# 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
#### 4.3.3 Summarizing the Benefits of UWB and SDM-Based Systems  
Before we conclude this sub-section, we wish to highlight the need to exploit all available capacity scaling dimensions (polarization, amplitude, phase, and number of spectral and spatial channels) and tailor them to specific applications and network domains. On one hand, SDM systems based on bundles of SSMFs are an already-adopted solution in submarine networks and intra-datacenter networks. However, the need for their deployment in terrestrial network segments is yet unclear. SDM also has very good potential to support the capacity and connectivity demands of the fronthaul. On the other hand, UWB (i.e., with mainly C and L-band transmission) systems have been a reality for over a decade now. The main question is if UWB is more economically feasible than, e.g., a 3-fiber SDM system that achieves the same capacity. To answer this question, one needs to delineate the most cost-effective solutions between SDM, UWD, and their combination, accounting for fiber availability, maturity--both at the component and system levels--the target total capacity, capacity per lane/channel, transmission reach, and the targeted specifications for cost/power consumption per transmitted bit. To reach this target, a technoeconomic analysis on the basis of a network planning study is missing from the literature and needs to be performed for indicative network types, e.g., submarine, access, etc. Regarding fiber availability, UWB can reach up to 100% availability as there is at least one fiber present in all network links. On the other hand, SDM in terrestrial links shows a lower fiber availability as the existence of bunches of dark fibers is not always guaranteed. Next, SDM is advantageous over UWB, as it exploits a well-operated and mature technology for various components, such as transceivers, amplifiers, filters, etc., since it focuses on the third transmission window. Moreover, the physical layer performance of Bu-SMF is significantly higher when it is compared with UWB systems (beyond C and L-bands), allowing to attain a longer transmission reach, prohibiting the use of UWB applicability in submarine and long-haul transmission systems as they require a large number of regenerators, especially for the channels in the lower bands. However, in the core domain, as shown in [88], four out of five bands of an UWB system can interconnect even the most distant nodes of a core network, while the O-band can be used to interconnect less distant nodes (e.g., up to 600 km), showing that UWB is a very good candidate for the terrestrial domain. This diversity of UWB systems is highly plausible, as different bands can be assigned to different network paths and/or carry different data rates, in this way balancing the cost/transparent reach ratio. Merging the advantages of both worlds, we can say that a combination of UWB and SDM systems seems to be the most efficient solution to maximize the terrestrial network connectivity, allowing it to transport a multi-thousand number of channels through each optical link. Next, both UWB and SDM systems have excellent upgradeability capabilities, as a new band or fiber can be engaged when the already active bands or fibers reach a utilization threshold, e.g., 60 or 70%, allowing it to follow a pay-as-you-grow policy, reducing the first-day capital expenditure. Another important advantage of SDM systems compared with UWB ones is the possibility of using common components for a large number of cores/fibers/modes, reducing the overall costs and power consumption. On the other hand, UWB can exploit amplifiers in multiple bands, such as C + L bands EDFA, UWB SOAs, and Raman amplifiers; however, these components are confined to two or three bands at most.