Maximum a Posteriori Decoding of Turbo Codes

The process of turbo-code decoding starts with the formation of a posteriori probabilities (APPs) for each data bit, which is followed by choosing the data-bit value that corresponds to the maximum a posteriori (MAP) probability for that data bit. Upon reception of a corrupted code-bit sequence, the process of decisionmaking with APPs allows the MAP algorithm to determine the most likely information bit to have been transmitted at each bit time. The metrics needed for the implementation of a MAP decoder are presented here, along with an example to illustrate how these metrics are used.