enge of transmitting at 100 Gbit/s however, phase modulation and coherent detection could become a key technology and we will analyze a few applications in Chapter 9. Among other alternative modulation formats we can remember single side band (SSB) modulation and duobinary modulation. 6.4.1 Single Side Band Modulation The spectrum of an IM-DD signal is constituted by two sidebands and a carrier. Due to the real nature of the baseband signal, the sidebands are symmetric; thus, in line of principle, one of them is enough to reconstruct the signal at the receiver. An SSB optical signal is generally obtained by generating a standard signal and filtering it before transmission SO to eliminate one of the side bands. An SSB optical signal ideally allows both a 3 dB SNo to be gained due to the reduced bandwidth occupation and a more dense DWDM transmission [44]. In practice, this transmission technique has not found commercial application due to the fact that at 10 Gbit/s it is not easy to control the shape of optical filters about 10 GHz wide, SO to eliminate one sideband completely without affecting the carrier or the other sideband. Moreover, it is to be noted that an SSB signal is more sensible to distortion with respect to standard IM, due to the lack of sideband redundancy that gives robustness to the standard modulation format [45]. The situation could change at 100 Gbit/s and with the evolution of electronic equaliza- tion to control signal distortions. 6.4.2 Duobinary Modulation Duobinary modulation has been proposed for optical communications but has not experi- mented wide diffusion. As a matter of fact, although as any proposed method, duobinary modulation has its own advantages, it is also true that it requires more complex transmit- ters and the greater global cost of the system has to be compared to the economic impact of the advantages in the network design. The duobinary modulation is a type of amplitude modulation where the bit stream is manipulated to reduce the bandwidth of the modulated signal with r