nt of accommodating 100 Gbit/s systems on transmission lines designed for lower rates is natural. The great majority of the system we will consider will be designed to work on a trans- mission line originally conceived for 10 or 40 Gbit/s channels. For the parameters of such lines, we will consider the examples we have developed in Chapter 6. At the end of the section we will also present some elements on the design of 100 Gbit/s systems with a newly optimized transmission line. 9.5.1 Practical Multilevel Systems: Transmitting 100 Gbit/s on a 40 Gbit/s Line We will assume to have a line designed for 40 Gbit/s and to design a 100 Gbit/s transmis- sion system that can be substituted to the 40 Gbit/s channels by only changing the tran- sponder (in case accommodating the new transponders in a separated ad hoc subrack). We will require that the system can be scaled substituting one by one the 40 Gbit/s chan- nels, up to a complete substitution that increases the overall system capacity of a factor 2.5. In order to have available all the parameters of the original 40 Gbit/s system, we will refer to the system described in Section 6.3.2.6: this is a WDM 40 Gbit/s system 2000 km long with a maximum of 22 channels in the extended C band and a channel spacing of 200 GHz. Next Generation Transmission Systems The first point is that, in order to be easily multiplexed with the 40 Gbit/s channels, the 100 Gbit/s signal must have an optical bandwidth equal, or preferably smaller, to that of a 40 Gbit/s. Thus, to avoid useless system complications, we will select a 8-DPSK NRZ modulation whose unilateral optical spectrum is 33 GHz wide, well compatible with the 40 GHz of the original channels. Direct detection by signal self-beating is also chosen. The overall ratio between the transmission line optical bandwidth at 40 Gbit/s and the electrical bandwidth is = 1.5. Thus the available unilateral optical bandwidth is 60 GHz at 1/e. Since we have assumed to have Gaussian AWG, the cascade of multiplexer and demultiplexer is Gaussian a