4,325 research outputs found
Integer-Forcing Source Coding
Integer-Forcing (IF) is a new framework, based on compute-and-forward, for
decoding multiple integer linear combinations from the output of a Gaussian
multiple-input multiple-output channel. This work applies the IF approach to
arrive at a new low-complexity scheme, IF source coding, for distributed lossy
compression of correlated Gaussian sources under a minimum mean squared error
distortion measure. All encoders use the same nested lattice codebook. Each
encoder quantizes its observation using the fine lattice as a quantizer and
reduces the result modulo the coarse lattice, which plays the role of binning.
Rather than directly recovering the individual quantized signals, the decoder
first recovers a full-rank set of judiciously chosen integer linear
combinations of the quantized signals, and then inverts it. In general, the
linear combinations have smaller average powers than the original signals. This
allows to increase the density of the coarse lattice, which in turn translates
to smaller compression rates. We also propose and analyze a one-shot version of
IF source coding, that is simple enough to potentially lead to a new design
principle for analog-to-digital converters that can exploit spatial
correlations between the sampled signals.Comment: Submitted to IEEE Transactions on Information Theor
Achieving Secrecy Capacity of the Gaussian Wiretap Channel with Polar Lattices
In this work, an explicit wiretap coding scheme based on polar lattices is
proposed to achieve the secrecy capacity of the additive white Gaussian noise
(AWGN) wiretap channel. Firstly, polar lattices are used to construct
secrecy-good lattices for the mod- Gaussian wiretap channel. Then we
propose an explicit shaping scheme to remove this mod- front end and
extend polar lattices to the genuine Gaussian wiretap channel. The shaping
technique is based on the lattice Gaussian distribution, which leads to a
binary asymmetric channel at each level for the multilevel lattice codes. By
employing the asymmetric polar coding technique, we construct an AWGN-good
lattice and a secrecy-good lattice with optimal shaping simultaneously. As a
result, the encoding complexity for the sender and the decoding complexity for
the legitimate receiver are both O(N logN log(logN)). The proposed scheme is
proven to be semantically secure.Comment: Submitted to IEEE Trans. Information Theory, revised. This is the
authors' own version of the pape
Fermi-surface induced modulation in an optimally doped YBCO superconductor
We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO
superconductor with a wavevector along CuO chains, {\it i.e.} =(0,,0). The value of is twice the Fermi
wavevector () along {\bf b*} connecting nearly nested FS `ridges'.
The modulation exists only within O-vacancy-ordered islands
(characterized by = and persists well above and
below . Our results are consistent with the presence of a FS-induced
charge-density wave
Chromium at High Pressures: Weak Coupling and Strong Fluctuations in an Itinerant Antiferromagnet
The spin- and charge-density-wave order parameters of the itinerant
antiferromagnet chromium are measured directly with non-resonant x-ray
diffraction as the system is driven towards its quantum critical point with
high pressure using a diamond anvil cell. The exponential decrease of the spin
and charge diffraction intensities with pressure confirms the harmonic scaling
of spin and charge, while the evolution of the incommensurate ordering vector
provides important insight into the difference between pressure and chemical
doping as means of driving quantum phase transitions. Measurement of the charge
density wave over more than two orders of magnitude of diffraction intensity
provides the clearest demonstration to date of a weakly-coupled, BCS-like
ground state. Evidence for the coexistence of this weakly-coupled ground state
with high-energy excitations and pseudogap formation above the ordering
temperature in chromium, the charge-ordered perovskite manganites, and the blue
bronzes, among other such systems, raises fundamental questions about the
distinctions between weak and strong coupling.Comment: 11 pages, 9 figures (8 in color
Multiple-Description Coding by Dithered Delta-Sigma Quantization
We address the connection between the multiple-description (MD) problem and
Delta-Sigma quantization. The inherent redundancy due to oversampling in
Delta-Sigma quantization, and the simple linear-additive noise model resulting
from dithered lattice quantization, allow us to construct a symmetric and
time-invariant MD coding scheme. We show that the use of a noise shaping filter
makes it possible to trade off central distortion for side distortion.
Asymptotically as the dimension of the lattice vector quantizer and order of
the noise shaping filter approach infinity, the entropy rate of the dithered
Delta-Sigma quantization scheme approaches the symmetric two-channel MD
rate-distortion function for a memoryless Gaussian source and MSE fidelity
criterion, at any side-to-central distortion ratio and any resolution. In the
optimal scheme, the infinite-order noise shaping filter must be minimum phase
and have a piece-wise flat power spectrum with a single jump discontinuity. An
important advantage of the proposed design is that it is symmetric in rate and
distortion by construction, so the coding rates of the descriptions are
identical and there is therefore no need for source splitting.Comment: Revised, restructured, significantly shortened and minor typos has
been fixed. Accepted for publication in the IEEE Transactions on Information
Theor
Incommensurate antiferromagnetism in a pure spin system via cooperative organization of local and itinerant moments
Materials with strong correlations are prone to spin and charge instabilities, driven by Coulomb, magnetic, and lattice interactions. In materials that have significant localized and itinerant spins, it is not obvious which will induce order. We combine electrical transport, X-ray magnetic diffraction, and photoemission studies with band structure calculations to characterize successive anti-ferromagnetic transitions in GdSi. GdSi has both sizable local moments and a partially nested Fermi surface, without confounding contributions from orbital effects. We identify a route to incommensurate order where neither type of moment dominates, but is rooted in cooperative feedback between them. The nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, whereas the ordered local moments in turn provide the necessary coupling for a spin-density wave to form among the itinerant electrons. This mechanism echoes the cooperative interactions between electrons and ions in charge-density-wave materials, and should be germane across a spectrum of transition-metal and rare-earth intermetallic compounds
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