Research On Channel Impairment Equalization In High-speed Optical Fiber Communication Nyquist System

With the development of science and technology, in addition to increasing demand of video, cloud computing and mobile Internet, people also have increasing demand on new applications such as intelligent TV,VR and intelligent home gateway, which need to be broadband based.High-speed optical fiber communication, which can carry terabit bit and long distance transmission, has become the mainstream of the current development. An effective method to improve the broadband speed is to increase the spectral width of optical signal and to improve the spectrum efficiency. For the single channel, the increase of the signal spectral width is mainly dependent on the increase of the signal symbol rate. In order to improve the spectrum efficiency, the modulation format evolves from QPSK to M-QAM. PDM technology using two orthogonal polarized state to transmit two optical signals can make the transmission rate doubled.Multi-carrier multiplexing techniques such as OFDM, Nyquist WDM and electric and optical domain variants have also become a hot research topic and gradually to the commercial development.PDM signal transmission in fiber will suffer from channel impairments such as CD, PMD, RSOP, PDL and nonlinear effect, which is the main reason for restricting the further increase of the bandwidth and transmission distance. In recent years, due to the development of digital signal processing (DSP) technology, the above impairments can be compensated and equalized in the DSP module of the coherent optical communication system. However, with the increase of the symbol rate, the increase of the modulation format and the increase of the transmission distance, the channel impairments and the interaction between the various impairments will enhance. The equalization DSP technology, which is suitable for low-rate, low-order modulation format and single channel,gradually become stretched. In this paper, we focus on channel impairments in Nyquist systems for high-speed optical fiber communication. We study and analyze the channel impairments and channel equalization techniques for high-speed coherent optical Nyquist systems. The main research works are as follows:(1) We introduce the principle of sending and receiving of the coherent Nyquist PDM M-QAM communication system, analyze the principle of fiber channel impairment and mathematical model, and construct the coherent Nyquist PDM-16QAM communication system.(2) We analyze the principle, applicable scope, advantages and disadvantages of several commonly used fiber channel impairment equalization algorithms. We propose a novel two-stage extended Kalman filter scheme, which can effectively equalize the impairment of multi-polarization effects (RSOP, PMD and PDL) and analyze its mathematical model. We also compare its performance with other algorithms and analyze its complexity theoretically.The innovation of the scheme: The proposed multi-polarization impairment equalization scheme is implemented in two stages. The first stage is to converge the signal constellation points to multiple rings to equalize and compensate the PDL, PMD and part of the RSOP. The second stage is to converge the signal constellation points to discrete clusters to compensate the residual RSOP, while also compensating the laser phase noise. The simulation results show that the performance of the proposed scheme is better than the combination of CMA, MMA and BPS when the optical OSNR is over 21dB in the Nyquist PDM-16QAM system. The scheme can track the angle frequency of azimuth and phase angle of SOP to 110Mrad/s and 1200krad/s, respectively. Moreover, the scheme can be simply extended to higher order modulation formats in theory.