Study On Subcarrier Multiplexing And Other DSP Alogorithms For DSCM Systems

The rapid development of communication network promotes the further growth of optical network capacity.In large capacity and long distance coherent optical communication system,the nonlinear effect of optical fiber during transmission becomes one of the main factors affecting signal quality.Digital Subcarrier Multiplexing(DSCM)has higher dispersion and fiber nonlinear tolerance than singlecarrier systems with the same spectral efficiency.Traditional single-carrier receiving and sending DSP algorithm is not suitable for DSCM system.DSCM system needs subcarrier multiplexing and demultiplexing algorithms to realize its basic functions.The edge subcarrier of DSCM system is seriously affected by the low frequency response of electrical components,and even affects the performance of the whole DSCM system.The IQ imbalance at the origin will lead to crosstalk between subcarriers with respect to frequency 0 symmetry.In view of the above technical problems,the key DSP algorithm of DSCM coherent optical transmission system is studied in this paper.The main research contents are as follows.1.In order to overcome the adverse impact of insufficient frequency response of electrical components at the source on the performance of DSCM system,a scheme of frequency domain joint subcarrier multiplexing and frequency response preequalization suitable for DSCM system is designed,which can realize subcarrier filtering forming,frequency response preequalization and subcarrier multiplexing under the condition of low complexity.The analysis shows that for 32(subcarrier number)×4G Baud(subcarrier sign rate)PM-16QAM DSCM system,the complexity of the designed scheme is about 61%lower than that of the time-domain subcarrier multiplexing and pre-equalization scheme under the condition of the same system performance.The simulation results of 32×4,16×8GBaud PM-16QAM and 8×12,4×24GBaud PM64QAM DSCM systems show that the proposed scheme can effectively compensate the filtering damage caused by 60%Nyquist signal bandwidth of 3dB transmitter.Backto-back 4×10GBaud PM-16QAM DSCM off-line experiments verify the effectiveness of the proposed scheme.2.Aiming at the high complexity of dispersion compensation in DSCM system under multiplexed signals,a frequence-domain joint subcarrier demultiplexing and baseband subcarrier dispersion compensation scheme is designed.Compared with the dispersion compensation in the multiplexed signal,the dispersion compensation scheme on the subcarrier signal can effectively reduce the algorithm complexity.The 16×8GBaud PM-16QAM and 4×24GBaud PM-64QAM DSCM system simulation and 4×10GBaud PM-16QAM DSCM multi-span transmission off-line experiments verify the effectiveness of the proposed scheme.3.In order to solve the problem that the IQ imbalance at the originating end of DSCM system will cause crosspoilers between frequency symmetric subcarriers,which will lead to system performance degradation,a calibration scheme of IQ imbalance at the originating end based on 4×2MIMO equalizer is proposed.This scheme can not only calibrate and compensate the IQ imbalance at the originating end,but also balance the insufficient frequency response of transmitter.Simulation results of 4×32GBaud PM-16QAM and 4×24GBaud PM-64QAM DSCM systems show that the proposed calibration scheme can effectively calibrate 5dB IQ amplitude unbalance,25°IQ phase unbalance and 0.5 sign period IQ skew.