54,683 research outputs found
A network resource availability model for IEEE802.11a/b-based WLAN carrying different service types
The electronic version of this article is the complete one and can be found online at: http://jwcn.eurasipjournals.com/content/2011/1/103.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Operators of integrated wireless systems need to have knowledge of the resource availability in their different access networks to perform efficient admission control and maintain good quality of experience to users. Network availability depends on the access technology and the service types. Resource availability in a WLAN is complex to gather when UDP and TCP services co-exist. Previous study on IEEE802.11a/b derived the achievable throughput under the assumption of inelastic and uniformly distributed traffic. Further study investigated TCP connections and derived a model to calculate the effective transmission rate of packets under the assumption of saturated traffic flows. The assumptions are too stringent; therefore, we developed a model for evaluating WLAN resource availability that tries to narrow the gap to more realistic scenarios. It provides an indication of WLAN resource availability for admitting UDP/TCP requests. This article presents the assumptions, the mathematical formulations, and the effectiveness of our model
Hybrid finite difference/finite element immersed boundary method
The immersed boundary method is an approach to fluid-structure interaction that uses a Lagrangian
description of the structural deformations, stresses, and forces along with an Eulerian description of the
momentum, viscosity, and incompressibility of the fluid-structure system. The original immersed boundary
methods described immersed elastic structures using systems of flexible fibers, and even now, most
immersed boundary methods still require Lagrangian meshes that are finer than the Eulerian grid. This
work introduces a coupling scheme for the immersed boundary method to link the Lagrangian and Eulerian
variables that facilitates independent spatial discretizations for the structure and background grid. This
approach employs a finite element discretization of the structure while retaining a finite difference scheme
for the Eulerian variables. We apply this method to benchmark problems involving elastic, rigid, and actively
contracting structures, including an idealized model of the left ventricle of the heart. Our tests include cases
in which, for a fixed Eulerian grid spacing, coarser Lagrangian structural meshes yield discretization errors
that are as much as several orders of magnitude smaller than errors obtained using finer structural meshes.
The Lagrangian-Eulerian coupling approach developed in this work enables the effective use of these coarse
structural meshes with the immersed boundary method. This work also contrasts two different weak forms
of the equations, one of which is demonstrated to be more effective for the coarse structural discretizations
facilitated by our coupling approach
Towards Minimax Online Learning with Unknown Time Horizon
We consider online learning when the time horizon is unknown. We apply a
minimax analysis, beginning with the fixed horizon case, and then moving on to
two unknown-horizon settings, one that assumes the horizon is chosen randomly
according to some known distribution, and the other which allows the adversary
full control over the horizon. For the random horizon setting with restricted
losses, we derive a fully optimal minimax algorithm. And for the adversarial
horizon setting, we prove a nontrivial lower bound which shows that the
adversary obtains strictly more power than when the horizon is fixed and known.
Based on the minimax solution of the random horizon setting, we then propose a
new adaptive algorithm which "pretends" that the horizon is drawn from a
distribution from a special family, but no matter how the actual horizon is
chosen, the worst-case regret is of the optimal rate. Furthermore, our
algorithm can be combined and applied in many ways, for instance, to online
convex optimization, follow the perturbed leader, exponential weights algorithm
and first order bounds. Experiments show that our algorithm outperforms many
other existing algorithms in an online linear optimization setting
Exotic mesons from quantum chromodynamics with improved gluon and quark actions on the anisotropic lattice
Hybrid (exotic) mesons, which are important predictions of quantum
chromodynamics (QCD), are states of quarks and anti-quarks bound by excited
gluons. First principle lattice study of such states would help us understand
the role of ``dynamical'' color in low energy QCD and provide valuable
information for experimental search for these new particles. In this paper, we
apply both improved gluon and quark actions to the hybrid mesons, which might
be much more efficient than the previous works in reducing lattice spacing
error and finite volume effect. Quenched simulations were done at
and on a anisotropic lattice using our PC cluster. We
obtain MeV for the mass of the hybrid meson
in the light quark sector, and Mev in the
charm quark sector; the mass splitting between the hybrid meson in the charm quark sector and the spin averaged S-wave charmonium mass
is estimated to be MeV. As a byproduct, we obtain MeV for the mass of a P-wave or
meson and MeV for the mass of a P-wave meson, which are comparable to their experimental value 1426 MeV for the
meson. The first error is statistical, and the second one is
systematical. The mixing of the hybrid meson with a four quark state is also
discussed.Comment: 12 pages, 3 figures. Published versio
Two-loop Renormalization Group Equations in General Gauge Field Theories
The complete set of two-loop renormalization group equations in general gauge
field theories is presented. This includes the \beta functions of parameters
with and without a mass dimension
Massive Dirac surface states in topological insulator/magnetic insulator heterostructures
Topological insulators are new states of matter with a bulk gap and robust
gapless surface states protected by time-reversal symmetry. When time-reversal
symmetry is broken, the surface states are gapped, which induces a topological
response of the system to electromagnetic field--the topological
magnetoelectric effect. In this paper we study the behavior of topological
surface states in heterostructures formed by a topological insulator and a
magnetic insulator. Several magnetic insulators with compatible magnetic
structure and relatively good lattice matching with topological insulators
are identified, and the best
candidate material is found to be MnSe, an anti-ferromagnetic insulator. We
perform first-principles calculation in superlattices and
obtain the surface state bandstructure. The magnetic exchange coupling with
MnSe induces a gap of 54 meV at the surface states. In addition we tune
the distance between Mn ions and TI surface to study the distance dependence of
the exchange coupling.Comment: 8 pages, 7 figure
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