Method and Apparatus for Decision Threshold Control In An Optical Signal Receiver

Abstract

A method of adjusting a decision threshold in an optical signal receiver. A forward error correction (FEC) decoder provides a feedback signal representative of corrected errors. The decision threshold is adjusted in response to a polarity of the received signal and to balance a number of corrected ones and zeros.

1 . A method of adjusting a decision threshold in an optical signal receiver, said method comprising:
 receiving an input signal;   determining a polarity of said input signal;   providing an output signal representing logic ones and zeros in response to comparison of said input signal with a decision threshold;   decoding said output signal using a forward error correction (FEC) decoder; and   modifying said decision threshold in response to said polarity of said input signal and a number of said ones and a number of said zeros corrected by said FEC decoder over a time period.   
     
     
         2 . A method according to  claim 1 , said method further comprising identifying a total number of errors corrected by said FEC decoder over said time period, and wherein said modifying said decision threshold comprises modifying said decision threshold when said total number of errors exceeds a predetermined error threshold. 
     
     
         3 . A method according to  claim 1 , wherein said modifying comprises modifying said decision threshold to balance said number of said ones with said number of said zeros. 
     
     
         4 . A method according to  claim 1 , wherein said modifying comprises modifying said decision threshold in proportion to a ratio of said number of said ones to said number of said zeros. 
     
     
         5 . A method according to  claim 1 , said method further comprising identifying a total number of errors corrected by said FEC decoder over said time period, and wherein said modifying comprises modifying said decision threshold in proportion to a ratio of a difference between said number of said ones and said number of said zeros to said total number of errors. 
     
     
         6 . A method according to  claim 5 , wherein said modifying comprises modifying said decision threshold in a direction determined by a sign of said difference between said number of said ones and said number of said zeros. 
     
     
         7 . A method according to  claim 1 , wherein said time period comprises a sampling period. 
     
     
         8 . A method according to  claim 1 , wherein said time period comprises a plurality of sampling periods. 
     
     
         9 . A method of adjusting a decision threshold in an optical signal receiver, said method comprising:
 receiving an input signal   determining a polarity of said input signal;   providing an output signal representing logic ones and zeros in response to comparison of said input signal with a decision threshold;   decoding said output signal using a forward error correction (FEC) decoder;   providing feedback representative of a number of errors corrected by said FEC decoder and indicating said polarity of said input signal; and   adjusting said decision threshold in response to said feedback to balance a number of said ones corrected by said decoder with a number of said zeros corrected by said FEC decoder, said adjusting comprising adjusting said decision threshold in a direction determined by a sign of a difference between said number of said ones and said number of said zeros and said polarity of said input signal indicated by said feedback.   
     
     
         10 . A method according to  claim 9 , wherein said adjusting comprises adjusting said decision threshold in proportion to a ratio of said number of said ones corrected by said FEC decoder to said number of said zeros corrected by said FEC decoder. 
     
     
         11 . A method according to  claim 9 , wherein said number of said ones corrected by said FEC decoder comprises a total number of said ones corrected by said FEC decoder over a plurality of sampling periods, and wherein said number of said zeros corrected by said FEC decoder comprises a total number of said zeros corrected by said FEC decoder over said plurality of sampling periods. 
     
     
         12 . A method according to  claim 9 , said method further comprising determining a total number of said errors corrected by said FEC decoder over one or more sampling periods by adding said number errors corrected by said FEC decoder over said one or more sampling periods, and wherein adjusting comprises adjusting said decision threshold when said total number of said errors exceeds a predetermined error threshold. 
     
     
         13 . A method according to  claim 12 , wherein said total number of said errors comprises a total number of said ones corrected by said FEC decoder over said one or more sampling periods and a total number of said zeros corrected by said FEC decoder over said one or more sampling periods. 
     
     
         14 . A method according to  claim 13 , wherein said adjusting comprises adjusting said decision threshold in proportion to a ratio of a difference between said total number of said ones and said total number of said zeros to said total number of errors. 
     
     
         15 . A method according to  claim 14 , wherein said adjusting comprises adjusting said decision threshold in a direction determined by a sign of said difference between said total number of said corrected ones and said total number of corrected zeros.