D) system, the main noise term contributing to the electrical cur- rent fluctuations is the beat term between the incoming signal and the ASE noise. In an ideal coherent detection, the current is the frequency downshifted replica both in amplitude and phase of the incoming optical field: as a matter of fact, an ideal coherent detector has to track the variations of the incoming polarization to always optimize the signal detection. Since no beat term is present, the main electrical noise term is proportional, in module and phase, to the ASE noise. In this situation, the detection problem is practically moved in the electrical domain, where Shannon sampling can be used to reduce the noise bandwidth from 2R to R thus gaining 3 dB. This is almost what is needed to achieve the performances of a 40 Gbit/s transmission, at least from a noise point of view. Next Generation Transmission Systems Even if an ideal homodyne coherent detection would need an optical phase-lock loop (PLL), a very complex circuit even for a laboratory demonstration, we will see in Section 9.4 that it is possible to avoid the use of such a complex optical system, at the cost of more com- plex electronics to be used at the receiver. 9.2.2 Fiber Chromatic Dispersion 9.2.2.1 Impact of Chromatic Dispersion on 100 Gbit/s Transmission Chromatic dispersion is SO a bad problem even at 10 Gbit/s that uncompensated systems are dispersion limited at about 36km. However, due to the deterministic nature of this phenomenon, a carefully designed compensation map can almost completely cancel the problem at 10 Gbit/s, but for the limitation posed to the span length from the interplay between dispersion and nonlinear effects. At 100 Gbit/s the situation is completely different. As a matter of fact, not only the second order dispersion has a big impact on transmission, but also the third order dispersion (the so-called dispersion slope) cannot be neglected. The effect of the dispersion slope is evident from Figures 9.1 and 9.2. The result of the simulation of a perf