142 research outputs found
Adaptive Linear Programming Decoding of Polar Codes
Polar codes are high density parity check codes and hence the sparse factor
graph, instead of the parity check matrix, has been used to practically
represent an LP polytope for LP decoding. Although LP decoding on this polytope
has the ML-certificate property, it performs poorly over a BAWGN channel. In
this paper, we propose modifications to adaptive cut generation based LP
decoding techniques and apply the modified-adaptive LP decoder to short
blocklength polar codes over a BAWGN channel. The proposed decoder provides
significant FER performance gain compared to the previously proposed LP decoder
and its performance approaches that of ML decoding at high SNRs. We also
present an algorithm to obtain a smaller factor graph from the original sparse
factor graph of a polar code. This reduced factor graph preserves the small
check node degrees needed to represent the LP polytope in practice. We show
that the fundamental polytope of the reduced factor graph can be obtained from
the projection of the polytope represented by the original sparse factor graph
and the frozen bit information. Thus, the LP decoding time complexity is
decreased without changing the FER performance by using the reduced factor
graph representation.Comment: 5 pages, 8 figures, to be presented at the IEEE Symposium on
Information Theory (ISIT) 201
J021659-044920: a relic giant radio galaxy at z ~ 1.3
We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at
redshift that exhibits large-scale extended, nearly co-spatial,
radio and X-ray emission from radio lobes, but no detection of Active Galactic
Nuclei core, jets and hotspots. The total angular extent of the GRG at the
observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is
found to be 2.4 arcmin, that corresponds to a total projected linear
size of 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz
shows high spectral curvature ( 1.19) with sharp steepening above
0.325 GHz, consistent with relic radio emission that is 8
10 yr old. The radio spectral index map between observed frame 0.325 and
1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from
outer-end to inner-end, indicating backflow of plasma in the lobes. The
extended X-ray emission characterized by an absorbed power-law with photon
index 1.86 favours inverse-Compton scattering of the Cosmic Microwave
Background (ICCMB) photons as the plausible origin. Using both X-ray and radio
fluxes under the assumption of ICCMB we estimate the magnetic field in the
lobes to be 3.3 G. The magnetic field estimate based on energy
equipartition is 3.5 G. Our work presents a case study of a rare
example of a GRG caught in dying phase in the distant Universe.Comment: 10 pages, 5 figures, 3 tables. Published in MNRAS. Corrected typos
and added a referenc
On the nature of infrared-faint radio sources in the SXDF and VLA-VVDS fields
Infrared-Faint Radio Sources (IFRSs) are an unusual class of objects that are
relatively bright at radio wavelengths but have faint or undetected infrared
counterparts even in deep surveys. We identify and investigate the nature of
IFRSs using deep radio (S 100 Jy beam at
5), optical (m 26 - 27.7 at 5), and near-IR
(S 1.3 - 2.0 Jy beam at 5) data
available in two deep fields namely the Subaru X-ray Deep Field (SXDF) and the
Very Large Array - VIMOS VLT Deep Survey (VLA-VVDS) field. In 1.8 deg of
the two fields we identify a total of nine confirmed and ten candidate IFRSs.
We find that our IFRSs are high-redshift radio-loud AGN, with 12/19 sources
having redshift estimates in the range of 1.7 - 4.3, while a limit
of 2.0 is placed for the remaining seven sources. Notably, our study
finds, for the first time, IFRSs with measured redshift 3.0, and also, the
redshift estimates for IFRSs in the faintest 3.6 m flux regime i.e.,
S 1.3 Jy. Radio observations show that our IFRSs
exhibit both compact unresolved as well as extended double-lobe morphologies,
and have predominantly steep radio spectra between 1.4 GHz and 325 MHz. The
non-detection of all but one IFRSs in the X-ray band and the optical-to-MIR
colour (m - m) suggest that a significant fraction
of IFRSs are likely to be hosted in dusty obscured galaxies.Comment: 20 pages, 8 figures, 4 tables, accepted for publication in MNRA
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