Research On Phase Noise Compensation Algorithms In Coherent Optical OFDM Systems

Coherent optical orthogonal frequency division multiplexing(CO-OFDM)is a multicarrier technology suitable for long-distance high-speed optical transmission system.As a result,the transmission capacity of the whole optical communication system is greatly improved.CO-OFDM system has strong ability of resisting polarization mode dispersion(PMD)and chromatic dispersion(CD),but it is also more sensitive to phase noise,especially the phase noise caused by the laser mismatch between the transmitter and the receiver.The influence of phase noise on the system can be divided into common phase error(CPE)which causes constellation rotation and inter carrier interference(ICI)which causes constellation divergence.The interference of phase noise to the system can be reduced by the laser with narrower linewidth or by the compensation algorithm.However,the narrower the laser linewidth,the higher the hardware cost and the more complex the equipment renewal and maintenance.Therefore,the phase noise compensation algorithm is the main research direction because it can be updated in time and has low cost.The main research contents in this thesis are as follows:1.Combined with the structure and basic principle of CO-OFDM system,the causes of phase noise are analyzed in detail,and the mathematical model of phase noise is established according to the main factors.Then,the influence of the main parameters and phase noise on the transmission performance of the system is theoretically derived and simulated.This thesis mainly analyzes the classification and improvement direction of the current phase noise compensation algorithm,and discusses the problems of the compensation algorithm,which provides a theoretical basis for the phase noise algorithm proposed in this thesis.2.In the case of large phase noise,the tolerance of laser linewidth in CO-OFDM system is low,and a novel phase noise compensation algorithm based on volume Kalman is proposed.With a small number of pilot subcarriers,the algorithm uses the extended Kalman filter(EKF)algorithm and linear interpolation to compensate the noise of CPE phase noise.After the signal is processed by fast Fourier transformation(FFT)and pre-determined,the phase noise is precisely compensated by using the cubature Kalman filter(CKF)algorithm with the processed pre-determined signal and the FFT signal.And the compensation accuracy is enhanced by the second iteration.The improved algorithm can effectively compensate the ICI when the phase noise is large.The improved algorithm can improve the tolerance of CO-OFDM system to laser linewidth and improve the performance of the system.3.Based on the advantage of low complexity of self cancellation(SC)algorithm,a novel phase noise compensation algorithm is proposed,which combines linear combination SC(LCSC)algorithm and decision feedback(DF)algorithm.The algorithm improves the subcarrier utilization of SC algorithm by linear combination,and replaces the virtual subcarrier required by DF algorithm by redundant information of LCSC algorithm at the receiver,which improves the error rate performance of compensation algorithm.The improved algorithm takes the BER performance and subcarrier utilization into account,which makes the LCSC-DF algorithm have better transmission performance.