High Spectral Efficiency 400 Gb/s Transmission Using PDM Time-Domain Hybrid 32–64 QAM and Training-Assisted Carrier Recovery

We report the successful transmission of ten 494.85 Gbit/s DWDM signals on the standard 50 GHz ITU-T grid over 32 100 km of ultra-large-area (ULA) fiber. A net spectral efficiency (SE) of 8.25 b/s/Hz was achieved, after excluding the 20% soft-decision forward-error-correction (FEC) overhead. Such a result was accomplished by the use of a recently proposed polarization-division-multiplexed (PDM) time-domain hybrid 32-64 quadrature-amplitude-modulation (QAM) format, along with improved carrier frequency and phase recovery algorithms. It is shown that time-domain hybrid QAM provides a new degree of design freedom to optimize the transmission performance by fine tuning the SE of the modulation format for a specific channel bandwidth and FEC redundancy requirement. In terms of carrier recovery, we demonstrate that 1) hardware efficient estimation and tracking of the frequency offset between the signal and local-oscillator (LO) can be achieved by using a new feedback-based method, and 2) a training-assisted two-stage phase estimation algorithm effectively mitigates cyclic phase slipping problems. This new phase recovery algorithm not only improves the receiver sensitivity by eliminating the need for differential coding and decoding, but also enables an additional equalization stage following the phase recovery. We have shown that the introduction of this additional equalization stage (with larger number of taps) helps reduce the implementation penalty. This paper also presents the first experimental study of the impact of inphase (I) and quadrature (Q) correlation for a high-order QAM. It is shown that an adaptive equalizer could exploit the correlation between I and Q signal components to artificially boost the performance by up to 0.7 dB for a PDM time-domain hybrid 32-64 QAM signal when the equalizer length is significantly longer than I/Q de-correlation delay.