Enhanced resilience towards ROADM-induced optical filtering using subcarrier multiplexing and optimized bit and power...

In flexible optical networks, lightpaths are routed from source to destination thanks to reconfigurable optical add-drop multiplexers (ROADMs) placed in every node connected by a line system. As ROADMs are based on wavelength selective switches (WSSs), which imply channel filtering, a non negligible performance reduction can be observed when lightpaths traverse many network nodes. Control plane software network controllers that base their decision on the quality of transmission of the physical layer must also take into account the impact of ROADMs cascading. The penalties caused by WSS cascading in single-carrier systems are mostly driven by uncompensated inter-symbol interference (ISI), and therefore their evaluation cannot be based on simple analytical solutions as required for real-time network control. In this paper, we consider a 200G optical system based on multi-subcarrier (MSC) modulation with hybrid modulation formats. Using a fully analytical framework, we show that it is possible to predict ROADM-induced penalties offline and accurately estimate the required settings for their transmitter-side mitigation through bit and/or power-loading. Moreover, resorting to both simulations and experiments, we also demonstrate that properly optimized MSC signals can provide an increased resilience to ROADM filtering with respect to standard single-carrier systems: applying analytically-driven bit-loading over 8×4 Gbaud and 16×2 Gbaud MSC signals after crossing 8 cascaded WSSs in a noise loading scenario, we experimentally found ∼2 dB gain in required SNR with respect to a traditional 32 Gbaud single-carrier signal.