Study On Frequency Offset Estimation Algorithms For Subcarrier Multiplexing Systems

With the arrival of 5G,cloud computing and the Internet of Things,network traffic continues to grow,promoting the development of coherent optical transmission technology towards higher spectral efficiency,larger capacity and longer distance.At present,Fiber nonlinearity is a major factor limiting long-distance transmission of large capacity optical transmission systems.However,Subcarrier Multiplexing(SCM)technology can improve the nonlinearity tolerance of optical fiber by optimizing the subcarrier symbol rate.And it can flexibly configure each subcarrier symbol rate and modulation format to achieve fine adjustment of system transmission rate.Different from single-carrier systems,offset estimation algorithms at the SCM systems need to be completed under multiplexed signals to ensure the integrity of subcarrier signals,and it needs to tolerate damage such as dispersion,polarization crosstalk and phase noise.Existing undata-assisted frequency offset estimation algorithms for SCM systems have some shortcomings such as low estimation accuracy and high computational complexity.This paper carries out the study of SCM frequency offset estimation algorithms,and the main work is as follows.1.Aiming at the high complexity of "spectrum dip search" frequency offset estimation algorithm in Digital Subcarrier Multiplexing(DSCM)systems,the fast Fourier transform is simplified by filling zeros to reduce the algorithm complexity under the same performance.The simulation results of 2(subcarrier number)x64GBaud(subcarrier symbol rate),4×32,8×16,16×8,32×4GBaud Polarization Multiplexing(PM)-M Quadrature Amplitude Modulation(MQAM)DSCM systems show that the improved algorithm can tolerate dispersion damage.With the same performance as the original algorithm,the complexity of the improved algorithm is reduced by 13%.4×l0GBaud PM-16QAM DSCM 8×80km transmission off-line experiment verifies the effectiveness of the improved algorithm.2.Aiming at the characteristics of the Analog Subcarrier Multiplexing(ASCM)systems that completes demultiplexing of subcarriers in the analog domain and each subcarrier performs frequency offset compensation separately,put forward a frequency offset compensation scheme based on singlet carrier frequency offset estimation and local vibration frequency adjustment of each subcarrier.The simulation results of 4×32 and 16×8GBaud PM-MQAM ASCM systems show that the proposed scheme can tolerate dispersion damage,and the complexity is reduced by about 60%compared with the improved algorithm of "spectrum dip search".3.Aiming at the low accuracy and high complexity of non-dataassisted frequency offset estimation algorithm in DSCM system,design a frequency offset estimation algorithm based on insertion training sequence on the multiplexed signal.The simulation results of 16×8GBaud PMMQAM DSCM system show that when the training cost is 0.03%,the accuracy of the proposed algorithm is equal to non-data-assisted frequency offset estimation algorithm,but the complexity is reduced by 36%.In addition,put forward a frequency offset estimation algorithm of DSCM system based on "direct current pilot",and the direct current pilot is generated by adjusting the direct current bias voltage in the transmitter Mahzander modulator.The simulation results of 16x8GBaud PM-MQAM DSCM system show that when the driving voltage linear interval is reduced by 5%,the best estimation accuracy of the proposed algorithm is improved by 5 times compared with non-data-assisted frequency offset estimation algorithm,and the complexity is reduced by 49%under the same performance.4×10GBaud PM-16QAM DSCM 8×80km transmission off-line experiment verifies the effectiveness of the proposed algorithm.