221,264 research outputs found
Channel Uncertainty in Ultra Wideband Communication Systems
Wide band systems operating over multipath channels may spread their power
over bandwidth if they use duty cycle. Channel uncertainty limits the
achievable data rates of power constrained wide band systems; Duty cycle
transmission reduces the channel uncertainty because the receiver has to
estimate the channel only when transmission takes place. The optimal choice of
the fraction of time used for transmission depends on the spectral efficiency
of the signal modulation. The general principle is demonstrated by comparing
the channel conditions that allow different modulations to achieve the capacity
in the limit. Direct sequence spread spectrum and pulse position modulation
systems with duty cycle achieve the channel capacity, if the increase of the
number of channel paths with the bandwidth is not too rapid. The higher
spectral efficiency of the spread spectrum modulation lets it achieve the
channel capacity in the limit, in environments where pulse position modulation
with non-vanishing symbol time cannot be used because of the large number of
channel paths
Exterior complex scaling as a perfect absorber in time-dependent problems
It is shown that exterior complex scaling provides for complete absorption of
outgoing flux in numerical solutions of the time-dependent Schr\"odinger
equation with strong infrared fields. This is demonstrated by computing high
harmonic spectra and wave-function overlaps with the exact solution for a
one-dimensional model system and by three-dimensional calculations for the H
atom and a Ne atom model. We lay out the key ingredients for correct
implementation and identify criteria for efficient discretization
Raman-assisted Rabi resonances in two-mode cavity QED
The dynamics of a vibronic system in a lossy two-mode cavity is studied, with
the first mode being resonant to the electronic transition and the second one
being nearly resonant due to Raman transitions. We derive analytical solutions
for the dynamics of this system. For a properly chosen detuning of the second
mode from the exact Raman resonance, we obtain conditions that are closely
related to the phenomenon of Rabi resonance as it is well known in laser
physics. Such resonances can be observed in the spontaneous emission spectra,
where the spectrum of the second mode in the case of weak Raman coupling is
enhanced substantially.Comment: 6 pages, 5 figure
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