C-band 120-Gb/s PAM-4 transmission over 50-km SSMF with improved weighted decision-feedback equalizer

An improved weighted decision-feedback equalizer (IWDFE) is adopted and experimentally demonstrated to mitigate chromatic dispersion (CD) induced power fading for C-band four-level pulse amplitude modulation (PAM-4) system. Based on a compressed sigmoid nonlinear function, the IWDFE is superior to the conventional DFE, rule-1 and rule-2 based WDFEs in reducing both the error propagation probability and percentage of errors. The IWDFE is combined with a feed-forward equalizer (FFE) to deal with both pre-cursor and post-cursor interference simultaneously. We also introduce 2nd-order polynomial nonlinear terms in both FFE and IWDFE at receiver side, constructing a nonlinear FFE-IWDFE (N-FFE-IWDFE) to simultaneously equalize both the CD-induced power fading and nonlinear distortions. Experimental results show that compared with nonlinear FFE-DFE (N-FFE-DFE), rule-1 and rule-2 based nonlinear FFE-WDFEs (N-FFE-WDFEs), the N-FFE-IWDFE can significantly reduce the bit error ratio (BER), which is close to that of the error-propagation-free N-FFE-DFE. By utilizing the N-FFE-IWDFE, 120-Gb/s PAM-4 transmission system over 50-km standard single-mode fiber (SSMF) is realized with the BER below 7% hard-decision forward error correction (HD-FEC) limit of 3.8 × 10−3, achieving around 1.1-dB improvement of the receiver sensitivity over the N-FFE-DFE. To the best of our knowledge, this is the first time that an IWDFE is employed for C-band IM/DD optical transmission system.