Can multilevel signaling improve the spectral efficiency of ASK optical FDM systems

TLDR: The question of whether multilevel signaling can improve the spectral efficiency of optical frequency division multiplexing (FDM) systems is investigated and it is shown that there is no advantage to going beyond binary signaling.

Abstract: The question of whether multilevel signaling can improve the spectral efficiency of optical frequency division multiplexing (FDM) systems is investigated. Multilevel amplitude-shift keying (ASK) and a receiver consisting of an optical channel selection filter (specifically, a single-stage Fabry-Perot), a pin diode for direct detection, and an integrator are assumed. It is also assumed that additive noise is negligible and that the major impairments are adjacent channel interference and intersymbol interference. The analysis shows that, for such a receiver, there is no advantage to going beyond binary signaling. It is also shown that this finding holds up when additive and phase noises and alternative approaches to filtering are considered. >