l will be filtered at the bit rate with an electrical filter after detection the SN will be larger. After the draft design, it has to be verified by the penalties analysis. The summary of this design phase is presented in Table 6.13. At the end, after the FEC gain, the required SNo is 18.1 dB, smaller than the design value of 18.4 dB. A final test has to be done by simulation, whose result completely confirms the draft design. Thus, a link 320 km long (280 km SSMF + km DCF) without intermediate sites can be realized using RZ modulation and Raman amplification even if it is short of margins out of the 3 dB standard added to the penalty analysis. It is to be noted that, since pumping of the DCF is always operated from the system end, the DCF itself can be cabled and used also for transmission. As in all the examples of this chapter, the system is not completely optimized; thus, it to expect that better performances can be attained with a careful optimization of all the design steps. 6.3.4.2 Single-Span Systems with Differential Phase Shift Keying Transmission and Raman Amplification In this section, we will try to improve the design we performed in the last section by changing the modulation format. Essentially, we will use differential phase modulation. The phase of the transmitted signal is modulated using a simple phase modulator and adopting differential encoding. Let us call bj the jth transmitted bit, the signal in the jth bit interval writes (6.91) where nt(t,j) is the Heaviside function that is equal to one for te -1)T,jT) and zero elsewhere. The name differential comes from the fact that the transmitted bit is encoded into the phase difference between the two consecutive symbol intervals SO that ((j) - o(j - 1) = birt In terms of quadrature, components (6.91) writes Transmission Systems Architectures and Performances TABLE 6.13 Penalty Analysis of the All Raman Single Span Intensity Modulation and Direct Detection System All Raman Single Span System at 10 Gbit/s 10 Gbit/s Notes System reach 320 km 280 km S