The Study Of Modulation Format And Equalization Algorithm In CO-OFDM System

Coherent optical orthogonal frequency division multiplexing(CO-OFDM)technology,as an efficient multi-carrier modulation technology,combines the advantages of OFDM and coherent optical communication systems.It has the advantages of high spectrum efficiency,strong dispersion tolerance and easy realization of digital signal processing,is one of the potential solutions for the next generation of ultra-high speed and long distance optical transmission system.However,CO-OFDM system due to the introduction of laser linewidth non-ideal factors will inevitably cause phase noise.And for a variety of signal damage exists in the signal transmission process,the CO-OFDM system in order to recover the original signal better,the receiver needs to further enhance the effect of equalization algorithm.Therefore,based on a Zero-Guard Interval(ZGI)CO-OFDM system with higher spectral efficiency,this paper proposes a modulation format based on the circle 16QAM(C-16QAM),and analyzes its transmission performance with the traditional square 16QAM.Then,the algorithm of channel estimation and phase noise compensation is improved in ZGI CO-OFDM system.The main research work and achievements of this paper can be divided into the following points:(1)CO-OFDM system has the problem of phase noise sensitivity,compared with the traditional square 16QAM,this paper uses a C-16QAM with the same minimum Euclidean distance as the modulation format.Furthermore,a large number of cyclic prefix(CP)added at the transmitter increase the system overhead and reduce the spectrum utilization in CO-OFDM system.In this paper,ZGI CO-OFDM system is used to solve the problem.Then,the performance differences between the two modulation formats in ZGI CO-OFDM system are analyzed from channel estimation,phase noise tolerance and dispersion tolerance.And simulation results show that C-16QAM has better phase noise tolerance and dispersion tolerance than 16QAM.(2)Aiming at ZGI CO-OFDM systems,this paper proposes a channel estimation algorithm based on intra-symbol frequency domain averaging(ISFA)and a two-stage phase noise compensation algorithm based on Viterbi-Viterbi(V-V)and maximum likelihood(ML).The simulation results show that the improved channel estimation algorithm based on ISFA and the improved algorithm of two-stage phase noise compensation based on V-V and ML can effectively improve the performance of equalization algorithm in ZGI CO-OFDM system.