Research On Carrier Frequency Offset And Phase Recovery Algorithm In Coherent Optical Communication System

In recent years,with the rapid development of emerging services such as high definition video,Internet,and cloud computing,people's bandwidth capacity requirements for backbone networks have doubled.Traditional fiber optic communication systems based on intensity modulation and direct detection cannot meet the actual needs.High speed coherent optical communication systems based on high order modulation,coherent detection and digital signal processing technology have the advantages of high speed and high spectrum utilization,and satisfy people's broadband,large capacity business needs.The coherent optical communication system uses high order modulation format to improve spectral efficiency and communication capacity,and compensates caused by transmission process signals through digital signal processing techniques such as IQ orthogonalization and dispersion compensation.Becase high order modulation format is more sensitive for frequency offset and phase noise,it is important to find fast,low complexity frequency estimation and phase recovery algorithms.Based on the theory of coherent optical communication,the causes of frequency offset and phase noise are studied.The joint model of frequency offset and phase noise is established.The influence of frequency offset jitter on carrier recovery is analyzed.A frequency offset estimation algorithm based on convex optimization is proposed.The relationship between the rotation angle and the first-order distance is analyzed,and the phase recovery algorithm based on the minimum first-order distance is proposed.The main work of this paper is as follows:(1)Frequency offset and phase noise joint modelThe causes and the probability distribution of phase noise are analyzed.The joint signal model of frequency offset and phase noise is established.The simulation shows that both frequency offset and phase noise will cause the signal to rotate in the complex plane,thus affecting the signal decision.I analyze the effects of different types of frequency offset jitter on carrier recovery.The simulation results show that the amplitude of jitter has a greater impact on carrier recovery than jitter frequency,and the impact of jitter on higher order modulation format is lower than that of low order modulation format.(2)Frequency offset estimation algorithm based on convex optimizationThe existing frequency offset estimation algorithm is analyzed.Aiming at the shortcomings of high complexity based on fast Fourier transform algorithm,a frequency offset estimation method based on convex optimization is proposed.The algorithm can quickly find the most worthy features of the convex function by using the three point method,and reduce the complexity of the frequency offset estimation.The simulation results show that the proposed algorithm is more accurate than the fast Fourier transform algorithm,and the algorithm complexity is only 51%of the fast Fourier transform algorithm.(3)Phase recovery algorithm based on minimum first order distanceThe advantages and disadvantages of the existing phase recovery algorithm is analyzed.I propose a phase recovery algorithm based on the minimum first order distance.The algorithm first finds the rough estimation interval of phase noise by blind phase search algorithm,then finds the phase noise quickly by using the minimum first order distance.The simulation results show that the phase recovery algorithm based on the minimum first order distance and the blind phase search algorithm have the same linewidth tolerance,but the complexity is 68%lower than the blind phase search algorithm.