Research On Parameter Identification And Equalization Of Polarization Impairments For Elastic Optical Network

To meet the global data traffic requirements in a growth level of exponential order,the next generation 400Gb/s or 1 Tb/s high-speed optical fiber communication system will utilize flexible transceivers and powerful digital signal processing(DSP)technologies of coherent receivers to transmit optical signals adaptively with adjustable parameters,such as bandwidth,number of subcarriers and modulation formats,in order to enable the flexible deployment of bandwidth resources.Accordingly,the next generation high-speed optical communication system will evolve towards the digital,software-based and dynamic reconfigurable elastic optical network.As prerequisites for stable operation of EON,techniques of parameter identification and equalizations of polarization impairments are two of important research topics in the field of EON reliable transmission.The parameters of flexible transmitter are always dynamically adjusting according to the fiber transmission distance,channel noise characteristics and user traffic changes in EON system.Therefore,the intelligent identification of parameters using DSP algorithm at the coherent receiver of EON is the basis and precondition for performing optical performance monitoring and correct reception of EON signals.In addition,EON system utilizes polarization division multiplexing(PDM)technology to achieve higher spectral efficiency,which is highly susceptible to various polarization impairments derived from EON systems and fiber transmission.Hence,the joint equalization of polarization impairments using DSP algorithms plays crucial role during the coherent reception process of EON.To sum up,it has great theoretical and practical significance to carry out the researches of EON parameter identification and joint equalization of polarization impairments,for the purpose of guaranteeing the reliable operation of the next generation high-speed optical fiber communication backbone network and the early realization and popularization of EON system.Focusing on these two topics of parameter identification and equalization of polarization impairments in EON system,this dissertation mainly carries out the research works of intelligent parameter identification of EON,joint equalization technology of polarization impairments,pre-equalization of EON transmitter.The main research contents and innovations in this dissertation are as follows:(1)Research on intelligent identifications of EON bandwidth parameters and modulation formats of subcarriersIn this thesis,we propose a new intelligent bandwidth identification scheme for orthogonal frequency division multiplexing(OFDM)-EON.The proposed scheme can be divided into 3 sub-stages:estimation of power spectrum density(PSD),noise filtering based on empirical mode decomposition(EMD),and bandwidth identification based on a sliding window.It can filter out noise adaptively and locate the start and termination positions of bandwidth intelligently.Simulation and experimental results show that this scheme can successfully identify EON bandwidths within a range of 2GHz to 40GHz with a resolution of 39MHz,which can distinguish Gbit/s bandwidth variable transmitters with enough accuracy.At almost the same recognition accuracy,the sampling points that the proposed scheme needs are only 1/16 of the cyclostationary characteristics scheme.In addition,the proposed scheme has a good tolerance to chromatic dispersion(CD),and the bandwidth identification accuracy would not be affected by the number of subcarriers and the frequency offset.Meanwhile,in order to identify the subcarrier modulation format of OFDM-EON,this dissertation proposes a modulation format identification scheme based on Binary Phase Shift Keying(BPSK)training symbols.Simulation results show that the proposed scheme can correctly identify all the modulation formats of subcarriers when OSNR is higher than 15dB,and it has the features of high OSNR tolerance and high recognition success rate.(2)The two-stage joint equalization research of polarization impairments based on extended Kalman filter(EKF)In this thesis,we propose a new two-stage joint equalization scheme of polarization impairments based on EKF.The proposed scheme constructs a two-stage compensation matrix in the digital DSP domain of flexible receiver and performs symbol-by-symbol estimation of unknown parameters in the compensation matrix based on the fast convergence characteristics of EKF.It can carry out the joint equalization of polarization impairments,such as Rotation of State of Polarization(RSOP),first-order Polarization Mode Dispersion(PMD)and second-order PMD(SOPMD).Besides that,it overcomes the shortcoming that the traditional constant modulus algorithm(CMA)and multiple modulus algorithm(MMA)cannot perform equalizations of polarization impairments under fast RSOP.The experimental results of received PDM-Quadrature Phase Shift Keying(QPSK)and PDM-16 Quadrature Amplitude Modulation(QAM)EON signals show that the fastest RSOPs that the scheme can track up are 120Mrad/s and 11 OMrad/s,respectively.The RSOP tracking capability of the proposed scheme is 35 times and 50 times fast as that of CMA and CMA/MMA,respectively.In addition,it gains good joint equalization performance of RSOP tracking,first-order PMD and SOPMD compensation,and obtains higher Q factors and better equalization effects than CMA/MMA.Furthermore,the convergence speed of the proposed scheme is about 50 symbols(equals to 5ns),which is much faster than CMA/MMA.The calculation complexities of each symbol that the proposed scheme consumes are just about 44%of CMA for PDM-QPSK,and 31%of CMA/MMA for PDM-16QAM.(3)Research on pre-equalization of EON transmitter supporting arbitrary modulation formatsTo solve the distortion problem of the output signals of EON transmitter supporting any modulation formats,the dissertation performs the research of DSP pre-equalization technology for EON transmitter in detail and proposes the schemes of frequency response pre-compensation of RF cables and digital-to-analog converter(DAC),skew calibrations of IQ channels,and the nonlinear modulation pre-compensation of Mach-Zehnder modulators(MZMs).These schemes are verified by corresponding experiments and high-quality EON transmitted signals are obtained.In addition,the experimental results show that after DSP pre-equalizations of EON transmitter,skew values between IQ channels are reduced to femtosecond level,Q factors can be overall improved by approximately 2dB,and the constellation points of transmitted signals are more evenly arranged.