans, unless they can satisfy nearly all of the queries posed to them by a single clustered index [7], or unless they have available specialized data structures - like bitmap indices, materialized views, or cubes - to answer queries directly. This thesis presents a new index data structure called a correlation index (CI) that enables OLAP databases to answer a wider range of queries from a single clustered index or sorted file. The CI exploits correlations between the key attribute of a clustered index and other unclustered attributes in the table. In order to predict when CIs will exhibit wins over alternative access methods, the thesis describes an analytical cost model that is suitable for integration with existing query optimizers. An implementation compares CI performance against sequential scans and unclustered B+Tree indices in the popular Berkeley DB [22] library. Experimental results over three different data sets validate the accuracy of the cost model and establish numerous cases where CIs accelerate lookup times by 5 to 20 times over both unclustered B+Trees and sequential scans. The strong experimental results suggest that CIs offer practical and substantial benefits in a variety of useful query scenarios.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
A channel characterization experiment for the underwater acoustic communication channel was carried out at Scripps Pier in May 1999. The experiment investigated acoustic transmission in very shallow water and breaking waves. In analyzing the data, several questions arose. The majority of the acoustic channel probe data was corrupted by crosstalk in the receiver array cable. This thesis investigates methods to correct for the effects of the crosstalk, to attempt to recover the channel probe data. In selected regions, the crosstalk could be removed quite effectively using a linear least-squares method to estimate the crosstalk coefficients. The bulk of the data could not be corrected, however, primarily due to crosstalk from a receiver channel which was not recorded, and hence could not be well estimated. A second question addressed by this thesis is concerned with acoustic propagation in shallow water under bubble clouds. The breaking waves injected air deep into the water column. The resulting bubble clouds heavily attenuated acoustic signals, effectively causing total dropouts of the acoustic communication channel. Due to buoyancy, the bubbles gradually rise, and the communication channel clears. The channel clearing was significantly slower than predicted by geometric ray acoustic propagation models, however. Proposed explanations included secondary, unobserved, breaking events causing additional bubble injection; delayed rising of bubbles due to turbulent currents; or failure of the geometric ray model due to suppression by bubble clouds of acoustic signals which are not along the geometric ray paths. This thesis investigated the final hypothesis, modeling the acoustic propagation in Scripps Pier environment, using the full wave equation modeling package OASES. It was determined that the attenuation of the propagating acoustic signal is not accurately predicted by the bubble-induced attenuation along the geometric ray path.
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
by ( Book )
1 edition published in in English and held by 2 libraries worldwide
by ( Book )
2 editions published between and 2009 in English and held by 1 library worldwide
In the ocean, low frequency acoustic waves propagate with low attenuation and cylindrical spreading loss over long-ranges, making them an effective tool for underwater source localization, tomography, and communications. Underwater mountains, or seamounts, are ubiquitous throughout the world's oceans and can absorb and scatter acoustic energy, offering many interesting acoustic modeling challenges. The goal of the research performed in support of this thesis is to measure the acoustic scattered field of a large, conical seamount at long-range, and reconcile observations with 2-D range-dependent acoustic models, for the purpose of understanding the effects of highly range-dependent bathymetry. The Basin Acoustic Seamount Scattering Experiment (BASSEX) was conducted to measure the scattered fields of the two seamounts which form the Kermit-Roosevelt Seamount Complex in the Northeast Pacific Ocean during September and October of 2004. The experiment used fixed and ship-deployed acoustic sources transmitting m-sequence signals at 68.2 and 250 Hz carrier frequencies, with 35 and 83 Hz bandwidth, respectively. The receiver was a towed hydrophone array with 3 m sensor spacing, cut for 250 Hz. BASSEX is the first experiment to measure acoustic arrival patterns in the scattered field of a seamount at many locations at sound path ranges of order 500 km, utilizing a rich bathymetry and sound velocity database. Convergence zones in the forward-scattered field of seamounts at long-range are observed, created by higher order mode coupling and blockage. Acoustic ray arrival angles, travel times, and amplitudes show good agreement with parabolic equation (PE) acoustic modeling results inside the forward-scattered fields; in particular, simulated results are fairly accurate for weak surface-reflected-bottom-reflected acoustic rays. The width of the forward-scattered field is shown to span the projected width of a seamount.
by ( Book )
2 editions published in in English and held by 1 library worldwide
 
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English (2,638)