46,674 research outputs found
Myths and Realities of Rateless Coding
Fixed-rate and rateless channel codes are generally treated separately in the related research literature and so, a novice in the field inevitably gets the impression that these channel codes are unrelated. By contrast, in this treatise, we endeavor to further develop a link between the traditional fixed-rate codes and the recently developed rateless codes by delving into their underlying attributes. This joint treatment is beneficial for two principal reasons. First, it facilitates the task of researchers and practitioners, who might be familiar with fixed-rate codes and would like to jump-start their understanding of the recently developed concepts in the rateless reality. Second, it provides grounds for extending the use of the well-understood code design tools — originally contrived for fixed-rate codes — to the realm of rateless codes. Indeed, these versatile tools proved to be vital in the design of diverse fixed-rate-coded communications systems, and thus our hope is that they will further elucidate the associated performance ramifications of the rateless coded schemes
An iterative algorithm for parametrization of shortest length shift registers over finite rings
The construction of shortest feedback shift registers for a finite sequence
S_1,...,S_N is considered over the finite ring Z_{p^r}. A novel algorithm is
presented that yields a parametrization of all shortest feedback shift
registers for the sequence of numbers S_1,...,S_N, thus solving an open problem
in the literature. The algorithm iteratively processes each number, starting
with S_1, and constructs at each step a particular type of minimal Gr\"obner
basis. The construction involves a simple update rule at each step which leads
to computational efficiency. It is shown that the algorithm simultaneously
computes a similar parametrization for the reciprocal sequence S_N,...,S_1.Comment: Submitte
How supernova feedback turns dark matter cusps into cores
We propose and successfully test against new cosmological simulations a novel
analytical description of the physical processes associated with the origin of
cored dark matter density profiles. In the simulations, the potential in the
central kiloparsec changes on sub-dynamical timescales over the redshift
interval 4 > z > 2 as repeated, energetic feedback generates large underdense
bubbles of expanding gas from centrally-concentrated bursts of star formation.
The model demonstrates how fluctuations in the central potential irreversibly
transfer energy into collisionless particles, thus generating a dark matter
core. A supply of gas undergoing collapse and rapid expansion is therefore the
essential ingredient. The framework, based on a novel impulsive approximation,
breaks with the reliance on adiabatic approximations which are inappropriate in
the rapidly-changing limit. It shows that both outflows and galactic fountains
can give rise to cusp-flattening, even when only a few per cent of the baryons
form stars. Dwarf galaxies maintain their core to the present time. The model
suggests that constant density dark matter cores will be generated in systems
of a wide mass range if central starbursts or AGN phases are sufficiently
frequent and energetic.Comment: 9 pages, 6 figures, accepted by MNRAS. No change in results. Expanded
discussion and more reference
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