645 research outputs found
Minimizing the effect of sinusoidal trends in detrended fluctuation analysis
The detrended fluctuation analysis (DFA) [Peng et al., 1994] and its
extensions (MF-DFA) [Kantelhardt et al., 2002] have been used extensively to
determine possible long-range correlations in self-affine signals. While the
DFA has been claimed to be a superior technique, recent reports have indicated
its susceptibility to trends in the data. In this report, a smoothing filter is
proposed to minimize the effect of sinusoidal trends and distortion in the
log-log plots obtained by DFA and MF-DFA techniques
Is a multiple excitation of a single atom equivalent to a single excitation of an ensemble of atoms?
Recent technological advances have enabled to isolate, control and measure
the properties of a single atom, leading to the possibility to perform
statistics on the behavior of single quantum systems. These experiments have
enabled to check a question which was out of reach previously: Is the
statistics of a repeatedly excitation of an atom N times equivalent to a single
excitation of an ensemble of N atoms? We present a new method to analyze
quantum measurements which leads to the postulation that the answer is most
probably no. We discuss the merits of the analysis and its conclusion.Comment: 3 pages, 3 figure
Multiple-Resampling Receiver Design for OFDM Over Doppler-Distorted Underwater Acoustic Channels
Cataloged from PDF version of article.In this paper, we focus on orthogonal frequency-divisionmultiplexing
(OFDM) receiver designs for underwater acoustic
(UWA) channels with user- and/or path-specific Doppler scaling
distortions. The scenario is motivated by the cooperative communications
framework, where distributed transmitter/receiver
pairs may experience significantly different Doppler distortions, as
well as by the single-user scenarios, where distinct Doppler scaling
factors may exist among different propagation paths. The conventional
approach of frontâend resampling that corrects for common
Doppler scalingmay not be appropriatein such scenarios, rendering
a post-fast-Fourier-transform (FFT) signal that is contaminated by
user- and/or path-specific intercarrier interference. To counteract
this problem, we propose a family of frontâend receiver structures
thatutilizemultiple-resampling (MR)branches,eachmatched to the
Doppler scaling factor of a particular user and/or path. Following
resampling, FFT modules transform the Doppler-compensated
signals into the frequency domain for further processing through
linear or nonlinear detection schemes. As part of the overall receiver
structure, a gradientâdescent approachis also proposed to refine the
channel estimates obtained by standard sparse channel estimators.
The effectiveness and robustness of the proposed receivers are
demonstrated via simulations, as well as emulations based on real
data collected during the 2010 Mobile Acoustic Communications
Experiment (MACE10, Marthaâs Vineyard, MA) and the 2008
Kauai Acomms MURI (KAM08, Kauai, HI) experiment
Quantum storage on subradiant states in an extended atomic ensemble
A scheme for coherent manipulation of collective atomic states is developed
such that total subradiant states, in which spontaneous emission is suppressed
into all directions due to destructive interference between neighbor atoms, can
be created in an extended atomic ensemble. The optimal conditions for creation
of such states and suitability of them for quantum storage are discussed. It is
shown that in order to achieve the maximum signal-to-noise ratio the shape of a
light pulse to be stored and reconstructed using a homogeneously broadened
absorbtion line of an atomic system should be a time-reversed regular part of
the response function of the system. In the limit of high optical density, such
pulses allow one to prepare collective subradiant atomic states with near flat
spatial distribution of the atomic excitation in the medium.Comment: V2: considerably revised (title, text). V3: minor changes - final
version as published in PR
A Joint Solution to Scheduling and Power Control for Multicasting in Wireless Ad Hoc Networks
This paper jointly addresses the problem of power control and scheduling in ad hoc networks supporting multicast traffic. First, we present a distributed algorithm which, given the set of multicast transmitters and their corresponding receivers, provides an optimal solution to the power control problem, if there is any. The transmit power levels obtained by solving the optimization problem minimize the network power expenditure while meeting the requirements on the SINR at the receivers. Whenever no optimal solution can be found for the given set of multicast transmitters, we introduce a joint scheduling and power control algorithm which eliminates the strong interferers, thus allowing the other transmitters to solve the power control problem. The algorithm can be implemented in a distributed manner. Although the proposed scheme provides a suboptimal solution, simulation results show that the obtained solution is close to the global optimum, when it exists. When instead there is no optimal solution, our algorithm allows for a high number of successful multicast transmissions
Kinetic Enhancement of Raman Backscatter, and Electron Acoustic Thomson Scatter
1-D Eulerian Vlasov-Maxwell simulations are presented which show kinetic
enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in
regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is
transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73,
025401 (2006)]. For the first time, a low phase velocity electron acoustic wave
(EAW) is seen developing from the self-consistent Raman physics. Backscatter of
the pump laser off the EAW fluctuations is reported and referred to as electron
acoustic Thomson scatter. This light is similar in wavelength to, although much
lower in amplitude than, the reflected light between the pump and SRBS
wavelengths observed in single hot spot experiments, and previously interpreted
as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev.
Lett. 87, 155001 (2001)]. The EAW is strongest well below the phase-matched
frequency for electron acoustic scatter, and therefore the EAW is not produced
by it. The beating of different beam acoustic modes is proposed as the EAW
excitation mechanism, and is called beam acoustic decay. Supporting evidence
for this process, including bispectral analysis, is presented. The linear
electrostatic modes, found by projecting the numerical distribution function
onto a Gauss-Hermite basis, include beam acoustic modes (some of which are
unstable even without parametric coupling to light waves) and a strongly-damped
EAW similar to the observed one. This linear EAW results from non-Maxwellian
features in the electron distribution, rather than nonlinearity due to electron
trapping.Comment: 15 pages, 16 figures, accepted in Physics of Plasmas (2006
Testing the assumptions of linear prediction analysis in normal vowels
This paper develops an improved surrogate data test to show experimental evidence, for all the simple vowels of US English, for both male and female speakers, that Gaussian linear prediction analysis, a ubiquitous technique in current speech technologies, cannot be used to extract all the dynamical structure of real speech time series. The test provides robust evidence undermining the validity of these linear techniques, supporting the assumptions of either dynamical nonlinearity and/or non-Gaussianity common to more recent, complex, efforts at dynamical modelling speech time series. However, an additional finding is that the classical assumptions cannot be ruled out entirely, and plausible evidence is given to explain the success of the linear Gaussian theory as a weak approximation to the true, nonlinear/non-Gaussian dynamics. This supports the use of appropriate hybrid linear/nonlinear/non-Gaussian modelling. With a calibrated calculation of statistic and particular choice of experimental protocol, some of the known systematic problems of the method of surrogate data testing are circumvented to obtain results to support the conclusions to a high level of significance
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