improve the performance of the hybrid solution SO increasing the difference with the all EDFA architecture. 6.3.3.3 Ultra-Long-Haul Transmission at 40 Gbit/s At 40 Gbit/s, all the distortions depending on the signal speed are more severe. The draft design is carried out with the same method used in the previous section start- ing from a system with 18 channels at 40 Gbit/s and spaced at GHz. The large spacing is due to the fact that, to control dispersion and the other sources of ISI, RZ is very useful. Transmission Systems Architectures and Performances Even if the duty cycle is reduced to 33% with respect to the 25% used at 10 Gbit/s, the optical signal spectrum is 120 GHz wide SO that, in order to reduce linear interference, a very large channel spacing is needed. A very simple inspection of the FWM and XPM penalty curves in this case shows that 10,000 km are too much. For this reason we will try to design a 6500 km long system. The draft design can be carried out with the same procedure followed in the 10 Gbit/s case and the parameters are the same, but for the transmitter loss that, due to the 40 Gbit/s modulator, is 2 dB higher and the sensitivity of the PIN photodiode that substitutes the APD used for 10 Gbit/s is -12 dBm. The PIN photodiode is used to have a higher saturation power; as a matter of fact the power per channel arriving on the PIN is -2 dBm. The final value of the SNo derived from the draft design is 26.8 The summary of the penalty analysis for this system is reported in Table 6.10, from which a required SNo equal to 25.6dB is obtained. The system is thus within the required per- formances, if the last test via simulation does not differ too much from the draft penalty analysis. In this case, the simulation has to take into account both nonlinear propagation and the complete model of Raman amplification. In order to simplify the simulation, we have simulated all the signal distortion, by coupling the nonlinear propagation in the SSMF with the complete pumps plus signal Raman equations, elimin