42,602 research outputs found
Maximum-likelihood estimation of delta-domain model parameters from noisy output signals
Fast sampling is desirable to describe signal transmission
through wide-bandwidth systems. The delta-operator provides an ideal discrete-time modeling description for such fast-sampled systems. However, the estimation of delta-domain model parameters is usually biased by directly applying the delta-transformations to a sampled signal corrupted by additive measurement noise. This problem is solved here by expectation-maximization, where the delta-transformations of the true signal are estimated and then used to obtain the model parameters. The method is
demonstrated on a numerical example to improve on the accuracy of using a shift operator approach when the sample rate is fast
Substructures in hydrodynamical cluster simulations
The abundance and structure of dark matter subhalos has been analyzed
extensively in recent studies of dark matter-only simulations, but
comparatively little is known about the impact of baryonic physics on halo
substructures. We here extend the SUBFIND algorithm for substructure
identification such that it can be reliably applied to dissipative
hydrodynamical simulations that include star formation. This allows, in
particular, the identification of galaxies as substructures in simulations of
clusters of galaxies, and a determination of their content of gravitationally
bound stars, dark matter, and hot and cold gas. Using a large set of
cosmological cluster simulations, we present a detailed analysis of halo
substructures in hydrodynamical simulations of galaxy clusters, focusing in
particular on the influence both of radiative and non-radiative gas physics,
and of non-standard physics such as thermal conduction and feedback by galactic
outflows. We also examine the impact of numerical nuisance parameters such as
artificial viscosity parameterizations. We find that diffuse hot gas is
efficiently stripped from subhalos when they enter the highly pressurized
cluster atmosphere. This has the effect of decreasing the subhalo mass function
relative to a corresponding dark matter-only simulation. These effects are
mitigated in radiative runs, where baryons condense in the central subhalo
regions and form compact stellar cores. However, in all cases, only a very
small fraction, of the order of one percent, of subhalos within the cluster
virial radii preserve a gravitationally bound hot gaseous atmosphere.
(abridged)Comment: improved manuscript, to appear in MNRA
Generalized time-frequency coherency for assessing neural interactions in electrophysiological recordings
Time-frequency coherence has been widely used to quantify statistical dependencies in bivariate data and has proven to be vital for the study of neural interactions in electrophysiological recordings. Conventional methods establish time-frequency coherence by smoothing the cross and power spectra using identical smoothing procedures. Smoothing entails a trade-off between time-frequency resolution and statistical consistency and is critical for detecting instantaneous coherence in single-trial data. Here, we propose a generalized method to estimate time-frequency coherency by using different smoothing procedures for the cross spectra versus power spectra. This novel method has an improved trade-off between time resolution and statistical consistency compared to conventional methods, as verified by two simulated data sets. The methods are then applied to single-trial surface encephalography recorded from human subjects for comparative purposes. Our approach extracted robust alpha- and gamma-band synchronization over the visual cortex that was not detected by conventional methods, demonstrating the efficacy of this method
Dynamic Compressive Sensing of Time-Varying Signals via Approximate Message Passing
In this work the dynamic compressive sensing (CS) problem of recovering
sparse, correlated, time-varying signals from sub-Nyquist, non-adaptive, linear
measurements is explored from a Bayesian perspective. While there has been a
handful of previously proposed Bayesian dynamic CS algorithms in the
literature, the ability to perform inference on high-dimensional problems in a
computationally efficient manner remains elusive. In response, we propose a
probabilistic dynamic CS signal model that captures both amplitude and support
correlation structure, and describe an approximate message passing algorithm
that performs soft signal estimation and support detection with a computational
complexity that is linear in all problem dimensions. The algorithm, DCS-AMP,
can perform either causal filtering or non-causal smoothing, and is capable of
learning model parameters adaptively from the data through an
expectation-maximization learning procedure. We provide numerical evidence that
DCS-AMP performs within 3 dB of oracle bounds on synthetic data under a variety
of operating conditions. We further describe the result of applying DCS-AMP to
two real dynamic CS datasets, as well as a frequency estimation task, to
bolster our claim that DCS-AMP is capable of offering state-of-the-art
performance and speed on real-world high-dimensional problems.Comment: 32 pages, 7 figure
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