Parity frame

Abstract

A method of forward error correction in an optical communications system. A signal to be transmitted is logically defined as a super-frame comprising a plurality of frames including a parity frame and a predetermined set of data frames. Each frame of the super-frame is processed in accordance with a first FEC scheme having a known error correlation characteristic. At least the set of data frames is processed in accordance with a second FEC scheme which is selected based on the error correlation characteristic of the first FEC scheme.

We claim: 
     
         1 . A method of forward error correction in an optical communications system, the method comprising:
 logically defining a signal as a super-frame comprising a plurality of frames including a parity frame and a predetermined set of data frames; and   a signal processor performing the steps of:
 processing each frame of the super-frame in accordance with a first FEC scheme having a known error correlation characteristic; and 
 processing at least the set of data frames in accordance with a second FEC scheme which is adapted to compensate at least the error correlation characteristic of the first FEC scheme. 
   
     
     
         2 . The method as claimed in  claim 1 , wherein the signal processor is associated with a transmitter of the optical communications system, and wherein processing at least the set of data frames in accordance with a second FEC scheme comprises computing the parity frame across the predetermined set of data frames. 
     
     
         3 . The method as claimed in  claim 2 , wherein computing the parity frame comprises accumulating a bit-wise XOR across the predetermined set of data frames. 
     
     
         4 . The method as claimed in  claim 2 , wherein the parity frame is computed before the step of processing each frame of the super-frame in accordance with the first FEC scheme. 
     
     
         5 . The method as claimed in  claim 1 , wherein the signal processor is associated with a receiver of the optical communications system, and wherein processing each frame of the super-frame in accordance with the first FEC scheme comprises:
 decoding received frames of the super-frame to generate corresponding decoded frames.   determining, for each received frame, whether or not the decoding step was successful.   
     
     
         6 . The method as claimed in  claim 5 , wherein processing at least the set of data frames in accordance with a second FEC scheme comprises computing a local parity frame across the decoded frames. 
     
     
         7 . The method as claimed in  claim 6 , wherein processing each frame of the super-frame further comprises determining, for each received frame, whether or not the decoding step was successful, and wherein the step of computing a local parity frame is performed across only decoded frames corresponding to received frames that were successfully decoded. 
     
     
         8 . The method as claimed in  claim 6 , wherein computing the local parity frame comprises accumulating a bit-wise XOR. 
     
     
         9 . The method as claimed in  claim 8 , wherein the known error correlation characteristic of the first FEC scheme comprises that an error burst due to channel effects will affect at most one frame within the super-frame. 
     
     
         10 . The method as claimed in  claim 9 , wherein processing at least the set of data frames in accordance with a second FEC scheme further comprises:
 identifying an erroneous decoded frame corresponding to a received frame that was not successfully decoded; and   correcting the erroneous decoded frame using the local parity frame.   
     
     
         11 . The method as claimed in  claim 10 , wherein correcting the erroneous decoded frame comprises replacing the erroneous decoded frame with the local parity frame. 
     
     
         12 . The method as claimed in  claim 6 , wherein computing the local parity frame comprises accumulating a bit-wise soft metric across the decoded frames corresponding to received frames that were successfully decoded. 
     
     
         13 . The method as claimed in  claim 12 , wherein the bit-wise soft metric is a probability of a predetermined bit value at each frame location. 
     
     
         14 . The method as claimed in  claim 12 , wherein the known error correlation characteristic of the first FEC scheme comprises that an error burst due to channel effects will affect at most two frames within the super-frame. 
     
     
         15 . The method as claimed in  claim 14 , wherein processing at least the set of data frames in accordance with a second FEC scheme further comprises:
 identifying each erroneous decoded frame corresponding to a received frame that was not successfully decoded; and   correcting each identified erroneous decoded frame using the local parity frame.   
     
     
         16 . The method as claimed in  claim 15 , wherein there are two erroneous decoded frames, and wherein correcting each identified erroneous decoded frame comprises:
 considering a set comprising the two erroneous frames and the local parity frame; and   identifying each bit position for which the sum of the corresponding bit values across the set results in odd parity; and   for each identified bit position, swapping the value of the least reliable bit among the set.   
     
     
         17 . The method as claimed in  claim 5 , further comprising:
 stopping the step of decoding received frames when a number of frames for which the decoding step was successful is sufficient that remaining data frames of the super-frame can be reconstructed using the second FEC scheme; and   reconstructing the remaining data frames of the super-frame using the second FEC scheme.   
     
     
         18 . A signal processor for implementing forward error correction of a signal in an optical communications system, the signal being logically formatted as a super-frame comprising a plurality of frames including a parity frame and a predetermined set of data frames, the signal processor comprising:
 means for processing each frame of the super-frame in accordance with a first FEC scheme having a known error correlation characteristic; and   means for processing at least the set of data frames in accordance with a second FEC scheme which is adapted to compensate at least the error correlation characteristic of the first FEC scheme.   
     
     
         19 . The signal processor as claimed in  claim 18 , wherein the signal processor is implemented as a FEC encoder in a transmitter of the optical communications system. 
     
     
         20 . The signal processor as claimed in  claim 18 , wherein the signal processor is implemented as a FEC decoder in a receiver of the optical communications system. 
     
     
         21 . The signal processor as claimed in  claim 18 , wherein the signal processor is implemented in hardware.