Research On Equalization Algorithms In Coherent Optical Transmission Systems

In recent years,high-order modulation formats such as 16QAM,combined with coherent detection and digital signal processing(DSP)techniques,are of great importance for improving communication transmission capacity.As for the 16QAM signals are more sensitive to laser phase noise and amplitude noise than lower order modulation format systems.The main disadvantage of traditional carrier phase recovery algorithms is that they cannot eliminate the amplitude noise.The performances of decision feedback algorithms are limited by the large linewidth cases.In this paper,various digital signal processing(DSP)algorithms for coherent optical transmission system are studied.The emphasis is on carrier phase recovery combined with Kalman filter for adaptive equalization in 16QAM system.The main parts of the research and innovations of the dissertation are shown as followed:The blind carrier phase recovery algorithms(such as VVPE,VVPE-ML)for 16QAM optical transmission cannot achieve the optimal estimation or the amplitude noise elimination.In this paper,the VVPE combined with Kalman filter for optimal carrier phase recovery was proposed,referred as VVPE-DFEKF.At the first stage,The VVPE algorithm is used for a wide range of carrier phase noise compensation.At the second stage,the feedback decision-aided Kalman filter is applied for adaptive equalization and residual phase noise and amplitude noise compensation.First of all,the 25 Gbaud DP-16QAM simulation systems were established.A 2000km transmission distance was achieved,which outperforms the traditional solutions such as VVPE and VVPE-ML about more than 20%.In order to demonstrate the nonlinear compensation performance for VVPE-DFEKF,the nonlinear phase noise model was designed in 16QAM coherent optical transmission simulation system.For 100kHz linewidth,the nonlinear tolerance of VVEP-DFEKF is 1.4 dB larger than the VVPE-LMS scheme at BER=le-3.And the results become larger with the linewidth increasing.The 11.2GBd,28GBd 16QAM coherent BTB experimental platform was built.The offline DSP algorithms were written in MATLAB programming language.For 11.2GBd,the required OSNR of VVPE-DFEKF at BER=3.8e-3 is 0.6dB lower than the VVPE-ML scheme.For 28GBd,the required OSNR of VVPE-DFEKF is 1.3dB lower than VVPE-ML at BER=3.8e-3.For the 28GBd DP-16QAM coherent BTB system,the BER of X,Y polarization can achieve?1e-4,which lower than FEC threshold.