# 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  
According to Cisco, the traffic demands will keep increasing at a rate of approximately 100% every 3 years (Figure 16). The reason for this traffic increase, as mentioned in Section 2, is the need to meet the ever-increasing demand for \"fresh\" services, which has been accelerated due to the increased tele-working following the COVID-19 pandemic. So, in order to satisfy the requirements of the upcoming F6G services while at the same time avoiding the 100 Tb/s capacity crunch of the Standard Single Mode Fiber (SSMF), network providers and vendors have to investigate and deploy novel techniques aiming at increasing the overall transportation capacity by at least one order of magnitude compared with F5G systems. The general trend of _transportation capacity per single fiber_ over time can be seen in Figure 17, according to which during the past five decades, the transportation capacity within an optical fiber started from approximately tens of Mb/s and has now reached approximately 100 Tb/s. This value is the maximum nominal rate of the Single Mode Fiber (SMF) in long-haul transmission. This figure highlights an increase of six orders of magnitude in less than fifty years and clearly underlines the need to find ways to exceed 1 Pb/s by 2030. Recently, various ways have been proposed in order to overcome this 100 Tb/s capacity crunch, such as (a) exploiting more spectrally efficient modulation formats, like probabilistic shaping and coded modulation, (b) exploiting the entire low-loss attenuation spectrum of the optical fiber (<0.4 dB/km), which extends from 1260-1625 nm by transmitting either a higher number of channels or by increasing the bandwidth of each channel (migrating e.g.,  
Figure 16: Global IP traffic demands (source: Cisco). to higher baud rates), and (c) using SDM, mainly by either considering bundles of SMFs or by using multi-core/mode fibers. In all three cases, the overall transportation capacity can be extended beyond 100 Tb/s and eventually satisfy the target of 1 Pb/s for 2030 for commercial systems and the target of 10 Pb/s for research works by 2030, as it is illustrated in Figure 17.  
Another observation that can be extracted from this illustration is that the research technology is approximately 8-10 years ahead of commercial systems. This is expected, as the results of the huge effort of researchers around the globe need to be refined and distilled in order to produce the most cost-efficient and, at the same time, performance-efficient methods/technologies/components, which will then proceed to commercialization. In order to attain Pb/s rates, novel optical components need to be introduced, such as band filters, cost-efficient optical amplifiers (especially for O and E-bands), band switches, and new generation pump sharing SDM-based amplifiers, which can provide both increased redundancy and reduced costs [63]. Another key technology, which can aid the F6G networks to transmit data at even larger distances and/or exploit higher cardinality modulation formats, increasing the transported capacity, is ML-based signal processing [64]. For example, Deep Neural Networks (DNNs) using digital backpropagation (DBP) methods can achieve a low-complexity nonlinearity compensation. Their performance is similar to those of conventional DBP modules, but they encounter relaxed computational complexity. Next, we study the evolution of the _capacity per channel_ over the time interval of 1970-2020 in order to understand the general trend and elaborate on the capacity needs for the next decade. Figure 18 highlights the need for fresh commercial systems able to reach at least 2 Tb/s per port by the end of the decade, while in [68], the need for up to 6.4 Tb/s by 2031 is also underlined. These rates can be achieved using multiple optical channels packed together forming a super-channel. Today, the highest commercially available rate is 800 Gb/s (Table 4) and can be realized mainly with two options.