3,732 research outputs found
Geometric Approximations of Some Aloha-like Stability Regions
Most bounds on the stability region of Aloha give necessary and sufficient
conditions for the stability of an arrival rate vector under a specific
contention probability (control) vector. But such results do not yield
easy-to-check bounds on the overall Aloha stability region because they
potentially require checking membership in an uncountably infinite number of
sets parameterized by each possible control vector. In this paper we consider
an important specific inner bound on Aloha that has this property of difficulty
to check membership in the set. We provide ellipsoids (for which membership is
easy-to-check) that we conjecture are inner and outer bounds on this set. We
also study the set of controls that stabilize a fixed arrival rate vector; this
set is shown to be a convex set.Comment: Presented at IEEE ISIT 2010 (Austin, TX
Rotational Behaviors and Magnetic Field Evolution of Radio Pulsars
The observed long-term spin-down evolution of isolated radio pulsars cannot
be explained by the standard magnetic dipole radiation with a constant braking
torque. However how and why the torque varies still remains controversial,
which is an outstanding problem in our understanding of neutron stars. We have
constructed a phenomenological model of the evolution of surface magnetic
fields of pulsars, which contains a long-term decay modulated by short-term
oscillations; a pulsar's spin is thus modified by its magnetic field evolution.
The predictions of this model agree with the precisely measured spin evolutions
of several individual pulsars; the derived parameters suggest that the Hall
drift and Hall waves in the NS crusts are probably responsible for the
long-term change and short-term quasi-periodical oscillations, respectively.
Many statistical properties of the timing noise of pulsars can be well
re-produced with this model, including correlations and the distributions of
the observed braking indices of the pulsars, which span over a range of more
than 100 millions. We have also presented a phenomenological model for the
recovery processes of classical and slow glitches, which can successfully model
the observed slow and classical glitch events without biases.Comment: 6 pages, 9 figures, submitted to conference proceedings of SMFNS2013
(Strong electromagnetic field and neutron stars 2013
Mean Square Capacity of Power Constrained Fading Channels with Causal Encoders and Decoders
This paper is concerned with the mean square stabilization problem of
discrete-time LTI systems over a power constrained fading channel. Different
from existing research works, the channel considered in this paper suffers from
both fading and additive noises. We allow any form of causal channel
encoders/decoders, unlike linear encoders/decoders commonly studied in the
literature. Sufficient conditions and necessary conditions for the mean square
stabilizability are given in terms of channel parameters such as transmission
power and fading and additive noise statistics in relation to the unstable
eigenvalues of the open-loop system matrix. The corresponding mean square
capacity of the power constrained fading channel under causal encoders/decoders
is given. It is proved that this mean square capacity is smaller than the
corresponding Shannon channel capacity. In the end, numerical examples are
presented, which demonstrate that the causal encoders/decoders render less
restrictive stabilizability conditions than those under linear
encoders/decoders studied in the existing works.Comment: Accepted by the 54th IEEE Conference on Decision and Contro
- …