23,457 research outputs found
Study of a microcanonical algorithm on the spin glass model in d=3
We consider a microcanonical local algorithm to be applied on the
spin glass model. We have compared the results coming from a microcanonical
Monte Carlo simulation with those from a canonical one: Thermalization times,
spin glass susceptibilities and Binder parameters. For a fixed lattice size we
found different results between the two thermodynamic ensembles, which tend to
vanish at bigger volumes. Moreover, microcanonical thermalization times are
longer than the canonical ones. Finally we have checked that one of the Guerra
relations is satisfied with good precision for the two largest lattices.Comment: Revised version. Latex 14 pages, 6 figures. To be published in
Comput. Phys. Commu
Large deviations for a stochastic model of heat flow
We investigate a one dimensional chain of harmonic oscillators in which
neighboring sites have their energies redistributed randomly. The sites
and are in contact with thermal reservoirs at different temperature
and . Kipnis, Marchioro, and Presutti \cite{KMP} proved that
this model satisfies {}Fourier's law and that in the hydrodynamical scaling
limit, when , the stationary state has a linear energy density
profile , . We derive the large deviation
function for the probability of finding, in the stationary
state, a profile different from . The function
has striking similarities to, but also large differences from, the
corresponding one of the symmetric exclusion process. Like the latter it is
nonlocal and satisfies a variational equation. Unlike the latter it is not
convex and the Gaussian normal fluctuations are enhanced rather than suppressed
compared to the local equilibrium state. We also briefly discuss more general
model and find the features common in these two and other models whose
is known.Comment: 28 pages, 0 figure
FDD massive MIMO channel spatial covariance conversion using projection methods
Knowledge of second-order statistics of channels (e.g. in the form of
covariance matrices) is crucial for the acquisition of downlink channel state
information (CSI) in massive MIMO systems operating in the frequency division
duplexing (FDD) mode. Current MIMO systems usually obtain downlink covariance
information via feedback of the estimated covariance matrix from the user
equipment (UE), but in the massive MIMO regime this approach is infeasible
because of the unacceptably high training overhead. This paper considers
instead the problem of estimating the downlink channel covariance from uplink
measurements. We propose two variants of an algorithm based on projection
methods in an infinite-dimensional Hilbert space that exploit channel
reciprocity properties in the angular domain. The proposed schemes are
evaluated via Monte Carlo simulations, and they are shown to outperform current
state-of-the art solutions in terms of accuracy and complexity, for typical
array geometries and duplex gaps.Comment: Paper accepted on 29/01/2018 for presentation at ICASSP 201
Downlink channel spatial covariance estimation in realistic FDD massive MIMO systems
The knowledge of the downlink (DL) channel spatial covariance matrix at the
BS is of fundamental importance for large-scale array systems operating in
frequency division duplexing (FDD) mode. In particular, this knowledge plays a
key role in the DL channel state information (CSI) acquisition. In the massive
MIMO regime, traditional schemes based on DL pilots are severely limited by the
covariance feedback and the DL training overhead. To overcome this problem,
many authors have proposed to obtain an estimate of the DL spatial covariance
based on uplink (UL) measurements. However, many of these approaches rely on
simple channel models, and they are difficult to extend to more complex models
that take into account important effects of propagation in 3D environments and
of dual-polarized antenna arrays. In this study we propose a novel technique
that takes into account the aforementioned effects, in compliance with the
requirements of modern 4G and 5G system designs. Numerical simulations show the
effectiveness of our approach.Comment: [v2] is the version accepted at GlobalSIP 2018. Only minor changes
mainly in the introductio
Quantum cryptographic ranging
We present a system to measure the distance between two parties that allows
only trusted people to access the result. The security of the protocol is
guaranteed by the complementarity principle in quantum mechanics. The protocol
can be realized with available technology, at least as a proof of principle
experiment.Comment: 2 pages, 1 figure. Contribution to the proceedings of the IV edition
of the Garda Lake Workshop "Mysteries, Puzzles and Paradoxes in Quantum
Mechanics
How the orbital period of a test particle is modified by the Dvali-Gabadadze-Porrati gravity?
In addition to the pericentre \omega, the mean anomaly M and, thus, the mean
longitude \lambda, also the orbital period Pb and the mean motion of a test
particle are modified by the Dvali-Gabadadze-Porrati gravity. While the
correction to Pb depends on the mass of the central body and on the geometrical
features of the orbital motion around it, the correction to is independent
of them, up to terms of second order in the eccentricity . The latter one
amounts to about 2\times 10^-3 arcseconds per century. The present-day accuracy
in determining the mean motions of the inner planets of the Solar System from
radar ranging and differential Very Long Baseline Interferometry is
10^-2-5\times 10^-3 arcseconds per century, but it should be improved in the
near future when the data from the spacecraft to Mercury and Venus will be
available.Comment: LaTex, 7 pages, 13 references, no tables, no figures. Section 2.3
added. To appear in JCA
Overview of rocket engine control
The issues of Chemical Rocket Engine Control are broadly covered. The basic feedback information and control variables used in expendable and reusable rocket engines, such as Space Shuttle Main Engine, are discussed. The deficiencies of current approaches are considered and a brief introduction to Intelligent Control Systems for rocket engines (and vehicles) is presented
Soft-gluon resummation effects on parton distributions
We gauge the impact of soft-gluon resummation on quark distributions by
performing a simple fit of Deep Inelastic Scattering structure function data
using next-to-leading order (NLO) and next-to-leading-logarithmic
(NLL)-resummed coefficient functions. We make use of NuTeV charged-current
data, as well as New Muon Collaboration (NMC) and
Bologna-CERN-Dubna-Munich-Saclay (BCDMS) neutral-current results, which probe
large values of x. Our results suggest that the inclusion of resummation
effects in global fits of parton distributions is both feasible and desirable,
in order to achieve at large x the accuracy goals of the LHC physics program.Comment: 19 pages, 10 figures. Few changes after referee report, one figure
and references added, published versio
Gaia Data Release 2: Validation of the classification of RR Lyrae and Cepheid variables with the Kepler and K2 missions
The second data release of the Gaia mission includes an advance catalog of
variable stars. The classification of these stars are based on sparse
photometry from the first 22 months of the mission. We set out to investigate
the purity and completeness of the all-sky Gaia classification results with the
help of the continuous light curves of the observed targets from the Kepler and
K2 missions, focusing specifically on RR Lyrae and Cepheid pulsators, outside
the Galactic Bulge region. We crossmatched the Gaia identifications with the
observations collected by the Kepler space telescope. We inspected the light
curves visually, then calculated the relative Fourier coefficients and period
ratios for the single- and double-mode K2 RR Lyrae stars to further classify
them. We identified 1443 and 41 stars classified as RR Lyrae or Cepheid
variables in Gaia DR2 in the targeted observations of the two missions and 263
more RR Lyre targets in the Full-Frame Images (FFI) of the original mission. We
provide the crossmatch of these sources. We conclude that the RR Lyrae catalog
has a completeness between 70-78%, and provide a purity estimate between 92-98%
(targeted observations) with lower limits of 75% (FFI stars) and 51% (K2
worst-case scenario). The low number of Cepheids prevents us from drawing
detailed conclusions but the purity of the DR2 sample is estimated to be around
66%.Comment: 15 pages, 12 figures, 10 tables, accepted into the Gaia DR2 Special
Issue in A&A. V1: submitted version, v2: accepted version. Lists available at
http://konkoly.hu/~lmolnar/gaiakepler_datafiles_R1.zi
Spin precession in the Dvali-Gabadadze-Porrati braneworld scenario
In this letter we work out the secular precession of the spin of a gyroscope
in geodesic motion around a central mass in the framework of the
Dvali-Gabadadze-Porrati multidimensional gravity model. Such an effect, which
depends on the mass of the central body and on the orbit radius of the
gyroscope, contrary to the precessions of the orbital elements of the orbit of
a test body, is far too small to be detected.Comment: Latex, 5 pages, no figures, no tables, 10 reference
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