A connection between the Baum-Welch algorithm and turbo decoding

It is shown that the problem of APP decoding of single and parallel concatenated codes can be recast in terms of parameter estimation for one or more nonstationary hidden Markov processes. It is then shown that simple extensions of the Baum-Welch (1966, 1968, 1970) algorithm provide solutions to these problems, including solutions that duplicate the forward-backward and turbo decoding algorithms.