Performance Analysis Of NB-QC-LDPC Code For High-Speed Optical Communication Systems

In order to search the performance of the non binary quasi-cyclic low-density parity check (NB-QC-LDPC) code with the high speed optical communication system, we construct the NB-QC-LDPC code over the Galois field GF(4). We find that appling the NB-QC-LDPC code will increase the transmission distance and forward error correction performance of the system. Furthermore the NB-QC-LDPC code will provide some dispersion and nonlinear tolerance, therefore, NB-QC-LDPC code can improve the performance of the fiber channel without increasing the complexity of the system. Because the NB-QC-LDPC code is over the GF(4), it is suitable for the system with QPSK modulation formats. There are three advantages with the NB-QC-LDPC code,(1)cancelling the multi-level modulation encoder;(2)removing the conversion interface between binary and non binary;(3)directing mapping can eliminate the data transfer between the encoding and decoding.First, we study the binary quasi-cyclic low-density parity check (QC-LDPC) code. On this basis, we study and discuss several construction algorithm with the NB-QC-LDPC code:random assignment algorithm, optimize hardware structure algorithm and optimize performance algorithm. And then, we find that the three construction algorithm have their own advantages for different purposes. Besides we also propose an improved Gaussian elimination can be applied to the full rank or non full rank check matrix to construct the generator matrix.We further study the decoding algorithm for the code and we study the traditional SPA, the LSPA and the LFSPA which is the improved decoding algorithm with the complexity significantly reduced. However, during the research on the LFSPA, we find that the decoding performance will greatly drop in the case of large signal-to-noise ratio. In this paper, we propose the truncated approximation algorithm to solve this data overflow and improve the decoding performance.In the end of the paper, we find the NB-QC-LDPC code (500,0.8) with500code length and0.8coding rate can provide about5.3dB coding gain at the BER of10-8by Mote Carlo simulation. We also find that the NB-QC-LDPC code (5055,08) can provide up to8dB coding gain at the BER of10-8. Furthermore, we study the dispersion and nonlinear tolerance of the NB-QC-LDPC code in the fiber channel and we find that NB-QC-LDPC code can tolerate certain residual dispersion and nonlinear phase noise without increasing the complexity of the system.