
dispersion-compensating fiber modules and optical amplifiers are shared by all
channels simultaneously. As WDM channels are usually transmitted on a fixed
frequency grid, adding more and more channels at low speeds has the disadvantage
that the spectral efficiency, measured in bit/s/Hz, is limiting the overall transmission
capacity. For example, 10-Gbit/s WDM transmission systems reach a spectral effi-
ciency of 0.1–0.2 bit/s/Hz, depending on the used channel grid, e.g., 100 or 50 GHz.
The use of higher channel data rates, achieved by applying ETDM technique,
offers a way to upgrade optical connections with high spectral efficiency, reduced
efforts ...