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IEEE International Symposium on Information Theory (ISIT)

Decoding of expander codes at rates close to capacity

Alexei Ashikhmin, Vitaly Skachek

Published Jan 1, 2005• 1 citation
Publisher

Abstract

The concatenation of nearly-MDS expander codes of Roth and Skachek, "On Nearly-MDS Expander Codes," Proc. IEEE ISIT'04, with 'typical' LDPC codes is investigated. It is shown that for the rates R = (1 - epsi)C (C is the capacity of the binary symmetric channel (BSC)), under certain condition on the parameters of LDPC codes, these concatenated codes have decoding time linear in their length and polynomial in 1/epsi, and the decoding error probability decays exponentially. These codes are compared to the recently presented codes of Barg and Zemor, "Error Exponents of Expander Codes," IEEE Trans. Inform. Theory, 2002, and "Concatenated Codes: Serial and Parallel," IEEE Trans. Inform. Theory, 2005. It is shown that the latter families can not be tuned to have all the aforementioned properties

DOI
10.1109/isit.2005.1523346

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The concatenation of nearly-MDS expander codes of Roth and Skachek, "On Nearly-MDS Expander Codes," Proc. IEEE ISIT'04, with 'typical' LDPC codes is investigated. It is shown that for the rates R = (1 - epsi)C (C is the capacity of the binary symmetric channel (BSC)), under certain condition on the parameters of LDPC codes, these concatenated codes have decoding time linear in their length and polynomial in 1/epsi, and the decoding error probability decays exponentially. These codes are compared to the recently presented codes of Barg and Zemor, "Error Exponents of Expander Codes," IEEE Trans. Inform. Theory, 2002, and "Concatenated Codes: Serial and Parallel," IEEE Trans. Inform. Theory, 2005. It is shown that the latter families can not be tuned to have all the aforementioned properties
The decoding error probability of codes is studied as a function of their block length. It is shown that the existence of codes with a polynomially small decoding error probability implies the existence of codes with an exponentially small decoding error probability. Specifically, it is assumed that there exists a family of codes of length N and rate R=(1-ε)C (C is a capacity of a binary symmetric channel), whose decoding probability decreases polynomially in 1/N. It is shown that if the decoding probability decreases sufficiently fast, but still only polynomially fast in 1/N, then there exists another such family of codes whose decoding error probability decreases exponentially fast in N. Moreover, if the decoding time complexity of the assumed family of codes is polynomial in N and 1/ε, then the decoding time complexity of the presented family is linear in N and polynomial in 1/ε. These codes are compared to the recently presented codes of Barg and Zemor, ``Error Exponents of Expander Codes,'' IEEE Trans. Inform. Theory, 2002, and ``Concatenated Codes: Serial and Parallel,'' IEEE Trans. Inform. Theory, 2005. It is shown that the latter families can not be tuned to have exponentially decaying (in N) error probability, and at the same time to have decoding time complexity linear in N and polynomial in 1/ε.
The decoding error probability of codes is studied as a function of their block length. It is shown that the existence of codes with a polynomially small decoding error probability implies the existence of codes with an exponentially small decoding error probability. Specifically, it is assumed that there exists a family of codes of length N and rate R=(1-epsiv)C (C is a capacity of a binary-symmetric channel), whose decoding probability decreases inverse polynomially in N. It is shown that if the decoding probability decreases sufficiently fast, but still only inverse polynomially fast in N, then there exists another such family of codes whose decoding error probability decreases exponentially fast in N. Moreover, if the decoding time complexity of the assumed family of codes is polynomial in N and 1/epsiv, then the decoding time complexity of the presented family is linear in N and polynomial in 1/epsiv. These codes are compared to the recently presented codes of Barg and Zemor, "Error Exponents of Expander Codes", IEEE Transactions on Information Theory, 2002, and "Concatenated Codes: Serial and Parallel", IEEE Transactions on Information Theory, 2005. It is shown that the latter families cannot be tuned to have exponentially decaying (in N) error probability, and at the same time to have decoding time complexity linear in N and polynomial in 1/epsiv
Recently, G. Zemor (see IEEE Trans. Inf. Theory, vol.47, p.835-7, 2001) proposed an improvement on the Sipser-Spielman analysis of expander codes (Sipser, M. and Spielman, D.A., IEEE Trans. Inf. Theory, vol.42 , p.1710-22, 1996) and presented a linear-time iterative decoder that can correct a number of errors up to approximately 1/4 the known lower bound on the minimum distance of the code. We propose an improvement on Zemor's decoder for F=GF(2), with the number of correctable errors becoming close to half the lower bound on the minimum distance. The improvement is obtained by inserting into the decoding algorithm features akin to generalized minimum distance decoding of concatenated codes.