498 research outputs found
Symbol synchronization in convolutionally coded systems
Alternate symbol inversion is sometimes applied to the output of convolutional encoders to guarantee sufficient richness of symbol transition for the receiver symbol synchronizer. A bound is given for the length of the transition-free symbol stream in such systems, and those convolutional codes are characterized in which arbitrarily long transition free runs occur
On the inherent intractability of certain coding problems
The fact that the general decoding problem for linear codes and the general problem of finding the weights of a linear code are both NP-complete is shown. This strongly suggests, but does not rigorously imply, that no algorithm for either of these problems which runs in polynomial time exists
Two-dimensional burst identification codes and their use in burst correction
A new class of codes, called burst identification codes, is defined and studied. These codes can be used to determine the patterns of burst errors. Two-dimensional burst correcting codes can be easily constructed from burst identification codes. The resulting class of codes is simple to implement and has lower redundancy than other comparable codes. The results are pertinent to the study of radiation effects on VLSI RAM chips, which can cause two-dimensional bursts of errors
On the existence of optimum cyclic burst-correcting codes
It is shown that for each integer b >= 1 infinitely many optimum cyclic b-burst-correcting codes exist, i.e., codes whose length n, redundancy r, and burst-correcting capability b, satisfy n = 2^{r-b+1} - 1. Some optimum codes for b = 3, 4, and 5 are also studied in detail
Femtosecond Spectroscopy with Vacuum Ultraviolet Pulse Pairs
We combine different wavelengths from an intense high-order harmonics source
with variable delay at the focus of a split-mirror interferometer to conduct
pump-probe experiments on gas-phase molecules. We report measurements of the
time resolution (<44 fs) and spatial profiles (4 {\mu}m x 12 {\mu}m) at the
focus of the apparatus. We demonstrate the utility of this two-color,
high-order-harmonic technique by time resolving molecular hydrogen elimination
from C2H4 excited into its absorption band at 161 nm
Age Related Visual Pathologies among Nursing Home Residents: An Evaluation of Light Conditions and Recording in Client Files
Objective: Reflection on visual problems in nursing homes. Data Sources: Eye examinations, documented visual problems and illuminance levels. Study design: The optometric examinations and recorded visual problems were combined with illuminance data. Data collection: In seven nursing homes, 259 residents underwent an optometric examination. Their client records were analyzed for information regarding visual functioning. The illuminance data were ranked to set the quality of the lighting conditions. Principal findings: 50% of the referred residents had problems with cataracts, retinal problems (21%), suspected glaucoma (13%), and other pathologies (16%). The information was not current in 56% of the records. The quality of lighting conditions was low or moderate. Conclusion: The finding of poor lighting conditions in nursing homes in combination with a high prevalence of visual problems (with cataract found to be the most common age related pathology), stretches the need of enhanced awareness of eye care by professional caregivers
On production and asymmetric focusing of flat electron beams using rectangular capillary discharge plasmas
A method for the asymmetric focusing of electron bunches, based on the active
plasma lensing technique is proposed. This method takes advantage of the strong
inhomogeneous magnetic field generated inside the capillary discharge plasma to
focus the ultrarelativistic electrons. The plasma and magnetic field parameters
inside the capillary discharge are described theoretically and modeled with
dissipative magnetohydrodynamic computer simulations enabling analysis of the
capillaries of rectangle cross-sections. Large aspect ratio rectangular
capillaries might be used to transport electron beams with high emittance
asymmetries, as well as assist in forming spatially flat electron bunches for
final focusing before the interaction point.Comment: 16 pages, 7 figures, 1 tabl
Overview of Plasma Lens Experiments and Recent Results at SPARC_LAB
Beam injection and extraction from a plasma module is still one of the
crucial aspects to solve in order to produce high quality electron beams with a
plasma accelerator. Proper matching conditions require to focus the incoming
high brightness beam down to few microns size and to capture a high divergent
beam at the exit without loss of beam quality. Plasma-based lenses have proven
to provide focusing gradients of the order of kT/m with radially symmetric
focusing thus promising compact and affordable alternative to permanent magnets
in the design of transport lines. In this paper an overview of recent
experiments and future perspectives of plasma lenses is reported
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