t one of the main problems in NRZ serial trans- mission of 100 Gbit/s signals is the huge optical spectral width. The NRZ 100 Gbit/s spectrum is 200 GHz wide (unilateral bandwidth 100 GHz), causing a great amount of ASE noise to enter the receiver and a great sensitivity both to chromatic dispersion slope and to PMD. Moreover, nonlinear Kerr effect depends also on the signal bandwidth, worsening the problem of transmitting NRZ 100 Gbit/s signals. Several alternative modulation formats can be used to reduce the 100 Gbit/s spectral width, such as single side band modulation or duobinary modulation (see description and References in Chapter 6), but the most effective and flexible way to reduce signal band- width is multilevel modulation [28,29]. In general, multilevel modulation consists in coding the binary signal to be transmitted in a signal that consists of a series of symbols extracted by an alphabet A of M symbols SO that the two signals are equivalent under an information point of view. This means that an invertible code must exist that establishes a correspondence between the input bit stream and the stream of elements from the alphabet A. In this condition, if the stream of transmitted symbols is correctly received, the original bit stream can be univocally reconstructed. If this is true, it is also possible to evaluate, starting from the correlation function of the bit stream, the resulting correlation function of the symbol stream to be transmit- ted [30]. In the case of 100 Gbit/s application, all electronic coding and decoding operations have to be carried out by very high speed electronics; thus, it is realistic to restrict the analysis of multilevel modulation to a particular case of modulation formats, allowing simple coding/ decoding and modulation/demodulation operations to be performed. Telecommunication Networks This is the class of the so-called "instantaneous" modulation formats, characterized by the property that, if the incoming bits are statistically independent, also the symbol stream deriv