# 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
In particular, the number of parallel cores can reach up to 32 for fiber diameters of \(\leq\)250 \(\upmu\)m, where glass remains flexible, transporting a capacity of 1 Pb/s over more than 200 km [126]. Moreover, with the use of the 8D-16 QAM format, the transmission distance can exceed 1200 km with a capacity of 0.75 Pbit/s. Further, in [127], it is demonstrated that the efficient operation of an SDM network is feasible, as the channels are adaptively (re)configured, taking into account the inter-core crosstalk with the aid of a software-defined network (SDN) controller. Regarding the introduction of SDM in terrestrial networks, the availability of multi-/core/fiber/mode amplifiers is a prerequisite. In particular, a Multi-Core Erbium/Ytterbium-Doped Fiber Amplifier (MC-EYDFA) can be a promising solution for the power restoration of the channels in multiple cores; however, some challenges need to be addressed, such as the reduction of its size and power consumption. Authors in [120] showed that a 32-core EYDFA can reduce the overall power consumption when benchmarked against a multiple EDFA scheme. This efficiency improvement for cladding pumping originates from increasing the effective area of active cores. Authors in [128] showed increased cladding pump efficiency for 19-core EDFA in C or L-bands while a cost analysis designates that the common amplifier brings long-term cost savings of 33% and up to 55% power savings [129]. In order to realize SDM within the limits of a terrestrial network, the existence of Optical Cross Connects (OXC) with SDM capabilities is a prerequisite. Scalability is the most important issue in order to accommodate several types of switching granularity demands, and a layered switching architecture is mandated. In future SDM networks, when the capacity per path will exceed several Tb/s, the introduction of spatial OXC-based architectures [130, 131] will be a must. The successful migration from current Wavelength Selective Switches (WSS)-based networks towards layered and scalable SDM switches [132, 133] will be key to the success of SDM-based networks. The era of SDM deployments in submarine networks has already started. The first generation of SDM-based submarine systems was announced and started its deployment phase in 2020, before any terrestrial SDM network of any kind had been deployed. It is characterized by early technology advancements that implement the concept of \"spatial integration of network elements\" with optical amplifiers. The spatial integration/sharing of resources is implemented in the submarine optical amplifiers via the so-called \"Repeater Pump Farming\" (RPF) technique. RPF consists of a group (named \"farm\") of repeaters, which are cross-connected to each other. Each RPF farm supports a certain group of Fiber Pairs (FPs) and utilizes a group of optical pumps that are shared among groups of FPs [134, 135]. A significant advantage of RPFs is that they can continue pumping FPs even in cases in which one or more pumps fail, offering a promising solution for network survivability by assuring redundancy. A schematic of an RPF system is illustrated in Figure 25.  
Figure 25: RPF systems that are expected to be in use in all future subsea cable systems [136]. tabulated in Table 5. As it is evident, the SDM can support a significantly larger number of fiber pairs along with very high-power repeaters to rectify the power of the optical signals, which are expected to be significantly larger in number compared with traditional cables. Moreover, the transmission within SDM cables is restricted in the C-band, while PCS can be employed to enhance the channel performance. To understand the potential of SDM systems in increasing the transportation capacity deeper, Figure 27 illustrates the total capacity of deployed submarine cables with and without the use of SDM.