Overcoming filtering penalties in flexi-grid long-haul optical systems

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 in the receiver, either the reach of Nyquist WDM flexi-grid networks can be significantly extended, or a higher spectral efficiency (SE) is possible at equal reach. By adopting 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 with 12.5GHz granularity are employed.