gain. This means that for the 8-DPSK channel, Moe==1.81. The ideal SN needed to reach a BER of 10-12 with this value of M is 20.5 dB. Using the relation between optical and electrical signal to noise ratio (9.83) we obtain the ideal optical signal to noise ratio required for BER = 10-12 to be about 18.5 dB. Carrying out the evaluation of the ASE limited performances, using parameters of Tables 9.8 and 9.9, an ASE limited signal to noise ratio of SNo o = 22.3 dB is obtained. At this point, the first design step, that is, the power budget, can be carried out. 9.5.1.1 Power Budget It is not difficult to see that the power budget is verified both at the transmitter and at the receiver since the power at the booster input is sufficient to saturate it but below its maxi- mum input power and the power at the preamplifier out is well above the limit power that matches after the receiver chain the sensitivity of the photodiode, but it is also below the power needed to saturate the considered avalanche photo-detector (APD). In this condition, the total power exiting from the amplifier at every span input is 18 dBm overall, that means 4.57 dBm per channel. TABLE 9.8 Parameters of the Transmitter and the Receiver of the 8-DPSK System Dimensioned Transmit 100 Gbit/s on a 40 Gbit/s Line Receiver Parameters Photodiode sensitivity -15 dBm AWG loss Si-SiO2 chip Patchecords and other losses Transmitter Laser emitter power 10 dBm Modulator loss AWG loss Other losses Telecommunication Networks TABLE 9.9 Characteristics of the Preexisting 40 Gbit/s Line Where the 100 Gbit/s Signal Has to Be Accommodated Span length 50,000 m Spans number Attenuation 0.25 dB/km Power at the input of a span 18 dBm Power per channel at the span input Power at the span end -2dBm Power at the span end per channel -15.4 dBm Noise Figure (all amplifiers) 9.5.1.2 Penalty Analysis 9.5.1.2.1 Residual Chromatic Dispersion The residual chromatic dispersion at the link end is 500 ps/nm, which is 275 ps in the band- width of 0.55 nm of the 8-DPSK signal. The chroma