CAN MULTILEVEL SIGNALING IMPROVE THE SPECTRAL EFFICIENCY OF ASK OPTICAL FDM SYSTEMS

With the tremendous advances in lightwave technology in recent years, the question arises whether multilevel signaling, which has greatly in creased the capabilities of voice-band and point-to-point radio system s, can be used to enhance the capabilities of optical communication sy stems. In particular, can multilevel signaling improve the spectral ef ficiency of optical FDM systems? In our study, we assumed multilevel a mplitude-shift-keying and a receiver consisting of an optical channel selection filter (specifically, a single-stage Fabry-Perot), a pin dio de for direct detection, and an integrator. We also assumed that addit ive noise is negligible and that the major impairments are adjacent ch annel interference and intersymbol interference. Our analysis shows th at, for such a receiver, there is no advantage to going beyond binary signaling. We also show that this finding holds up when we consider ad ditive and phase noises and alternative approaches to filtering.