Research On Dispersion And Polarization Rotation Damage Compensation Algorithm In Digital Coherent Optical Communication System

With the rapid development of the Internet,the emergence of big data,cloud computing,5G and other emerging technologies,people's demand for data traffic has sharply increased,both for long-distance optical transmission backbone network and short-distance interconnection data center,are facing the transmission capacity limit and other challenges.The digital coherent optical communication system relies on high-order modulation format,coherent detection,digital signal processing and other technologies to greatly improve the transmission performance of optical communication network.With its characteristics of large capacity,high speed,long distance,etc.,it gradually becomes the development direction of high-speed optical network in the future.However,optical signals transmitted in optical fibers are inevitably affected by optical fiber channels,The methods include Chromatic Dispersion(CD),polarization effect damage,Carrier Frequency Offset(CFO),Carrier Phase Noise(CPN)and nonlinear effect.The Digital Signal Processing(DSP)module integrated in the coherent receiver can effectively affect the balance part of the electrical domain.However,with the increase of the transmission rate and transmission distance of the optical fiber communication system,the damage in the optical fiber channel is aggravated,and the performance of the existing Signal Processing algorithm tends to bottleneck.Therefore,It is of great significance to improve the performance of optical fiber transmission system to improve the signal damage equalization algorithm in DSP module of digital coherent receiver.In this paper,part of the channel damage in the fiber channel is studied,and the relevant algorithm is improved.The main work of this paper is as follows:1.The performance of the existing Frequency Domain Equalizer(FDE)and Time Domain Equalizer(TDE)is degraded due to the excessive number of pins when the signal transmission bandwidth is too high.In this paper,a Momentum decision-directed Least means-square(MDD-LMS)algorithm based on Momentum factor is proposed to optimize the influence of dispersion equalizer.And through the construction of 56 GBaud Polarizaton Division Multiplexing-Quadrature Phase Shift Keying,PDM-QPSK)simulation platform of coherent optical transmission system to verify the feasibility of this scheme.The simulation results show that MDD-LMS algorithm can reach steady state when the iteration number is about 4000.Adding this algorithm in the TDE back-end can effectively eliminate the influence of dispersion on the signal.2.In view of the dispersion fluctuation in the actual optical fiber communication system,the Kalman Filter(KF)has the advantages of fast tracking speed and high convergence accuracy.In this paper,an Extended Kalman Filter(EKF)based on overlapping retention method was used to balance the effects of Residual Dispersion(RCD)in optical fibers.The feasibility of the scheme was explored through the simulation platform of 56 GBaud PDM-QPSK coherent Optical transmission system.When the Optical signal-to-noise Ratio(OSNR)is 15 dB and the length of compensation Signal block is 4096,This scheme can eliminate the influence of the accumulated dispersion value of384 Ps /nm.On the other hand,a time-domain EKF equalization algorithm based on the Rotation of State of Polarization(RSOP)model is used to study the effects of different parameters in the RSOP model on the EKF performance under extreme conditions.Experimental results show that this scheme can track the RSOP of about 13Mrad/s.When one parameter changes from 10Mrad/s to 20Mrad/s and the other two parameters change at a rate of 13Mrad/s,This algorithm can meet the threshold requirements of Forward Error Correction(FEC)when the Bit Error Rate(BER)is 2.4E-2.Compared with the traditional Constant Modulus Algorithm(CMA),the EKF Algorithm has more advantages in the tolerance and computational complexity of fiber channel damage.