# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 4 Optical Transmission: Key Challenges and Proposed Directions
### Capacity Scaling: What to Expect in the F6G
The first one is to employ a baud rate much larger than those of 100/200G cases, e.g., 90 Gbaud, and the second is to employ more efficient modulation formats, e.g., probabilistic constellation shaping and/or employ a higher cardinality modulation. Each of these alternatives has its own trade-offs, e.g., lower cardinality modulation formats such as 16 QAM, require a lower Optical Signal to Noise plus Interference Ratio (OSNIR) in order to attain the same Bit Error Rate (BER) compared with higher cardinality formats, e.g., 64 QAM. This eventually leads to a higher reach for the lower cardinality modulation formats; however, this reach is traded for a higher number of channels as the lower cardinality formats consume a wider bandwidth for the same data rate. In this case, capacity is traded for connectivity, and the optimal solution is based solely on the high-level design set by the network engineer. Figure 17: Evolution of transported capacity within a single mode fiber over the last 50 years along with forecasts for up to 2030 [65, 66, 67]. Another observation that can be derived from Table 4 is that the evolution of the baud rate and modulation format from 100G to 800G clearly indicates that F6G systems need to adopt transceivers with baud rates of 256 Gbaud and even higher in order to exceed 2 Tb/s. The most recent research works demonstrate baud rates of 168 Gbaud with PM-16 QAM format [69] and 220 Gbaud with OOK format [70]. Further, the interplay between attainable reach and bit rate is shown in [71]. In particular, in [71] it is shown that when using 130 Gbaud while considering different modulation formats, rates of 930 Gb/s can be transmitted over 1105 km and rates of 1.28 Tb/s can be transmitted over 452.4 km. Finally, rates of 1.6 Tb/s can be transmitted in a link length of 153.4 km [71]. This analysis clearly indicates that in the near future, technology will show significant progress in this direction both by stressing the electronics to exceed 512 Gbaud and by using DSPs, which will be more tailored to system parameters than current DSPs, allowing it to increase the transmission reach (for longer links). However, for short-reach communication, the use of oDAC approaches, as presented in the previous section, seems an excellent option to scale the data rates in a low cost and low power consumption way. As the rates per channel can reach up to 800 Gb/s with currently available solutions, the attainable rates in the access part of the network are significantly smaller. For example, the most recent PON standard, which is 50G-EPON [75], can provide access to the network with a downstream rate of 50 Gb/s. In the F6G, there is a strong need to advance to the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Modulation Format** & **Baud Rate** & **Channel Spacing** & **Data Rate** &
\begin{tabular}{c} **Required OSNIR** \\ **(for BER = 10\({}^{-3}\))** \\ \end{tabular} \\ \hline
100G & PM-QPSK & 32 Gbaud & 37.5 GHz & 100 Gb/s & 9.8 dB \\ \hline
200G & PM-16 QAM & 32 Gbaud & 37.5 GHz & 200 Gb/s & 16.55 dB \\ \hline
400G & PM-16 QAM & 63 Gbaud & 75 GHz & 400 Gb/s & 16.55 dB \\  & PM-64 QAM & 42 Gbaud & 50 GHz & 400 Gb/s & 22.5 dB \\  & PCS-16 QAM & 80-95 Gbaud & 100 GHz & 400 Gb/s & varies \\ \hline
800G & PM-16 QAM & 128 Gbaud & 150 GHz & 800 Gb/s & 16.55 dB \\  & PM-32 QAM & 96 Gbaud & 112.5 GHz & 800 Gb/s & 19.5 dB \\  & PM-64 QAM & 80 Gbaud & 100 GHz & 800 Gb/s & 22.5 dB \\  & PCS-64 QAM & 90 Gbaud & 100 GHz & 800 Gb/s & N/A \\ \hline
200–800G & Probabilistic Shaping & 60–95 Gbaud & 75–100 GHz & 200–800 Gb/s & varies \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Details of commercially available transceivers for regional and long-haul transmission [74]. Figure 18: Evolution of transported capacity per single channel for commercially available systems over the last 50 years along with forecasts for up to 2030 [65, 66, 67, 72, 73]. 100G-PON standard or directly to 200G-PON in order to support the various capacity demanding services, which were presented in Section 2.