244 research outputs found
Variance Reduction of Resampling for Sequential Monte Carlo
A resampling scheme provides a way to switch low-weight particles for
sequential Monte Carlo with higher-weight particles representing the objective
distribution. The less the variance of the weight distribution is, the more
concentrated the effective particles are, and the quicker and more accurate it
is to approximate the hidden Markov model, especially for the nonlinear case.
We propose a repetitive deterministic domain with median ergodicity for
resampling and have achieved the lowest variances compared to the other
resampling methods. As the size of the deterministic domain (the size
of population), given a feasible size of particles, our algorithm is faster
than the state of the art, which is verified by theoretical deduction and
experiments of a hidden Markov model in both the linear and non-linear cases.Comment: 13 pages, 6 figure
Recommendations and illustrations for the evaluation of photonic random number generators
The never-ending quest to improve the security of digital information
combined with recent improvements in hardware technology has caused the field
of random number generation to undergo a fundamental shift from relying solely
on pseudo-random algorithms to employing optical entropy sources. Despite these
significant advances on the hardware side, commonly used statistical measures
and evaluation practices remain ill-suited to understand or quantify the
optical entropy that underlies physical random number generation. We review the
state of the art in the evaluation of optical random number generation and
recommend a new paradigm: quantifying entropy generation and understanding the
physical limits of the optical sources of randomness. In order to do this, we
advocate for the separation of the physical entropy source from deterministic
post-processing in the evaluation of random number generators and for the
explicit consideration of the impact of the measurement and digitization
process on the rate of entropy production. We present the Cohen-Procaccia
estimate of the entropy rate as one way to do this. In order
to provide an illustration of our recommendations, we apply the Cohen-Procaccia
estimate as well as the entropy estimates from the new NIST draft standards for
physical random number generators to evaluate and compare three common optical
entropy sources: single photon time-of-arrival detection, chaotic lasers, and
amplified spontaneous emission
Effect of stenosis geometry on the Doppler-catheter gradient relation in vitro: A manifestation of pressure recovery
AbstractObjectives. This study investigated the effect of stenosis geometry on the Doppler-catheter gradient relation.Background. Although gradient estimation by Doppler ultrasound has been shown to be accurate in various clinical and in vitro settings, there have also been reports of substantial discrepancies between Doppler and catheter gradients. These conflicting results may be due to differences in geometry and hemodynamic characteristics of flow obstructions.Methods. Stenoses of various geometry were simultaneously studied with continuous wave Doppler and catheter technique in a well controlled pulsatile flow model.Results. Doppler and catheter gradients correlated very well regardless of stenosis geometry and site of distal catheter measurement (r = 0.98 to 0.99, SEE = 1.8 to 5.3 mm Hg). When the catheter was pulled back through the stenosis, the highest gradients were found in or close to the stenosis. When these catheter gradients were compared with Doppler gradients, the agreement between the two techniques was excellent regardless of stenosis geometry (slope 0.97; mean difference 0.6 ± 2.0 mm Hg). However, when distal pressures were measured 10 cm downstream from the stenotic segment, the slope of the regression line, and therefore the agreement between Doppler and catheter gradients, differed for the different stenosis types (slopes from 0.98 to 1.69). In stenoses with abrupt narrowing and abrupt expansion, agreement was acceptable. Doppler gradients were only slightly greater than catheter gradients (mean difference 4.5 ± 5.2 mm Hg). In stenoses with a gradually tapering inlet and outlet, the Doppler-catheter gradient relation was dependent on the outflow angle. Good agreement was found for an angle of 60 ° (mean difference 0.6 ± 1.8 mm Hg). In stenoses with a 40 ° outflow angle, Doppler gradients exceeded the catheter gradients by 13% on average; for stenoses with a 20 ° outflow angle, Doppler gradients exceeded catheter gradients by 46 ± 11.4%, with differences as great as 65 mm Hg. These results were identical for stenoses gradually tapering outward to the distal tubing diameter and those with abrupt expansion after 2 cm of gradual expansion. The results were also not affected by changing the inflow angle from 20 ° to 60 °. However, an abrupt narrowing instead of a tapering inlet significantly altered the Doppler-catheter gradient relation (p < 0.001); Doppler gradients exceeded the catheter gradients by 34 ± 10% for this stenosis type.Conclusions. Doppler gradients accurately reflect the highest gradients across flow obstructions that occur in the vena contracta. However, these gradients may be significantly greater than catheter gradients that are measured farther downstream, as is usually the case in clinical catheterization studies. These discrepancies are due to pressure recovery. The magnitude of pressure recovery is highly dependent on the stenosis geometry, which therefore significantly affects the Doppler-catheter gradient relation. It is the outflow geometry that predominantly influences this relation, but the shape of the inlet may affect the results as well. Although pressure recovery occurs even in stenoses with abrupt narrowing and abrupt expansion, the phenomenon is most likely to become clinically relevant in stenoses with a gradually tapering inlet and outlet with an outflow angle ≥20 °
Modeling Quantum Optical Components, Pulses and Fiber Channels Using OMNeT++
Quantum Key Distribution (QKD) is an innovative technology which exploits the
laws of quantum mechanics to generate and distribute unconditionally secure
cryptographic keys. While QKD offers the promise of unconditionally secure key
distribution, real world systems are built from non-ideal components which
necessitates the need to model and understand the impact these non-idealities
have on system performance and security. OMNeT++ has been used as a basis to
develop a simulation framework to support this endeavor. This framework,
referred to as "qkdX" extends OMNeT++'s module and message abstractions to
efficiently model optical components, optical pulses, operating protocols and
processes. This paper presents the design of this framework including how
OMNeT++'s abstractions have been utilized to model quantum optical components,
optical pulses, fiber and free space channels. Furthermore, from our toolbox of
created components, we present various notional and real QKD systems, which
have been studied and analyzed.Comment: Published in: A. F\"orster, C. Minkenberg, G. R. Herrera, M. Kirsche
(Eds.), Proc. of the 2nd OMNeT++ Community Summit, IBM Research - Zurich,
Switzerland, September 3-4, 201
FINITE ELEMENT METHOD FOR NONLINEAR EDDY CURRENT PROBLEMS IN POWER TRANSFORMERS
An efficient finite element method to take account of the nonlinearity of the magnetic materials
when analyzing three dimensional eddy current problems is presented in this paper. The problem is
formulated in terms of vector and scalar potentials approximated by edge and node based finite element
basis functions. The application of Galerkin techniques leads to a large, nonlinear system of ordinary
differential equations in the time domain.
The excitations are assumed to be time-periodic and the steady state periodic solution is of
interest only. This is represented in the frequency domain as a Fourier series for each finite element
degree of freedom and a finite number of harmonics is to be determined, i.e. a harmonic balance method
is applied. Due to the nonlinearity, all harmonics are coupled to each other, so the size of the equation
system is the number of harmonics times the number of degrees of freedom.
The harmonics would be decoupled if the problem were linear, therefore, a special nonlinear
iteration technique, the fixed-point method is used to linearize the equations by selecting a timeindependent
permeability distribution, the so called fixed-point permeability in each nonlinear iteration
step. This leads to uncoupled harmonics within these steps resulting in two advantages. One is that each
harmonic is obtained by solving a system of algebraic equations with only as many unknowns as there
are finite element degrees of freedom. A second benefit is that these systems are independent of each
other and can be solved in parallel. The appropriate selection of the fixed point permeability accelerates
the convergence of the nonlinear iteration.
The method is applied to the analysis of a large power transformer. The solution of the
electromagnetic field allows the computation of various losses like eddy current losses in the massive
conducting parts (tank, clamping plates, tie bars, etc.) as well as the specific losses in the laminated parts
(core, tank shielding, etc.). The effect of the presence of higher harmonics on these losses is
investigated
FINITE ELEMENT METHOD FOR NONLINEAR EDDY CURRENT PROBLEMS IN POWER TRANSFORMERS
An efficient finite element method to take account of the nonlinearity of the magnetic materials
when analyzing three dimensional eddy current problems is presented in this paper. The problem is
formulated in terms of vector and scalar potentials approximated by edge and node based finite element
basis functions. The application of Galerkin techniques leads to a large, nonlinear system of ordinary
differential equations in the time domain.
The excitations are assumed to be time-periodic and the steady state periodic solution is of
interest only. This is represented in the frequency domain as a Fourier series for each finite element
degree of freedom and a finite number of harmonics is to be determined, i.e. a harmonic balance method
is applied. Due to the nonlinearity, all harmonics are coupled to each other, so the size of the equation
system is the number of harmonics times the number of degrees of freedom.
The harmonics would be decoupled if the problem were linear, therefore, a special nonlinear
iteration technique, the fixed-point method is used to linearize the equations by selecting a timeindependent
permeability distribution, the so called fixed-point permeability in each nonlinear iteration
step. This leads to uncoupled harmonics within these steps resulting in two advantages. One is that each
harmonic is obtained by solving a system of algebraic equations with only as many unknowns as there
are finite element degrees of freedom. A second benefit is that these systems are independent of each
other and can be solved in parallel. The appropriate selection of the fixed point permeability accelerates
the convergence of the nonlinear iteration.
The method is applied to the analysis of a large power transformer. The solution of the
electromagnetic field allows the computation of various losses like eddy current losses in the massive
conducting parts (tank, clamping plates, tie bars, etc.) as well as the specific losses in the laminated parts
(core, tank shielding, etc.). The effect of the presence of higher harmonics on these losses is
investigated
Simulation and evaluation of freeze-thaw cryoablation scenarios for the treatment of cardiac arrhythmias
BACKGROUND: Cardiac cryoablation is a minimally invasive procedure to treat cardiac arrhythmias by cooling cardiac tissues responsible for the cardiac arrhythmia to freezing temperatures. Although cardiac cryoablation offers a gentler treatment than radiofrequency ablation, longer interventions and higher recurrence rates reduce the clinical acceptance of this technique. Computer models of ablation scenarios allow for a closer examination of temperature distributions in the myocardium and evaluation of specific effects of applied freeze-thaw protocols in a controlled environment. METHODS: In this work multiple intervention scenarios with two freeze-thaw cycles were simulated with varying durations and starting times of the interim thawing phase using a finite element model verified by in-vivo measurements and data from literature. To evaluate the effects of different protocols, transmural temperature distributions and iceball dimensions were compared over time. Cryoadhesion durations of the applicator were estimated in the interim thawing phase with varying thawing phase starting times. In addition, the increase of cooling rates was compared between the freezing phases, and the thawing rates of interim thawing phases were analyzed over transmural depth. RESULTS: It could be shown that the increase of cooling rate, the regions undergoing additional phase changes and depths of selected temperatures depend on the chosen ablation protocol. Only small differences of the estimated cryoadhesion duration were found for ablation scenarios with interim thawing phase start after 90 s freezing. CONCLUSIONS: By the presented model a quantification of effects responsible for cell death is possible, allowing for the analysis and optimization of cryoablation scenarios which contribute to a higher clinical acceptance of cardiac cryoablation
NSs protein of Schmallenberg virus counteracts the antiviral response of the cell by inhibiting its transcriptional machinery
Bunyaviruses have evolved a variety of strategies to counteract the antiviral defence systems of mammalian cells. Here we show that the NSs protein of Schmallenberg virus (SBV) induces the degradation of the RPB1 subunit of RNA polymerase II and consequently inhibits global cellular protein synthesis and the antiviral response. In addition, we show that the SBV NSs protein enhances apoptosis in vitro and possibly in vivo, suggesting that this protein could be involved in SBV pathogenesis in different ways
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