h speed long haul optical systems [67,69]. Naturally, OFDM also presents critical aspects, which have to be faced to design effec- tive systems. Among the technical criticalities of OFDM, there are the sensitivity to phase noise [69] and to unbalance between the quadratures at the receiver, for example, caused by differ- ences between receiver hybrids [66]. As a matter of fact, it is easy to imagine that if the different carriers of an OFDM sig- nal are constructed selecting amplitude and phase SO that nearby carriers results to be orthogonal, any phenomenon introducing an uncontrolled phase element can alter this balance causing strong interference between nearby channels at the receiver, when they are demultiplexed in the frequency domain. Next Generation Transmission Systems Since SPM introduces a nonlinear form of phase noise, the SPM sensitivity is another critical aspect of OFDM modulation. The fact that the OFDM system has to be designed with a great care for the linearity both of transmitter and receiver and of fiber propagation is also due to the characteristics of the OFDM signal to have a great ratio between peaks and average value of the amplitude. These big fluctuations immediately create distortion if nonlinearity is encountered [70]. Distortion of this kind can be classified as Out Of Band Power and In Band Distortion. Out of band power not only creates a power penalty due to the loss of a part of the avail- able power, but also could provoke interference if this power is reflected again in the signal bandwidth during the nonlinear propagation through the system [66]. Band distortion creates interference between the carriers. Another potential critic aspect of OFDM is the cost of the transmitter and the receiver compared with a traditional transmitter or receiver of an IM-DD system. However, this element tends to disappear more and more while traditional WDM sys- tems are forced to use complex maximum likelihood sequence estimation (MLSE) or back propagation systems to compensate for ISI. As a matt