334,019 research outputs found
Faraday Rotation Measure Synthesis
We extend the rotation measure work of Burn (1966) to the cases of limited
sampling of lambda squared space and non-constant emission spectra. We
introduce the rotation measure transfer function (RMTF), which is an excellent
predictor of n-pi ambiguity problems with the lambda squared coverage. Rotation
measure synthesis can be implemented very efficiently on modern computers.
Because the analysis is easily applied to wide fields, one can conduct very
fast RM surveys of weak spatially extended sources. Difficult situations, for
example multiple sources along the line of sight, are easily detected and
transparently handled. Under certain conditions, it is even possible to recover
the emission as a function of Faraday depth within a single cloud of ionized
gas. Rotation measure synthesis has already been successful in discovering
widespread, weak, polarized emission associated with the Perseus cluster (De
Bruyn and Brentjens, 2005). In simple, high signal to noise situations it is as
good as traditional linear fits to polarization angle versus lambda squared
plots. However, when the situation is more complex or very weak polarized
emission at high rotation measures is expected, it is the only viable option.Comment: 17 pages, 14 figures, accepted by A&A, added references, corrected
typo
A 90 nm CMOS 16 Gb/s Transceiver for Optical Interconnects
Interconnect architectures which leverage high-bandwidth optical channels offer a promising solution to address the increasing chip-to-chip I/O bandwidth demands. This paper describes a dense, high-speed, and low-power CMOS optical interconnect transceiver architecture. Vertical-cavity surface-emitting laser (VCSEL) data rate is extended for a given average current and corresponding reliability level with a four-tap current summing FIR transmitter. A low-voltage integrating and double-sampling optical receiver front-end provides adequate sensitivity in a power efficient manner by avoiding linear high-gain elements common in conventional transimpedance-amplifier (TIA) receivers. Clock recovery is performed with a dual-loop architecture which employs baud-rate phase detection and feedback interpolation to achieve reduced power consumption, while high-precision phase spacing is ensured at both the transmitter and receiver through adjustable delay clock buffers. A prototype chip fabricated in 1 V 90 nm CMOS achieves 16 Gb/s operation while consuming 129 mW and occupying 0.105 mm^2
A Sensitivity and Array-Configuration Study for Measuring the Power Spectrum of 21cm Emission from Reionization
Telescopes aiming to measure 21cm emission from the Epoch of Reionization
must toe a careful line, balancing the need for raw sensitivity against the
stringent calibration requirements for removing bright foregrounds. It is
unclear what the optimal design is for achieving both of these goals. Via a
pedagogical derivation of an interferometer's response to the power spectrum of
21cm reionization fluctuations, we show that even under optimistic scenarios,
first-generation arrays will yield low-SNR detections, and that different
compact array configurations can substantially alter sensitivity. We explore
the sensitivity gains of array configurations that yield high redundancy in the
uv-plane -- configurations that have been largely ignored since the advent of
self-calibration for high-dynamic-range imaging. We first introduce a
mathematical framework to generate optimal minimum-redundancy configurations
for imaging. We contrast the sensitivity of such configurations with
high-redundancy configurations, finding that high-redundancy configurations can
improve power-spectrum sensitivity by more than an order of magnitude. We
explore how high-redundancy array configurations can be tuned to various
angular scales, enabling array sensitivity to be directed away from regions of
the uv-plane (such as the origin) where foregrounds are brighter and where
instrumental systematics are more problematic. We demonstrate that a
132-antenna deployment of the Precision Array for Probing the Epoch of
Reionization (PAPER) observing for 120 days in a high-redundancy configuration
will, under ideal conditions, have the requisite sensitivity to detect the
power spectrum of the 21cm signal from reionization at a 3\sigma level at
k<0.25h Mpc^{-1} in a bin of \Delta ln k=1. We discuss the tradeoffs of low-
versus high-redundancy configurations.Comment: 34 pages, 5 figures, 2 appendices. Version accepted to Ap
Optimal sub-arraying of compromise planar arrays through an innovative ACO-weighted procedure
In this paper, the synthesis of sub-arrayed monopulse planar arrays providing an optimal sum pattern and best compromise difference patterns is addressed by means of an innovative clustering approach based on the Ant Colony Optimizer. Exploiting the similarity properties of optimal and independent sum and difference excitation sets, the problem is reformulated into a combinatorial one where the definition of the sub-array configuration is obtained through the search of a path within a weighted graph. Such a weighting strategy allows one to effectively sample the solution space avoiding bias towards sub-optimal solutions. The sub-array weight coefficients are then determined in an optimal way by exploiting the convexity of the problem at hand by means of a convex programming procedure. Representative results are reported to assess the effectiveness of the weighted global optimization and its advantages over previous implementations. (c) The Electromagnetics Academy - The final version of this article is available at the url of the journal PIER (Progress In Electromagnetics Research): http://www.jpier.org/PIER/pier.php?paper=1009200
(k,q)-Compressed Sensing for dMRI with Joint Spatial-Angular Sparsity Prior
Advanced diffusion magnetic resonance imaging (dMRI) techniques, like
diffusion spectrum imaging (DSI) and high angular resolution diffusion imaging
(HARDI), remain underutilized compared to diffusion tensor imaging because the
scan times needed to produce accurate estimations of fiber orientation are
significantly longer. To accelerate DSI and HARDI, recent methods from
compressed sensing (CS) exploit a sparse underlying representation of the data
in the spatial and angular domains to undersample in the respective k- and
q-spaces. State-of-the-art frameworks, however, impose sparsity in the spatial
and angular domains separately and involve the sum of the corresponding sparse
regularizers. In contrast, we propose a unified (k,q)-CS formulation which
imposes sparsity jointly in the spatial-angular domain to further increase
sparsity of dMRI signals and reduce the required subsampling rate. To
efficiently solve this large-scale global reconstruction problem, we introduce
a novel adaptation of the FISTA algorithm that exploits dictionary
separability. We show on phantom and real HARDI data that our approach achieves
significantly more accurate signal reconstructions than the state of the art
while sampling only 2-4% of the (k,q)-space, allowing for the potential of new
levels of dMRI acceleration.Comment: To be published in the 2017 Computational Diffusion MRI Workshop of
MICCA
Modeling the ionizing spectra of H ii regions: individual stars versus stellar ensembles
Aims. We study how IMF sampling affects the ionizing flux and emission line
spectra of low mass stellar clusters. Methods. We performed 2 x 10^6 Monte
Carlo simulations of zero-age solar-metallicity stellar clusters covering the
20 - 10^6 Mo mass range. We study the distribution of cluster stellar masses,
Mclus, ionizing fluxes, Q(H0), and effective temperatures, Tclus. We compute
photoionization models that broadly describe the results of the simulations and
compare them with photoionization grids. Results. Our main results are: (a) A
large number of low mass clusters (80% for Mclus = 100 Mo) are unable to form
an H ii region. (b) There are a few overluminous stellar clusters that form H
ii regions. These overluminous clusters preserve statistically the mean value
of obtained by synthesis models, but the mean value cannot be used as a
description of particular clusters. (c) The ionizing continuum of clusters with
Mclus < 10^4 Mo is more accurately described by an individual star with
self-consistent effective temperature(T*) and Q(H0) than by the ensemble of
stars (or a cluster Tclus) produced by synthesis models. (d)Photoionization
grids of stellar clusters can not be used to derive the global properties of
low mass clusters. Conclusions. Although variations in the upper mass limit,
mup, of the IMF would reproduce the effects of IMF sampling, we find that an ad
hoc law that relates mup to Mclus in the modelling of stellar clusters is
useless, since: (a) it does not cover the whole range of possible cases, and
(b) the modelling of stellar clusters with an IMF is motivated by the need to
derive the global properties of the cluster: however, in clusters affected by
sampling effects we have no access to global information of the cluster but
only particular information about a few individual stars.Comment: A&A in pres
Statistical Methods for Thermonuclear Reaction Rates and Nucleosynthesis Simulations
Rigorous statistical methods for estimating thermonuclear reaction rates and
nucleosynthesis are becoming increasingly established in nuclear astrophysics.
The main challenge being faced is that experimental reaction rates are highly
complex quantities derived from a multitude of different measured nuclear
parameters (e.g., astrophysical S-factors, resonance energies and strengths,
particle and gamma-ray partial widths). We discuss the application of the Monte
Carlo method to two distinct, but related, questions. First, given a set of
measured nuclear parameters, how can one best estimate the resulting
thermonuclear reaction rates and associated uncertainties? Second, given a set
of appropriate reaction rates, how can one best estimate the abundances from
nucleosynthesis (i.e., reaction network) calculations? The techniques described
here provide probability density functions that can be used to derive
statistically meaningful reaction rates and final abundances for any desired
coverage probability. Examples are given for applications to s-process neutron
sources, core-collapse supernovae, classical novae, and big bang
nucleosynthesis.Comment: Accepted for publication in J. Phys. G Focus issue "Enhancing the
interaction between nuclear experiment and theory through information and
statistics
Distributed Event-Based State Estimation for Networked Systems: An LMI-Approach
In this work, a dynamic system is controlled by multiple sensor-actuator
agents, each of them commanding and observing parts of the system's input and
output. The different agents sporadically exchange data with each other via a
common bus network according to local event-triggering protocols. From these
data, each agent estimates the complete dynamic state of the system and uses
its estimate for feedback control. We propose a synthesis procedure for
designing the agents' state estimators and the event triggering thresholds. The
resulting distributed and event-based control system is guaranteed to be stable
and to satisfy a predefined estimation performance criterion. The approach is
applied to the control of a vehicle platoon, where the method's trade-off
between performance and communication, and the scalability in the number of
agents is demonstrated.Comment: This is an extended version of an article to appear in the IEEE
Transactions on Automatic Control (additional parts in the Appendix
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