Hybrid modulation formats enabling elastic fixed-grid optical networks

In this paper, we analyze hybrid modulation formats as an effective technology for the implementation of flexible transponders that are capable of trading off the delivered data rate by the lightpath quality of transmission with fine granularity. Flexible transponders are an enabling technology that can introduce the elastic paradigm in state-of-the-art networks while maintaining compatibility with the installed equipment, including fibers, mux-demux, and reconfigurable optical add-drop multiplexers, as required by telecom operators willing to exploit fixed-grid wavelength-division multiplexed (WDM) transmission. We consider two solutions achieving different levels of flexibility and employing different hybridization approaches: time-division (TDHMF) and quadrature-division (Flex-PAM) hybrid modulation formats. We introduce a comprehensive theoretical assessment of back-to-back performances, analyzing different transmitter operating conditions, and we provide an extensive simulation analysis on the propagation of a Nyquist-WDM channel comb over an uncompensated and amplified fiber link. After assessing the impact of nonlinear propagation on the maximum signal reach, we present simple countermeasures for non-linear mitigation and discuss their effectiveness for both TDHMF and Flex-PAM.