System and method for forward error correction decoding using soft information

A forward error correction (FEC) decoding system for use in a receiver capable of demodulating a modulated FEC encoded signal into a constructive signal and a destructive signal and producing a hard decision differential detection signal, said FEC decoding system comprising:

• first and second hard decision detectors operable to receive said constructive and said destructive signals, respectively, and to produce an error locating signal indicating possible errors in said hard decision differential detection signal;
• at least first and second mapping tables for mapping multi-level soft values to said hard decision differential detection signal, wherein said mapping tables are configured to be selected in response to said error locating signal;
• and
• a soft decision decoder operable to decode said multi-level soft values mapped to said hard decision differential detection signal.

A forward error correction (FEC) decoding system for use in a receiver capable of demodulating a modulated FEC encoded signal into a constructive signal and a destructive signal and producing a hard decision differential detection signal, said FEC decoding system comprising:

• first and second hard decision detectors operable to receive said constructive and destructive signals and to produce an error locating signal indicating possible errors in said hard decision differential detection signal;
• a soft information adjuster operable to replace at least one value in said hard decision differential detection signal with an erasure when said error locating signal indicates that said at least one value is a possible error, thereby producing an adjusted differential detection signal;
• and
• an erasure decoder operable to decode said adjusted differential detection signal.

An optical communications system, comprising:

• a transmitter comprising an encoder for forward error correction (FEC) encoding a data stream and a modulator for modulating said FEC encoded data stream to produce a modulated FEC encoded signal;
• and
• a receiver comprising:
• a demodulator operable to receive said modulated FEC encoded signal and to produce a constructive signal and a destructive signal;
• a differential detector operable to receive said constructive and destructive signals and to produce a hard decision differential detection signal corresponding to said FEC encoded data stream;
• and
• a soft decision FEC decoding system operable to receive said constructive and destructive signals and said differential detection signal, said FEC decoding system being operable to determine a possible error in said differential detection signal using hard-decision detection of each of said constructive and destructive signals, to convert said hard decision differential detection signal into a soft decision signal including multi-level soft values, and to decode said soft decision signal.

A method of forward error correction (FEC) decoding, said method comprising:

• receiving a modulated FEC encoded signal containing a FEC encoded data stream;
• demodulating the modulated FEC encoded signal to produce a constructive signal and a destructive signal;
• differentially detecting said constructive signal and said destructive signal to produce a hard decision differential detection signal corresponding to said FEC encoded data stream;
• detecting each of said constructive signal and said destructive signal to produce an error locating signal indicating possible errors in said differential detection signal;
• converting said hard decision differential detection signal into a soft decision signal using said error locating signal, said soft decision signal including multi-level soft values;
• and
• decoding said soft decision signal to produce an output data stream.