26,168 research outputs found
Experimental demonstration of digital predistortion for orthogonal frequency-division multiplexing-radio over fibre links near laser resonance
Radio over fibre (RoF), an enabling technology for distribution of wireless broadband service signals through analogue optical links, suffers from non-linear distortion. Digital predistortion has been demonstrated as an effective approach to overcome the RoF non-linearity. However, questions remain as to how the approach performs close to laser resonance, a region of significant dynamic non-linearity, and how resilient the approach is to changes in input signal and link operating conditions. In this work, the performance of a digital predistortion approach is studied for directly modulated orthogonal frequency-division multiplexing RoF links operating from 2.47 to 3.7 GHz. It extends previous works to higher frequencies, and to higher quadrature amplitude modulation (QAM) levels. In addition, the resilience of the predistortion approach to changes in modulation level of QAM schemes, and average power levels are investigated, and a novel predistortion training approach is proposed and demonstrated. Both memoryless and memory polynomial predistorter models, and a simple off-line least-squares-based identification method, are used, with excellent performance improvements demonstrated up to 3.0 GHz
Supercurrent on a vortex core in 2H-NbSe: current driven scanning tunneling spectroscopy
We report current driven scanning tunneling spectroscopy (CDSTS) measurements
at very low temperatures on vortices in 2H-NbSe2. We find that a current
produces an increase of the density of states at the Fermi level in between
vortices, and a reduction of the zero bias peak at the vortex center. This
occurs well below the de-pairing current. We conclude that a supercurrent
affects the low energy part of the superconducting gap structure of 2H-NbSe2.Comment: 5 pages, 5 figure
Phase diagram of random lattice gases in the annealed limit
An analysis of the random lattice gas in the annealed limit is presented. The
statistical mechanics of disordered lattice systems is briefly reviewed. For
the case of the lattice gas with an arbitrary uniform interaction potential and
random short-range interactions the annealed limit is discussed in detail. By
identifying and extracting an entropy of mixing term, a correct physical
expression for the pressure is explicitly given. As an application, the
one-dimensional lattice gas with uniform long-range interactions and random
short-range interactions satisfying a bimodal annealed probability distribution
is discussed. The model is exactly solved and is shown to present interesting
behavior in the presence of competition between interactions, such as the
presence of three phase transitions at constant temperature and the occurrence
of triple and quadruple points.Comment: Final version to be published in the Journal of Chemical Physic
Stabilized jellium model and structural relaxation effects on the fragmentation energies of ionized silver clusters
Using the stabilized jellium model in two schemes of `relaxed' and `rigid',
we have calculated the dissociation energies and the fission barrier heights
for the binary fragmentations of singly-ionized and doubly-ionized Ag clusters.
In the calculations, we have assumed spherical geometries for the clusters.
Comparison of the fragmentation energies in the two schemes show differences
which are significant in some cases. This result reveals the advantages of the
relaxed SJM over the rigid SJM in dynamical processes such as fragmentation.
Comparing the relaxed SJM results and axperimental data on fragmentation
energies, it is possible to predict the sizes of the clusters just before their
fragmentations.Comment: 9 pages, 12 JPG figure
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