Research On Clock Recovery Joint Channel Equalization Technology In Coherent Optical Communication Systems

The coherent optical communication transmission system has been a research focus in the field of optical communication. In the previous communication system, the intensity modulation mode is often be used.However, the phase modulation is generally used as modulation method in long haul optical fiber communication system. Phase modulation is that the baseband signal is modulated to the phase of the optical signal. The receiver generally use the differential receiver technology or coherent receiver. Therefore, the system designed in this paper also belongs to the field of coherent receiver. Because NRZ has good signal integrity, simple implementation, high spectral efficiency, and technology maturity, so it has been widely used in long distance optical transmission system. But the disadvantage of NRZ is that it is not good to resist dispersion and polarization mode dispersion. Therefore, it is needed to compensate the chromatic dispersion and polarization mode dispersion in the system.Synchronization is the premise of information transmission in digital communication system. In order to ensure the reliable transmission of information, the timing recovery algorithm should have very high reliability and stability. In the communication system, the timing information is accurate or not plays an important role in the communication system. This is directly related to the bit error rate performance of the whole system, so it is very important to study the clock synchronization algorithm. But the computation of clock recovery algorithm is very large and the complexity is very high, therefore, high speed high speed optical fiber communication system needs a specific algorithm, in order to meet the demand of optical fiber communication system.A new feedback symbol timing recovery technique using timing estimation joint equalization is presented for digital receivers with two samples/symbol or higher sampling rate. Different from the traditional method, the clock recovery algorithm in this paper adopts another algorithm by distinguishing the phase of adjacent symbols, so as to use the adjacent signals with same phase to estimating the timing offset accurately.It is also shown that by adding the part of eliminating phase modulation interference before timing estimation, the variance can be further reduced,resulting in a smoothed timing estimate. The MSE and BER of the resulting timing estimate are simulated, supporting a satisfactory estimation performance. Good clock tone performance is obtained for MQAM modulation formats and roll-off factor (ROF) values close to 0. In back-to-back system, when ROF=0, the maximum of MSE in the proposed approach reaches 0.0125. BER down to 10-3 with ROF=0 and SNR?11 db after 100km fiber transmission. As the increase of ROF, MSE and BER shows better performance.The research contents and main innovations of this paper are as follows:Firstly, in this paper, based on the coherent optical communication transmission system, the key modules of the transmitter and receiver are designed, that is the modulation/demodulation module, polarization multiplexing/de-multiplexing module. Including the research of key devices such as polarization multiplexing. The design of timing recovery loop based on coherent optical communication is studied, and the structure of feed forward and feedback timing recovery is studied deeply. The key modules of the clock recovery loop, namely the interpolation filter, the digital control oscillator, the loop filter and the timing detector module are studied in depth.In addition, the damage factors on the link transmission system, such as the influence of chromatic dispersion and polarization mode dispersion on the useful signal caused by the system, and link analysis were carried out based on the formula, the theoretical model of the transmission link in the system, and the related simulation verification.Furthermore, in this paper, the commonly used Gardner algorithm and other classical algorithms are studied, and the novel algorithm by numerical simulation of their performance is analyzed. In this paper, we propose a method of feedback dual phase timing recovery algorithm, which is different from the classical method, using the adjacent symbol phase splitting algorithm, using the adjacent signal to accurately estimate the timing error. The timing detection algorithm is applied to the Nyquist system, which greatly reduces the sensitivity of the roll off factor, and has better performance.At last, this paper based on the analysis of the transmission system link introduces a joint channel equalization clock recovery loop. At the receiving end of optical communication system, clock recovery loop with dispersion compensation and MIMO equilibrium, in order to reduce the influence of clock chromatic dispersion and polarization mode dispersion of the signal recovery. At the same time, polarization estimation is added to the coherent receiver, which can reduce the interference of the clock recovery algorithm.