The turbo code decoder is an iterative Maximum A posteriori (MAP) algorithm. MAP algorithms use a Bayesian analysis of the channel transition probabilities and the state diagram of the encoder to estimate the most likely transmitted sequence of data bits based on the received sequence of data and code bits. The decoder sequentially decodes the results from each constituent encoder.
The results of each of these steps are fed to the next step as a-priori information called extrinsic data. After processing the results of all constituent codes, the algorithm goes back to the first constituent code and repeats the process see Figure below.
