# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 4 Optical Transmission: Key Challenges and Proposed Directions
### Capacity Increase Employing a Greater Number of Channels
#### 4.3.1 Ultra-Wideband Transmission
As it is evident from this figure, by using either EDFAs alone or in combination with Raman amplifiers, UWB transmission is feasible in long-haul and submarine networks, while for shorter distances, e.g., access/DCI and metro, there is a broader gamut of available options for optical amplification. This allows one to select the most cost-effective technology between DFAs (e.g., Thulium DFA and EDFAs in parallel), hybrid-Raman amplifiers, and SOAs. SOA technology particularly is a very cost-effective option for short distances as it can offer a large amplification bandwidth, e.g., 100 nm, with only one amplifier [113, 100]. Hollow-Core Fibers for Further Increasing the Transmission Spectrum  
The low-loss attenuation spectrum of SMF spans over approximately 50 THz (O to L bands). However, signal transmission beyond this range, in particular from 1800 to 2350 nm, is possible, allowing an approximately 37 THz additional range using hollow-core fibers [67]. In  
Figure 22: Gain and noise figure of indicative available doped fiber amplifiers assuming a total input power in each amplifier equal to 0 dBm [74]. the hollow-core fiber, the light propagates within a hollow region, in such a way that only a small portion of the optical signal propagates in the solid fiber material. Hollow-core fibers with proper manufacturing can attain a low fiber loss within a range that spans around 2000 nm. However, the transmission within hollow-core fibers requires the deployment of new fibers, which leads to high installation costs and buying prices. This fiber alternative, as well as the SDM concept, is included in the category of \"green-field\", where new fiber types need to be deployed while the UWB transmission is part of the \"brown-field\" category, where the existing fiber infrastructure is used as the basis for installing new fiber subsystems and elements, such as amplifiers, filters, etc. An important metric that shows the current state of the technological progress in the optical transmission is the \"relative bandwidth\", which is equal to the total system bandwidth divided by the central frequency [67]. This metric relates the system bandwidth to its central frequency, and the ultimate target is to approach as close as possible to 100%. For example, a C-band-only system shows a relative bandwidth of less than 3%, while a theoretically fully populated UWB system can attain a relative bandwidth of approximately 25%. Finally, when the transmission spectrum is extended using hollow-core fibers, the relative bandwidth can exceed 65%. As the value of this metric increases, the overall system costs become higher as novel components such as amplifiers, transceivers, band filters, etc., need to be deployed. These components are more costly than those used in C and L-bands because the technology in C and L-bands is more mature, and the economy of scale can lead to reduced costs, compared, e.g., with E and S-bands. This high cost is a significant constraint for network operators that desire to upgrade their infrastructure in order to engage more bands on optical transmission and can steer them towards exploiting SDM techniques, which are analyzed in the next section.