8,937 research outputs found
A new bound and algorithm for star 16-QAM carrier phase estimation
Copyright © 2003 IEEEThe true Cramer-Rao lower bound (CRLB) is derived and evaluated for the estimation of carrier phase of Star 16-quadrature amplitude modulation (QAM) and can be simply applied to carrier frequency estimation. Different geometries are investigated by varying the ring ratio (RR). For signal-to-noise ratios (SNRs) between 6-15 dB, the CRLB with RR=3 is lower than that of Square 16-QAM. A modified phase estimator is presented, which closely follows the new CRLB. Investigation of symbol error performance in short packet length reveals Star 16-QAM to be superior to Square 16-QAM for SNR<13 dB, which is a reasonable operating range for a coded system. Although Square 16-QAM and Star RR=1.8 are optimum for a perfect receiver, when the effect of phase estimation is considered, we find Star RR=3 to be better for SNR below 10 dB.Feng Rice, Mark Rice, and Bill Cowle
The role of the energy equation in the fragmentation of protostellar discs during stellar encounters
In this paper, we use high-resolution smoothed particle hydrodynamics (SPH)
simulations to investigate the response of a marginally stable self-gravitating
protostellar disc to a close parabolic encounter with a companion discless
star. Our main aim is to test whether close brown dwarfs or massive planets can
form out of the fragmentation of such discs. We follow the thermal evolution of
the disc by including the effects of heating due to compression and shocks and
a simple prescription for cooling and find results that contrast with previous
isothermal simulations. In the present case we find that fragmentation is
inhibited by the interaction, due to the strong effect of tidal heating, which
results in a strong stabilization of the disc. A similar behaviour was also
previously observed in other simulations involving discs in binary systems. As
in the case of isolated discs, it appears that the condition for fragmentation
ultimately depends on the cooling rate.Comment: 9 pages, 10 figures, accepted in MNRA
The threshold of detection of vegetative canopies using remotely sensed data
There are no author-identified significant results in this report
Global properties of the nearby spiral M101
M101 (NGC 5457) is a classic Sc I spiral galaxy located suffiently nearby, 6.8 Mpc, that its structure can be studied even with the coarse angular resolution of the Infrared Astronomy Satellite (IRAS). The global infrared properties of M101 are addressed including the radial dependence of its infrared emission
Current-driven vortex dynamics in untwinned superconducting single crystals
Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue
Coarse-Grained Picture for Controlling Complex Quantum Systems
We propose a coarse-grained picture to control ``complex'' quantum dynamics,
i.e., multi-level-multi-level transition with a random interaction. Assuming
that optimally controlled dynamics can be described as a Rabi-like oscillation
between an initial and final state, we derive an analytic optimal field as a
solution to optimal control theory. For random matrix systems, we numerically
confirm that the analytic optimal field steers an initial state to a target
state which both contains many eigenstates.Comment: jpsj2.cls, 2 pages, 3 figure files; appear in J. Phys. Soc. Jpn.
Vol.73, No.11 (Nov. 15, 2004
The Future of Mass Tort Claims: Comparison of Settlement Class Action to Bankruptcy Treatment of Mass Tort Claims
Coherent open-loop optimal control of light-harvesting dynamics
We apply theoretically open-loop quantum optimal control techniques to
provide methods for the verification of various quantum coherent transport
mechanisms in natural and artificial light-harvesting complexes under realistic
experimental constraints. We demonstrate that optimally shaped laser pulses
allow to faithfully prepare the photosystem in specified initial states (such
as localized excitation or coherent superposition, i.e. propagating and
non-propagating states) and to probe efficiently the dynamics. These results
provide a path towards the discrimination of the different transport pathways
and to the characterization of environmental properties, enhancing our
understanding of the role that coherent processes may play in biological
complexes.Comment: 12 pages, 15 figure
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