25,576 research outputs found
Diversity Backpressure Scheduling and Routing with Mutual Information Accumulation in Wireless Ad-hoc Networks
We suggest and analyze algorithms for routing in multi-hop wireless ad-hoc
networks that exploit mutual information accumulation as the physical layer
transmission scheme, and are capable of routing multiple packet streams
(commodities) when only the average channel state information is present and
that only locally. The proposed algorithms are modifications of the Diversity
Backpressure (DIVBAR) algorithm, under which the packet whose commodity has the
largest "backpressure metric" is chosen to be transmitted and is forwarded
through the link with the largest differential backlog (queue length). In
contrast to traditional DIVBAR, each receiving node stores and accumulates the
partially received packet in a separate "partial packet queue", thus increasing
the probability of successful reception during a later possible retransmission.
We present two variants of the algorithm: DIVBAR-RMIA, under which all the
receiving nodes clear the received partial information of a packet once one or
more receiving nodes firstly decode the packet; and DIVBAR-MIA, under which all
the receiving nodes retain the partial information of a packet until the packet
has reached its destination. We characterize the network capacity region with
RMIA and prove that (under certain mild conditions) it is strictly larger than
the network capacity region with the repetition (REP) transmission scheme that
is used by the traditional DIVBAR. We also prove that DIVBAR-RMIA is
throughput-optimum among the polices with RMIA, i.e., it achieves the network
capacity region with RMIA, which in turn demonstrates that DIVBAR-RMIA
outperforms traditional DIVBAR on the achievable throughput. Moreover, we prove
that DIVBAR-MIA performs at least as well as DIVBAR-RMIA with respect to
throughput. Simulations also confirm these results.Comment: 56 pages, 6 figure
Holographic entanglement entropy and the internal space
We elaborate on the role of extremal surfaces probing the internal space in
AdS/CFT. Extremal surfaces in AdS quantify the "geometric" entanglement between
different regions in physical space for the dual CFT. This, however, is just
one of many ways to split a given system into subsectors, and extremal surfaces
in the internal space should similarly quantify entanglement between subsectors
of the theory. For the case of AdSS, their area was interpreted
as entanglement entropy between U(n) and U(m) subsectors of U(n+m) N=4 SYM.
Making this proposal precise is subtle for a number of reasons, the most
obvious being that from the bulk one usually has access to gauge-invariant
quantities only, while a split into subgroups is inherently gauge variant. We
study N=4 SYM on the Coulomb branch, where some of the issues can be mitigated
and the proposal can be sharpened. Continuing back to the original
AdSS geometry, we obtain a modified proposal, based on the
relation of the internal space to the R-symmetry group.Comment: 11 pages, 6 figures; to appear in PR
Chemical bond and entanglement of electrons in the hydrogen molecule
We theoretically investigate the quantum correlations (in terms of
concurrence of indistinguishable electrons) in a prototype molecular system
(hydrogen molecule). With the assistance of the standard approximations of the
linear combination of atomic orbitals and the configuration interaction methods
we describe the electronic wavefunction of the ground state of the H2 molecule.
Moreover, we managed to find a rather simple analytic expression for the
concurrence (the most used measure of quantum entanglement) of the two
electrons when the molecule is in its lowest energy. We have found that
concurrence does not really show any relation to the construction of the
chemical bond.Comment: 4 figures, 11 page
Calabi-Yau manifolds realizing symplectically rigid monodromy tuples
We define an iterative construction that produces a family of elliptically
fibered Calabi-Yau -folds with section from a family of elliptic Calabi-Yau
varieties of one dimension lower. Parallel to the geometric construction, we
iteratively obtain for each family with a point of maximal unipotent monodromy,
normalized to be at t=0, its Picard-Fuchs operator and a closed-form expression
for the period holomorphic at t=0, through a generalization of the classical
Euler transform for hypergeometric functions. In particular, our construction
yields one-parameter families of elliptically fibered Calabi-Yau manifolds with
section whose Picard-Fuchs operators realize all symplectically rigid
Calabi-Yau differential operators with three regular singular points classified
by Bogner and Reiter, but also non-rigid operators with four singular points.Comment: 65 page
Orthogonal colorings of the sphere
An orthogonal coloring of the two-dimensional unit sphere , is
a partition of into parts such that no part contains a pair of
orthogonal points, that is, a pair of points at spherical distance
apart. It is a well-known result that an orthogonal coloring of
requires at least four parts, and orthogonal colorings with exactly four parts
can easily be constructed from a regular octahedron centered at the origin. An
intriguing question is whether or not every orthogonal 4-coloring of
is such an octahedral coloring. In this paper we address this
question and show that if every color class has a non-empty interior, then the
coloring is octahedral. Some related results are also given
Static and dynamic properties of Josephson weak links with singlet and triplet coupling
We theoretically study static and dynamic properties of short Josephson
junctions (JJ) with singlet and triplet Josephson coupling. In singlet
Josephson weak links, two singlet superconductors S are connected with each
other by a normal film (N) or wire. Triplet JJs, which we denote
S/N(F)/S, are formed by two singlet BCS
superconductors covered by a thin layer of a weak ferromagnet F.
These superconductors S are separated from the N (or F) layer by
spin filters, which pass electrons with only one spin orientation. The triplet
Cooper pairs propagating from the left (right) superconductors S
differ from each other not only by polarizations, but also by chiralities. The
latter is determined by the magnetization orientation in weak ferromagnets
F. We obtain analytical formulas for the critical Josephson
current in both types of the JJs. If chiralities of the triplet Cooper pairs
penetrating into the N film in S/N(F)/S JJs from the
left and right S are different, the Josephson current is not zero
in the absence of the phase difference (spontaneous Josephson current). We also
calculate the admittance for arbitrary frequencies in the
case of singlet JJs and for low frequencies in the case of triplet JJs. At low
temperatures , the real part of the admittance in
singlet JJs starts to increase from zero at , but at , it has a peak at
low frequencies the magnitude of which is determined by inelastic processes.
The subgap depends on transparencies of the S/N interfaces
and on the phase difference . The low-frequency peak in
in triplet JJs disappears.Comment: 11+ pages; 4 figures; published versio
Colorful theorems for strong convexity
We prove two colorful Carath\'eodory theorems for strongly convex hulls,
generalizing the colorful Carat\'eodory theorem for ordinary convexity by Imre
B\'ar\'any, the non-colorful Carath\'eodory theorem for strongly convex hulls
by the second author, and the "very colorful theorems" by the first author and
others. We also investigate if the assumption of a "generating convex set" is
really needed in such results and try to give a topological criterion for one
convex body to be a Minkowski summand of another
Soft-Output Sphere Decoder for Multiple-Symbol Differential Detection of Impulse-Radio Ultra-Wideband
Power efficiency of noncoherent receivers for impulse-radio ultra-wideband
(IR-UWB) transmission systems can significantly be improved, on the one hand,
by employing multiple-symbol differential detection (MSDD), and, on the other
hand, by providing reliability information to the subsequent channel decoder.
In this paper, we combine these two techniques. Incorporating the computation
of the soft information into a single-tree-search sphere decoder (SD), the
application of this soft-output MSDD in a typical IR-UWB system imposes only a
moderate complexity increase at, however, improved performance over hard-output
MSDD, and in particular, over conventional symbol-by-symbol noncoherent
differential detection.Comment: accepted for presentation at 2010 IEEE International Symposium on
Information Theory (ISIT 2010), Austin (TX), USA, as paper #106
Microrheology of supercooled liquids in terms of a continuous time random walk
Molecular dynamics simulations of a glass-forming model system are performed
under application of a microrheological perturbation on a tagged particle. The
trajectory of that particle is studied in its underlying potential energy
landscape. Discretization of the configuration space is achieved via a
metabasin analysis. The linear and nonlinear responses of drift and diffusive
behavior can be interpreted and analyzed in terms of a continuous time random
walk. In this way the physical origin of linear and nonlinear response can be
identified. Critical forces are determined and compared with predictions from
literature
Individual Preference Aware Caching Policy Design in Wireless D2D Networks
Cache-aided wireless device-to-device (D2D) networks allow significant
throughput increase, depending on the concentration of the popularity
distribution of files. Many studies assume that all users have the same
preference distribution; however, this may not be true in practice. This work
investigates whether and how the information about individual preferences can
benefit cache-aided D2D networks. We examine a clustered network and derive a
network utility that considers both the user distribution and channel fading
effects into the analysis. We also formulate a utility maximization problem for
designing caching policies. This maximization problem can be applied to
optimize several important quantities, including throughput, energy efficiency
(EE), cost, and hit-rate, and to solve different tradeoff problems. We provide
a general approach that can solve the proposed problem under the assumption
that users coordinate, then prove that the proposed approach can obtain the
stationary point under a mild assumption. Using simulations of practical
setups, we show that performance can improve significantly with proper
exploitation of individual preferences. We also show that different types of
tradeoffs exist between different performance metrics and that they can be
managed through caching policy and cooperation distance designs.Comment: Accepted by IEEE Transactions on Wireless Communication
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