A system and method for soft decision forward error correction (FEC)
decoding may be used to determine a possible error in a differential
detection signal, for example, in a DPSK system. The system and method
uses the constructive and destructive signals from a demodulator to
provide an error locating signal. Using the error locating signal, the
system and method converts the differential detection signal into a soft
decision signal including multi-level soft values.