Research On Joint LDPC And Differential Iterative Decoding In High-Speed PDM-16QAM Optical Coherent Systems

With the demand for the high-speed long-haul optical communication growing,the coherent optical communication has received much attention because of high receiver sensitivity and multiple modulation formats.However,It exists phase ambiguity in coherent detection system.A lot of errors occur while demodulating.In order to solve the problem,differential encoding is proposed.But,differential encoding,which is memory modulation,will cause error spreading and performance degradation.Channel encoding can be adopted to compensate the performance degradation.In this paper,A LDPC combined with differential iterative decoding scheme is proposed for high-speed PDM-16QAM coherent optical communication system to improve the transmission performance.In the case of low redundant of channel encoding,LDPC combined with differential iterative decoding scheme can further enhance the performance of the system while compensating the error spreading by differential encoding.The main achievements are as follows:1.16QAM differential coding modulation is investigated and a soft demodulation althogrithm are proposed.At first,the realizaiton of 16QAM differential coding modulation is analyzed.It is including 16QAM differential encoder and a unique 16QAM modulator.Then,the soft demodulation algorithm and SISO differential decoding algorithm are derived with mathematical analysis.The computation complexity of tradional soft demodulation algorithms is too high,therefore,an improved soft demodulation algorithm with low computational complexity for the unique 16QAM modulation is proposed.Simulation demonstrates the performance of the improved soft demodulation algorithm is quite closely compared with the traditional soft demodulation algorithm.2.A LDPC combined with differential decoder iterative scheme is proposed to improve transmission performance for high-speed PDM-16QAM coherent optical communication systems.Firstly,the model structure of LDPC combined with differential decoder iterative scheme is introduced.Next,the key technology of each module in the model is discussed.And then,the processing of the iterative decoding is introduced in detail.At last,a high-speed PDM-16QAM coherent optical communication simulation system with LDPC combined differential iterative decoder is setup for evaluating the decoding performance.The simulation results reveal that phase ambiguity can be efficiently resolved by LDPC combined with differential decoder iterative scheme,at the same time,the performance of the system can be further enhanced without any additional redundant.The simulation result shows that the scheme has prospects for application in high-speed and long-reach coherent optical transmissions.