ts' substitutions and maintenance in general. 6.3.3.4 Ultra-Long-Haul Systems with Electronic Pre-Compensation The case studies we have carried out in the previous sections are intended to show the key points of long-haul and ultra-long-haul systems design. In all the considered design variations, the compensation of the transmission line is car- ried out by optical means, but for the electronic compensation at the receiver. A radically different way to solve the problem is to design a system leveraging on elec- tronic pre-distortion via a combination of FFE-DFE and look-up table in order to deploy along the line only single-stage EDFA amplifiers to compensate losses (compare Chapter 5 for description and bibliography). The potentialities of this strategy are all in the ability of the pre-distortion to correctly simulate and reproduce the signal propagation along the line. From different experimental pre-distortion implementations, it results that at 10 Gbit/s the solutions based on optical compensations are quite more effective. Today, it seems that there is no viable implementation of pre-distortion circuit that at 10 Gbit/s allows about 10,000 km of SSMF to be traveled by a comb of 40 channels in extended C band. On the other hand, the situation at 40 Gbit/s is quite different. The reach of 40 Gbit/s systems is shorter, and nonlinear evolution is more evident. In this situation, electronic pre-distortion is competitive from a performance point of view with optical compensation techniques [41]. Electronic pre-distortion can also be used not in alternative, but as a complement to optical compensation techniques. In this case the algorithm determining pre- distortion is a bit more complex, since all the optical compensation devices such as DCF and pre-chirping have to be simulated in back propagation, but the results can be significant. Integrating pre-distortion with the contemporary use of look-up tables and FFE-DFE pre-compensation in the ultra-long-haul system at 10 Gbit/s analyzed in the previous sec- tion a