35,709 research outputs found
Network Coding Over SATCOM: Lessons Learned
Satellite networks provide unique challenges that can restrict users' quality
of service. For example, high packet erasure rates and large latencies can
cause significant disruptions to applications such as video streaming or
voice-over-IP. Network coding is one promising technique that has been shown to
help improve performance, especially in these environments. However,
implementing any form of network code can be challenging. This paper will use
an example of a generation-based network code and a sliding-window network code
to help highlight the benefits and drawbacks of using one over the other.
In-order packet delivery delay, as well as network efficiency, will be used as
metrics to help differentiate between the two approaches. Furthermore, lessoned
learned during the course of our research will be provided in an attempt to
help the reader understand when and where network coding provides its benefits.Comment: Accepted to WiSATS 201
Living with flexible exchange rates:
This overview paper examines two main issues. The first is why the exchange rate matters, especially for emerging market economies. The second is under what circumstances and how countries have dealt with the challenges posed by the exchange rate in recent years in the context of inflation targeting. We find that emerging market economies, being more exposed to the influence of the exchange rate, are likely to accord the exchange rate a bigger role in policy assessment and decision-making. However, even with the greater emphasis on the exchange rate, the emerging market economies under review have not acted in contradiction to their announced inflation targets. Furthermore, recent experience shows that having to keep an eye on the exchange rate is also a fact of life in industrial economies, inflation targeting or not.inflation targeting emerging markets exchange rate
Buoyancy and g-modes in young superfluid neutron stars
We consider the local dynamics of a realistic neutron star core, including
composition gradients, superfluidity and thermal effects. The main focus is on
the gravity g-modes, which are supported by composition stratification and
thermal gradients. We derive the equations that govern this problem in full
detail, paying particular attention to the input that needs to be provided
through the equation of state and distinguishing between normal and superfluid
regions. The analysis highlights a number of key issues that should be kept in
mind whenever equation of state data is compiled from nuclear physics for use
in neutron star calculations. We provide explicit results for a particular
stellar model and a specific nucleonic equation of state, making use of cooling
simulations to show how the local wave spectrum evolves as the star ages. Our
results show that the composition gradient is effectively dominated by the
muons whenever they are present. When the star cools below the superfluid
transition, the support for g-modes at lower densities (where there are no
muons) is entirely thermal. We confirm the recent suggestion that the g-modes
in this region may be unstable, but our results indicate that this instability
will be weak and would only be present for a brief period of the star's life.
Our analysis accounts for the presence of thermal excitations encoded in
entrainment between the entropy and the superfluid component. Finally, we
discuss the complete spectrum, including the normal sound waves and, in
superfluid regions, the second sound.Comment: 29 pages, 9 figures, submitted to MNRA
Seismology of adolescent neutron stars: Accounting for thermal effects and crust elasticity
We study the oscillations of relativistic stars, incorporating key physics
associated with internal composition, thermal gradients and crust elasticity.
Our aim is to develop a formalism which is able to account for the
state-of-the-art understanding of the complex physics associated with these
systems. As a first step, we build models using a modern equation of state
including composition gradients and density discontinuities associated with
internal phase-transitions (like the crust-core transition and the point where
muons first appear in the core). In order to understand the nature of the
oscillation spectrum, we carry out cooling simulations to provide realistic
snapshots of the temperature distribution in the interior as the star evolves
through adolescence. The associated thermal pressure is incorporated in the
perturbation analysis, and we discuss the presence of -modes arising as a
result of thermal effects. We also consider interface modes due to
phase-transitions and the gradual formation of the star's crust and the
emergence of a set of shear modes.Comment: 27 pages, 14 figure
Implications of an r-mode in XTE J1751-305: Mass, radius and spin evolution
Recently Strohmayer and Mahmoodifar presented evidence for a coherent
oscillation in the X-ray light curve of the accreting millisecond pulsar XTE
J1751-305, using data taken by RXTE during the 2002 outburst of this source.
They noted that a possible explanation includes the excitation of a non-radial
oscillation mode of the neutron star, either in the form of a g-mode or an
r-mode. The r-mode interpretation has connections with proposed spin-evolution
scenarios for systems such as XTE J1751-305. Here we examine in detail this
interesting possible interpretation. Using the ratio of the observed
oscillation frequency to the star's spin frequency, we derive an approximate
neutron star mass-radius relation which yields reasonable values for the mass
over the range of expected stellar radius (as constrained by observations of
radius-expansion burst sources). However, we argue that the large mode
amplitude suggested by the Strohmayer and Mahmoodifar analysis would inevitably
lead to a large spin-down of the star, inconsistent with its observed spin
evolution, regardless of whether the r-mode itself is in a stable or unstable
regime. We therefore conclude that the r-mode interpretation of the observed
oscillation is not consistent with our current understanding of neutron star
dynamics and must be considered unlikely. Finally we note that, subject to the
availability of a sufficiently accurate timing model, a direct
gravitational-wave search may be able to confirm or reject an r-mode
interpretation unambiguously, should such an event, with a similar inferred
mode amplitude, recur during the Advanced detector era.Comment: 8 pages, 3 figures; submitted to MNRA
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