Research On IQ Imbalance And Phase Noise Suppression Algorithm In CO-OFDM System

Coherent Optical Orthogonal Frequency Division Multiplexing(CO-OFDM)technology has been widely studied because of its fast transmission rate,high spectral efficiency,large channel capacity,and suitable for long-distance transmission.However,due to the non-ideal introduction of the imbalance between the in-phase/quadrature-phase(IQ)during the production of the modulator and the phase noise(PN)which is introduced by the linewidth of the lasers,the performance of the system is deteriorated.This paper is mainly aimed at solving the problems of IQ imbalance at the transmitter of the CO-OFDM system and the transmitter(Tx)IQ imbalance when phase noise is considered,the details are as follows:(1)A training symbol structure is proposed to estimate the IQ imbalance and channel information in the CO-OFDM system.The structure contains two training symbols with the same structure,each of which is divided into two subsets:the zero-value subset,and its mirror image position subset.Simulation results show that,when the transmission rate is 100Gb/s,the transmission distance is 640km,the IQ phase mismatch is ?=30°,the tolerance of amplitude mismatch can be improved about 2dB by the proposed method;when the amplitude mismatch ?=3dB,the tolerance of phase mismatch can be improved about 5°;when the amplitude mismatch?=0.7dB,the phase mismatch ?=15°,the transmission distance is increased by about 160km.(2)A new training symbol structure is proposed to estimate transmitter IQ mismatch and channel information in CO-OFDM system with the effect of phase noise.The proposed scheme is composed of three adjacent training symbols.The first training symbol has two subsets:the zero-value subset,and its mirror image position subset.The second and the third training symbols have three subsets.Simulation results show that,in the condition of the 50Gb/s transmission rate,the laser power is 3dBm,after 640km transmission in fiber,the tolerance to the amplitude mismatch can be improved about 1.5dB in the present of 200kHz linewidth and the tolerance to phase mismatch can be improved about 5°in the present of 120kHz linewidth;when?=3dB,?=15°,the transmission distance is increased with different values of linewidth compared with the reference method,and the OSNR penalty of proposed method is 21dB,which is decreased 2dB than the reference method.