Nonlinear Mitigation of a 400G Frequency-Hybrid Superchannel for the 62.5-GHz Slot

We propose a 400G frequency-hybrid superchannel solution based on three carriers, two edge PM-16QAM, and a central PM-64QAM carrier, compatible with the 62.5-GHz grid slot (spectral efficiency of 6.4 b/s/Hz). The proposed superchannel is experimentally assessed in long-haul transmission by copropagation with other eight similar superchannels. The optimum power ratio between superchannel carriers is analytically determined in linear and nonlinear operation regimes using the enhanced Gaussian noise model and validated by experimental and simulation results. The 400G superchannel performance is evaluated in terms of maximum reach determining the optimum launch power and considering three distinct forward-error correction (FEC) paradigms: superchannel FEC (SC-FEC) where a single FEC is applied to the entire superchannel, independent carrier FEC (IC-FEC) where an independent FEC with fixed overhead is applied to each superchannel carrier, and independent carrier flexible FEC (Flex-FEC) where optimized FEC overheads are applied independently to each superchannel carrier with the constraint of a given total overhead. When compared to the IC-FEC approach, the SC-FEC or Flex-FEC approaches enable to extend the maximum transmission distance by more than 60%, while reducing the optimum power ratio by $\sim$ 6 dB, at the cost of 2 dB higher launched power. The system performance is also analyzed for the case of nonlinear compensation via digital backpropagation (DBP) techniques, assessing the improvement in reach and evaluating their impact on the optimum power ratio and launch power. For the proposed frequency-hybrid superchannel, we demonstrate that the application of DBP can be restricted to the carrier with higher QAM cardinality, thereby significantly reducing the overall computational effort, with a maximum reach reduction of only $\sim$2% over the application of DBP to all three carriers individually.