561 research outputs found
Signal recovery from partial fractional fourier domain information and pulse shape design using iterative projections
Cataloged from PDF version of article.Signal design and recovery problems come up in a wide variety of applications in signal
processing. In this thesis, we first investigate the problem of pulse shape design
for use in communication settings with matched filtering where the rate of communication,
intersymbol interference, and bandwidth of the signal constitute conflicting
themes. In order to design pulse shapes that satisfy certain criteria such as bit rate,
spectral characteristics, and worst case degradation due to intersymbol interference,
we benefit from the wellknown Projections Onto Convex Sets. Secondly, we investigate
the problem of signal recovery from partial information in fractional Fourier
domains. Fractional Fourier transform is a mathematical generalization of the ordinary
Fourier transform, the latter being a special case of the first. Here, we assume
that low resolution or partial information in different fractional Fourier transform
domains is available in different intervals. These information intervals define convex
sets and can be combined within the Projections Onto Convex Sets framework. We
present generic scenarios and simulation examples in order to illustrate the use of
the method.Güven, H EmreM.S
p-brane Solitons in Maximal Supergravities
In this paper, we give a construction of -brane solitons in all maximal
supergravity theories in dimensions that are obtainable from
supergravity by dimensional reduction. We first obtain the full bosonic
Lagrangians for all these theories in a formalism adapted to the -brane
soliton construction. The solutions that we consider involve one dilaton field
and one antisymmetric tensor field strength, which are in general linear
combinations of the basic fields of the supergravity theories. We also study
the supersymmetry properties of the solutions by calculating the eigenvalues of
the Bogomol'nyi matrices, which are derived from the commutators of the
supercharges. We give an exhaustive list of the supersymmetric -brane
solutions using field strengths of all degrees , and the
non-supersymmetric solutions for . As well as studying elementary and
solitonic solutions, we also discuss dyonic solutions in and . In
particular, we find that the Bogomol'nyi matrices for the supersymmetric
massless dyonic solutions have indefinite signature.Comment: 31 pages, Latex, no figure
Simultaneous use of Individual and Joint Regularization Terms in Compressive Sensing: Joint Reconstruction of Multi-Channel Multi-Contrast MRI Acquisitions
Purpose: A time-efficient strategy to acquire high-quality multi-contrast
images is to reconstruct undersampled data with joint regularization terms that
leverage common information across contrasts. However, these terms can cause
leakage of uncommon features among contrasts, compromising diagnostic utility.
The goal of this study is to develop a compressive sensing method for
multi-channel multi-contrast magnetic resonance imaging (MRI) that optimally
utilizes shared information while preventing feature leakage.
Theory: Joint regularization terms group sparsity and colour total variation
are used to exploit common features across images while individual sparsity and
total variation are also used to prevent leakage of distinct features across
contrasts. The multi-channel multi-contrast reconstruction problem is solved
via a fast algorithm based on Alternating Direction Method of Multipliers.
Methods: The proposed method is compared against using only individual and
only joint regularization terms in reconstruction. Comparisons were performed
on single-channel simulated and multi-channel in-vivo datasets in terms of
reconstruction quality and neuroradiologist reader scores.
Results: The proposed method demonstrates rapid convergence and improved
image quality for both simulated and in-vivo datasets. Furthermore, while
reconstructions that solely use joint regularization terms are prone to
leakage-of-features, the proposed method reliably avoids leakage via
simultaneous use of joint and individual terms.
Conclusion: The proposed compressive sensing method performs fast
reconstruction of multi-channel multi-contrast MRI data with improved image
quality. It offers reliability against feature leakage in joint
reconstructions, thereby holding great promise for clinical use.Comment: 13 pages, 13 figures. Submitted for possible publicatio
An augmented lagrangian method for sparse SAR imaging
In this paper, we present a solution to the constrained l1-norm minimization problem for sparse SAR imaging. The technique we present relies on recent advances in the solution of optimization problems, based on Augmented Lagrangian Methods (ALMs), namely the Alternating Direction Method of Multipliers. Here, we present an application of C-SALSA (an ALM for constrained optimization problems) to SAR imaging, and introduce a new weighting scheme to improve the sparsity of the reconstructions. We then compare the performances of several techniques to understand the effectiveness of ALMs in the context of SAR imaging
On the nature of compact stars determined by gravitational waves, radio-astronomy, x-ray emission and nuclear physics
We investigate the question of the nature of compact stars, considering they
may be neutron stars or hybrid stars containing a quark core, within the
present constraints given by gravitational waves, radio-astronomy, X-ray
emissions from millisecond pulsars and nuclear physics. A Bayesian framework is
used to combine together all these constraints and to predict tidal
deformabilities and radii for a 1.4~M compact star. We find that
present gravitation wave and radio-astronomy data favors asy-stiff EoS
compatible with nuclear physics and that GW170817 waveform is best described
for binary hybrid stars. In addition, this data favors stiff quark matter,
independently of the nuclear EoS. Combining this result with constraints from
X-ray observation supports the existence of canonical ~M mass
hybrid star, with a radius predicted to be ~km.Comment: 5 pages, 3 figure
Kaluza-Klein electrically charged black branes in M-theory
We present a class of Kaluza-Klein electrically charged black p-brane
solutions of ten-dimensional, type IIA superstring theory. Uplifting to eleven
dimensions these solutions are studied in the context of M-theory. They can be
interpreted either as a p+1 extended object trapped around the eleventh
dimension along which momentum is flowing or as a boost of the following
backgrounds: the Schwarzschild black (p+1)-brane or the product of the
(10-p)-dimensional Euclidean Schwarzschild manifold with the (p+1)-dimensional
Minkowski spacetime.Comment: 16 pages, uses latex and epsf macro, figures include
An augmented Lagrangian method for autofocused compressed SAR imaging
We present an autofocus algorithm for Compressed SAR Imaging. The technique estimates and corrects for 1-D phase errors in the phase history domain, based on prior knowledge that the reflectivity field is sparse, as in the case of strong scatterers against a weakly-scattering background. The algorithm relies on the Sparsity Driven Autofocus (SDA) method and Augmented Lagrangian Methods (ALM), particularly Alternating Directions Method of Multipliers (ADMM). In particular, we propose an ADMM-based algorithm that we call Autofocusing Iteratively Re-Weighted Augmented Lagrangian Method (AIRWALM) to solve a constrained formulation of the sparsity driven autofocus problem with an ℓp-norm, p ≤ 1 cost function. We then compare the performance of the proposed algorithm's performance to Phase Gradient Autofocus (PGA) and SDA [2] in terms of autofocusing capability, phase error correction, and computation time
Instanton Moduli and Brane Creation
We obtain new intersecting 5-brane, string and pp-wave solutions in the
heterotic string on a torus and on a K3 manifold. In the former case the
5-brane is supported by Yang-Mills instantons, and in the latter case both the
5-brane and the string are supported by the instantons. The instanton moduli
are parameterised by the sizes and locations of the instantons. We exhibit two
kinds of phase transition in which, for suitable choices of the instanton
moduli, a 5-brane and/or a string can be created. One kind of phase transition
occurs when the size of an instanton vanishes, while the other occurs when a
pair of Yang-Mills instantons coalesce. We also study the associated
five-dimensional black holes and the implications of these phase transitions
for the black-hole entropy. Specifically, we find that the entropy of the
three-charge black holes is zero when the instantons are separated and of
non-zero scale size, but becomes non-zero (which can be counted miscrospically)
after either of the phase transitions.Comment: 18 pages, Late
Vertical versus Diagonal Dimensional Reduction for p-branes
In addition to the double-dimensional reduction procedure that employs
world-volume Killing symmetries of -brane supergravity solutions and acts
diagonally on a plot of versus spacetime dimension , there exists a
second procedure of ``vertical'' reduction. This reduces the transverse-space
dimension via an integral that superposes solutions to the underlying Laplace
equation. We show that vertical reduction is also closely related to the
recently-introduced notion of intersecting -branes. We illustrate this with
examples, and also construct a new solution describing four intersecting
membranes, which preserves of the supersymmetry. Given the two reduction
schemes plus duality transformations at special points of the scalar modulus
space, one may relate most of the -brane solutions of relevance to
superstring theory. We argue that the maximum classifying duality symmetry for
this purpose is the Weyl group of the corresponding Cremmer-Julia supergravity
symmetry . We also discuss a separate class of duality-invariant
-branes with .Comment: Latex, 21 pages, no figures. References adde
Self-consistent local-equilibrium model for density profile and distribution of dissipative currents in a Hall bar under strong magnetic fields
Recent spatially resolved measurements of the electrostatic-potential
variation across a Hall bar in strong magnetic fields, which revealed a clear
correlation between current-carrying strips and incompressible strips expected
near the edges of the Hall bar, cannot be understood on the basis of existing
equilibrium theories. To explain these experiments, we generalize the
Thomas-Fermi--Poisson approach for the self-consistent calculation of
electrostatic potential and electron density in {\em total} thermal equilibrium
to a {\em local equilibrium} theory that allows to treat finite gradients of
the electrochemical potential as driving forces of currents in the presence of
dissipation. A conventional conductivity model with small values of the
longitudinal conductivity for integer values of the (local) Landau-level
filling factor shows that, in apparent agreement with experiment, the current
density is localized near incompressible strips, whose location and width in
turn depend on the applied current.Comment: 9 pages, 7 figure
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