Multi-amplitude Minimum-shift Keying System Based On Low Density Parity Check Code With Coherent Detection

With the rapid development of information society,the demand for communication capacity is increasing,serious spectrum congestion will lead to a substantial deterioration in the quality of information transmission,especially in the limited bandwidth of the channel.In order to improve bandwidth utilization,further increase the system capacity,and to achieve long distance transmission of optical signal,the research of high spectrum efficiency transmission technology of multi-dimensional and multi-level modulation format has been paid more attention.Minimum Shift Keying(MSK)as a special kind of continuous phase modulation(CPM),with the advantages of continuous phase shifting,constant envelope,spectrum energy concentrated,band weakened quickly and so on,has been widely used in the wireless digital communication system.This paper presents a Multi-Amplitude Minimum Shift Keying(MAMSK)modulation format,which is more suitable for coherent detection,and build a coherent optical detection system based on single carrier MAMSK modulation,at the same time,using irregular quasi-cyclic low density parity check code(LDPC)as the channel coding to improve the bit error rate performance of the system.The main work of this paper includes as follows:1.Based on a hybrid method combined with phase and amplitude modulation,a kind of MAMSK modulation scheme suitable for coherent detection is proposed.It combines ASK modulation and MSK modulation,and both are placed in an electric field to acheive.MAMSK signal is generated in the electric field,and modulated on the optical carrier through the optical I/Q modulator.It can simplify the configuration of transmitter and reduce the complexity of system.Then the MAMSK coherent optical communication system is build,which is mainly composed of MAMSK transmitter,optical I/Q modulator,optical fiber link and MAMSK receiver.In the simulation,it compared and analyzed the transmission performance of MAMSK with 2ADQPSK and 4ADPSK.Simulation results show that MAMSK has higher spectrum efficiency,higher dispersion tolerance,higher nonlinear tolerance and lower bit error rate.2.The coded modulation scheme based on LDPC codes and MAMSK modulation is proposed.using irregular quasi-cyclic LDPC codes as channelcoding to improve the error correction capability of symbols.Due to its sparse parity check matrix,the number of information bits participated with the same parity equation is few and scattered,so that the code itself have the anti continuous burst error characteristics.In this paper,the LDPC coding technology and MAMSK modulation technology are combined,and LDPC-MAMSK coherent optical system is set up.In the simulation,it makes some comparison according to the influence of different parameters on the performance of LDPC code,and the simulation results prove that the LDPC codes can effectively reduce the error rate of MAMSK optical coherent system,and it can obtain higher coding gain in the long distance transmission compared with the uncoded scheme.