1,852 research outputs found
Quantum Versus Classical Decay Laws in Open Chaotic Systems
We study analytically the time evolution in decaying chaotic systems and
discuss in detail the hierarchy of characteristic time scales that appeared in
the quasiclassical region. There exist two quantum time scales: the Heisenberg
time t_H and the time t_q=t_H/\sqrt{\kappa T} (with \kappa >> 1 and T being the
degree of resonance overlapping and the transmission coefficient respectively)
associated with the decay. If t_q < t_H the quantum deviation from the
classical decay law starts at the time t_q and are due to the openness of the
system. Under the opposite condition quantum effects in intrinsic evolution
begin to influence the decay at the time t_H. In this case we establish the
connection between quantities which describe the time evolution in an open
system and their closed counterparts.Comment: 3 pages, REVTeX, no figures, replaced with the published version
(misprints corrected, references updated
Optimized puncturing distributions for irregular non-binary LDPC codes
In this paper we design non-uniform bit-wise puncturing distributions for
irregular non-binary LDPC (NB-LDPC) codes. The puncturing distributions are
optimized by minimizing the decoding threshold of the punctured LDPC code, the
threshold being computed with a Monte-Carlo implementation of Density
Evolution. First, we show that Density Evolution computed with Monte-Carlo
simulations provides accurate (very close) and precise (small variance)
estimates of NB-LDPC code ensemble thresholds. Based on the proposed method, we
analyze several puncturing distributions for regular and semi-regular codes,
obtained either by clustering punctured bits, or spreading them over the
symbol-nodes of the Tanner graph. Finally, optimized puncturing distributions
for non-binary LDPC codes with small maximum degree are presented, which
exhibit a gap between 0.2 and 0.5 dB to the channel capacity, for punctured
rates varying from 0.5 to 0.9.Comment: 6 pages, ISITA1
Analysis of Quasi-Cyclic LDPC codes under ML decoding over the erasure channel
In this paper, we show that Quasi-Cyclic LDPC codes can efficiently
accommodate the hybrid iterative/ML decoding over the binary erasure channel.
We demonstrate that the quasi-cyclic structure of the parity-check matrix can
be advantageously used in order to significantly reduce the complexity of the
ML decoding. This is achieved by a simple row/column permutation that
transforms a QC matrix into a pseudo-band form. Based on this approach, we
propose a class of QC-LDPC codes with almost ideal error correction performance
under the ML decoding, while the required number of row/symbol operations
scales as , where is the number of source symbols.Comment: 6 pages, ISITA1
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