em margin Polarization effects Taken into account in detail. Required an unbalance between the quadrature amplitudes not larger than 2.5% Total SN post code Code gain (dB) Total SN pre-code 9.6 Summary of Experimental 100 Gbit/s Systems Characteristics Since currently there is no diffused offering of products for long distance transmission at 100 Gbit/s we cannot provide the list of realistic characteristics generated from a synthesis of the characteristics of real products. Thus, we will do a sort of summary of a few experimental implementations of high speed optical transmission. In Table 9.17, a few experiments using multilevel modulation are summarized. In all the cases, but the first, PDM is used, that is, two channels are sent on orthogonal polarizations and on the same bandwidth. It is easy to observe that the signal produced, just for an example, using PDM 16-QAM is not SO different from a 256-4QAM obtained by the Cartesian product of two 16-QAM constellations, but for the fact that the signals on the coordinate plans (e1, e2) and (e3, e4) are synchronous in time in the 256-4QAM transmission, while they are asynchronous in the case of PDM 16-QAM. For 256-4QAM obtained with full QAM constellations, we see that the normalized mini- mum distance is 0,111 while it is 0,471 for one of the QAM channels. As a matter of fact, the absence of correlation between the transmitted constellations destroy the capability of 256-4QAM to compress the signal bandwidth of a factor eight TABLE 9.17 Review of the Main Features of a Few 100 Gbit/s Experiments Using Instantaneous Multilevel Modulation Serial Multilevel Transmission Modulation Channel Format Capacity Parameters Distance Link Characteristics Notes References NRZ PMD 100 Gbit/s 16 (C band) 2417 km Recirculation over installed SSMF link 63 km Coherent homodyne detection, long, all EDFA amplification manual polarization control, phase diversity No in line DCF NRZ PMD 100 Gbit/s 164 (C + L 2550 km Recirculation over SSMF link 65 km long, Coherent homodyne detection, b