case under analysis where the power per channel at the span input is 2 dBm, the depolarization due to nonlinear effects is 0.0065. The plateau due to depolarization is located for this value of SPOL at SN, = 21.8 dB, thus it is very near to the system working point, even if both the dispersion map and the transmit- ted power are optimized. However, taking into account the gain of a super FEC of 11 dB and the value of 17.5 dB of ASE-limited required SN, a careful design should manage to make the system work. Telecommunication Networks L = 9000 km FIGURE 9.64 Nonlinear depolarization factor in the 9000km 16-4QAM system under analysis versus the Power per channel at the span input (dBm) power at the input of the span. The penalty derived from the nonlinear polarization evolution is 3 dB and it is by far the largest penalty. In the presence of SO large a penalty, the penalty addition rule in principle does not hold. However in this case, this is the only penalty that is SO high; SO we will go on assuming that the penalties can be added, using the simulation of signal propagation as validation test. 9.5.4.2.6 Polarization-Dependent Losses A slight amount of polarization dependent loss is always present in transmission systems due to both slightly misaligned connections and to components characteristics [83]. Due to the critical system design, this point has to be considered in detail and not included into the margins. As a matter of fact, polarization dependent losses have a strong impact on all the very high performance systems where part of the information is coded into the field polariza- tion [85,86]. The penalty due to the polarization dependent loss can be analyzed as in [84] as a func- tion of the unbalance between the average amplitudes of the received field components along the reference polarization unit vectors. The analysis shows that in order to maintain the penalty within 0.5 dB it is needed to maintain the unbalance within 2.5% with typical components parameters and system assembly accuracy, that is, to