Spectral Efficiency Optimization in Flexi-Grid Long-Haul Optical Systems

TL;DR

This paper demonstrates that limited-complexity trellis processing at the receiver can significantly enhance spectral efficiency or extend reach in flexi-grid long-haul optical systems, overcoming ISI challenges.

Contribution

It introduces a novel trellis processing method that improves spectral efficiency or reach in flexi-grid optical networks with narrow filters.

Findings

Trellis processing extends system reach under Nyquist spacing.

It enables denser channel packing beyond Nyquist limits.

Significant spectral efficiency gains are quantified.

Abstract

Flexible grid optical networks allow a better exploitation of fiber capacity, by enabling a denser frequency allocation. A tighter channel spacing, however, requires narrower filters, which increase linear intersymbol interference (ISI), and may dramatically reduce system reach. Commercial coherent receivers are based on symbol by symbol detectors, which are quite sensitive to ISI. In this context, Nyquist spacing is considered as the ultimate limit to wavelength-division multiplexing (WDM) packing. In this paper, we show that by introducing a limited-complexity trellis processing at the receiver, either the reach of Nyquist WDM flexi-grid networks can be significantly extended, or a denser-than-Nyquist channel packing (i.e., a higher spectral efficiency (SE)) is possible at equal reach. By adopting well-known information-theoretic techniques, we design a limited-complexity trellis processing and quantify its SE gain in flexi-grid architectures where wavelength selective switches over a frequency grid of 12.5GHz are employed.