Research On DSP Algorithms For Signal Damage Comsenpations In Flexible Coherent Optical Transmission Systems

With the emergence of emerging network services such as big data,cloud computing,smart medical care,VR/AR,high-definition video,and online education,the huge data traffic and various traffic patterns pose huge challenge to the capacity and flexibility of the underlying optical transmission system.The original semi-static continuous transmission method based on a single modulation format and fixed connection configuration is very difficult to meet what the communication needs of emerging services with strong dynamic burstiness and high rate/bandwidth.High-efficiency compensation and support for modulation and demodulation technology that adapts to dynamic burst transmission.Correspondingly,dynamic burst coherent reception,probabilistic shaping(PS)with flexible spectral efficiency/performance,and high spectral efficiency faster-than-Nyquist(FTN)large-capacity optical transmission technologies have become flexible supporting technology for coherent optical transmission systems.However,the digital signal processing(DSP)algorithm,which is a system enabling technology,still faces technical problems such as poor damage compensation effect for fast-changing burst transmission,difficulty in adapting to flexible adjustment of spectral efficiency,and conflicting algorithm complexity/overhead and performance.There is an urgent need to research and design flexible,efficient,low-complexity,and low-overhead DSP algorithms suitable for burst coherent reception,flexible PS,and highly spectrally efficient FTN optical transmission.Focusing on the above problems,this paper conducts in-depth research on DSP algorithms in flexible and dynamic coherent optical transmission systems.The main research contents and innovative research results are as follows.1.Aiming at the transient effect caused by wavelength channel switching in optical burst coherent optical transmission,a DSP scheme for optical burst coherent reception based on training sequence is proposed in the paper.This scheme effectively deals with the difficult problems that optical burst data is aliased after chromatic dispersion(CD)compensation using the overlapping frequency domain algorithm when transient frequency offset and CD damage coexist,which makes it difficult for subsequent DSP algorithms to complete damage compensation.In the scheme,the training sequence is used for interpolation fitting to complete the transient frequency offset and power estimation.Besides,the polarization demultiplexing and adaptive equalization on least mean square algorithm and the phase offset estimation are used for joint processing.The simulation and offline experimental results of 128 Gbps polarization multiplexing quadrature phase shift keying system for optical back-to-back and 500 km transmission show that,the frequency offset and power during the transient effect can be accurately estimated and compensated by the proposed DSP scheme,and the system can tolerate the large linewidth of the laser in the order of MHz,so that the data can actively handle the transmission link during the transient effect of wavelength channel switching.Compared with the semi-blind DSP scheme based on constant modulus algorithm,the burst packet overhead of the proposed scheme is reduced by 73%.2.In view of the failure of the traditional frequency offset estimation algorithm of PS system and the high complexity of the existing carrier frequency offset estimation algorithm suitable for PS system,a lowcomplexity two-stage frequency offset estimation algorithm is proposed suitable for probability shaping high-order quadrature amplitude modulation(PS-QAM)coherent optical transmission systems.The first stage of the algorithm is based on the multi-symbol interval phase difference in polar coordinates for coarse frequency offset estimation.In this stage,the polar coordinate transformation is used to extract the signal amplitude and phase to participate in the frequency offset estimation operation,which significantly reduces the computational complexity.The second stage completes high-precision frequency offset estimation based on local constellation point modulus amplification combined with maximum spectral line search after sparse operation.The modulus amplification operation introduced in this stage ensures the stability of the algorithm under large shaping factors.Meanwhile,the sparse operation effectively reduces the computational complexity of the fast Fourier transform(FFT)process in the spectral line search.The simulation results of 128GBaud PM-PS-16QAM and 108GBaud PM-PS-16QAM and 96GBaud PM-PS-64QAM systems show that the performance of the proposed algorithm is comparable to that of the traditional radius-assisted quadratic FFT algorithm for frequency offset estimation,and the computational complexity is reduced by about 71%.At the same time,the offline experimental results of 40 GBaud PM-PS-16QAM and 32 GBaud PM-PS-32QAM systems further verify the above conclusions.3.Aiming at the problem that the strong inter symbols interference in the FTN optical transmission system seriously degrades the performance of DSP algorithms such as polarization demultiplexing and carrier recovery,a pilot-based FTN system with high reliability,low pilot overhead and low complexity DSP scheme is proposed.In the scheme,the pilot frequency does not occupy spectrum resources,and is directly inserted into the highfrequency roll-off of the signal spectrum.This scheme uses the pilot frequency estimation channel Jones matrix to realize polarization demultiplexing,and combines the pilot frequency estimation with the maximum likelihood algorithm.Not only the performance of polarization demultiplexing is stable and reliable,but also the computational complexity in a position to be significantly reduced compared with the blind phase estimation algorithm.The simulation results of the threecarrier 128 GBaud FTN PM-16QAM system show that the acceleration factor is 0.95/0.9/0.85,and the maximum linewidth that can be tolerated at the 1dB optical signal to noise ratio(OSNR)penalty is 1.9/2.5/3.2 MHz,respectively.In the simulation of transmitting 800 km and passing through 10 reconfigurable optical add/drop multiplexers,the acceleration factor of 0.9 is 26.1 dB in the OSNR tolerance of the 2E-2 BER system.Compared with the full-blind scheme that does not occupy spectrum over-head,the OSNR tolerance is improved by～1 dB,and compared with the OSNR tolerance of the pilot scheme that occupies spectrum overhead.