Research On The Timing Synchronization And Carrier Frequency Offset Estimation Algorithm For CO-OFDM System

In recent years,with the increasing demand for Internet traffic,higher channel capacity and transmission rate are highly demanded in the optical fiber communication networks.Coherent optical orthogonal frequency division multiplexing?CO-OFDM?technology is considered as a promising solution for future high-capacity optical fiber transmission systems thanks to its significant advantages such as high spectral efficiency,good dispersion resistance,and ease of channel compensation.However,the CO-OFDM technique is easily affected by synchronization errors.Carrier frequency offset and timing synchronization errors can cause inter-symbol interference?ISI?and inter-carrier interference?ICI?,leading to serious degradation of system performance.Therefore,this paper will mainly focus on timing synchronization and carrier frequency offset estimation algorithms.Firstly,based on the basic principles and key technologies of the CO-OFDM system,this paper has completed the construction of the CO-OFDM synchronization model.The formulas of timing synchronization and carrier frequency offset are deduced and analyzed,and their effects on the performance of CO-OFDM systems are discussed in detail.Furthermore,the verification of the simulation results based on the MATALB platform provide a theoretical basis for subsequent research on timing synchronization and frequency offset estimation algorithms.Secondly,study of the timing synchronization algorithm based on training symbol is deeply investigated in this paper.The formulas of the three classical algorithms such as Schmidl&Cox algorithm,Minn algorithm and Park algorithm are deduced and analyzed,and the timing performance of these three algorithms have been simulated for the CO-OFDM system.Meanwhile,based on the three classical algorithms,an improved timing synchronization algorithm is proposed in this paper,the problem that the conventional algorithm is not sharp enough for the timing measurement function curve and the sidelobe is well solved.The simulation results show that the improved algorithm can not only accurately time the fiber channel with a OSNR as low as 2dB,but also has better timing performance than the above three traditional algorithms within 1600 km transmission for CO-OFDM system.Thirdly,study of the carrier frequency offset estimation algorithm based on the training sequence is deeply investigated in this paper,the principle of frequency offset estimation of classic algorithms such as P.H.Moose algorithm,Schmidl&Cox algorithm and Molrelli&Mengali algorithm are discussed in detail,and the performance of these three algorithms is verified by simulation.On the basis of analyzing the advantages and disadvantages of these three algorithms,a carrier frequency offset estimation algorithm based on the constant modulus algorithm?CMA?is proposed in the paper.The frequency offset estimation range of proposed algorithm can achieve±N/2 using two training symbols with lower computational complexity.Finally,based on the MATLAB simulation platform,a polarization-division multiplexing?PDM?CO-OFDM system is built.Under the QPSK and 16-QAM modulation formats,the transmission rate of proposed timing and frequency synchronization algorithm are 95.77 Gbit/s and 191.54 Gbit/s,respectively.The simulation results show that a bit error rate of 10^-3 can be achieved at 16 dB OSNR for 4-QAM signal with 1600 km transmission,and the required OSNR for a bit error rate of 10^-3 is about 26 d B with 800 km transmission distance for 16-QAM.The mean normalized carrier frequency offset estimation error can keep under 12×10^-3 even under the OSNR as low as 10 dB.