Research On DSP Algorithm For Carrier Recovery In Faster-than-Nyquist Coherent Optical Transmission Systems

With the rapid development of 5G and other technologies,users’demand for high-capacity communication is rising.However,because optical fiber C+L band frequency resource shortage,it is difficult for the traditional wavelength division multiplexing(WDM)system to further improve the system capacity.In recent years,fast-than-Nyquist(FTN),which breaks the orthogonal criterion,is a promising development direction in the field of ultra-high spectral efficiency and large capacity coherent optical transmission.However,the serious inter symbol interference(ISI)introduced by FTN strong filter makes the system more sensitive to various damage factors.The carrier frequency offset estimation algorithm in the receiver DSP algorithm faces the problems of prominent contradiction between the effect and complexity.The phase estimation algorithm is faced with serious phase slip(CS)caused by ISI.At present,although relevant research results are reported,DSP technical problems such as low complexity and high-precision carrier frequency offset estimation algorithm and carrier phase estimation algorithm with cycle slip suppression need to be studied.Aiming at the above technical problems,the research work is as follows.1、According to the demand of FTN for high-precision,reliable and low complexity frequency offset estimation algorithm,a frequency offset estimation algorithm based on ’training sequence phase difference+CZT’is proposed.The algorithm completes the first-order rough estimation through the training sequence and the second-order fine estimation through CZT based on the coarse estimation value of frequency offset,which can realize low complexity and high-precision frequency offset estimation.The performance of the algorithm is simulated and analyzed in a three carrier 128GBaud PM-16QAM FTN-WDM system.When the acceleration factor is 0.95/0.9/0.85,the proposed algorithm can achieve stable frequency offset estimation in the range of[-1.6,1.6]GHz,and the residual frequency offset is about 2/2.5/3MHz respectively.Under the typical 1 GHz frequency offset and the acceleration factor of 0.95/0.9/0.85,the OSNR required by the FTN system under the soft decision FEC threshold with bit error rate of 2×10-2 is 23.5/23.8/24.5dB respectively.Compared with the traditional blind quartic FFT frequency offset estimation algorithm with the same accuracy order,the computational complexity of the proposed algorithm is only 8%of the latter,and the overhead is only 1.56%.2、Aiming at the serious phase cycle slip problem in carrier phase estimation of FTN system,a cycle slip suppression carrier phase estimation algorithm based on training sequence is proposed.The algorithm estimates the reference phase through the training symbols inserted into the effective signal,and detects and compensates the cycle slip based on the reference phase.In the three carrier 128GBaud PM-16QAM FTN-WDM system,the performance simulation analysis and performance evaluation of the algorithm are carried out.The results show that when the acceleration factor is 0.95/0.9/0.85 and the laser linewidth is 100KHz,the corresponding OSNR required under the soft decision FEC threshold of BER=2×10-2 is 23.4/23.7/24.3dB respectively.When the acceleration factor is 0.95/0.9/0.85,the maximum linewidth that can be tolerated at the cost of 1 dB OSNR(compared with the OSNR corresponding to 0 linewidth)is 3.8/3.2/2.2MHz respectively.The proposed phase estimation algorithm has a cost of 0.39%,which realizes the reliable and efficient detection and compensation of cycle slip of carrier phase estimation in FTN system.Compared with QPSK partitioning algorithm,the increase of computational complexity is negligible,but it can effectively solve the problem of phase cycle slip.3、Using the three carrier 40GBaud PM-16QAM FTN-WDM coherent optical transmission system offline experimental platform,the performance of the proposed "phase difference+CZT" frequency offset estimation algorithm based on training sequence and cycle slip suppression algorithm based on training sequence are verified through experiments.By adjusting the center frequency of the receiving laser to form different system frequency offsets,the performance of the proposed carrier frequency offset estimation algorithm under different frequency offsets is experimentally verified;In order to verify the performance of cycle slip suppression algorithm under different linewidth,phase noise equivalent to laser linewidth noise is added in MATALB programming when generating originator data.In the frequency offset range of[-1.6,1.6]GHz,when the acceleration factor is 0.9,the absolute value of the maximum frequency offset estimation error of the proposed frequency offset estimation algorithm is about 3MHz.When the acceleration factor of FTN system is 0.9,the OSNR tolerance under the soft decision FEC threshold of BER=2×10-2 is 20dB.By analyzing and comparing the phase noise tracking curve,compared with the traditional QPSK partitioning algorithm,the proposed cycle slip suppression algorithm based on training sequence can effectively suppress cycle slip.When the acceleration factor is 0.9,the maximum linewidth tolerated by 1dB OSNR cost is 1.6MHz.