Research On Waveform Processing And Carrier Phase Estimation Algorithms For Coherent Optocal Communication System

To meet the rapidly increasing traffic demand of communication network,optical fiber communication technology is constantly upgrading.Currently,backbone network has been upgraded from 10Gbit/s,40Gbit/s to 100Gbit/s optical fiber communication systems,but in order to meet future demand for the rapid growth of network traffic in recent years,it is an inevitable trend that the optical fiber communication system need upgrade to 400Gbit/s even Terabit transmission system.During this process,how to maximize system spectral efficiency(SE)and the transmission performance of optical fiber communication system is one of the critical challenges for researchers.Advanced modulation formats like QPSK,16QAM can improve spectral efficiency(SE)by 2 and 4 times,but they need 3dB or 6dB OSNR additional requirement,and hence transmission distance is limited.Efficient waveform processing technology is another way to increase spectral efficiency,such as Nyquist signaling make the signals with the symbol rate of 50 Gbaud hold in a 50GHz WDM channel without the inter-channel interference(ICI).Faster than Nyquist signaling can compress the occupation bandwidth to be a fraction of bandwidth that Nyquist signaling occupies,so it has highter spectral efficiency than Nyquist signaling.Thus,the waveform processing technology is an important research scheme for the future high-capacity transmission systems.The optical signal suffer from a series of linear and nolinear injuries during transmission in the fiber,thus the transmission performance of the system will be seriously influenced.In coherent optical communication system,lots of impairments compensation can be implemented digitally such as chromatic dispersion(CD),polarization mode dispersion(PMD),frequency offset(FO)and laser phase noise(PN).Carrier phase estimation(CPE)is a significant part for coherent transmission system,whose function is to compensate for the laser phase noise induced by the lasers located at the transmitter(Tx)and at the receiver(Rx)ends and ensure the the reliability of the coherent optical communication system.In a FTN system,there existence the ISI introduced by FTN signaling,so how to compensate for the carrier phase noise is a problem we need to solve.This paper focuses on the waveform processing and carrier phase estimation in coherent optical communication system.Furthermore,we have investigated the carrier phase estimation algorithms in Nyquist and Faster than Nyquist system.The main research efforts are summarized as follow:(1)The effectively waveform processing technology of Nyquist and Faster than Nyquist is investigated.Furthermore,different method is used to generate the Nyquist and Faster than Nyquist signal.In Faster than Nyquist system,two kinds of algorithms to detect the signal and eliminate ISI is realized.(2)Different carrier phase estimation algorithms in Nyquist system have been researched and the performance between this algorithms is compared,Viterbi&Viterbi phase estimation(VVPE)algorithm,Constelation Transformation(CT)algorithm,Maximum Likelihood(ML)algorithm and Bland Phase Search(BPS)algorithm is included.As the traditional Constelation Transformation(CT)algorithm is only suited for the rectangular constellation of QAM modulation format,so a Crossed Constelation Transformation(CCT)algorithm that can be applied to non-rectangular constellation of 32QAM and 128QAM modulation formats is proposed.Finally,a two-stage carrier phase estimation scheme using crossed constellation transformation(CCT)algorithm followed by optical pilot aided algorithm is proposed.Performance of the PA+CCT carrier phase estimation method is studied and compared with other carrier phase estimation methods such as only PA,PA+ML and BPS in double-carrier 32QAM/128QAM Nyquist system.Analysis results showed that the proposed PA+CCT scheme has the best linewidth tolerant and lower computation complexity.PA+CCT shows good line-width tolerance of 2.5 MHz for 32-QAM and 800 kHz for 128-QAM at the bit error rate(BER)of 3.8×10-3 with 1dB OSNR penalty.(3)In this work,the model of phase noise caused by laser in Faster than Nyquist QPSK system is built.Then,the challenges for estimating the phase noise in Faster than Nyquist system is discussed.In order to compensate the phase noise induced by the lasers and achieve high linewidth tolerance with low hardware complexity,two different carrier phase estimation schemes in Faster than Nyquist optical communication system is proposed,which can effectively overcome the effect of the strong inter-symbol interference(ISI)intruding by FTN signaling."In chapter 5.2,the optical domain pilot-aided method can reach larger linewidth tolerance and lower bandwidth overhead.In chapter 5.3,the proposed system is easy to implement in hardware and can avoid the problem of phase synchronization.