7,394 research outputs found
One-Bit Quantization Design and Adaptive Methods for Compressed Sensing
There have been a number of studies on sparse signal recovery from one-bit
quantized measurements. Nevertheless, little attention has been paid to the
choice of the quantization thresholds and its impact on the signal recovery
performance. This paper examines the problem of one-bit quantizer design for
sparse signal recovery. Our analysis shows that the magnitude ambiguity that
ever plagues conventional one-bit compressed sensing methods can be resolved,
and an arbitrarily small reconstruction error can be achieved by setting the
quantization thresholds close enough to the original data samples without being
quantized. Note that unquantized data samples are unaccessible in practice. To
overcome this difficulty, we propose an adaptive quantization method that
adaptively adjusts the quantization thresholds in a way such that the
thresholds converges to the optimal thresholds. Numerical results are
illustrated to collaborate our theoretical results and the effectiveness of the
proposed algorithm
Fast Asynchronous Parallel Stochastic Gradient Decent
Stochastic gradient descent~(SGD) and its variants have become more and more
popular in machine learning due to their efficiency and effectiveness. To
handle large-scale problems, researchers have recently proposed several
parallel SGD methods for multicore systems. However, existing parallel SGD
methods cannot achieve satisfactory performance in real applications. In this
paper, we propose a fast asynchronous parallel SGD method, called AsySVRG, by
designing an asynchronous strategy to parallelize the recently proposed SGD
variant called stochastic variance reduced gradient~(SVRG). Both theoretical
and empirical results show that AsySVRG can outperform existing
state-of-the-art parallel SGD methods like Hogwild! in terms of convergence
rate and computation cost
Super-Resolution Compressed Sensing: An Iterative Reweighted Algorithm for Joint Parameter Learning and Sparse Signal Recovery
In many practical applications such as direction-of-arrival (DOA) estimation
and line spectral estimation, the sparsifying dictionary is usually
characterized by a set of unknown parameters in a continuous domain. To apply
the conventional compressed sensing to such applications, the continuous
parameter space has to be discretized to a finite set of grid points.
Discretization, however, incurs errors and leads to deteriorated recovery
performance. To address this issue, we propose an iterative reweighted method
which jointly estimates the unknown parameters and the sparse signals.
Specifically, the proposed algorithm is developed by iteratively decreasing a
surrogate function majorizing a given objective function, which results in a
gradual and interweaved iterative process to refine the unknown parameters and
the sparse signal. Numerical results show that the algorithm provides superior
performance in resolving closely-spaced frequency components
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Pattern Coupled Sparse Bayesian Learning for Recovery of Time Varying Sparse Signals
We consider the problem of recovering block-sparse signals whose structures
are unknown \emph{a priori}. Block-sparse signals with nonzero coefficients
occurring in clusters arise naturally in many practical scenarios. However, the
knowledge of the block structure is usually unavailable in practice. In this
paper, we develop a new sparse Bayesian learning method for recovery of
block-sparse signals with unknown cluster patterns. Specifically, a
pattern-coupled hierarchical Gaussian prior model is introduced to characterize
the statistical dependencies among coefficients, in which a set of
hyperparameters are employed to control the sparsity of signal coefficients.
Unlike the conventional sparse Bayesian learning framework in which each
individual hyperparameter is associated independently with each coefficient, in
this paper, the prior for each coefficient not only involves its own
hyperparameter, but also the hyperparameters of its immediate neighbors. In
doing this way, the sparsity patterns of neighboring coefficients are related
to each other and the hierarchical model has the potential to encourage
structured-sparse solutions. The hyperparameters, along with the sparse signal,
are learned by maximizing their posterior probability via an
expectation-maximization (EM) algorithm. Numerical results show that the
proposed algorithm presents uniform superiority over other existing methods in
a series of experiments
On the Convergence of Memory-Based Distributed SGD
Distributed stochastic gradient descent~(DSGD) has been widely used for
optimizing large-scale machine learning models, including both convex and
non-convex models. With the rapid growth of model size, huge communication cost
has been the bottleneck of traditional DSGD. Recently, many communication
compression methods have been proposed. Memory-based distributed stochastic
gradient descent~(M-DSGD) is one of the efficient methods since each worker
communicates a sparse vector in each iteration so that the communication cost
is small. Recent works propose the convergence rate of M-DSGD when it adopts
vanilla SGD. However, there is still a lack of convergence theory for M-DSGD
when it adopts momentum SGD. In this paper, we propose a universal convergence
analysis for M-DSGD by introducing \emph{transformation equation}. The
transformation equation describes the relation between traditional DSGD and
M-DSGD so that we can transform M-DSGD to its corresponding DSGD. Hence we get
the convergence rate of M-DSGD with momentum for both convex and non-convex
problems. Furthermore, we combine M-DSGD and stagewise learning that the
learning rate of M-DSGD in each stage is a constant and is decreased by stage,
instead of iteration. Using the transformation equation, we propose the
convergence rate of stagewise M-DSGD which bridges the gap between theory and
practice
Generation of high-energy clean multicolored ultrashort pulses and their application in single-shot temporal contrast measurement
We demonstrate the generation of 100-{\mu}J-level multicolored femtosecond
pulses based on a single-stage cascaded four-wave mixing (CFWM) process in a
thin glass plate. The generated high-energy CFWM signals can shift the central
wavelength and have well-enhanced temporal contrast because of the third-order
nonlinear process. They are innovatively used as clean sampling pulses of a
cross-correlator for single-shot temporal contrast measurement. With a simple
home-made setup, the proof-of-principle experimental results demonstrate the
single-shot cross-correlator with dynamic range of 1010, temporal resolution of
about 160 fs and temporal window of 50 ps. To the best of our knowledge, this
is the first demonstration in which both the dynamic range and the temporal
resolution of a single-shot temporal contrast measurement are comparable to
those of a commercial delay-scanning cross-correlator.Comment: 20 pages, 8 figure
Pattern-Coupled Sparse Bayesian Learning for Recovery of Block-Sparse Signals
We consider the problem of recovering block-sparse signals whose structures
are unknown \emph{a priori}. Block-sparse signals with nonzero coefficients
occurring in clusters arise naturally in many practical scenarios. However, the
knowledge of the block structure is usually unavailable in practice. In this
paper, we develop a new sparse Bayesian learning method for recovery of
block-sparse signals with unknown cluster patterns. Specifically, a
pattern-coupled hierarchical Gaussian prior model is introduced to characterize
the statistical dependencies among coefficients, in which a set of
hyperparameters are employed to control the sparsity of signal coefficients.
Unlike the conventional sparse Bayesian learning framework in which each
individual hyperparameter is associated independently with each coefficient, in
this paper, the prior for each coefficient not only involves its own
hyperparameter, but also the hyperparameters of its immediate neighbors. In
doing this way, the sparsity patterns of neighboring coefficients are related
to each other and the hierarchical model has the potential to encourage
structured-sparse solutions. The hyperparameters, along with the sparse signal,
are learned by maximizing their posterior probability via an
expectation-maximization (EM) algorithm. Numerical results show that the
proposed algorithm presents uniform superiority over other existing methods in
a series of experiments
A New Class of Parametrization for Dark Energy without Divergence
In this paper, we propose a new class of parametrization of the equation of
state of dark energy. In contrast with the famous CPL parametrization, these
new parametrization of the equation of state does not divergent during the
evolution of the Universe even in the future. Also, we perform a observational
constraint on two simplest dark energy models belonging to this new class of
parametrization, by using the Markov Chain Monte Carlo (MCMC) method and the
combined latest observational data from the type Ia supernova compilations
including Union2(557), cosmic microwave background, and baryon acoustic
oscillation.Comment: 8 pages, 4 figure
Scalable Stochastic Alternating Direction Method of Multipliers
Stochastic alternating direction method of multipliers (ADMM), which visits
only one sample or a mini-batch of samples each time, has recently been proved
to achieve better performance than batch ADMM. However, most stochastic methods
can only achieve a convergence rate on general convex
problems,where T is the number of iterations. Hence, these methods are not
scalable with respect to convergence rate (computation cost). There exists only
one stochastic method, called SA-ADMM, which can achieve convergence rate
on general convex problems. However, an extra memory is needed for
SA-ADMM to store the historic gradients on all samples, and thus it is not
scalable with respect to storage cost. In this paper, we propose a novel
method, called scalable stochastic ADMM(SCAS-ADMM), for large-scale
optimization and learning problems. Without the need to store the historic
gradients, SCAS-ADMM can achieve the same convergence rate as the best
stochastic method SA-ADMM and batch ADMM on general convex problems.
Experiments on graph-guided fused lasso show that SCAS-ADMM can achieve
state-of-the-art performance in real application
Controllable growth of centimeter-size 2D perovskite heterostructural single crystals for highly narrow dual-band photodetectors
Two-dimensional (2D) organic-inorganic perovskites have recently attracted
increasing attention due to their great environmental stability, remarkable
quantum confinement effect and layered characteristic. Heterostructures
consisting of 2D layered perovskites are expected to exhibit new physical
phenomena inaccessible to the single 2D perovskites and can greatly extend
their functionalities for novel electronic and optoelectronic applications.
Herein, we develop a novel solution method to synthesize 2D perovskite
single-crystals with the centimeter size, high phase purity, controllable
junction depth, high crystalline quality and great stability for highly narrow
dual-band photodetectors. On the basis of the different lattice constant,
solubility and growth rate between different n number, the newly designed
synthesis method allows to first grow n=1 perovskite guided by the
self-assembled layer of the organic cations at the water-air interface and
subsequently n=2 layer is formed via diffusion process. Such growth process
provides an efficient away for us to readily obtain 2D perovskite
heterostructural single-crystals with various thickness and junction depth by
controlling the concentration, reaction temperature and time. Photodetectors
based on such heterostructural single crystal plates exhibit extremely low dark
current, high on-off current ratio, and highly narrow dual-band spectral
response with a full-width at half-maximum of 20 nm at 540 nm and 34 nm at 610
nm. In particular, the synthetic strategy is general for other 2D perovskites
and the narrow dual-band spectral response with all full-width at half-maximum
below 40 nm can be continuously tuned from red to blue by properly changing the
halide compositions
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