| In the process of optical communication,the transmission characteristics such as dispersion and nonlinear effect have always affected the transmission quality of optical signal,so reasonable and effective compensation is an important means to improve the transmission quality of optical fiber.The new optical modulation format can significantly improve the spectral efficiency of digital coherent optical communication system,so this thesis combines the optical QAM modulation format and coherent optical communication,and completely builds the coherent optical communication transmission system of three modulation formats.The modulation,transmission and demodulation techniques of QPSK,16 QAM and 64 QAM signals are mainly studied,and the dispersion and nonlinear effects of optical fiber on signals are mainly studied.DCF and DSP are used for dispersion compensation,and BP algorithm is used for nonlinear effect compensation.In this thesis,QPSK signal is modulated,transmitted and demodulated by constructing QPSK coherent optical communication system.The results of uncompensated,DCF compensation and DSP dispersion compensation and BP compensation are obtained by simulation.The influence of dispersion and nonlinear effects on signal transmission in optical fiber transmission is analyzed,and the compensated effects are compared.Then the relationship between EVM and transmission distance is studied under different transmission rates and different dispersion compensation methods.Based on the research of QPSK coherent optical communication system,a similar research method is used to further study the high order modulation scheme.For 16 QAM modulation format,the relationship between transmission performance and transmission distance under different dispersion compensation modes is simulated,and EVM and BER performance indexes are used to analyze the results.Then,the relationship between bit error rate and transmission distance is analyzed after the dispersion compensation of 56Gbit/s,112Gbit/s and 224Gbit/s coherent optical system,and the constellation map of the 16 QAM coherent optical system with the dispersion compensation and the constellation map without the dispersion compensation are compared and analyzed.In addition,the results of the system with or without nonlinear suppression were compared,and the factors affecting the nonlinear compensation effect--compensation step size and filter selection were simulated analysis,and finally through the constellation diagram intuitive analysis of BP compensation step size is set to 20 km and 100 km compensation comparison diagram.Finally,for the 64 QAM modulation format,the constellation map with no compensation and with different compensation methods is obtained by simulation.In addition,the relationship between bit error rate and transmission distance of 64 QAM modulation format with different compensation methods is analyzed.The results show that the dispersion compensation effect of DCF is better than that of DSP when there is no nonlinear effect.Using DSP for dispersion compensation and nonlinear suppression is the best.The larger the transmission distance,the larger BER and EVM,but DCF is always better than DSP dispersion compensation effect at the same rate.When DSP is used for dispersion compensation,the higher the system transmission rate,the larger the EVM is.After nonlinear suppression by BP algorithm,the bit error rate of the system is reduced by at least one order of magnitude.When the compensation step size is 20 km,the nonlinear compensation effect is the best.The higher the order of modulation format,the larger the bit error rate of the system. |
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