Reverse Concatenated Coded Modulation for High-Speed Optical Communication

We propose the reverse concatenated code as a forward error correction (FEC) scheme suitable for beyond 100-Gb/s optical transmission. In this scheme, BCH code is used as inner code and low-density parity-check (LDPC) code as outer code. The BCH decoder is implemented based on maximum a posteriori (MAP) decoding such as the BCJR/Ashikmin´s algorithm, and an LDPC decoder is based on a min-sum-with-correction-term algorithm. Because maximum a posteriori (MAP) decoding is used as the inner decoder, it provides high accuracy reliabilities to be used in LDPC decoding. We show that proposed FEC scheme performs comparably with much longer LDPC codes of girth 12 for a smaller number of LDPC decoder iterations. Because the outer LDPC code is of medium length and the number of required iterations is low, the proposed concatenated scheme represents an interesting candidate to be used in beyond 100-Gb/s optical transmission. The net coding gain (NCG) of concatenated LDPC(16935,14819)-BCH(64,57) code is 9.62 dB at a bit error rate (BER) of 10^-9, whereas the expected NCG at BER of 10^-13 is 11.38 dB. This concatenated code outperforms the corresponding turbo-product counterpart with a Chase II decoding algorithm by 0.94 dB at BER of 10^-9.