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Abstract-This paper describes the problematic of filter narrowing effect in the context of next generation elastic optical networks. First, three possible scenarios are introduced: the transition from actual fixed-grid to a flexi-grid network; the generic full flexi-grid network; and a proposal for filterless optical network. Next, we investigate different transmission techniques and evaluate the penalty introduced by the filtering effect when considering: Nyquist WDM, SSB DD-OFDM and symbol-rate variable DP-4QAM. Also, different approaches to compensate for the filter narrowing effect are discussed. Results show that the specific needs per each scenario can be fulfilled by the aforementioned technologies and techniques, or a combination of them, when balancing performance, network reach and cost.Index Terms-Networks, optical communications, elastic optical networks, flexi-grid, WSS. I. INTRODUCTIONThe future adoption of elastic optical network (EON), mainly fostered by the advent of next technologies (e.g., media, HDTV, 5G, Internet of Things, etc.) and backed by the considerable advances of transmission techniques in terms of flexibility and capacity, is heading to undertake new challenges and goals. In fact, when adopting the flexi-grid paradigm [1], optical channels with different bandwidth occupation can coexist within the same fiber. Some of these channels, denominated as super-channels, are wider in frequency and comprise multiple sub-channels transmitted

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