39,628 research outputs found
Chung-Li, Taiwan dual mode (Doppler and spaced antenna) VHF radar: Preliminary specifications
A major unresolved question in the field of atmospheric research using VHF radar techniques is the relative merit of the two most widely used systems. These systems are the Doppler method and the spaced antenna method. It has been suggested that one radar of each type be operated side by side for a direct comparison of the two techniques. This duplication of effort is not cost effective. The major components of both systems are identical, and one radar could be operated in both modes by proper design of a suitable antenna system and by proper data analysis. The Chung-Li radar will be able to switch between modes on a time scale of seconds and is the first VHF radar to be able to directly compare the Doppler data with spaced antenna data. The system will have performance comparable with the present SOUSY spaced antenna system and will provide mesospheric data in addition to stratospheric and tropospheric data. The major specifications of the Chung-Li radar are given
Performance of binary block codes at low signal-to-noise ratios
The performance of general binary block codes on an unquantized additive white Gaussian noise (AWGN) channel at low signal-to-noise ratios is considered. Expressions are derived for both the block error and the bit error probabilities near the point where the bit signal-to-noise ratio is zero. These expressions depend on the global geometric structure of the code, although the minimum distance still seems to play a crucial role. Examples of codes such as orthogonal codes, biorthogonal codes, the (24,12) extended Golay code, and the (15,6) expurgated BCH code are discussed. The asymptotic coding gain at low signal-to-noise ratios is also studied
Photonics-enabled sub-Nyquist radio frequency sensing based on temporal channelization and compressive sensing
A novel approach to sensing broadband radio frequency (RF) spectrum beyond the Nyquist limit based on photonic temporal channelization and compressive sensing is proposed. A spectrally-sparse RF signal with unknown frequencies is modulated onto a highly chirped optical pulse. An optical channelizer slices the modulated pulse spectrum, which is equivalent to temporally sampling the RF waveform thanks to the dispersion-induced wavelength-to-time mapping. This serial-to-parallel conversion avoids the use of a high-speed detector and digitizer. Furthermore, compressive sensing with optical random demodulation is achieved using a spatial light modulator, enabling the system to capture the wideband multi-tone RF signal with a sampling rate far lower than the Nyquist rate. It is demonstrated that the temporal channelization system with a channel spacing of 20 GHz achieves RF spectrum sensing with a high resolution of 196 MHz. With an equivalent sampling rate of only 25 GHz, a 50-GHz broadband two-tone RF signal can be captured and reconstructed by the system thanks to compressive sensing with a compression ratio of 4
Influence of quantum fluctuations on the superfluid critical velocity of a one-dimensional Bose gas
The mean-field Gross-Pitaevskii equation with repulsive interactions exhibits
frictionless flow when stirred by an obstacle below a critical velocity. Here
we go beyond the mean-field approximation to examine the influence of quantum
fluctuations on this threshold behaviour in a one-dimensional Bose gas in a
ring. Using the truncated Wigner approximation, we perform simulations of
ensembles of trajectories where the Bose gas is stirred with a repulsive
obstacle below the mean-field critical velocity. We observe the probabilistic
formation of grey solitons which subsequently decay, leading to an increase in
the momentum of the fluid. The formation of the first soliton leads to a
soliton cascade, such that the fluid rapidly accelerates to minimise the speed
difference with the obstacle. We measure the initial rate of momentum transfer,
and relate it to macroscopic tunnelling between quantised flow states in the
ring.Comment: 10 pages, 9 figure
Electroweak Baryogenesis, Electric Dipole Moments, and Higgs Diphoton Decays
We study the viability of electroweak baryogenesis in a two Higgs doublet
model scenario augmented by vector-like, electroweakly interacting fermions.
Considering a limited, but illustrative region of the model parameter space, we
obtain the observed cosmic baryon asymmetry while satisfying present
constraints from the non-observation of the permanent electric dipole moment
(EDM) of the electron and the combined ATLAS and CMS result for the Higgs boson
diphoton decay rate. The observation of a non-zero electron EDM in a next
generation experiment and/or the observation of an excess (over the Standard
Model) of Higgs to diphoton events with the 14 TeV LHC run or a future
collider would be consistent with generation of the observed baryon asymmetry
in this scenario.Comment: 22 pages, 3 figure
Bosonic Super Liouville System: Lax Pair and Solution
We study the bosonic super Liouville system which is a statistical
transmutation of super Liouville system. Lax pair for the bosonic super
Liouville system is constructed using prolongation method, ensuring the Lax
integrability, and the solution to the equations of motion is also considered
via Leznov-Saveliev analysis.Comment: LaTeX, no figures, 11 page
- …