fiber loss is lowest. The higher losses in the L-band can be detrimental in very long systems. Moreover, it is normally cheaper to employ 80 channels at 50-GHz spacing within the gain bandwidth of one C-band EDFA than to use a C-band EDFA for one group of 40 channels and an L-band device for another. However, this argument must be stated with caution because the modulators for alternative modulation formats are more compli- cated and the lasers and filters must operate to more exacting standards when the channel spacing is low, all of which increase costs. There has been much research on multilevel coding, in which the objective has been to define more than two logic levels and therefore achieve narrower spectral bandwidths. Figure 6.29 shows that, if we take the difference between the first two Os as our definition, the bandwidth of an NRZ channel is 2fAT. However, it follows from the Fourier transform that the value can be reduced to fcAT if we use four logic levels. Although we can achieve greater channel spectral densities in WDM systems in this way, the approach requires more complex electronics at both the transmitter and the receiver. Furthermore, the required channel powers are high, which can worsen nonlinear crosstalk. A potentially useful compromise is a format called "duobinary transmission," in which there are three logic levels and a band- width of fcAT. In this way, it is possible to extend transmission distances at 10 Gbit/s beyond the values shown in Figure 6.19 (Gu et al., 1994; Walklin & Conradi, 1999). Another approach to reducing the bandwidth is to recognize that the spectra shown in Figure 6.29 have two symmetrical sets of sidebands and therefore carry twice the informa- tion necessary. It is possible to use very sharp filtering to eliminate one of the sides of the spectra and therefore reduce the overall bandwidth. The technique is called "Single- Sideband" (SSB) modulation (Kobayashi et al, 2000). When the suppression is partial, we refer to "Vestigial-Sideband" (VSB) modulation (Bigo et 