640-Gbps/Carrier WDM Transmission over 6,400 km Based on PS-16QAM at 106 Gbaud Employing Advanced DSP

We experimentally demonstrated a 5-channel 125-GHz-grid wavelength division multiplexing (WDM) transmission over 6,400-km Raman amplified ultra large effective area fiber (ULAF) with 848-Gbps/carrier line-rate and 640-Gbps/carrier net-bit-rate. Our transmission is based on probabilistically shaped (PS) dual-polarization 16-ary quadrature-amplitude-modulation (16QAM) considering 25%-overhead low-density parity-check threshold of 4.6 × 10
^-2
bit-error-rate. By utilizing a high-speed digital-analog-converter (DAC) with a 3-dB bandwidth of 35 GHz and innovative pre-equalization based on 4-level pulse-amplitude-modulation (PAM4), 106-Gbaud 16QAM signal is reliably generated and transmitted. Besides, the transmission distance is improved obviously by using ULAF and Raman amplifier. Our results show that the PS-16QAM outperforms the regular-16QAM by around 1.5-dB sensitivity gain and 33% transmission-distance improvement. Furthermore, we have successfully applied Volterra nonlinear equalization (VNLE) to ultra-long-haul coherent optical transmission and proposed a modified offline digital-signal-processing process. By utilizing VNLE, another 15% reach improvement is obtained for PS-16QAM signal. Our work can provide an effective solution for a 600G+ ultra-long-haul transmission. As we know, this is the first time to achieve 640-Gbps/carrier WDM transmission over 6,400 km based on 106-Gbaud PS-16QAM.