20 research outputs found
Multireference Alignment is Easier with an Aperiodic Translation Distribution
In the multireference alignment model, a signal is observed by the action of
a random circular translation and the addition of Gaussian noise. The goal is
to recover the signal's orbit by accessing multiple independent observations.
Of particular interest is the sample complexity, i.e., the number of
observations/samples needed in terms of the signal-to-noise ratio (the signal
energy divided by the noise variance) in order to drive the mean-square error
(MSE) to zero. Previous work showed that if the translations are drawn from the
uniform distribution, then, in the low SNR regime, the sample complexity of the
problem scales as . In this work, using a
generalization of the Chapman--Robbins bound for orbits and expansions of the
divergence at low SNR, we show that in the same regime the sample
complexity for any aperiodic translation distribution scales as
. This rate is achieved by a simple spectral algorithm.
We propose two additional algorithms based on non-convex optimization and
expectation-maximization. We also draw a connection between the multireference
alignment problem and the spiked covariance model
Estimation in the group action channel
We analyze the problem of estimating a signal from multiple measurements on a
\mbox{group action channel} that linearly transforms a signal by a random
group action followed by a fixed projection and additive Gaussian noise. This
channel is motivated by applications such as multi-reference alignment and
cryo-electron microscopy. We focus on the large noise regime prevalent in these
applications. We give a lower bound on the mean square error (MSE) of any
asymptotically unbiased estimator of the signal's orbit in terms of the
signal's moment tensors, which implies that the MSE is bounded away from 0 when
is bounded from above, where is the number of observations,
is the noise standard deviation, and is the so-called
\mbox{moment order cutoff}. In contrast, the maximum likelihood estimator is
shown to be consistent if diverges.Comment: 5 pages, conferenc
Multi-target detection with rotations
We consider the multi-target detection problem of estimating a
two-dimensional target image from a large noisy measurement image that contains
many randomly rotated and translated copies of the target image. Motivated by
single-particle cryo-electron microscopy, we focus on the low signal-to-noise
regime, where it is difficult to estimate the locations and orientations of the
target images in the measurement. Our approach uses autocorrelation analysis to
estimate rotationally and translationally invariant features of the target
image. We demonstrate that, regardless of the level of noise, our technique can
be used to recover the target image when the measurement is sufficiently large.Comment: 20 pages, 5 figure