The Study Of LDPC Code In BICM-8QAM Optical Fiber Communication System

With the development of Internet technology,people’s demand for communication bandwidth has increased dramatically.High speed rate,high frequency spectral efficiency in the large capacity optical communication transmission network has become the mainstream of the next generation of communication network standards.In order to improve the spectrum efficiency of the optical communication system,high order modulation format,coherent reception,multi-dimensional multiplexing technology are widely used.Moreover,the forward error correction technology(FEC)is widely used in encoding technology to improve the transmission quality and reliability of optical fiber communication system.In the past decades,low density parity check codes(LDPC)is proven to have an excellent error correction performance in both theoretical and practical systems.With careful design of the code structure,the LDPC encoding transmission performance can be very close to the Shannon limit,and has a low bit error rate compared to other forward error correction codes.In recent years,the application of LDPC code in the high frequency spectral efficiency of large capacity optical fiber communication system has attracted the attention of the industry circles,and has become a hot spot in the field of optical communication.In this paper,we briefly introduce the history of optical communication system and LDPC codes,and then we describe the components of optical communication system,including lasers,modulators,receivers,and so on.To finish performance analysis of LDPC in optical communication system,we also set up the optical fiber communication system simulation environment,and then test the LDPC bit error rate(BER)performance in coherent optical communication system,which is mainly based on decoding formula and Voronoi diagram.Detail analysis is made on the LDPC decoding in PM-BICM-8QAM,which suggest that the ideal domain decision and the actual decision domain is not in accordance with each other.Finally we put forward an improvement scheme and have achieved good results.The main contents and innovations of this paper are as follows:(1)A coherent detection and transmission system with 8QAM modulation format based on MATLAB is built.The effects of different types of LDPC codes and different parameters of SNR codes on bit error rate are simulated and verified.(2)We finish the design of the Gallager type,Mackay type,PEG type LDPC encoding and PEG construction method based on the QC-LDPC encoding.The performance of PEG-QC-LDPC code in BICM-8QAM system is verified by simulation.(3)We analyzes the shortcomings of the original information initializing method according to the optical transmission system PM-BICM-8QAM gray mapping.A new decoding algorithm is proposed to reduce the decoding of error rate and decoding bit error rate(BER).The results show that the new decoding method can reduce the pre bit error rate to 96%,and the post bit error rate is reduced to 50%.From the SNR-BER diagram,the bit error rate can be reduced by 0.1-0.2dB.