1,683 research outputs found
On the Computation of EXIT Characteristics for Symbol-Based Iterative Decoding
In this paper we propose an efficient method for computing index-based extrinsic information transfer (EXIT) charts, which are useful for estimating the convergence properties of non-binary iterative decoding. A standard method is to apply <i>a priori</i> reliability information to the <i>a posteriori</i> probability (APP) constituent decoder and compute the resulting average extrinsic information at the decoder output via multidimensional histogram measurements. However, this technique is only reasonable for very small index lengths as the complexity of this approach grows exponentially with the index length. We show that by averaging over a function of the extrinsic APPs for a long block the extrinsic information can be estimated with very low complexity. In contrast to using histogram measurements this method allows to generate EXIT charts even for larger index alphabets. Examples for a non-binary serial concatenated code and for turbo trellis-coded modulation, resp., demonstrate the capabilities of the proposed approach
Chiral structures of lander molecules on Cu(100)
Supramolecular assemblies of lander molecules (CH) on Cu(100)
are investigated with low-temperature scanning tunneling microscopy. The
energetically most favourable conformation of the adsorbed molecule is found to
exist in two mirror symmetric enantiomers or conformers. At low coverage, the
molecules align in enantiomerically pure chains along the chiral directions
and . The arrangement is proposed to be
mainly governed by intermolecular van-der-Waals interaction. At higher
coverages, the molecular chains arrange into chiral domains, for which a
structural model is presented.Comment: to appear in Nanotechnology vol. 15 (2004
Turbo-Detected Unequal Error Protection Irregular Convolutional Codes Designed for the Wideband Advanced Multirate Speech Codec
Abstract—since the different bits of multimedia information, such as speech and video, have different error sensitivity, efficient unequalprotection channel coding schemes have to be used to ensure that the perceptually more important bits benefit from more powerful protection. Furthermore, in the context of turbo detection the channel codes should also match the characteristics of the channel for the sake of attaining a good convergence performance. In this paper, we address this design dilemma by using irregular convolutional codes (IRCCs) which constitute a family of different-rate subcodes. we benefit from the high design flexibility of IRCCs and hence excellent convergence properties are maintained while having unequal error protection capabilities matched to the requirements of the source. An EXIT chart based design procedure is proposed and used in the context of protecting the different-sensitivity speech bits of the wideband AMR speech codec. As a benefit, the unequalprotection system using IRCCs exhibits an SNR advantage of about 0.4dB over the equal-protection system employing regular convolutional codes, when communicating over a Gaussian channel
Trapping Set Enumerators for Repeat Multiple Accumulate Code Ensembles
The serial concatenation of a repetition code with two or more accumulators
has the advantage of a simple encoder structure. Furthermore, the resulting
ensemble is asymptotically good and exhibits minimum distance growing linearly
with block length. However, in practice these codes cannot be decoded by a
maximum likelihood decoder, and iterative decoding schemes must be employed.
For low-density parity-check codes, the notion of trapping sets has been
introduced to estimate the performance of these codes under iterative message
passing decoding. In this paper, we present a closed form finite length
ensemble trapping set enumerator for repeat multiple accumulate codes by
creating a trellis representation of trapping sets. We also obtain the
asymptotic expressions when the block length tends to infinity and evaluate
them numerically.Comment: 5 pages, to appear in proc. IEEE ISIT, June 200
Focussing quantum states
Does the size of atoms present a lower limit to the size of electronic
structures that can be fabricated in solids? This limit can be overcome by
using devices that exploit quantum mechanical scattering of electron waves at
atoms arranged in focussing geometries on selected surfaces. Calculations
reveal that features smaller than a hydrogen atom can be obtained. These
structures are potentially useful for device applications and offer a route to
the fabrication of ultrafine and well defined tips for scanning tunneling
microscopy.Comment: 4 pages, 4 figure
Picosecond time-resolved pure-rotational coherent anti-Stokes Raman spectroscopy for N-2 thermometry
This paper was published in Optics Letters and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/abstract.cfm?URI=ol-34-23-3755. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.Peer reviewedPublisher PD
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