123 research outputs found
Constructing packings in Grassmannian manifolds via alternating projection
This paper describes a numerical method for finding good packings in
Grassmannian manifolds equipped with various metrics. This investigation also
encompasses packing in projective spaces. In each case, producing a good
packing is equivalent to constructing a matrix that has certain structural and
spectral properties. By alternately enforcing the structural condition and then
the spectral condition, it is often possible to reach a matrix that satisfies
both. One may then extract a packing from this matrix.
This approach is both powerful and versatile. In cases where experiments have
been performed, the alternating projection method yields packings that compete
with the best packings recorded. It also extends to problems that have not been
studied numerically. For example, it can be used to produce packings of
subspaces in real and complex Grassmannian spaces equipped with the
Fubini--Study distance; these packings are valuable in wireless communications.
One can prove that some of the novel configurations constructed by the
algorithm have packing diameters that are nearly optimal.Comment: 41 pages, 7 tables, 4 figure
Coherence Optimization and Best Complex Antipodal Spherical Codes
Vector sets with optimal coherence according to the Welch bound cannot exist
for all pairs of dimension and cardinality. If such an optimal vector set
exists, it is an equiangular tight frame and represents the solution to a
Grassmannian line packing problem. Best Complex Antipodal Spherical Codes
(BCASCs) are the best vector sets with respect to the coherence. By extending
methods used to find best spherical codes in the real-valued Euclidean space,
the proposed approach aims to find BCASCs, and thereby, a complex-valued vector
set with minimal coherence. There are many applications demanding vector sets
with low coherence. Examples are not limited to several techniques in wireless
communication or to the field of compressed sensing. Within this contribution,
existing analytical and numerical approaches for coherence optimization of
complex-valued vector spaces are summarized and compared to the proposed
approach. The numerically obtained coherence values improve previously reported
results. The drawback of increased computational effort is addressed and a
faster approximation is proposed which may be an alternative for time critical
cases
Subspace Packings : Constructions and Bounds
The Grassmannian is the set of all -dimensional
subspaces of the vector space . K\"{o}tter and Kschischang
showed that codes in Grassmannian space can be used for error-correction in
random network coding. On the other hand, these codes are -analogs of codes
in the Johnson scheme, i.e., constant dimension codes. These codes of the
Grassmannian also form a family of -analogs of block
designs and they are called subspace designs. In this paper, we examine one of
the last families of -analogs of block designs which was not considered
before. This family, called subspace packings, is the -analog of packings,
and was considered recently for network coding solution for a family of
multicast networks called the generalized combination networks. A subspace
packing - is a set of -subspaces from
such that each -subspace of is
contained in at most elements of . The goal of this work
is to consider the largest size of such subspace packings. We derive a sequence
of lower and upper bounds on the maximum size of such packings, analyse these
bounds, and identify the important problems for further research in this area.Comment: 30 pages, 27 tables, continuation of arXiv:1811.04611, typos
correcte
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Mini-Workshop: Algebraic, Geometric, and Combinatorial Methods in Frame Theory
Frames are collections of vectors in a Hilbert space which have reconstruction properties similar to orthonormal bases and applications in areas such as signal and image processing, quantum information theory, quantization, compressed sensing, and phase retrieval. Further desirable properties of frames for robustness in these applications coincide with structures that have appeared independently in other areas of mathematics, such as special matroids, Gel’Fand-Zetlin polytopes, and combinatorial designs. Within the past few years, the desire to understand these structures has led to many new fruitful interactions between frame theory and fields in pure mathematics, such as algebraic and symplectic geometry, discrete geometry, algebraic combinatorics, combinatorial design theory, and algebraic number theory. These connections have led to the solutions of several open problems and are ripe for further exploration. The central goal of our mini-workshop was to attack open problems that were amenable to an interdisciplinary approach combining certain subfields of frame theory, geometry, and combinatorics
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