Exact thresholds and optimal codes for the binary-symmetric channel and Gallager's decoding algorithm A

  • Authors:
  • L. Bazzi;T. J. Richardson;R. L. Urbanke

  • Affiliations:
  • Fac. of Eng. & Archit., American Univ. of Beirut, Lebanon;-;-

  • Venue:
  • IEEE Transactions on Information Theory
  • Year:
  • 2006

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Abstract

We show that for the case of the binary-symmetric channel and Gallager's decoding algorithm A the threshold can, in many cases, be determined analytically. More precisely, we show that the threshold is always upper-bounded by the minimum of (1-λ2ρ'(1))/(λ'(1)ρ'(1)-λ2ρ'(1)) and the smallest positive real root τ of a specific polynomial p(x) and we observe that for most cases this bound is tight, i.e., it determines the threshold exactly. We also present optimal degree distributions for a large range of rates. In the case of rate one-half codes, for example, the threshold x0* of the optimal degree distribution is given by x*0∼0.0513663. Finally, we outline how thresholds of more complicated decoders might be determined analytically.