146,131 research outputs found
Statistical Traffic State Analysis in Large-scale Transportation Networks Using Locality-Preserving Non-negative Matrix Factorization
Statistical traffic data analysis is a hot topic in traffic management and
control. In this field, current research progresses focus on analyzing traffic
flows of individual links or local regions in a transportation network. Less
attention are paid to the global view of traffic states over the entire
network, which is important for modeling large-scale traffic scenes. Our aim is
precisely to propose a new methodology for extracting spatio-temporal traffic
patterns, ultimately for modeling large-scale traffic dynamics, and long-term
traffic forecasting. We attack this issue by utilizing Locality-Preserving
Non-negative Matrix Factorization (LPNMF) to derive low-dimensional
representation of network-level traffic states. Clustering is performed on the
compact LPNMF projections to unveil typical spatial patterns and temporal
dynamics of network-level traffic states. We have tested the proposed method on
simulated traffic data generated for a large-scale road network, and reported
experimental results validate the ability of our approach for extracting
meaningful large-scale space-time traffic patterns. Furthermore, the derived
clustering results provide an intuitive understanding of spatial-temporal
characteristics of traffic flows in the large-scale network, and a basis for
potential long-term forecasting.Comment: IET Intelligent Transport Systems (2013
Modeling open nanophotonic systems using the Fourier modal method: Generalization to 3D Cartesian coordinates
Recently, an open geometry Fourier modal method based on a new combination of
an open boundary condition and a non-uniform -space discretization was
introduced for rotationally symmetric structures providing a more efficient
approach for modeling nanowires and micropillar cavities [J. Opt. Soc. Am. A
33, 1298 (2016)]. Here, we generalize the approach to three-dimensional (3D)
Cartesian coordinates allowing for the modeling of rectangular geometries in
open space. The open boundary condition is a consequence of having an infinite
computational domain described using basis functions that expand the whole
space. The strength of the method lies in discretizing the Fourier integrals
using a non-uniform circular "dartboard" sampling of the Fourier space. We
show that our sampling technique leads to a more accurate description of the
continuum of the radiation modes that leak out from the structure. We also
compare our approach to conventional discretization with direct and inverse
factorization rules commonly used in established Fourier modal methods. We
apply our method to a variety of optical waveguide structures and demonstrate
that the method leads to a significantly improved convergence enabling more
accurate and efficient modeling of open 3D nanophotonic structures
Seizure-onset mapping based on time-variant multivariate functional connectivity analysis of high-dimensional intracranial EEG : a Kalman filter approach
The visual interpretation of intracranial EEG (iEEG) is the standard method used in complex epilepsy surgery cases to map the regions of seizure onset targeted for resection. Still, visual iEEG analysis is labor-intensive and biased due to interpreter dependency. Multivariate parametric functional connectivity measures using adaptive autoregressive (AR) modeling of the iEEG signals based on the Kalman filter algorithm have been used successfully to localize the electrographic seizure onsets. Due to their high computational cost, these methods have been applied to a limited number of iEEG time-series (< 60). The aim of this study was to test two Kalman filter implementations, a well-known multivariate adaptive AR model (Arnold et al. 1998) and a simplified, computationally efficient derivation of it, for their potential application to connectivity analysis of high-dimensional (up to 192 channels) iEEG data. When used on simulated seizures together with a multivariate connectivity estimator, the partial directed coherence, the two AR models were compared for their ability to reconstitute the designed seizure signal connections from noisy data. Next, focal seizures from iEEG recordings (73-113 channels) in three patients rendered seizure-free after surgery were mapped with the outdegree, a graph-theory index of outward directed connectivity. Simulation results indicated high levels of mapping accuracy for the two models in the presence of low-to-moderate noise cross-correlation. Accordingly, both AR models correctly mapped the real seizure onset to the resection volume. This study supports the possibility of conducting fully data-driven multivariate connectivity estimations on high-dimensional iEEG datasets using the Kalman filter approach
Path-tracing Monte Carlo Library for 3D Radiative Transfer in Highly Resolved Cloudy Atmospheres
Interactions between clouds and radiation are at the root of many
difficulties in numerically predicting future weather and climate and in
retrieving the state of the atmosphere from remote sensing observations. The
large range of issues related to these interactions, and in particular to
three-dimensional interactions, motivated the development of accurate radiative
tools able to compute all types of radiative metrics, from monochromatic, local
and directional observables, to integrated energetic quantities. In the
continuity of this community effort, we propose here an open-source library for
general use in Monte Carlo algorithms. This library is devoted to the
acceleration of path-tracing in complex data, typically high-resolution
large-domain grounds and clouds. The main algorithmic advances embedded in the
library are those related to the construction and traversal of hierarchical
grids accelerating the tracing of paths through heterogeneous fields in
null-collision (maximum cross-section) algorithms. We show that with these
hierarchical grids, the computing time is only weakly sensitivive to the
refinement of the volumetric data. The library is tested with a rendering
algorithm that produces synthetic images of cloud radiances. Two other examples
are given as illustrations, that are respectively used to analyse the
transmission of solar radiation under a cloud together with its sensitivity to
an optical parameter, and to assess a parametrization of 3D radiative effects
of clouds.Comment: Submitted to JAMES, revised and submitted again (this is v2
Benchmarking five numerical simulation techniques for computing resonance wavelengths and quality factors in photonic crystal membrane line defect cavities
We present numerical studies of two photonic crystal membrane microcavities,
a short line-defect cavity with relatively low quality () factor and a
longer cavity with high . We use five state-of-the-art numerical simulation
techniques to compute the cavity factor and the resonance wavelength
for the fundamental cavity mode in both structures. For each method,
the relevant computational parameters are systematically varied to estimate the
computational uncertainty. We show that some methods are more suitable than
others for treating these challenging geometries.Comment: Revised and final version for publication. 28 pages, 10 figures, 7
table
Open geometry Fourier modal method: Modeling nanophotonic structures in infinite domains
We present an open geometry Fourier modal method based on a new combination
of open boundary conditions and an efficient -space discretization. The open
boundary of the computational domain is obtained using basis functions that
expand the whole space, and the integrals subsequently appearing due to the
continuous nature of the radiation modes are handled using a discretization
based on non-uniform sampling of the -space. We apply the method to a
variety of photonic structures and demonstrate that our method leads to
significantly improved convergence with respect to the number of degrees of
freedom, which may pave the way for more accurate and efficient modeling of
open nanophotonic structures
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