On the Entropy of a Hidden Markov Process

  • Authors:
  • Philippe Jacquet;Gadiel Seroussi;Wojciech Szpankowski

  • Affiliations:
  • -;-;-

  • Venue:
  • DCC '04 Proceedings of the Conference on Data Compression
  • Year:
  • 2004

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Abstract

We study the entropy rate of a binary hidden Markov process (HMP) definedby observing the output of a binary symmetric channel whose input is a first-order binaryMarkov process. Despite the simplicity of the models involved, the characterization of thisentropy is a long standing open problem. By presenting the probability of a sequence underthe model as a product of random matrices, we show that the entropy rate sought is a topLyapunov exponent of the product, which explains the difficulty in its explicit computation.We apply the same product of random matrices to derive an explicit expression for a firstorder Taylor approximation of the entropy rate with respect to the parameter of the binary symmetric channel. The accuracy of the approximation is validated against empiricalsimulation results. We also extend our results to Rényi's entropy of any order.