Researches On Concatenated Codes For Coherent Optical OFDM Communication Systems

Coherent optical orthogonal frequency division multiplexing (CO-OFDM)technology is the highlight candidate scheme for the100Gb/s and beyond100Gb/s optical communication systems. Although there are a lot of maturedigital processing algorithms to overcome the impairments of fiber channel, itstill need the forward error correction (FEC) technology to achieve the biterror ratio (BER) requirement of the International Telecommunication UnionTelecommunication Standardization Sector (ITU-T). Basing on the theoreticresearch of CO-OFDM systems, low density parity check (LDPC) code andconcatenated code, this dissertation accomplishes seveal work for designingthe LDPC-based concatenated code in CO-OFDM communication systems.Firstly, after the probe into the single-level type and interleaved typeconcatenated codes, the concatenated code is demonstrated to be an efficientmethod of combating the error floor of LDPC code. And according to theanalysis on the error floor of LDPC code, we propose a concatenated codingscheme called random interleaved concatenated code (RICC). Also a jointiterative decoding algorithm, which combines the sum product algorithm (SPA)and Berlekamp-Messey (BM) algorithm, is proposed for the RICC. Thesimulation results show that when the BER is10^-8the net code gain (NCG) ofRICC-4P with only four set-partitions is0.15dB more than that ofsingle-level type concatenated code without set-partition. Also the NCG ofRICC-4P is3dB,1.5dB and0.4dB more than those of the RS(255,239),RS(255,239)+CSOC(n/k=7/6, J=8) and25%-mode orthogonally concatenatedBCH code which are the recommended codes in ITU-T G.709and G.975.1.Basing on the work refered aboved, a concatenated coding scheme for100Gb/s16-QAM CO-OFDM system, including the encoding and decodingalgorithms, are proposed. The ITU-T reqiement of100Gb/s opticaltransmission systems is considered in this scheme. The simulation resultsshow that when BER is10^-8the NCG of proposed scheme is2.8dB more thanRS(255,239). And its NCG are1.3dB and0.2dB more than those ofRS(255,239)+CSOC(n/k=7/6, J=8) and orthogonally concatenated BCH coderespectively which are two of FEC schemes with similar code-rate in ITU-T G.975.1. At the same time, the redundancy of proposed scheme is lower thanthat of those two schemes. On the other hand, the spectral efficiency (SE) ofproposed scheme is3.28bits/symbol. Comparing with the NCG of otherschemes with lower SE such as4-QAM,8-QAM and8-QAM-RS(255,239), theproposed scheme also has3.5dB,6.4dB and2dB additional gain respectively.Therefore the proposed scheme has advantage of both NCG and SE. And it is asuper FEC technology which is suitable for the application in100Gb/s opticalcommunication systems.