3,026 research outputs found
Achievable Outage Rate Regions for the MISO Interference Channel
We consider the slow-fading two-user multiple-input single-output (MISO)
interference channel. We want to understand which rate points can be achieved,
allowing a non-zero outage probability. We do so by defining four different
outage rate regions. The definitions differ on whether the rates are declared
in outage jointly or individually and whether the transmitters have
instantaneous or statistical channel state information (CSI). The focus is on
the instantaneous CSI case with individual outage, where we propose a
stochastic mapping from the rate point and the channel realization to the
beamforming vectors. A major contribution is that we prove that the stochastic
component of this mapping is independent of the actual channel realization.Comment: Accepted for publication in IEEE Wireless Communications Letter
Efficient Computation of Pareto Optimal Beamforming Vectors for the MISO Interference Channel with Successive Interference Cancellation
We study the two-user multiple-input single-output (MISO) Gaussian
interference channel where the transmitters have perfect channel state
information and employ single-stream beamforming. The receivers are capable of
performing successive interference cancellation, so when the interfering signal
is strong enough, it can be decoded, treating the desired signal as noise, and
subtracted from the received signal, before the desired signal is decoded. We
propose efficient methods to compute the Pareto-optimal rate points and
corresponding beamforming vector pairs, by maximizing the rate of one link
given the rate of the other link. We do so by splitting the original problem
into four subproblems corresponding to the combinations of the receivers'
decoding strategies - either decode the interference or treat it as additive
noise. We utilize recently proposed parameterizations of the optimal
beamforming vectors to equivalently reformulate each subproblem as a
quasi-concave problem, which we solve very efficiently either analytically or
via scalar numerical optimization. The computational complexity of the proposed
methods is several orders-of-magnitude less than the complexity of the
state-of-the-art methods. We use the proposed methods to illustrate the effect
of the strength and spatial correlation of the channels on the shape of the
rate region.Comment: Accepted for publication in IEEE Transactions on Signal Processin
Improved Time-Domain Accuracy Standards for Model Gravitational Waveforms
Model gravitational waveforms must be accurate enough to be useful for
detection of signals and measurement of their parameters, so appropriate
accuracy standards are needed. Yet these standards should not be unnecessarily
restrictive, making them impractical for the numerical and analytical modelers
to meet. The work of Lindblom, Owen, and Brown [Phys. Rev. D 78, 124020 (2008)]
is extended by deriving new waveform accuracy standards which are significantly
less restrictive while still ensuring the quality needed for gravitational-wave
data analysis. These new standards are formulated as bounds on certain norms of
the time-domain waveform errors, which makes it possible to enforce them in
situations where frequency-domain errors may be difficult or impossible to
estimate reliably. These standards are less restrictive by about a factor of 20
than the previously published time-domain standards for detection, and up to a
factor of 60 for measurement. These new standards should therefore be much
easier to use effectively.Comment: 10 pages, 5 figure
Effect of hyperon bulk viscosity on neutron-star r-modes
Neutron stars are expected to contain a significant number of hyperons in
addition to protons and neutrons in the highest density portions of their
cores. Following the work of Jones, we calculate the coefficient of bulk
viscosity due to nonleptonic weak interactions involving hyperons in
neutron-star cores, including new relativistic and superfluid effects. We
evaluate the influence of this new bulk viscosity on the gravitational
radiation driven instability in the r-modes. We find that the instability is
completely suppressed in stars with cores cooler than a few times 10^9 K, but
that stars rotating more rapidly than 10-30% of maximum are unstable for
temperatures around 10^10 K. Since neutron-star cores are expected to cool to a
few times 10^9 K within seconds (much shorter than the r-mode instability
growth time) due to direct Urca processes, we conclude that the gravitational
radiation instability will be suppressed in young neutron stars before it can
significantly change the angular momentum of the star.Comment: final PRD version, minor typos etc correcte
R-Modes in Superfluid Neutron Stars
The analogs of r-modes in superfluid neutron stars are studied here. These
modes, which are governed primarily by the Coriolis force, are identical to
their ordinary-fluid counterparts at the lowest order in the small
angular-velocity expansion used here. The equations that determine the next
order terms are derived and solved numerically for fairly realistic superfluid
neutron-star models. The damping of these modes by superfluid ``mutual
friction'' (which vanishes at the lowest order in this expansion) is found to
have a characteristic time-scale of about 10^4 s for the m=2 r-mode in a
``typical'' superfluid neutron-star model. This time-scale is far too long to
allow mutual friction to suppress the recently discovered gravitational
radiation driven instability in the r-modes. However, the strength of the
mutual friction damping depends very sensitively on the details of the
neutron-star core superfluid. A small fraction of the presently acceptable
range of superfluid models have characteristic mutual friction damping times
that are short enough (i.e. shorter than about 5 s) to suppress the
gravitational radiation driven instability completely.Comment: 15 pages, 8 figure
The r-modes in accreting neutron stars with magneto-viscous boundary layers
We explore the dynamics of the r-modes in accreting neutron stars in two
ways. First, we explore how dissipation in the magneto-viscous boundary layer
(MVBL) at the crust-core interface governs the damping of r-mode perturbations
in the fluid interior. Two models are considered: one assuming an
ordinary-fluid interior, the other taking the core to consist of superfluid
neutrons, type II superconducting protons, and normal electrons. We show,
within our approximations, that no solution to the magnetohydrodynamic
equations exists in the superfluid model when both the neutron and proton
vortices are pinned. However, if just one species of vortex is pinned, we can
find solutions. When the neutron vortices are pinned and the proton vortices
are unpinned there is much more dissipation than in the ordinary-fluid model,
unless the pinning is weak. When the proton vortices are pinned and the neutron
vortices are unpinned the dissipation is comparable or slightly less than that
for the ordinary-fluid model, even when the pinning is strong. We also find in
the superfluid model that relatively weak radial magnetic fields ~ 10^9 G (10^8
K / T)^2 greatly affect the MVBL, though the effects of mutual friction tend to
counteract the magnetic effects. Second, we evolve our two models in time,
accounting for accretion, and explore how the magnetic field strength, the
r-mode saturation amplitude, and the accretion rate affect the cyclic evolution
of these stars. If the r-modes control the spin cycles of accreting neutron
stars we find that magnetic fields can affect the clustering of the spin
frequencies of low mass x-ray binaries (LMXBs) and the fraction of these that
are currently emitting gravitational waves.Comment: 19 pages, 8 eps figures, RevTeX; corrected minor typos and added a
referenc
Shear viscosity of neutron matter from realistic nucleon-nucleon interactions
The calculation of transport properties of Fermi liquids, based on the
formalism developed by Abrikosov and Khalatnikov, requires the knowledge of the
probability of collisions between quasiparticles in the vicinity of the Fermi
surface. We have carried out a numerical study of the shear viscosity of pure
neutron matter, whose value plays a pivotal role in determining the stability
of rotating neutron stars, in which these processes are described using a
state-of-the-art nucleon-nucleon potential model. Within our approach medium
modifications of the scattering cross section are consistently taken into
account, through an effective interaction obtained from the matrix elements of
the bare interaction between correlated states. Inclusion of medium effects
lead to a large increase of the viscosity at densities larger than
fm^{-3}.Comment: 4 pages, 4 figures. Corrected typo
Nonlinear r-Modes in Neutron Stars: Instability of an unstable mode
We study the dynamical evolution of a large amplitude r-mode by numerical
simulations. R-modes in neutron stars are unstable growing modes, driven by
gravitational radiation reaction. In these simulations, r-modes of amplitude
unity or above are destroyed by a catastrophic decay: A large amplitude r-mode
gradually leaks energy into other fluid modes, which in turn act nonlinearly
with the r-mode, leading to the onset of the rapid decay. As a result the
r-mode suddenly breaks down into a differentially rotating configuration. The
catastrophic decay does not appear to be related to shock waves at the star's
surface. The limit it imposes on the r-mode amplitude is significantly smaller
than that suggested by previous fully nonlinear numerical simulations.Comment: Published in Phys. Rev. D Rapid Comm. 66, 041303(R) (2002
Effect of a neutron-star crust on the r-mode instability
The presence of a viscous boundary layer under the solid crust of a neutron star dramatically increases the viscous damping rate of the fluid r-modes. We improve previous estimates of this damping rate by including the effect of the Coriolis force on the boundary-layer eigenfunction and by using more realistic neutron-star models. If the crust is assumed to be perfectly rigid, the gravitational radiation driven instability in the r-modes is completely suppressed in neutron stars colder than about 1.5 x 10^8 K. Energy generation in the boundary layer will heat the star, and will even melt the crust if the amplitude of the r-mode is large enough. We solve the heat equation explicitly (including the effects of thermal conduction and neutrino emission) and find that the r-mode amplitude needed to melt the crust is approximately a_c = 5 x 10^{-3} for maximally rotating neutron stars. If the r-mode saturates at an amplitude larger than a_c, the heat generated is sufficient to maintain the outer layers of the star in a mixed fluid-solid state analogous to the pack ice on the fringes of the Arctic Ocean. We argue that in young, rapidly rotating neutron stars this effect considerably delays the formation of the crust. By considering the dissipation in the ice flow, we show that the final spin frequency of stars with r-mode amplitude of order unity is close to the value estimated for fluid stars without a crust
Möglichkeiten und Probleme bei der Anwendung der Klebtechnik
Nur wenn eine klebgerecht ausgeführte Konstruktion mit dem richtigen Klebstoff nach optimaler Oberflächenbehandlung und mit angepssten Abbindebedingungen gefertigt wird, sind Klebverbindungen von maximaler Festigkeit und Alterungsbeständigkeit zu erzielen. Am Beispiel von Bremsbelägen wird gezeigt, dass bei einer entsprechenden Erprobung auch sogenannte Sicherheitsteile durch Kleben hergestellt werden könne
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