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 decision-making 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.
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