Iterative Source-Channel Decoding: Improved System Design Using EXIT Charts
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Iterative Source-Channel Decoding: Improved System Design Using EXIT Charts Marc Adrat Institute of Communication Systems and Data Processing, Aachen University of Technology (RWTH), 52056 Aachen, Germany Email: [email protected]
Peter Vary Institute of Communication Systems and Data Processing, Aachen University of Technology (RWTH), 52056 Aachen, Germany Email: [email protected] Received 1 October 2003; Revised 5 April 2004 The error robustness of digital communication systems using source and channel coding can be improved by iterative sourcechannel decoding (ISCD). The turbo-like evaluation of natural residual source redundancy and of artificial channel coding redundancy makes step-wise quality gains possible by several iterations. The maximum number of profitable iterations is predictable by an EXIT chart analysis. In this contribution, we exploit the EXIT chart representation to improve the error correcting/concealing capabilities of ISCD schemes. We propose new design guidelines to select appropriate bit mappings and to design the channel coding component. A parametric source coding scheme with some residual redundancy is assumed. Applying both innovations, the new EXIT-optimized index assignment as well as the appropriately designed recursive nonsystematic convolutional (RNSC) code allow to outperform known approaches to ISCD by far in the most relevant channel conditions. Keywords and phrases: iterative source-channel decoding, turbo principle, soft-input/soft-output decoding, softbit source decoding, extrinsic information, EXIT charts.
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INTRODUCTION
The design and development guidelines for today’s digital communication systems are inspired by the information theoretic considerations of C. E. Shannon. His fundamental statements indicate that, in order to find the most error resistant realization of a communication system, the transmit, respectively, receive operations are in principle separable into source coding and channel coding. However, the achievement of the global optimum using this two-stage process is possibly subject to impractical computational complexity, to unlimited signal delay, and to stationary source signals. Taking realistic constraints of real-world communication systems into account, a separate treatment of source and channel coding usually inflicts a loss of optimality. Joint sourcechannel coding allows to narrow the gap to the global optimum. The present contribution addresses a novel concept for joint source-channel coding. A new method is proposed to This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
improve the error robustness of existing or emerging digital mobile communication systems like GSM (global system for mobile communications) or UMTS (universal mobile telecommunications system), or the digital audio/video broadcasting systems (DAB/DVB). In these systems the source coding part extracts char
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