7,517 research outputs found
Broadband Spectrum Survey Measurements for Cognitive Radio Applications
It is well known that the existing spectrum licensing system results in a gross under-utilization of the frequency spectrum. Spectrum background measurements – spectrum surveys – provide useful data for spectrum regulation, planning or finding frequency niches for spectrum sharing. Dynamic spectrum sharing as a main goal of cognitive radio (CR) is the modern option on how to optimize usage of the frequency spectrum. A spectrum survey measurement system is introduced with results obtained from a variety of markedly different scenarios allowing us, unlike other studies, to focus on wideband and fast spectrum scans. The sensitivity of the receiver is no worse than -113 dBm in the whole band. The utilization of the frequency spectrum is analyzed to prove its under-utilization and to show spectrum sharing opportunities. This was shown to be true in the frequency band higher than 2.5 GHz. A comparison with other spectrum survey campaigns is provided
Transient electrophoretic current in a nonpolar solvent
The transient electric current of surfactants dissolved in a nonpolar solvent
is investigated both experimentally and theoretically in the parallel-plate
geometry. Due to a low concentration of free charges the cell can be completely
polarized by an external voltage of several volts. In this state, all the
charged micelles are compacted against the electrodes. After the voltage is set
to zero the reverse current features a sharp discharge spike and a broad peak.
This shape and its variation with the compacting voltage are reproduced in a
one-dimensional drift-diffusion model. The model reveals the broad peak is
formed by a competition between an increasing number of charges drifting back
to the middle of the cell and a decreasing electric field that drives the
motion. After complete polarization is achieved, the shape of the peak stops
evolving with further increase of the compacting voltage. The spike-peak
separation time grows logarithmically with the charge content in the bulk. The
time peak is a useful measure of the micelle mobility. Time integration of the
peak yields the total charge in the system. By measuring its variation with
temperature, the activation energy of bulk charge generation has been found to
be 0.126 eV.Comment: 7 pages, 5 figure
Long range intermolecular forces in triatomic systems: connecting the atom-diatom and atom-atom-atom representations
The long-range forces that act between three atoms are analysed in both
atom-diatom and atom-atom-atom representations. Expressions for atom-diatom
dispersion coefficients are obtained in terms of 3-body nonadditive
coefficients. The anisotropy of atom-diatom C_6 dispersion coefficients arises
primarily from nonadditive triple-dipole and quadruple-dipole forces, while
pairwise-additive forces and nonadditive triple-dipole and
dipole-dipole-quadrupole forces contribute significantly to atom-diatom C_8
coefficients. The resulting expressions are applied to dispersion coefficients
for Li + Li_2 (triplet) and recommendations are made for the best way to obtain
global triatomic potentials that dissociate correctly both to three separated
atoms and to an atom and a diatomic molecule.Comment: To be published in a special issue of Molecular Physics in honour of
Mark Chil
Approximations of singular vertex couplings in quantum graphs
We discuss approximations of the vertex coupling on a star-shaped quantum
graph of edges in the singular case when the wave functions are not
continuous at the vertex and no edge-permutation symmetry is present. It is
shown that the Cheon-Shigehara technique using interactions with
nonlinearly scaled couplings yields a -parameter family of boundary
conditions in the sense of norm resolvent topology. Moreover, using graphs with
additional edges one can approximate the -parameter family of
all time-reversal invariant couplings.Comment: LaTeX source file, 33 pages, with 3 eps figure
Hidden Symmetries of Higher Dimensional Black Holes and Uniqueness of the Kerr-NUT-(A)dS spacetime
We prove that the most general solution of the Einstein equations with the
cosmological constant which admits a principal conformal Killing-Yano tensor is
the Kerr-NUT-(A)dS metric. Even when the Einstein equations are not imposed,
any spacetime admitting such hidden symmetry can be written in a canonical form
which guarantees the following properties: it is of the Petrov type D, it
allows the separation of variables for the Hamilton-Jacobi, Klein-Gordon, and
Dirac equations, the geodesic motion in such a spacetime is completely
integrable. These results naturally generalize the results obtained earlier in
four dimensions.Comment: 5 pages, no figure
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