# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 7 Economic Aspects of Submarine Networks  
SDM is evolving in submarine networks as it promises increased capacities. Apparently, these increased capacities will come along with a significant cost. Having in mind that the submarine environment is quite different from the terrestrial one, the economics of SDM-based submarine systems need to be investigated. Common sense dictates that we tend to seek increased capacities up to the point we can afford them or they are no longer worth the associated cost. There have been various economic aspects, models, and proposals for defining and limiting the cost of an SDM-based submarine cable system. We briefly discuss most of them in this section. In Figure 33a there is a demonstration of how global cable construction costs during the period from 2010 to 2020 derived a total investment cost of $7.6 billion for all the regions (Figure 33b). [PERSON], [PERSON] et al. in [76; 77] explored cost saving cases in submarine systems. They first noticed that cost/bit-optimized systems follow different design rules than those followed by capacity-optimized ones. The authors focused on cost-optimized systems and showed that  
Figure 32: Comparison of two cable systems for different power (from legacy to the latest generation) and capacity values [75]. SDM-based submarine systems with 50 FPs, long spans, and MCFs can achieve lower costs compared to the use of several single-fiber cables. They concluded that using SDM with 45 FPs could achieve a 44% cost saving compared with the current Pacific system, which incorporates 8 FPs of PSCF110 and C-band optical amplifiers. Moreover, the integrations of optical amplifiers (i.e., in pump-farming arrays) and transponders result in extra significant cost savings. An interesting finding regarding power supply is that even if we could supply increased power beyond known levels and achieve higher capacities, we would not result in a reduced cost/bit. This happens because the cable and deployment costs (which are responsible for a large percentage of the total cost) will not be affected. As SDM and MCFs are evolving and amplifiers represent a significant percentage of the total cost (especially in long-haul networks), methods of reducing amplifiers' cost are investigated. [PERSON], [PERSON] et al. in [78] investigated economic issues of the upcoming state of the art MC-EYDFA. MC-EYDFA's architecture is described in the amplifiers section. The authors performed a techno-economic analysis by using a 12-core EYDFA over a 2280-km terrestrial network in France. The network consisted of 7 links, has 8 ROADMs with a nodal degree varying from 2 to 5, and used 29 EDFAs. Results showed that in the future, the replacement of EDFAs by MC-EYDFAs will achieve significant savings in the range of 33-35% for amplifiers cost (by year 2038) and of 60% for energy consumption (by 2026). These savings lie in the range of tens of millions of euros in absolute values. In addition to the technological research and progress in optical hardware (single-core fibers, MCFs, MMFs, amplifiers, transponders) research has also been conducted on advanced architectures that will efficiently support the demanding SDM technology. [PERSON] and [PERSON] [79] performed a cost comparison of several proposed hierarchical optical cross-connect (HOXC) architectures which can be embedded in spatial core networks (SCNs). The HOXC architectures examined are the baseline-stacked wavelength cross connect (WXC) architecture, the full-size matrix (FS-MS)-based architecture, the sub-matrix switch-based (Sub MS) architecture, the full-size core-selective switch (FS-CSS)-based architecture, and the sub core selective switch (Sub-CSS)-based architecture. Although today there is an uncertainty regarding what exactly these OYAC architectures will cost, the authors concluded that the sub-MSs and CSSs SCx architectures will be more cost-effective over the well-known full-size matrix and the ROADM/WXC architectures.