DSP Algorithms And Simulation In Carrier Synchronization For Optical Coherent Detection

In recent years, optical coherent detection based on digital signal processing (DSP) has called wide attention, not only because it achieves high spectral efficiency, but also enables electronic compensation of fiber-optic transmission impairments such as chromatic dispersion, polarization mode dispersion and fiber nonlinearity. Therefore, the implementation cost is remarkably reduced. However, due to the asynchrony between transmit and receive lasers, the received data constellation suffers from a rotation, which would bring a huge degradation to the phase-modulated transport system (PSK and QAM). As a result, carrier synchronization is required in the receiver.At the beginning, the analysis and simulation of linear equalization for optical fiber channel are carried out, covering the influence of filter length for chromatic dispersion compensation as well as the importance of the selected step size in the tap update process for Jones Matrix estimation.The carrier synchronization algorithms are our research focus. For the problem of frequency offset estimation in carrier synchronization, we present an algorithm without removing modulated data phase in PSK system. Compared with all the algorithms requiring the erosion of modulated phase beforehand, our proposal is a big step forward. Then, we modify the proposed approach to accommodate QAM system by partial constellation rotation scheme, leading to a robust noise tolerance. For the problem of laser phase noise in carrier synchronization, we introduce a novel two-stage structure based on Test Phase algorithm and the computational complexity deceases nearly 50% at least. Meanwhile, by considering the different effects of additive noise on different region in high order QAM constellation, a group of weighted coefficients are introduced. Taking QPSK partition-based algorithm for example, the modified version gains a 40% increment of laser linewidth tolerance.In the end, a dual-polarization QPSK simulation platform is designed by using OptiSystem and MATLAB, which clearly shows the operation flow of DSP-based coherent detection technology.