roblem with respect to that faced in the last section. In order to match the signal bandwidth for which the system is designed, the optical bandwidth should be reduced of a factor 10, and this means using 1024 levels at least. However, similar to almost all the systems designed for 10 Gbit/s, our reference system also has an optical pass-band (essentially determined by the multiplexer and the demultiplexer) greater than the bandwidth of a 10 Gbit/s signal: as a matter of fact Moe = 2. Since the available unilateral optical bandwidth is 20 GHz (the channel shape is approxi- mately Gaussian) we can also accommodate a signal with a wider spectrum. As a trade-off between spectrum width and required signal to noise, we will choose a 64 levels 4QAM with a unilateral optical bandwidth of 16.6 GHz thus reducing to 1.2. The line data are summarized in Table 6.2. Next Generation Transmission Systems 9.5.2.1 Ideal Signal to Noise Ratio Requirements The ideal SN, required to achieve an error probability of 10-12 can be deduced from Figure 9.42 for constant power 64-4QAM obtaining 21.8 dB. The symbol rate is 16.6Gsymbol/s, thus using NRZ transmission, the optical unilateral signal bandwidth is 16.6 GHz. With the resulting value of M=1.2, = the required SN° is equal to 24 dB. 9.5.2.1.1 Power Budget The parameters of the components that are used at the transmitter and at the receiver are reported in Table 9.11, where it is imagined that the receiver is built using Si-SiO2 chips as shown in Section 9.3.2.4. At the transmitter, in order to overcome 15 dB of loss of modulator and transmitter pas- sives, a high power external cavity tunable laser is selected, to assure 13 dBm of emitted power and a very small linewidth, which in line with the best commercial lasers we will assume equal to 500 kHz. In this way, the power on the booster amplifier is -7 dBm per channel, that is 7.4 dBm total. This power needs an attenuation not to damage the booster, but it is good to face tolerances and aging. At the receiver, the total receiver loss