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

The spectral efficiency of optical FDM (frequency division multiplexing) systems when multilevel ASK signaling is used is determined. A direct detection receiver consisting of an optical channel selection filter (specifically, a single-stage Fabry-Perot), a pin diode, and an integrator is assumed. It is also assumed that additive noise is negligible and that the major impairments are the adjacent channel interference and intersymbol interference. The analysis shows that, for such a receiver structure, there is no advantage to going beyond binary signaling. Taking account of additive noise and phase noise does nothing to alter this conclusion; neither does trying to optimize the signal level spacings; and neither do any realistic alternative approaches to transmitter, predetector or postdetector filtering