Variable spectral efficiency optical modulation schemes

Abstract

A received signal is received over a noise channel. A stream of received symbols is recovered from the received signal. Each symbol is decoded into one or more bits, where a decoded bit of the symbol has a high probability of being in error, and a probability of being a ‘1’ substantially different from 0.5. An inverse mapping is performed on the decoded bits, where the output of the inverse mapping is a stream of bits with a very low probability of being in error and a probability of being a ‘1’ substantially equal to 0.5.

What is claimed is: 
     
         1 . A method of handling a received signal, the method comprising:
 receiving a received signal over a noisy channel;   recovering a stream of received symbols from the received signal;   decoding each symbol into one or more bits, where a decoded bit of the symbol has a high probability of being in error, and a probability of being a ‘1’ substantially different from 0.5; and   performing an inverse mapping on the decoded bits, where the output of the inverse mapping is a stream of bits with a very low probability of being in error and a probability of being a ‘1’ substantially equal to 0.5.   
     
     
         2 . The method as recited in  claim 1 , wherein the inverse mapping comprises error correction followed by inverse shaping. 
     
     
         3 . The method as recited in  claim 2 , wherein the error correction achieves a frame error rate that is much less than 10 −12 . 
     
     
         4 . The method as recited in  claim 1 , wherein the inverse mapping performed on the stream of bits operates on a block basis. 
     
     
         5 . The method as recited in  claim 1 , wherein the received signal is an optical received signal. 
     
     
         6 . A receiver in a communications system, the receiver comprising:
 a detector configured to recover a stream of received symbols from a signal received over a noisy channel;   a digital signal processor configured to decode each symbol into one or more bits, where a decoded bit of the symbol has a high probability of being in error, and a probability of being a ‘1’ substantially different from 0.5, and to perform an inverse mapping on the decoded bits, where the output of the inverse mapping is a stream of bits with a very low probability of being in error and a probability of being a ‘1’ substantially equal to 0.5.   
     
     
         7 . The receiver as recited in  claim 6 , wherein the inverse mapping comprises error correction followed by inverse shaping. 
     
     
         8 . The method as recited in  claim 7 , wherein the error correction achieves a frame error rate that is much less than 10 −12 . 
     
     
         9 . The receiver as recited in  claim 6 , wherein the inverse mapping performed on the stream of bits operates on a block basis. 
     
     
         10 . A method of transmitting a data signal using a transmitter of a communications system, the method comprising:
 applying a shaping operation to the data signal to generate a shaped data signal, the shaping operation creating bits with a probability of being a ‘1’ substantially different from 0.5;   encoding the shaped data signal according to a systematic forward error correction (FEC) scheme to generate an encoded signal, the systematic FEC scheme creating overhead bits with a probability of being a ‘1’ substantially equal to 0.5; and   modulating the encoded signal to symbol values according to a modulation scheme to generate a corresponding symbol stream, bits of the symbol values being categorized as belonging to a first category or a second category, where modulating the encoded symbol to the symbol values involves mapping substantially all of the bits with a probability of being a ‘1’ substantially equal to 0.5 to symbol value bits of the first category and mapping substantially all of the bits with a probability of being a ‘1’ substantially different from 0.5 to symbol value bits of the second category.   
     
     
         11 . The method as recited in  claim 10 , wherein the communications system is an optical communications system and modulating the encoded signal comprises modulating a carrier light according to the symbol stream to generate an optical signal for transmission through the optical communications system.