We investigated the reach increase obtained through non-linearity mitigation by means of transmission symbol rate optimization (SRO). First, we did this theoretically and simulatively. We showed that the non-linearity model that properly accounts for the phenomenon is the EGN model, in its version that specifically includes four-wave mixing. We then found that for PM-QPSK systems at full-C-band the reach increase may be substantial, on the order of 10%-25%, with optimum symbol rates on the order of 2-to-6 GBaud. We extended the investigation to PM-16QAM, where we found a qualitatively similar effect, although the potential reach increase is typically only about 50% to 60% of that of PM-QPSK. We show that, for C-band PM-QPSK systems over SMF, the potential mitigation due to SRO is greater than that ideally granted by digital back-propagation (the latter applied over a bandwidth of a 32-GBaud channel).We then set up an experiment to obtain confirmation of the theoretical and simulative predictions. It consisted of 19 PM-QPSK channels, operating at 128 Gbit/s per channel, over PSCF, with span length 108 km and EDFA-only amplification. We demonstrated a reach increase of about 13.5%, when going from single-carrier per channel transmission, at 32 GBaud, to 8subcarrier per channel, at 4 GBaud, in line with the EGN model predictions.

We report a comprehensive set of experimental, simulation and analytical results on the benefit of nonlinear mitigation strategies for multi-subcarrier (MSC) PM-16QAM transmission systems. First, we demonstrate ∼9% maximum reach gain enabled by symbol-rate optimization (SRO) of MSC-PM-16QAM in a 31 channels WDM transmission experiment. Then, we demonstrate that, in the considered experimental scenario, the gain provided by digital backpropagation (DBP) over single-carrier (SC) transmission is similar to that achieved by SRO over MSC transmission. Furthermore, we show that the SRO phenomenon can be weakened after self-channel interference (SCI) removal through DBP. As a result, and due to DBP performance limitations in the experiment, the combined effect of SRO and DBP was found to enable only an additional 4% gain in maximum reach. Finally, we address the impact and symbol-rate dependence of nonlinear phase noise (NLPN) in MSC-PM-16QAM transmission, discussing on the NLPN mitigation capability of standard carrier phase estimation (CPE) and on respective gains that could be achieved through its enhanced mitigation.

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Employing step-size randomization and multi-band quantization, we propose a reduced complexity time-domain (TD) digital backpropagation (DBP) and experimentally demonstrate penalty-free operation at an average number of ∼ 4 bits per FIR coefficient.

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