Subcarrier Reliability Aware Soft-Decision LDPC Code in CO-OFDM Systems

We propose a subcarrier reliability aware low-density parity-check (SRA-LDPC) soft-decision coding scheme for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The SRA-LDPC code takes consideration of varying signal-to-noise ratio performance among different OFDM subcarriers. At the SRA-LDPC encoder, flexible forward error correction is applied to different ranges of subcarriers. At the decoder, the algorithm keeps track of noise variance of each subcarrier and utilizes such variance for decoding. Our proposed algorithm is substantiated in a 120-Gb/s CO-OFDM system where near-dc noise originated from the local oscillator is detrimental to the system performance. The experimental results show that the SRA-LDPC code is effective in combating near-dc noise and enhancing the receiver sensitivity.