Research On DSP Algorithms Such As ISI Suppressions Of Faster-than-nyouist WDM Coherent Optical Transmissions Systems

In recent years,Faster-than-Nyquist Wavelength Division Multiplexing(FTN-WDM),a coherent optical transmission technology,has been used to significantly increase system capacity with limited optical fiber bandwidth resources due to its high spectral efficiency.However,due to the severe inter-symbol interference(ISI)introduced by FTN strict filtering,the system is more sensitive to various types of impairments.The reported digital signal processing(DSP)algorithms face difficulties in balancing computational complexity and effectiveness,degradation of the algorithm’s ability to equalize/compensate for impairments,and difficulties in suppressing inter-channel interference(ICI),which seriously hinder the development of future high-spectral-efficiency FTN-WDM systems.1.A low-complexity adaptive ISI suppression algorithm is proposed to address the problem that the effectiveness and complexity of conventional ISI suppression algorithms are difficult to combine in FTN-WDM systems.The proposed algorithm can adaptively adjust the PF tap coefficients and reduce the computational complexity by 90.4%compared with the conventional "2-tap Post-Filter(PF)+Maximum Likelihood Sequence Detection(MLSD)algorithm" scheme.Back-to-back(BTB)20GBaud FTN-16QAM offline experimental results verify that the proposed algorithm achieves a 0.6dB Optical Signal Noise Ratio(OSNR)tolerance improvement over the conventional "2-tap PF+MLSD" scheme at acceleration factors as low as 0.85.2.A two-stage receiver-side(Rx)IQ imbalance algorithm is designed to address the problem of severe ISI introduced by FTN strict filtering leading to the deterioration of the equalization capability of the conventional Rx IQ imbalance compensation algorithm.The algorithm uses a time domain equalizer based on the Decision Directed-Least Mean Square algorithm cascaded with a 4×2 frequency domain(FD)equalizer based on the Radius Directed Equalization(RDE)algorithm,which can effectively improve the compensation range compared to using only a 4×2 FD equalizer based on the RDE algorithm.128Gbaud-FTN-PM-16QAM system simulation results show that with an acceleration factor as low as 0.85 and a transmission of 800km,the designed algorithm has a 14.3%,100%and 40%improvement in the compensation range of Rx IQ amplitude,phase and skew imbalance compared to using only the 4×2 FD equalizer based on the RDE algorithm.In addition,BTB 40GBaud FTN-PM-16QAM offline experiment results also validate the effectiveness of the designed algorithm.3.For the problem of difficult ICI suppression in FTN-WDM systems where the channel spacing is less than the symbol rate,an ICI suppression algorithm based on Quadrature duobinary filtering at the receiver is designed.Simulation results of the three-channel 128Gbaud-PM-FTN-16QAM system show that the BER of the system is reduced by 66%when the channel spacing is 114GHz compared to using the conventional ICI suppression algorithm,and when the channel spacing is 112 GHz,the coherent receiver can demodulate correctly with a BER of 3.8E-2 compared to the conventional ICI suppression algorithm,which cannot be demodulated correctly with the designed algorithm.