599,274 research outputs found
A Fast Convergence Density Evolution Algorithm for Optimal Rate LDPC Codes in BEC
We derive a new fast convergent Density Evolution algorithm for finding
optimal rate Low-Density Parity-Check (LDPC) codes used over the binary erasure
channel (BEC). The fast convergence property comes from the modified Density
Evolution (DE), a numerical method for analyzing the behavior of iterative
decoding convergence of a LDPC code. We have used the method of [16] for
designing of a LDPC code with optimal rate. This has been done for a given
parity check node degree distribution, erasure probability and specified DE
constraint. The fast behavior of DE and found optimal rate with this method
compare with the previous DE constraint.Comment: This Paper is a draft of final paper which represented in 7th
International Symposium on Telecommunications (IST'2014
Spontaneous non-steady magnetic reconnection within the solar environment
This work presents a 2.5-dimensional simulation study of the instability of
current-sheets located in a medium with a strong density variation along the
current layer. The initial force-free configuration is observed to undergo a
two-stage evolution consisting of an abrupt regime transition from a slow to a
fast reconnection process leading the system to a final chaotic configuration.
Yet, the onset of the fast phase is not determined by the presence of any
anomalous enhancement in plasma's local resistivity, but rather is the result
of a new mechanism discovered in Lapenta (2008)* and captured only when
sufficient resolution is present. Hence, the effects of the global resistivity,
the global viscosity and the plasma beta on the overall dynamics are
considered. This mechanism allowing the transition from slow to fast
reconnection provides a simple but effective model of several processes taking
place within the solar atmosphere from the high chromosphere up to the low
corona. In fact, the understanding of a spontaneous transition to a
self-feeding fast reconnection regime as well as its macroscopic evolution is
the first and fundamental step to produce realistic models of all those
phenomena requiring fast (and high power) triggering events (* Lapenta G. 2008,
Phys. Rev. Lett., 100, 235001).Comment: 29 pages, 10 figure
The Mellin Transform Technique for the Extraction of the Gluon Density
A new method is presented to determine the gluon density in the proton from
jet production in deeply inelastic scattering. By using the technique of Mellin
transforms not only for the solution of the scale evolution equation of the
parton densities but also for the evaluation of scattering cross sections, the
gluon density can be extracted in next-to-leading order QCD. The method
described in this paper is, however, more general, and can be used in
situations where a repeated fast numerical evaluation of scattering cross
sections for varying parton distribution functions is required.Comment: 13 pages (LaTeX); 2 figures are included via epsfig; the
corresponding postscript files are uuencode
Multidimensional spectroscopy with entangled light; loop vs ladder delay scanning protocols
Multidimensional optical signals are commonly recorded by varying the delays
between time ordered pulses. These control the evolution of the density matrix
and are described by ladder diagrams. We propose a new non-time-ordered
protocol based on following the time evolution of the wavefunction and
described by loop diagrams. The time variables in this protocol allow to
observe different types of resonances and reveal information about intraband
dephasing not readily available by time ordered techniques. The time variables
involved in this protocol become coupled when using entangled light, which
provides high selectivity and background free measurement of the various
resonances. Entangled light can resolve certain states even when strong
background due to fast dephasing suppresses the resonant features when probed
by classical light
Derivation of the time dependent Gross-Pitaevskii equation without positivity condition on the interaction
Using a new method it is possible to derive mean field equations from the
microscopic body Schr\"odinger evolution of interacting particles without
using BBGKY hierarchies.
In this paper we wish to analyze scalings which lead to the Gross-Pitaevskii
equation which is usually derived assuming positivity of the interaction. The
new method for dealing with mean field limits presented in [6] allows us to
relax this condition. The price we have to pay for this relaxation is however
that we have to restrict the scaling behavior to and that we have
to assume fast convergence of the reduced one particle marginal density matrix
of the initial wave function to a pure state
Radio Observations of SN 1980K: Evidence for Rapid Presupernova Evolution
New observations of SN 1980K made with the VLA at 20 and 6 cm from 1994 April
through 1996 October show that the supernova (SN) has undergone a significant
change in its radio emission evolution, dropping by a factor of ~2 below the
flux density S \propto t^{-0.73} power-law decline with time t observed
earlier. However, although S at all observed frequencies has decreased
significantly, its current spectral index of \alpha= -0.42\pm0.15 (S \propto
\nu^{+\alpha}) is consistent with the previous spectral index of
\alpha=-0.60_{-0.07}^{+0.04}.
It is suggested that this decrease in emission may be due to the SN shock
entering a new region of the circumstellar material which has a lower density
than that expected for a constant speed (w), constant mass-loss rate (Mdot)
wind from the progenitor. If such an interpretation is correct, the difference
in wind and shock speeds appears to indicate a significant evolution in the
mass-loss history of the SN progenitor ~10^4 years before explosion, with a
change in circumstellar density (\propto Mdot/w) occurring over a time span of
\lesssim 4 kyr. Such features could be explained in terms of a fast
``blue-loop'' evolutionary phase of a relatively massive pre-SN progenitor
star. If so, we may, for the first time, provide a stringent constraint on the
mass of the SN progenitor based solely on the SN's radio emission.Comment: 22 pages, 3 figures, to appear in Ap
Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations
An incident fast ion in the electronic stopping regime produces a track of
excitations which can lead to particle ejection and cratering. Molecular
Dynamics simulations of the evolution of the deposited energy were used to
study the resulting crater morphology as a function of the excitation density
in a cylindrical track for large angle of incidence with respect to the surface
normal. Surprisingly, the overall behavior is shown to be similar to that seen
in the experimental data for crater formation in polymers. However, the
simulations give greater insight into the cratering process. The threshold for
crater formation occurs when the excitation density approaches the cohesive
energy density, and a crater rim is formed at about six times that energy
density. The crater length scales roughly as the square root of the electronic
stopping power, and the crater width and depth seem to saturate for the largest
energy densities considered here. The number of ejected particles, the
sputtering yield, is shown to be much smaller than simple estimates based on
crater size unless the full crater morphology is considered. Therefore, crater
size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see:
http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded
5/16/01, with minor text changes + new figure
Optimal Redshift Weighting For Redshift Space Distortions
The low statistical errors on cosmological parameters promised by future
galaxy surveys will only be realised with the development of new, fast,
analysis methods that reduce potential systematic problems to low levels. We
present an efficient method for measuring the evolution of the growth of
structure using Redshift Space Distortions (RSD), that removes the need to make
measurements in redshift shells. We provide sets of galaxy-weights that cover a
wide range in redshift, but are optimised to provide differential information
about cosmological evolution. These are derived to optimally measure the
coefficients of a parameterisation of the redshift-dependent matter density,
which provides a framework to measure deviations from the concordance
CDM cosmology, allowing for deviations in both geometric and/or
growth. We test the robustness of the weights by comparing with alternative
schemes and investigate the impact of galaxy bias. We extend the results to
measure the combined anisotropic Baryon Acoustic Oscillation (BAO) and RSD
signals.Comment: 10 pages, 5 figures, submitted to MNRA
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