# 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
#### 4.3.2 Space Division Multiplexing
attain diversity in connectivity as the lower bands can employ less costly components employed for shorter links, although these bands result in a shorter reach when they are compared with SDM. On the other hand, in UWB, only the C and L-bands can be exploited for the more distant links of a network, which cannot be the case in C-band SDM systems where the entire available spectrum is considered \"premium\" due to its high-quality transmission. Certainly, a combination of SDM/UWB is possible, e.g., by considering a multiple fiber transmission, e.g., Bu-SMFs, exploiting both C and L-bands. However, in order to select the optimal solution between (a) the various SDM variants, (b) the exploited amplification bands of a UWB system, and (c) a combination of them, we need to take into account a number of different factors, where many of them are antagonistic, such as the attainable capacity, node connectivity, cost/energy-effectiveness, transparent reach, node scalability, system upgradeability, and node/spectrum flexibility. Cost-effectiveness is obviously one of the most important factors when designing an optical network. SDM can reduce the overall component-associated costs by exploiting \"spatial integration of network elements\", e.g., amplifiers. Considering transmission within multiple modes and/or cores, the most important concern is if these modes/cores intermix. This is an important issue, as a possible energy transfer between them will result in a lower physical layer performance, increasing the BER of the transmitted channels. For example, in multi-mode fibers, degenerate spatial modes exhibit mixing during fiber propagation while bundles of SMF do not. Proper system design may suppress the impact of this mixing. Further, MCFs' coupling can be reduced by considering different neighboring core properties and increasing the pitch [117]. Note also that the placement of identical cores in proximity leads to mutual coupling [118], allowing more precise control. Mixing between the modes in an MMF can be reduced by breaking the mode group degeneracy [119]. Optical amplification in UWB systems, when DFA technology is adopted, can be achieved with the use of multiple amplifiers. More specifically, a single amplifier could be employed for each band, which is placed either in parallel or serial [90]. On the other hand, power feeding in SDM systems utilizing the C-band can be implemented by groups of EDFAs, e.g., one per guided mode, core, or fiber [120, 121]. A significant benefit of SDM over UWB here is the ability to share common pumping lasers [122], in this way reducing the number of laser pumps. SDM is also advantageous when compared with UWB as it possesses the ability to share the lasers in the transmitter and receiver parts [123]. In particular, if mode mixing exists, a common laser source leverages the post-detection signal processing required to unravel the original information, as the phase relationship is fixed. However, SDM shows higher costs than UWB in cases of limited fiber availability or in cases where, due to regulatory aspects, network operators are forced to pay rent to a government body/agency for any occupied fiber resources. For example, CAPEX aimed to roll-out a new fiber is \(\sim\)25 keuro/km in rural areas, and up to \(\sim\)500 keuro/km in metropolitan areas [124]. In particular, for a European country, the lease cost is approximately \(0.33\times\) (-\(\xi\)1308) per fiber/km/year for five years leasing package; but the lease cost in an Indian network is approximately \(0.007\times\) (\(\sim\)$29) per fiber/km/year [125]. These leasing costs need to be quantified during the selection of the optimal solution between UWB and SDM. The exploitation of SDM in terrestrial networks will not only aid in attaining the target of 10 Pb/s by 2030 (Figure 17) but will also significantly boost node connectivity through the introduction of additional channels, which are transmitted through the additional cores and fibers.