Joint chromatic dispersion, differential group delay compensation, and timing recovery scheme for Nyquist optical transmission systems

The complexity of chromatic dispersion (CD) and differential group delay (DGD) estimation and compensation algorithms is relatively high. Furthermore, residual CD and DGD impairments significantly increase the convergence cost required for timing recovery algorithms to establish synchronization. We propose a joint CD/DGD compensation and timing recovery scheme. The proposed scheme aims to estimate and compensate for the impairments of CD and DGD by periodically extracting the clock tone in the timing synchronization loop, thereby effectively reducing computational complexity. Meanwhile, thanks to the innovative scheme, which compensates for CD and DGD impairments during the feedback process of the loop, the convergence cost of timing recovery could be reduced compared with the conventional scheme. Theoretical analysis demonstrates that the proposed scheme’s computational complexity is only 2.2% of that required by conventional fractional Fourier transform–based time-domain reconstruction combined with multitask deep neural networks. The simulation results of 128 GBaud polarization multiplexing (PM) 16-quadrature amplitude modulation (QAM) Nyquist-wavelength-division multiplexing​​ transmission systems demonstrate that the proposed scheme could stably achieve CD estimation in the range of 20,480 ps/nm and DGD estimation in the range of 8 ps. Moreover, the estimation errors of CD and DGD are less than 60 ps/nm and 0.4 ps, respectively. In addition, compared with the conventional scheme, the proposed scheme can reduce the convergence cost by at least 38%. In addition, the three-carrier 40 GBaud PM-16-QAM Nyquist experimental results show that the proposed scheme can not only achieve CD estimation with errors below 45 ps/nm over 10,240 ps/nm but also attain DGD estimation with errors under 0.42 ps over 25 ps.