Study On Carrier Phase Estimation In Coherent Optical Communication

In recent years, coherent optical communication combined with digital signal processing(DSP) has attracted significant attention. The distortions of signal existed in optical link can be effectively compensated by using DSP algorithms. With the rapid growth of capacity demand in optical networks, high-order quadrature amplitude modulation(QAM) formats are considered to be promising candidates for their high spectral efficiency. However, high-order modulation formats suffer from phase noise induced by finite linewidths of both transmitter laser and receiver local oscillator. Therefore, it is important to design effective carrier phase estimation(CPE) algorithms.This dissertation first introduces the background of the digital coherent optical communication and the development of CPE algorithms. Then, the principle of optical coherent receiver and the whole DSP schemes are investigated. We research and analysis the present CPE algorithms from the signal model of phase noise induced by laser sources. And we propose a CPE algorithm for 32-QAM by using quasi-QPSK partitioning to replace conventional QPSK partitioning method. The proposed method improves the available constellation points for CPE from 25% to 75%, which enhances the ability of CPE to track changes of phase greatly. To evaluate the performance of proposed method, we conduct simulations to compare proposed quasi-QPSK partitioning and the conventional QPSK partitioning method with the help of MATLAB. The multi-stages CPE algorithms by using crossed constellation transform(CCT) and Maximum likelihood(ML) are also simulated. For a 1 dB penalty in optical signal-to-noise ratio at a bit error ratio of 1 10-2, the simulation results show that the proposed method can achieve an improvement of 85% in the linewidth symbol duration product. With the help of fine stages, the maximum linewidth symbol duration product can be up to 5.1 10-5, which means a total 1.63 MHz combined linewidth can be tolerated in 32 GBaud optical coherent systems.