Research On DSP Algorithms For Polarization Related Damage Cmpensation In Nyquist Coherent Optical Transmission Systems Based On Kalman Filter

Nowadays,6G,Smart Link,native AI and perceptual Internet technology are developing rapidly.The ultimate connection era of ground and non ground network integration is coming.With the rapid increase in the number of Internet of things devices,the throughput of 6G network increasing makes the research of high-capacity transmission technology more important.The development demand of ultra-high speed,ultra large capacity and ultra long distance transmission optical fiber communication is becoming more and more urgent.The high-capacity coherent optical transmission technology of wavelength division multiplexing combined with polarization multiplexing is widely used.However,with the development of optical fiber,there are few available frequencies in C+L band.So that only increasing the number of wavelength multiplexers to improve the system capacity has been difficult to meet the exponential growth of network capacity demand.Nyquist wavelength division multiplexing coherent optical transmission system improves the spectral efficiency of the system by compressing the signal bandwidth,together with the use of powerful digital signal processing(DSP)algorithm to recover all kinds of channel damage.It has become an important development direction in the field of high spectral efficiency and large capacity optical fiber transmission.However,the system DSP algorithm still faces problems such as more possibility of fast changing RSOP caused by lightning events,much harder to track RSOP due to DGD from long-distance transmission,high computational complexity of the algorithm and so on.Aiming at the above problems,this paper studies the key DSP algorithms in Nyquist coherent optical transmission system.The main research work is as follows.1.Aiming at the problem of lack of polarization damage model of fast changing RSOP caused by lightning events,in this paper,a fast changing RSOP model caused by lightning return current stage based on double exponential function is proposed.The model accurately describes the fast changing RSOP caused by the current intensity in the return current stage of lightning phenomenon.In the stage of return stroke current,the peak value of current intensity is the highest and changes the fastest,resulting in rapid change RSOP up to 8Mrad/s.The model is verified under 28GBaud PM-QPSK and PM-16QAM Nyquist coherent optical transmission simulation system.In this paper,the effectiveness of the proposed model is verified from the actual effect by observing the changes of signal constellation and Stokes space constellation before and after the introduction of fast changing RSOP model.By analyzing the corresponding situation of RSOP introduced into this model and parameters tracked by typical RSOP tracking algorithm,illustrating that the proposed rapid change RSOP model can accurately represent the magnitude and change of RSOP velocity caused by lightning.It can be used to indicate RSOP caused by Lightning events,so that to verify the tracking performance of RSOP tracking algorithm.2.Aiming at the problem that fast changing RSOP is difficult to track in longdistance Nyquist system,a reliable parameter-transfer tracking scheme of fast changing RSOP is proposed.The first level AKF algorithm is used to track the fast changing RSOP for its characteristics of fast convergence.The convergence of the second level RDE algorithm is promoted by parameter transferring from the first stage.The second level RDE algorithm is used to compensate DGD and improve the steady-state error of the algorithm.The ability of tracking fast changing RSOP in the presence of DGD of the scheme is verified under the 28GBaud Nyquist-PM-16QAM simulation system.This scheme can realize the tracking of 10Mrad/s constant speed RSOP under DGD of about 40ps and the tracking of 10Mrad/s fast changing RSOP under DGD of about 32ps.This scheme can compensate DGD up to 60ps and track RSOP up to 50Mrad/s.The RSOP tracking ability of this scheme is much better than the traditional RDE algorithm.When DGD and RSOP is tracking at the same time,the DGD compensation range of this scheme is more than three times that of AKF algorithm.The fast changing RSOP tracking performance of the scheme is verified under the three carrier 40GBaudNyquist-PM-16QAM offline experimental system.In the case of BTB,this scheme can track the fast changing RSOP of more than 40Mrad/s,while the traditional RDE algorithm can only track the fast changing RSOP of 5Mrad/s.In the verification of 240km optical fiber cross section transmission,when OSNR exceeds 24dB,the steadystate error performance of this scheme is equivalent to that of traditional RDE algorithm,which is obviously better than AKF algorithm.Through theoretical analysis,simulation and off-line experimental verification,it is considered that the proposed parametertransfer scheme can be used for tracking fast changing RSOP exceeding 10Mrad/s along with large DGD under long-distance transmission.3.Aiming at the high complexity of phase estimation scheme,a low complexity phase estimation scheme based on EKF is proposed.In this scheme,the phase offset between the two polarization signals is compensated first,and then the phase noise is compensated together.The real part(or imaginary part)of two polarization signals is considered as input information to ensure that it is equivalent to the input data of single polarization EKF.At the same time,in the process of parameter estimation,only the signals of big phase internals are considered,so as to further increase the phase offset compensation range.Further,this scheme considers the method of updating parameters with interval symbols,which further reduces the computational complexity while ensuring the estimation performance.When the number of interval symbols of the scheme is 1,the times of real multiplication and real addition per symbol is only 13.5 and 26,respectively.It is about 1/3 of the typical EKF algorithm,about half of the double polarization EKF algorithm.It is 1/30 of the traditional single polarization BPS algorithm and 1/4 of the QPSK parting algorithm.Under 28GBaud Nyquist-PM16QAM simulation system,the phase offset compensation range of this scheme is about 1.5MHz,which is better than that of single state EKF scheme.The steady-state error of the scheme with an interval of 1 or 2 symbols is equivalent to that of the dual polarization EKF algorithm,which is obviously better than the single polarization EKF algorithm.