Some Studies Of Digital Signal Processing Technologies For The Next Generation Coherent Optical Communication System

Coherent optical communication is the main stream of the current optical transmission technology. Compared to the traditional intensity modulation direct detection (IM-DD) systems, coherent detection is theoretically able to detect the whole optical field, and thus obtains information in more dimensions. To meet the ever increasing communication bandwidth requirement, the next generation optical communication technology has to improve the utilization efficiency of the optical field. Currently, as the available transmission spectral band has almost been exhausted by DWDM technology, one of the main approach to obtain higher capacity is to increase the spectral efficiency (SE). High order modulation format and narrow bandwidth, including Nyquist and Faster-Than-Nyquist (FTN), pulse shaping are two effective routines. However, they could raise great challenges to the currently employed digital signal processing (DSP) technologies. Starting from the current coherent optical communication system and DSP technology, this thesis consists of three parts, namely, time synchronization, frequency offset synchronization and carrier phase synchronization, to investigate the receiver side DSP technologies in the next generation coherent communication system employing high order modulation format and narrow bandwidth pulse shaping.The investigation of time synchronization consisted of two parts. In the frame synchronization part, when there exists narrow bandwidth interference, which could result from the finite modulation extinction ratio in the transmitter, a new algorithm was proposed to enhance the resistance to such interference, on condition of keeping the estimation accuracy. In the timing phase estimation part, the equivalence among four second order statistics based estimators was derived. While these estimators can not work properly in narrow bandwidth pulse shaping situation, a new estimator was proposed to apply the second order statistics based estimator to the nonlinearly pre-processed signal.The investigation of carrier frequency synchronization consisted of two parts. In frequency acquisition part, the performance of the traditional algorithms in narrow bandwidth pulse shaping condition was analyzed by simulation. And in the frequency tracking part, an estimator based on nonlinear pre-processing was introduced to work in narrow bandwidth pulse shape systems. Since this estimator can work before timing phase recovery, the estimation accuracy degradation caused by the frequency offset is removed.Finally, the carrier phase synchronization was sumerized. The carrier phase estimators were unified to the maximum a posteriori (MAP) probability framework. Three solutions were obtained through different methods, namely, one optimal approach by brutal searching, one feedforward approximation and one feedback approximation. Then a new algorithm based on constellation image processing was introduced. Compared to the optimal approach, the estimation accuracy was retained and meanwhile the computational complexity was greatly decreased. Finally, for high order format, it is point out by simulation that, a multi-stage carrier phase estimation strategy can be employed to further decrease the computational complexity.