Nonlinearity Mitigation in WDM Systems: Models, Strategies, and Achievable Rates

TL;DR

This work addresses the capacity limits of WDM optical-fiber channels by adopting the FRLP model, which treats interchannel nonlinear interference as phase and polarization noise with time-frequency coherence. It develops a mitigation framework that combines phase/polarization noise compensation with subcarrier multiplexing and symbol-rate optimization, evaluated through a particle-based AIR/SE computation against three auxiliary channels (AWGN, PN, PPN). Key contributions include a practical auxiliary-channel methodology for lower-bounding information rates, demonstration that SCM+PPN can yield substantial spectral-efficiency gains (up to ~1 bit/s/Hz/pol in ideal distributed amplification) and that gains depend on link type and dispersion management. The results challenge AWGN-based capacity limits and provide design guidance for high-SE WDM systems employing nonlinear mitigation, while pointing to future refinements in PPN modeling and constellation shaping to further boost performance.

Abstract

After reviewing models and mitigation strategies for interchannel nonlinear interference (NLI), we focus on the frequency-resolved logarithmic perturbation model to study the coherence properties of NLI. Based on this study, we devise an NLI mitigation strategy which exploits the synergic effect of phase and polarization noise compensation (PPN) and subcarrier multiplexing with symbol-rate optimization. This synergy persists even for high-order modulation alphabets and Gaussian symbols. A particle method for the computation of the resulting achievable information rate and spectral efficiency (SE) is presented and employed to lower-bound the channel capacity. The dependence of the SE on the link length, amplifier spacing, and presence or absence of inline dispersion compensation is studied. Single-polarization and dual-polarization scenarios with either independent or joint processing of the two polarizations are considered. Numerical results show that, in links with ideal distributed amplification, an SE gain of about 1 bit/s/Hz/polarization can be obtained (or, in alternative, the system reach can be doubled at a given SE) with respect to single-carrier systems without PPN mitigation. The gain is lower with lumped amplification, increases with the number of spans, decreases with the span length, and is further reduced by in-line dispersion compensation. For instance, considering a dispersion-unmanaged link with lumped amplification and an amplifier spacing of 60 km, the SE after 80 spans can be be increased from 4.5 to 4.8 bit/s/Hz/polarization, or the reach raised up to 100 spans (+25%) for a fixed SE.

Figures (6)

• Figure 1: Equivalent lowpass system model and corresponding WDM spectrum.
• Figure 2: Continuous-time FRLP model.
• Figure 3: XPM coherence: (a) IDA link; (b) 10x100km LA-DU link.
• Figure 4: 1000 km IDA link with 2-pol transmission and PPN detection: (a) SE vs. launch power for different number of subcarriers; (b) phase (and its autocorrelation in the inset) and (c) polarization rotations induced by fiber nonlinearity at $\unit[-7]{dBm}$ and $N=4;$ (d) estimated parameters of the PPN auxiliary channel vs. $N$ at $\unit[-7]{dBm}$.
• Figure 5: Dependence of the achievable SE on: (a) distance, for an IDA link; (b) amplifier spacing for a 1000 km LA link.