Theoretical And Simulation Research On Digital Nonlinear Compensation Of OFDM Optical Fiber Communication System

Orthogonal frequency-division multiplexed(OFDM)technology is widely used in optical communication system due to its high spectrum efficiency and strong restraining inter-symbol interference ability.For large-capacity coherent OFDM(CO-OFDM)system,four-wave-mixing(FWM)noise is one of the main factors limiting the performance of such systems.In order to better optimize the design of such systems,the impact of FWM noise on system performance should be evaluated accurately.Phase Conjugate Twin Waves(PCTW)is one of the effective methods of digital nonlinear mitigation which can be explained that the correlated nonlinear distortions can be coherently canceled by linear superposition in the digital signal processing at the receiver when a signal and its phase-conjugate copy on are transmitted along a fiber link on any two orthogonal dimensions.This paper focuses on the theoretical analysis and simulation research on the extent of nonlinear suppression in OFDM system by using the PCTW schemes which are implemented in time,subcarrier and polarization domains,respectively.Taken into account the important influencing factors such as phase matching,walk-off effect and randomness of the symbol sequences in the FWM process,a semi-analytical calculation model for FWM noise in the quadrature amplitude modulation(QAM)PCTW CO-OFDM system is derived.It is found that the variances of FWM noise have different component characteristics for different PCTW schemes.Some calculations are carried out by using this semi-analytical model.Based on the co-simulation of Optisystem and Matlab software,OFDM-WDM simulation platforms for different PCTW schemes are established.Optimized PCTW scheme is obtained.The performances of PCTW OFDM-WDM system for 16-ary QAM and 16-ary phase shift keying(PSK)are compared.The semi-analytical FWM model for CO-OFDM system with three PCTW implementation schemes derived in this paper is of great significance to efficiently evaluate the impact of FWM noise on the performance of such systems.The co-simulation platform based on Optisystem and Matlab software also has certain significant for optimizing such systems.