Research On Carrier Recovery Algorithm For High-speed Optical Coherent 16-QAM Receiver

With the development of the information era,the traditional low-speed optical communication system based on intensity modulation/direct detection has been unable to meet the growing needs for bandwidth.High-speed optical coherent communication network based on high-order modulation,coherent detection and digital signal processing(DSP)technologies has the characteristics of high rate,high spectrum utilization and low cost,which is one of the development directions of the next-generation high-speed communications networks.The digital coherent optical receiver with coherent detection and DSP technologies can not only retain the carrier amplitude,phase and polarization information,but also recover and compensate the the optical link impairments in the electrical domain.It has become a hotspot in the field of optical communication in recent years.In this paper,we mainly focus on the research of high-speed optical coherent communication system based on 16-QAM modulation format,coherent detection and DSP receiving technologies.The influence of the frequency offset and phase noise introduced between the transmitter and local laser is studied emphatically.The carrier frequency offset and carrier phase are accurately estimated and recovered in a digital optical coherent receiver.The main contributions of this thesis are listed as follows:(1)The generation method and principle of square 16-QAM optical signal are studied,and the simulation platform of 112Gb/s 16-QAM optical coherent communication system by four-level driving IQ modulator is built.With this platform,the carrier frequency offset estimation algorithm and carrier phase estimation algorithm are used to recover the received signal.(2)Carrier frequency offset estimation algorithms based on partial constellation and training sequences are studied.The estimation range,estimation precision and computational complexity of these two carrier frequency offset estimation algorithms are analyzed and compared in the 112Gb/s 16-QAM optical coherent communication system.The influence of the system residual frequency offset on the carrier phase compensation is discussed.(3)Many carrier phase estimation algorithms are studied.In this paper,several carrier phase estimation algorithms which are suitable for 16-QAM and other high-order modulation format are studied.The algorithms are divided into two classes,the improved Viterbi&Viterbi algorithms and the two-stages blind phase search(BPS)algorithms.The tolerances of linewidth and hardware complexity of the eight types of carrier phase estimation algorithms in 16-QAM system are compared and analyzed.(4)A linear approximation(LA)carrier phase estimation algorithm based on BPS is proposed.The basic principle and implementation flow of LA algorithm are expounded,and the best test phase point of the algorithm is obtained by simulation.The bit error rate,linewidth tolerance,hardware complexity and frequency offset tolerance of LA carrier phase estimation algorithm are also analyzed.The signal of 112Gb/s 16-QAM optical coherent communication system is recovered by the LA carrier phase estimation algorithm.Simulation results show that the linewidth tolerance of LA algorithm is similar to that of BPS algorithm,but the hardware complexity of the BPS algorithm is reduced by a factor of 2.13/2.18/2.91/2.06(multiplier/adder/comparator/decision device).When the residual frequency is more than 30MHz,the signal recovery performance through the LA algorithm is better than that through the BPS algorithm.