Hidden Markov models have been widely used to statistically characterize sources and channels in communication systems. In this paper we consider their application in the context of turbo codes. We describe simplified techniques for modifying a decoder for turbo codes to incorporate the hidden Markov model. This will allow the receiver to utilize the statistical characteristics of the source or channel during the decoding process, leading to significantly improved performance relative to systems in which the Markov properties are not exploited. For the case of binary hidden Markov sources and continuous hidden Markov channels, the lack of a priori knowledge of the parameters of the model does not degrade the performance of turbo decoding since these parameters can be jointly estimated with turbo decoding.
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