185 research outputs found
Density of Spherically-Embedded Stiefel and Grassmann Codes
The density of a code is the fraction of the coding space covered by packing
balls centered around the codewords. This paper investigates the density of
codes in the complex Stiefel and Grassmann manifolds equipped with the chordal
distance. The choice of distance enables the treatment of the manifolds as
subspaces of Euclidean hyperspheres. In this geometry, the densest packings are
not necessarily equivalent to maximum-minimum-distance codes. Computing a
code's density follows from computing: i) the normalized volume of a metric
ball and ii) the kissing radius, the radius of the largest balls one can pack
around the codewords without overlapping. First, the normalized volume of a
metric ball is evaluated by asymptotic approximations. The volume of a small
ball can be well-approximated by the volume of a locally-equivalent tangential
ball. In order to properly normalize this approximation, the precise volumes of
the manifolds induced by their spherical embedding are computed. For larger
balls, a hyperspherical cap approximation is used, which is justified by a
volume comparison theorem showing that the normalized volume of a ball in the
Stiefel or Grassmann manifold is asymptotically equal to the normalized volume
of a ball in its embedding sphere as the dimension grows to infinity. Then,
bounds on the kissing radius are derived alongside corresponding bounds on the
density. Unlike spherical codes or codes in flat spaces, the kissing radius of
Grassmann or Stiefel codes cannot be exactly determined from its minimum
distance. It is nonetheless possible to derive bounds on density as functions
of the minimum distance. Stiefel and Grassmann codes have larger density than
their image spherical codes when dimensions tend to infinity. Finally, the
bounds on density lead to refinements of the standard Hamming bounds for
Stiefel and Grassmann codes.Comment: Two-column version (24 pages, 6 figures, 4 tables). To appear in IEEE
Transactions on Information Theor
Nonintersecting Subspaces Based on Finite Alphabets
Two subspaces of a vector space are here called ``nonintersecting'' if they
meet only in the zero vector. The following problem arises in the design of
noncoherent multiple-antenna communications systems. How many pairwise
nonintersecting M_t-dimensional subspaces of an m-dimensional vector space V
over a field F can be found, if the generator matrices for the subspaces may
contain only symbols from a given finite alphabet A subseteq F? The most
important case is when F is the field of complex numbers C; then M_t is the
number of antennas. If A = F = GF(q) it is shown that the number of
nonintersecting subspaces is at most (q^m-1)/(q^{M_t}-1), and that this bound
can be attained if and only if m is divisible by M_t. Furthermore these
subspaces remain nonintersecting when ``lifted'' to the complex field. Thus the
finite field case is essentially completely solved. In the case when F = C only
the case M_t=2 is considered. It is shown that if A is a PSK-configuration,
consisting of the 2^r complex roots of unity, the number of nonintersecting
planes is at least 2^{r(m-2)} and at most 2^{r(m-1)-1} (the lower bound may in
fact be the best that can be achieved).Comment: 14 page
A Noncoherent Space-Time Code from Quantum Error Correction
In this work, we develop a space-time block code for noncoherent
communication using techniques from the field of quantum error correction. We
decompose the multiple-input multiple-output (MIMO) channel into operators from
quantum mechanics, and design a non-coherent space time code using the quantum
stabilizer formalism. We derive an optimal decoder, and analyze the former
through a quantum mechanical lens. We compare our approach to a comparable
coherent approach and a noncoherent differential approach, achieving comparable
or better performance.Comment: 6 pages, one figure, accepted at the 53rd annual Conference on
Information Sciences and System
Time-frequency Grassmannian signalling for MIMO multi-channel-frequency-flat systems
In this paper, we consider the application of non-coherent Grassmannian signalling in practical multi-channel-frequency-flat multiple-input multiple-output (MIMO) wireless communication systems. In these systems, Grassmannian signalling, originally developed for single-channel block-fading systems, is not readily applicable. In particular, in such systems, the channel coefficients are constant across time and frequency, which implies that spectrally-efficient signalling ought to be jointly structured over these domains. To approach this goal, we develop a concatenation technique that yields a spectrally-efficient time-frequency Grassmannian signalling scheme, which enables the channel coherence bandwidth to be regarded as an additional coherence time. This scheme is shown to achieve the high signal-to-noise ratio non-coherent capacity of MIMO channels when the fading coefficients are constant over a time-frequency block. This scheme is also applicable in fast fading systems with coherence bandwidth exceeding that of one subchannel. The proposed scheme is independent of the symbol duration, i.e., the channel use duration, and is thus compatible with the transmit filter designs in current systems.The work of the first and second authors is supported, in part, by the Natural Sciences and Engineering Research Council of Canada (NSERC). This work is also supported, in part, by Huawei Canada Co., Ltd., in part, by the Ontario Ministry of Economic Development and Innovation's ORF-RE (Ontario Research Fund-Research Excellence) program, and, in part, by the Ministerio de Ciencia e Innovacion (project number TEC2011-27723-C02-02). The associate editor coordinating the review of this paper and approving it for publication was Z. Wang.Fouad, YMM.; Gohary, RH.; Cabrejas Peñuelas, J.; Yanikomeroglu, H.; Calabuig Soler, D.; Roger Varea, S.; Monserrat Del RÃo, JF. (2015). Time-frequency Grassmannian signalling for MIMO multi-channel-frequency-flat systems. IEEE Communications Letters. 19(3):475-478. https://doi.org/10.1109/LCOMM.2014.2386873S47547819
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