“Turbo Decoding” Soft Iterative Decoding of Concatenated Codes
Berrou et al (1993) “Parallel Concatenated-Turbo” Codes
Benedetto et al (1996) “Serial Concatenated” Codes
Decode Inner Code generating Soft-decision Outputs by MAP Algorithm.
Use them as Inputs to Outer code, but also
generate Soft-decision Outputs from Outer Code;
feed these back to Inner Code and repeat -- typically 8 to 16 iterations.
Result is almost equivalent to optimum (maximum likelihood)
decoding of composite (overall) code! [Why?: Theory Incomplete]