| With the development of global infrastructure construction and network coverage,the increasing use of the Internet for streaming video and audio,cloud computing and online games has led to a huge increase in the demand for bandwidth.To meet this demand,optical communication systems need to operate at a higher data rate,which also promotes the research and development of coherent optical communication.The nonlinear effect is one of the important factors affecting the performance of coherent optical fiber communication system.Phase conjugated twin waves(PCTW)algorithm is a relatively simple and effective nonlinear suppression technology.PCTW constructs a pair of phase conjugated signals on the transmitting side to compensate the linear and nonlinear damages.Constellation shaping can also improve the nonlinear tolerance and obtain shaping gain.In this paper,theoretical and simulation research is carried out on PCTW OFDM system with constellation shaping.The main research innovations are as follows:(1)Previous studies on PCTW have explored several conjugate schemes in different dimensions to suppress nonlinear damage.The performance of optical communication system can be further improved by combing PCTW and probabilistic shaping(PS)technology.There are some differences of BER between in-phase sequence I and quadrature sequence Q for PCTW system,which is called IQ imbalance.However,the performance of PCTW-IQ imbalance in different types of fiber links has not been adequately studied.Simulation software is used to simulate the performance of PS-PCTW OFDM system under three dimensions(polarization,time domain and subcarrier)after transmission through two fiber links(conventional dispersion distribution and new dispersion distribution).The simulation results show that PCTW system using the new dispersion distribution link has better performance,and IQ imbalance phenomenon is observed after the transmission of the two links.(2)An IQ imbalance theory is proposed from three perspectives(PS encoding,linear crosstalk within and between channels,statistical properties of correlated noise)for PS-PCTW system.The simulation results show that a large pseudo-IQ imbalance occurs in the polarization PCTW system under the influence of simulation parameters.The causes of this pseudo-IQ imbalance are analyzed theoretically from the perspective of noise statistics,which has certain enlightenment for the future simulation research of PCTW technology.Aiming at PCTW-IQ imbalance,geometric shaping(GS)is combined with PS-PCTW system.The hybrid constellation shaping PCTW system is realized in the polarization,time domain and subcarrier dimensions.The simulation results show that geometric shaping can improve IQ imbalance and system performance for PCTW system to some extent without affecting the spectral efficiency.(3)The conventional PCTW schemes only realize the adjacent transmission of the original signal and conjugated copy in a single dimension.In order to further improve the ability of PCTW to suppress nonlinear and linear damage,optimized schemes of conjugated signal position in single and double dimensional PCTW are proposed and analyzed theoretically.The relevant simulation platform is built,simulation results show that the BER of the "subcarrier + time domain" conjugated signal position optimized scheme for two-dimensional PCTW is similar to that of the time domain conjugated copy interval insertion optimized scheme.They can both improve the system performance and suppress adverse effects of IQ imbalance effectively compared with conventional PCTW schemes.Among all the schemes discussed,the "polarization + time domain" two-dimensional PCTW conjugated signal position optimization scheme has the best performance. |
