Research On Channel Equalization And Compensation In High-Speed Optical Coherent System

Nowadays, with the rapid development of the data services on internet and multimedia fields, public’s demand on the large capacity, long haul transmission system is increasing as well. Optical fiber, the small-loss and large-bandwidth media, is able to transport the tremendous amount of data, which is subjected to a wide range of research and concern since it came out. The capacity of optical transmission system has gradually increased to40G even to100G compared with10G at the beginning. On one hand, the system upgrade meets our requirement on data transmission. On the other hand, it also brings some problems. The loss, chromatic dispersion, polarization mode dispersion and nonlinear effects sharply reduce the transmission efficiency. It’s imperative for us to find a receiver solution which is able to effectively compensate and restore these losses. In addition, the transmitter complexity is affected by the system upgrade, so to generate high-speed signal with a simple transmitter solution is essential as well.In recent years, the development of flexible coherent detection is evolving rapidly. Coherent receiver linearly transfers optical signals to electrical signals, completely retaining the phase information and state of polarization. In the digital signal processing module of the receiver, there are several algorithms can effectively compensate and restore the transmission loss. Accordingly, the transmitter may use high-order modulation combined with multiplexing technique to increase the spectral efficiency, such as phase shift keying or quadrature amplitude modulation.In this paper, I did research and simulation for high-speed coherent optical communication systems, as well as some compensation and equalization algorithms. It mainly consists of the following parts:1. Analysis of the generation mechanisms and theoretical models of several fiber characteristics are given, including the loss, chromatic dispersion, polarization mode dispersion and nonlinear effects.2. Detailed analysis of the structure and principle of the coherent receiver system are carried out. Two common receiver solutions, as well as the function and inner structure of each module are introduced. Explanation of modulation and demodulation process is given, and also the implementation principle of polarization division multiplexing. At last, a112Gbit/s PDM-16QAM signal is simulated in a coherent detection system.3. The description of the structure and principle of the equalizers in the receiver is given, as well as some typical compensating algorithms. For the lack of the conventional constant modulus algorithms, several modified algorithms are developed, and the theoretical models are given, respectively. Then the modified algorithms and the conventional one are simulated in the112Gb/s PDM-16QAM system, including the convergence rate, the convergence effect and the resistance of singularity problem.4. The necessity of carrier phase recovery is introduced, and the principles of the carrier phase recovery as well as the typical recovery solution are carried out. Then the solution is simulated in the QPSK system. According to the characteristics of16QAM format, an improved scheme is given, and also verified in the PDM-16QAM simulation system. Eventually, the impact of several parameters on the system bit error rate is studied.In summary, this thesis is mainly about the electrical equalization and compensation in high speed coherent optical communication systems. Based on high-order modulation format as well as the physical model, some modified algorithms and solutions are carried out, in order to improve the transmission efficiency of the system.