 is 21 dB, which requires the use of an APD with a sensitivity of -25 dBm over an optical bandwidth of 20 GHz. Fixing the power on the APD equal to -19.5 dBm to be able to neglect thermal and mul- tiplication noise in the power budget, the power per channel at the preamplifier output is 0.5 dBm, for a total power at the preamplifier output of 20 dBm, the same power exiting from the preamplifier when the system works at 10 Gbit/s. 9.5.2.1.2 Draft Design SN Estimation The estimation of the ASE limited optical signal to noise ratio can be carried out as in all the other cases: evaluating the ASE power in the line after the design. In our case, the ASE power is -16.9 dBm, which provides an SN, of 17.5 dB at the pream- plifier output in the assumed hypothesis for the channels power. TABLE 9.11 Transmitter and Receiver Data for 1024-QAM Designed to Be Used on a 10 Gbit/s Line Receiver Parameters Photodiode Sensitivity -28 dBm Polarization beam splitter loss AWG loss Integrated optics chiploss Patchecords and other losses TOTAL RECEIVER OPTICAL LOSS Transmitter Laser emitter power 13 dBm modulator loss AWG loss other losses Telecommunication Networks Adding an advanced FEC gain of 11 dB an SN, of about 28.5 dB is obtained, which is greater than the required value of 24 dB. However, the difference between the power budget value, comprising the FEC gain, and the theoretical value is only 3.5 dB. So small a value does not allow a normal amount of penalties to be allocated, thus even if theoretically the system can work, in practice, it is not feasible due to nonideal propagation effects and to the margins necessary to take into account components characteristics fluctuations and aging. In order to increase the SNo to allocate the penalties, it is needed to increase the power per channel along the line without increasing the noise. Moreover, the impact of nonlinear effects has to be carefully controlled. This can be done in two ways: either regulating the line amplifiers and the preamplifier to increase the output power or 