The Research Of Frequency Offest Estimation And Phase Recovery In Coherent Optical Communication

With the rapid growth of network traffic, global optical communication network is facing with growing demand for higher bandwidth capacity. In recent years,among many novel optical modulation transmission technology, CO-OFDM system, which has the advantages of high transmission rate, high spectrum efficiency,high immunity to chromatic dispersion and low cost,combines the advantages of wireless OFDM technology and optical communications.However, the local laser added in the receiver end for coherent detection makes CO-OFDM more sensitive to frequency offset and the phase noise caused by the laser linewidth.As a multicarrier technology OFDM signals show high PAPR(peak to average power), which result servere fiber nonlinearity to the OFDM signal in long distance transmission. In order to solve the frequency offset, phase noise, some major research are summarized as follow:(1) First the basic principle of OFDM technology are well researched, including the mathematical model and modulation/demodulation principle, the importance of cyclic prefix in OFDM system, Then introduces the the key signal processing module in OFDM from the sender to the receiver. Then,starting from the formula derivation the advantages and disadvantages of the Coherent-Optical OFDM(CO-OFDM) system is deeply analyzed.(2) A 100 Gbit/s CO-OFDM simulation platform is constructed using the MATLAB and VPI simulation tools.The platform is verified by comparing the simulation Q-factors with the theoretical values.(3) This paper investgates the theory and influence of the laser phase noise and chromatic dispersion(CD), To compensate laser phase noise in CO-OFDM, pilot aided phase compensation algorithm is presented,.simulation of pilot aided algorithm indicates that it can effectively compensate the phase noise caused by the laser linewidth.(4) Based on the simulation platform carrier frequency offest compensation algorithm and phase noise compensation algorithm are investigated.two kinds of carrier frequency offest compensation algorithm are described,the symbol timing and frequency deviation joint estimation algorithm by adopting FH conjugate symmetrical sequence is proposed on the basic Schmidl&Cox and Minn algoritm.The estimation of frequency deviation range will be expanded with rational training sequence length.Finally, we analyze synchronization algorithm based on cyclix prefix through a large number of simulations,analysis and performance comparsion on the tradtional ML algorithm and ML algorithm based on continuous symbols,correlation algorithm based on part of CP is used to estimated fraction frequency offest and improved the estimation accuracy.