3,539 research outputs found
Decentralized Constraint Satisfaction
We show that several important resource allocation problems in wireless
networks fit within the common framework of Constraint Satisfaction Problems
(CSPs). Inspired by the requirements of these applications, where variables are
located at distinct network devices that may not be able to communicate but may
interfere, we define natural criteria that a CSP solver must possess in order
to be practical. We term these algorithms decentralized CSP solvers. The best
known CSP solvers were designed for centralized problems and do not meet these
criteria. We introduce a stochastic decentralized CSP solver and prove that it
will find a solution in almost surely finite time, should one exist, also
showing it has many practically desirable properties. We benchmark the
algorithm's performance on a well-studied class of CSPs, random k-SAT,
illustrating that the time the algorithm takes to find a satisfying assignment
is competitive with stochastic centralized solvers on problems with order a
thousand variables despite its decentralized nature. We demonstrate the
solver's practical utility for the problems that motivated its introduction by
using it to find a non-interfering channel allocation for a network formed from
data from downtown Manhattan
Experimental implementation of optimal WLAN channel selection without communication
Proposed a simple decentralised algorithm for channel allocation that is provably correct and requires no message passing or common administrative control between interfering WLANs. In this paper we implement this algorithm using a standard 802.11 hardware testbed and demonstrate that it does indeed offer the potential for effective channel allocation in realistic environments. This includes environments with complex, spatially varying noise and channel dependent propagation behaviour and with time-varying load
Solution to the twin image problem in holography
While the invention of holography by Dennis Gabor truly constitutes an
ingenious concept, it has ever since been troubled by the so called twin image
problem limiting the information that can be obtained from a holographic
record. Due to symmetry reasons there are always two images appearing in the
reconstruction process. Thus, the reconstructed object is obscured by its
unwanted out of focus twin image. Especially for emission electron as well as
for x- and gamma-ray holography, where the source-object distances are small,
the reconstructed images of atoms are very close to their twin images from
which they can hardly be distinguished. In some particular instances only,
experimental efforts could remove the twin images. More recently, numerical
methods to diminish the effect of the twin image have been proposed but are
limited to purely absorbing objects failing to account for phase shifts caused
by the object. Here we show a universal method to reconstruct a hologram
completely free of twin images disturbance while no assumptions about the
object need to be imposed. Both, amplitude and true phase distributions are
retrieved without distortion
Differential approximation for Kelvin-wave turbulence
I present a nonlinear differential equation model (DAM) for the spectrum of
Kelvin waves on a thin vortex filament. This model preserves the original
scaling of the six-wave kinetic equation, its direct and inverse cascade
solutions, as well as the thermodynamic equilibrium spectra. Further, I extend
DAM to include the effect of sound radiation by Kelvin waves. I show that,
because of the phonon radiation, the turbulence spectrum ends at a maximum
frequency where
is the total energy injection rate, is the speed of sound and
is the quantum of circulation.Comment: Prepared of publication in JETP Letter
Nitrogen deposition does not enhance Sphagnum decomposition
Long-term additions of nitrogen (N) to peatlands have altered bryophyte growth, species dominance, N content in peat and peat water, and often resulted in enhanced Sphagnum decomposition rate. However, these results have mainly been derived from experiments in which N was applied as ammonium nitrate (NH4NO3), neglecting the fact that in polluted areas, wet deposition may be dominated either by NO3- or NH4+. We studied effects of elevated wet deposition of NO3- vs. NH4+ alone (8 or 56 kg N ha(-1) yr(-1) over and above the background of 8 kg N ha(-1) yr(-1) for 5 to 11 years) or combined with phosphorus (P) and potassium (K) on Sphagnum quality for decomposers, mass loss, and associated changes in hummock pore water in an ombrotrophic bog (Whim). Adding N, especially as NH4+, increased N concentration in Sphagnum, but did not enhance mass loss from Sphagnum. Mass loss seemed to depend mainly on moss species and climatic factors. Only high applications of N affected hummock pore water chemistry, which varied considerably over time. Overall, C and N cycling in this N treated bog appeared to be decoupled. We conclude that moss species, seasonal and annual variation in climatic factors, direct negative effects of N (NH4+ toxicity) on Sphagnum production, and indirect effects (increase in pH and changes in plant species dominance under elevated NO3- alone and with PK) drive Sphagnum decomposition and hummock C and N dynamics at Whim. (C) 2016 Elsevier B.V. All rights reserved.Peer reviewe
Decentralised Learning MACs for Collision-free Access in WLANs
By combining the features of CSMA and TDMA, fully decentralised WLAN MAC
schemes have recently been proposed that converge to collision-free schedules.
In this paper we describe a MAC with optimal long-run throughput that is almost
decentralised. We then design two \changed{schemes} that are practically
realisable, decentralised approximations of this optimal scheme and operate
with different amounts of sensing information. We achieve this by (1)
introducing learning algorithms that can substantially speed up convergence to
collision free operation; (2) developing a decentralised schedule length
adaptation scheme that provides long-run fair (uniform) access to the medium
while maintaining collision-free access for arbitrary numbers of stations
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