140 research outputs found
Slepian Spatial-Spectral Concentration on the Ball
We formulate and solve the Slepian spatial-spectral concentration problem on
the three-dimensional ball. Both the standard Fourier-Bessel and also the
Fourier-Laguerre spectral domains are considered since the latter exhibits a
number of practical advantages (spectral decoupling and exact computation). The
Slepian spatial and spectral concentration problems are formulated as
eigenvalue problems, the eigenfunctions of which form an orthogonal family of
concentrated functions. Equivalence between the spatial and spectral problems
is shown. The spherical Shannon number on the ball is derived, which acts as
the analog of the space-bandwidth product in the Euclidean setting, giving an
estimate of the number of concentrated eigenfunctions and thus the dimension of
the space of functions that can be concentrated in both the spatial and
spectral domains simultaneously. Various symmetries of the spatial region are
considered that reduce considerably the computational burden of recovering
eigenfunctions, either by decoupling the problem into smaller subproblems or by
affording analytic calculations. The family of concentrated eigenfunctions
forms a Slepian basis that can be used be represent concentrated signals
efficiently. We illustrate our results with numerical examples and show that
the Slepian basis indeeds permits a sparse representation of concentrated
signals.Comment: 33 pages, 10 figure
Multitaper estimation on arbitrary domains
Multitaper estimators have enjoyed significant success in estimating spectral
densities from finite samples using as tapers Slepian functions defined on the
acquisition domain. Unfortunately, the numerical calculation of these Slepian
tapers is only tractable for certain symmetric domains, such as rectangles or
disks. In addition, no performance bounds are currently available for the mean
squared error of the spectral density estimate. This situation is inadequate
for applications such as cryo-electron microscopy, where noise models must be
estimated from irregular domains with small sample sizes. We show that the
multitaper estimator only depends on the linear space spanned by the tapers. As
a result, Slepian tapers may be replaced by proxy tapers spanning the same
subspace (validating the common practice of using partially converged solutions
to the Slepian eigenproblem as tapers). These proxies may consequently be
calculated using standard numerical algorithms for block diagonalization. We
also prove a set of performance bounds for multitaper estimators on arbitrary
domains. The method is demonstrated on synthetic and experimental datasets from
cryo-electron microscopy, where it reduces mean squared error by a factor of
two or more compared to traditional methods.Comment: 28 pages, 11 figure
A Universal Framework for Holographic MIMO Sensing
This paper addresses the sensing space identification of arbitrarily shaped
continuous antennas. In the context of holographic multiple-input
multiple-output (MIMO), a.k.a. large intelligent surfaces, these antennas offer
benefits such as super-directivity and near-field operability. The sensing
space reveals two key aspects: (a) its dimension specifies the maximally
achievable spatial degrees of freedom (DoFs), and (b) the finite basis spanning
this space accurately describes the sampled field. Earlier studies focus on
specific geometries, bringing forth the need for extendable analysis to
real-world conformal antennas. Thus, we introduce a universal framework to
determine the antenna sensing space, regardless of its shape. The findings
underscore both spatial and spectral concentration of sampled fields to define
a generic eigenvalue problem of Slepian concentration. Results show that this
approach precisely estimates the DoFs of well-known geometries, and verify its
flexible extension to conformal antennas
Slepian concentration problem for polynomials on the unit Ball
We study the Slepian spatiospectral concentration problem for the space of
multi-variate polynomials on the unit ball in . We will discuss
the phenomenon of an asymptotically bimodal distribution of eigenvalues of the
spatiospectral concentration operators of polynomial spaces equipped with two
different notions of bandwidth: (a) the space of polynomials with a fixed
maximal overall polynomial degree, (b) the space of polynomials separated into
radial and spherical contributions, with fixed but separate maximal degrees for
the radial and spherical contributions, respectively. In particular, we
investigate the transition position of the bimodal eigenvalue distribution (the
so-called Shannon number) for both setups. The analytic results are illustrated
by numerical examples on the 3-D ball
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