Research On Time Synchronization Algorithm For CO-OFDM

In recent years, optical orthogonal frequency division multiplexing (OOFDM), which is characterized with high spectral efficiency, and high tolerance of fiber chromatic dispersion (CD) and polarization mode dispersion (PMD), has become a research hotspot at home and abroad. At the receiver, the signals are sampled into digital signals by ADC, the influence of chromatic dispersion, polarization mode dispersion, frequency offset and phase noise between the transceiver and transmitter is eliminated adaptively by dispersion compensation, channel estimation, frequency offset estimation and carrier phase recovery. Polarization Division Multiplexing Coherent Optical Orthogonal Frequency Division Multiplexing (PDM-CO-OFDM) systems can double the data rate and increase the channel capacity.CO-OFDM system is very sensitive to the synchronous error. Therefore, the research on the synchronization system is significant. This dissertation focuses on the analysis and research of the symbol timing synchronization algorithm and sampling clock synchronization algorithm.First, a simple exposition for the research background and evolvement in CO-OFDM transfer and synchronization technology are presented. The overall structure, working principle and key technologies of the CO-OFDM systems are investigated deeply. The 56 Gb/s CO-OFDM single-mode fiber system simulation platform is built by VPI optical simulation software and MATLAB software. The IQ modulator module, Fiber Channel and coherent receiver module are constructed and simulated by VPI to ensure the accuracy of the system channel conditions.Second, The analysis and simulation are focused on three classic symbol timing timing algorithms:Schmidl&Cox algorithms, Minn&Letaief algorithms, and Park&Cheon algorithms.Then a comparision and analysis for the four algorithms are carried out in the CO-OFDM single-mode fiber system simulation platform. Based on this study, the constant envelope zero auto-correlation sequence is introduced, then an improved timing synchronization algorithm of Chu sequence based on training sequence is proposed. Simulation results show that the proposed algorithm can achieve higher timing synchronization accuracy. And the synchronization algorithm proposed in this system is feasible. In order to achieve fine synchronization symbol, Thus the synchronization algorithm based on frequency domain is studied, Simulation results show that the algorithm can compensate synchronization error exactly.Third, the sampling frequency offset synchronization algorithm of the CO-OFDM systems is investigated, the frequency domain pilot algorithm is simulated and the strengths and weaknesses of the algorithm are analyzed. Finally, a synchronization algorithm based on timing loop sampling frequency offset is proposed, the analysis and simulation under the open-loop and closed-loop show that the algorithm of the system performance has been significantly improved.