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IEEE Communications Letters

Adaptive Combined Chase-GMD Algorithms for Block Codes

Ashraf M. Mahran, Mohammed Benaissa

Published Apr 1, 2004• 7 citations
Publisher

Abstract

This work proposes a new adaptive approach to the soft decision decoding of the so-called combined Chase-GMD class of algorithms (CGA). A reliability threshold is used to match the decoding performance to the channel conditions. As a result, a significant reduction in decoding complexity can be achieved by reducing the number of required algebraic decodings. The proposed adaptive application can achieve the upper performance limit of the CGA with significant complexity reduction.

DOI
10.1109/lcomm.2004.827979

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This work proposes a new adaptive approach to the soft decision decoding of the so-called combined Chase-GMD class of algorithms (CGA). A reliability threshold is used to match the decoding performance to the channel conditions. As a result, a significant reduction in decoding complexity can be achieved by reducing the number of required algebraic decodings. The proposed adaptive application can achieve the upper performance limit of the CGA with significant complexity reduction.
A well known soft decoding algorithm that can make use of algebraic decoding techniques is the GMD algorithm introduced by Forney. This algorithm is based on several error-erasure decoding trails where unreliable positions are successively erased. Chase proposed a closely related family of algorithms for binary codes where unreliable positions are switched', i.e. different estimates for the positions are used in intermediate steps during the decoding process. In our algorithm, suitable for non binary codes, we try to combine erasures and switchings in a favourable way. A version of the algorithm comparable in complexity with the GMD algorithm is investigated. Simulations show that our algorithm in many cases has a somewhat better error correcting capability then the GMD algorithm.
In this paper,the ChaseⅡ algorithm,which is well known as a reliability-based decoding algorithm,is used for the soft decoding of the(23,12,7)Golay code.This soft-decision decoding algorithm improves greatly the error-correcting capacity compared with algebraic hard-decision decoding algorithms,but also results in more computational complexity.To overcome this,a optimality condition is introduced to quickly terminate the ChaseⅡ decoding process.Computer simulation results got by VC6.0 show that soft decoding obtains 1.7 dB coding gain compared with hard decoding case at BER of 10-5,and the modified chase-II algorithm based on the optimality condition reduces the total simulation time by 87.64%,and thus improves greatly the decoding efficiency.