## 5 Physical Layer: Five Decades of Evolution (from 45 Mb/s to 100+ Tb/s)
### Physical Layer Evolution in Access
#### 5.3.1 Candidate Modulation Formats for Access  
The designation of the most cost-wise modulation format for the access domain is a complex problem. This happens as on one hand, NRZ, which is one of the less costly solutions, has a limited receiver sensitivity for a given bit-rate, and on the other hand, modulation formats such as PAM4, PAM8, electrical Duobinary (EDB) and DQPSK may provide a better receiver sensitivity, allowing for a greater transmission reach and/or number or users, but at a higher cost. Figure 33 illustrates a comparison of the power penalty vs fiber length for various modulation formats [87]. As a reference in order to estimate the power penalty, an OOK back-to-back system is considered. The bit rate is 50 Gb/s and the target BER is set at \(10^{-3}\) as FEC is employed. The main transmission impairments are fiber attenuation and dispersion. As expected, the power penalty for the OOK increases rapidly after 5 km, being the most physical layer degraded modulation format. Differential binary phase shift keying (DBPSK) performs slightly better than OOK, mainly due to the increased tolerance on dispersion. Next, PAM4 and PAM8 start with an increased power penalty compared with OOK due to the larger number of symbols; however, after 7 km, they outperform OOK. The large power penalty of PAM8 at the beginning of transmission makes this modulation format ineffective for practical links. Further, DQPSK shows a small increase in performance compared with PAM4. Finally, combined amplitude phase shift (CAPS), which is very similar to optical duobinary, qualifies as the best candidate for access networks spanning more than 10 km, mainly due to its high tolerance to chromatic dispersion. Coherent systems are a capable solution that can increase the data rate as well as the link distance. However, coherent systems, such as the ones discussed in Section 5.2, which were considered for core and long-haul transmission, are cost-ineffective solutions for access. For this reason, novel low-complexity coherent systems have to be introduced gradually in fiber-based short-haul networks in order to meet the capacity demands due to fixed/wireless network convergence (and intra-DCI), such as what happened at the backbone counterpart segments of the network, where a gradual phase of migration from direct detection systems to coherent ones was considered. To achieve this target, the P2 MP transmission via XR-optics, shown in Figure 13, can be employed in order to minimize the number of end-points or the direct digital to optical domain (D/O) conversion, which can  
Figure 33: Power penalty of various modulation formats versus link distance [87]. replace the electronic digital to analog converter (eDAC) intermediaries, increasing the overall cost-effectiveness. Another possible cost-effective solution for access is the simplified coherent reception proposed in [88]. In this case, the detection scheme is based on a 3 \(\times\) 3 fiber coupler 120\({}^{\circ}\) optical hybrid with 25G APD, cutting in half the overall costs compared with a fully coherent receiver. As it is shown in Table 11, the APD and the simplified coherent receivers can provide significant gains of 8-19 dB in terms of power budget, depending on the modulation format. This gain makes them very good candidates for longer networks and/or systems with a larger number of users, where a significant number of power splits is required. In order to weigh the gain-cost potential, a relative cost comparison among various modulation formats is shown in Figure 34. The assumption that the cost of a 10G system equals 1 has been used. As it is evident, when the data rate migrates from 10G to 25G, the cost is increased mainly due to the higher costs associated with the 25G APD compared with the 10G APD.