# 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  
Space Division Multiplexing is, in principle, a system that incorporates at least one subsystem (e.g., a transmission fiber, an amplifier, a switching node, or terminal equipment), which implements the concept of \"spatial integration of network elements\". SDM is of great interest as it promises to increase the overall transported capacity by multiple times compared with one SSMF. In particular, the main options that can be considered to increase the number of spatial channels within the transmission link are [16]:  
* Multiplication of the number of conventional fibers (thus implementing parallelism that consists of single-core/single-mode fibers), considering the existence of at least one element that performs spatial integration, e.g., an amplifier with sharing pumps, a switching node, or terminal equipment, named bundles of Single-Mode Fibers (Bu-SMFs). Figure 23: Experimental results on transported capacity vs. transparent reach for UWB transmission based on [92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106]. * Multiplication of the number of cores; within the fiber, multiple cores arranged within the cladding with each supporting a single spatial mode (Multi-Core Fiber--MCF), or multiple cores each supporting multiple modes (Multi-Core-Mode Fiber--MCMF). Coupled Core (CC) fibers. CC can provide strong mode coupling between the different cores, attaining shorter core-to-core distances and higher spatial density compared with the uncoupled MCFs. * Multiplication of the number of modes in MMF fibers within a single core, supporting a discrete number of spatial modes (Multi-Mode Fiber--MMF, Few-Mode Fiber--FMF). * A combination of the above categories, e.g., MCF/FMF, is also feasible. All these combinations are illustrated in Figure 24.  
As mentioned above, in the case of Bu-SMFs, a Bu-SMF considered an SDM needs to incorporate at least one sharing scheme, e.g., a pump-sharing scheme in the optical amplifiers/repeaters. It is worth mentioning that the main target of SDM, especially in submarine networks, is not to simply increase the number of spatial channels, targeting a higher attainable capacity, but to exploit multiple spatial channels and pump-sharing schemes to achieve a reduction in cost/bit and power/bit quantities while providing obvious modular capacity scaling [16]. The comparison between SDM and UWB can aid in designating three main advantages of SDM; the first is its better physical layer performance. In particular, when SDM transmission utilizing the C-band is compared against lower bands of a UWB system, the SDM system benefits from the (a) negligible impact of SRS, (b) lower attenuation, and (c) higher dispersion, leading to both decreased ASE and nonlinear effects and eventually to an improved physical layer performance. This not only increases the modulation format cardinality (and so the data rate per channel) but also boosts the transparent reach. The latter will lead to a lower number of 3R regenerators in the network (especially in long-haul networks), making SDM systems the ideal candidates for, e.g., long-range terrestrial and submarine networks, when compared with UWB. The second advantage of SDM is the use of \"spatial integration of network elements\", such as optical amplifiers, etc., leading to significantly lower OpEx and CapEx. The third advantage of SDM over UWB is the lower-component associated costs, e.g., costs for transceivers, amplifiers, and filters. However, SDM comes with two significant drawbacks. The first is the need to deploy new fibers in links where there is an insufficiency of available fibers; however, this is not the case in most links, where abundant dark fibers are present. The second is that SDM cannot attain diversity in connectivity compared with UWB. More specifically, UWB systems can  
Figure 24: Different SDM options in comparison with standard single mode fiber [16].