30,271 research outputs found
Mobility: a double-edged sword for HSPA networks
This paper presents an empirical study on the performance of mobile High Speed Packet Access (HSPA, a 3.5G cellular standard) networks in Hong Kong via extensive field tests. Our study, from the viewpoint of end users, covers virtually all possible mobile scenarios in urban areas, including subways, trains, off-shore ferries and city buses. We have confirmed that mobility has largely negative impacts on the performance of HSPA networks, as fast-changing wireless environment causes serious service deterioration or even interruption. Meanwhile our field experiment results have shown unexpected new findings and thereby exposed new features of the mobile HSPA networks, which contradict commonly held views. We surprisingly find out that mobility can improve fairness of bandwidth sharing among users and traffic flows. Also the triggering and final results of handoffs in mobile HSPA networks are unpredictable and often inappropriate, thus calling for fast reacting fallover mechanisms. We have conducted in-depth research to furnish detailed analysis and explanations to what we have observed. We conclude that mobility is a double-edged sword for HSPA networks. To the best of our knowledge, this is the first public report on a large scale empirical study on the performance of commercial mobile HSPA networks
Probing viscoelastic properties of a thin polymer film sheared between a beads layer and quartz crystal resonator
We report measurements of viscoelastic properties of thin polymer films of
10-100 nm at the MHz range. These thin films are confined between a quartz
crystal resonator and a millimetric bead layer, producing an increase of both
resonance frequency and dissipation of the quartz resonator. The shear modulus
and dynamic viscosity of thin films extracted from these measurements are
consistent with the bulk values of the polymer. This modified quartz resonator
provides an easily realizable and effective tool for probing the rheological
properties of thin films at ambient environment.Comment: submitted to ap
Gunrock: A High-Performance Graph Processing Library on the GPU
For large-scale graph analytics on the GPU, the irregularity of data access
and control flow, and the complexity of programming GPUs have been two
significant challenges for developing a programmable high-performance graph
library. "Gunrock", our graph-processing system designed specifically for the
GPU, uses a high-level, bulk-synchronous, data-centric abstraction focused on
operations on a vertex or edge frontier. Gunrock achieves a balance between
performance and expressiveness by coupling high performance GPU computing
primitives and optimization strategies with a high-level programming model that
allows programmers to quickly develop new graph primitives with small code size
and minimal GPU programming knowledge. We evaluate Gunrock on five key graph
primitives and show that Gunrock has on average at least an order of magnitude
speedup over Boost and PowerGraph, comparable performance to the fastest GPU
hardwired primitives, and better performance than any other GPU high-level
graph library.Comment: 14 pages, accepted by PPoPP'16 (removed the text repetition in the
previous version v5
Relaxed 2-D Principal Component Analysis by Norm for Face Recognition
A relaxed two dimensional principal component analysis (R2DPCA) approach is
proposed for face recognition. Different to the 2DPCA, 2DPCA- and G2DPCA,
the R2DPCA utilizes the label information (if known) of training samples to
calculate a relaxation vector and presents a weight to each subset of training
data. A new relaxed scatter matrix is defined and the computed projection axes
are able to increase the accuracy of face recognition. The optimal -norms
are selected in a reasonable range. Numerical experiments on practical face
databased indicate that the R2DPCA has high generalization ability and can
achieve a higher recognition rate than state-of-the-art methods.Comment: 19 pages, 11 figure
Interface Width and Bulk Stability: requirements for the simulation of Deeply Quenched Liquid-Gas Systems
Simulations of liquid-gas systems with extended interfaces are observed to
fail to give accurate results for two reasons: the interface can get ``stuck''
on the lattice or a density overshoot develops around the interface. In the
first case the bulk densities can take a range of values, dependent on the
initial conditions. In the second case inaccurate bulk densities are found. In
this communication we derive the minimum interface width required for the
accurate simulation of liquid gas systems with a diffuse interface. We
demonstrate this criterion for lattice Boltzmann simulations of a van der Waals
gas. When combining this criterion with predictions for the bulk stability we
can predict the parameter range that leads to stable and accurate simulation
results. This allows us to identify parameter ranges leading to high density
ratios of over 1000. This is despite the fact that lattice Boltzmann
simulations of liquid-gas systems were believed to be restricted to modest
density ratios of less than 20.Comment: 5 pages, 3 figure
The CJT calculation in studying nuclear matter beyond mean field approximation
We have introduced a CJT calculation in studying nuclear matter beyond mean
field approximation. Based on the CJT formalism and using Walecka model, we
have derived a set of coupled Dyson equations of nucleons and mesons.
Neglecting the medium effects of the mesons, the usual MFT results could be
reproduced. The beyond MFT calculations have been performed by thermodynamic
consistently determining the meson effective masses and solving the coupled gap
equations for nucleons and mesons. The numerical results for the nucleon and
meson effective masses at finite temperature and chemical potential in nuclear
matter are discussed.Comment: 8 pages, 8 figure
Electronic structure and bonding properties of cobalt oxide in the spinel structure
The spinel cobalt oxide Co3O4 is a magnetic semiconductor containing cobalt
ions in Co2+ and Co3+ oxidation states. We have studied the electronic,
magnetic and bonding properties of Co3O4 using density functional theory (DFT)
at the Generalized Gradient Approximation (GGA), GGA+U, and PBE0 hybrid
functional levels. The GGA correctly predicts Co3O4 to be a semiconductor, but
severely underestimates the band gap. The GGA+U band gap (1.96 eV) agrees well
with the available experimental value (~ 1.6 eV), whereas the band gap obtained
using the PBE0 hybrid functional (3.42 eV) is strongly overestimated. All the
employed exchange-correlation functionals predict 3 unpaired d electrons on the
Co2+ ions, in agreement with crystal field theory, but the values of the
magnetic moments given by GGA+U and PBE0 are in closer agreement with the
experiment than the GGA value, indicating a better description of the cobalt
localized d states. Bonding properties are studied by means of Maximally
Localized Wannier Functions (MLWFs). We find d-type MLWFs on the cobalt ions,
as well as Wannier functions with the character of sp3d bonds between cobalt
and oxygen ions. Such hybridized bonding states indicate the presence of a
small covalent component in the primarily ionic bonding mechanism of this
compound.Comment: 24 pages, 8 figure
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