span after the precompensation and preamplification, the signal power is 2 dBm per chan- nel, that is, 18.5 dBm total power. At the receiver, the EDFA preamplifier is not needed since the Raman stage provides all the needed gain of 17.5 dB, being dimensioned the same way of the in-line Raman amplifiers. Moreover, a good APD has the required sensitivity at 25 Gsymbols/s to satisfy the power budget. The pumps of the Raman amplifiers can be optimized both for flatness and for gain; however, also gain flattening filters with a very low loss (1.5 dB) are foreseen to improve the gain flatness. The design SN results to be 22.4 dB, with a great margin with respect to the ASE limited value of 17.5 dB. This is mainly due to the small value of the span bringing the system nearer to the opti- mum design that would be a continuous distributed amplification (see Chapter 6). Just to verify this fact, if the design is done again with a span of 70 km, the SN, goes down to 15 dB, rendering in practice the transmission impossible. It is to be observed that, under an SN point of view, the system could be much longer, up to 10,000 km and more. Moreover, a quite higher SN is achieved by slightly increasing the power at the span input. For example, bringing the power per channel at the span input at 3 dBm and the system at 10,000k the SN is 25.78, better than that we have now. Both these improvements are impossible due to nonlinear effects, mainly SPM and non- linear polarization rotation, which are the main limitations to the system performances. 9.5.4.2 Penalties Analysis The penalties analysis in this case presents several new elements with respect to what is done in Chapter 6 and in the previous sections. Telecommunication Networks 9.5.4.2.1 Linear Crosstalk The analysis of the impact of the linear crosstalk can be done in complete analogy to what was done in Chapter 6 for IM systems. In our case, the channels are quite near (100GHz with a symbol rate of 25 Gsymbols/), thus even if the Gaussian filters are quite tight, the linear c