# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 7 Economic Aspects of Submarine Networks
[PERSON] in [80] investigated maximum subsea cable capacities and the minimum cost/capacity for SC fibers by using C and C + L bands and MCFs using C-band. Basic assumptions are fixed power constraints, specific SNR requirements and similar fiber core main characteristics. The length of the test bed links were 6600 km and 10,000 km (suitable for transatlantic cables). Results revealed that the MCF and SC C + L minimum cost capacity were 20% and 5% higher than that for the SC C-band only for the 6600-km link and 37% and 12% higher for the 10,000-km link. The author concluded that cable systems using high count of SC fibers achieve lower cost/capacity values than those achieved by MCFs. [PERSON] et al. in [81] investigated costs between SMFs and MCFs for submarine systems. The authors compared costs based on a system model which counts for two different priorities: maximum cable capacity and minimum overall cost/capacity. They concluded that although MCF systems provide greater cable capacities for a small number of fibers, the cost/capacity ratios are significantly higher. When they focused on minimum cost/capacity the MCF systems get an easy victory over SC systems. It was also noted that submarine-deployed MCFs may need increased electrical power requirements (over the SMFs) for feeding the MC-EDFAs and may suffer from possible physical space limitations. Electrical power and physical sizes affect not only the electrical and deployment hardware issues but also the cost. Consequently, for a proper cost analysis, these factors should be taken into consideration. [PERSON] et al. in [82] presented a model that considers not only economics but also physical factors that describe a submarine connection. A techno-economic model was shown in Figure 5[79] and included all cost-affecting parameters/factors. By using a \"least amount of technological change\" approach, the authors concluded that a 16-FPs SDMbased system is a feasible yet economical solution for near-future submarine cable systems, approaching the cost (per unit) capacity limit. More FPs in a single cable seems to have serious drawbacks both for technology issues and for cost. Moreover, results for the cost ratio both for optimized SDM and non-SDM solutions were presented. Physical and initial costs are similar for both solutions, except for the operating optical power requirements. Results showed that SDM gives a minimal cost benefit at short distances but significant benefits (in the range of 14%) for longer distances and higher capacities. [PERSON] and [PERSON] et al. in [13, 83], performed an economic study including almost all of the equipment needed for a closed-sea submarine network. An analytical cost model was described which derives the total relative cost (TRC) of each ROADM design and total relative network cost (TRNC); thus the network cost includes the cost of network components plus site survey costs. The cost model took into account all three available WSS ROADM switching strategies (independent switching, fraction joint switching, and joint switching) [84], several common spatial dimensions (of 4, 8, 12, and 16), the number of add/drop (A/D) capability nodes, and the in-line amplifiers (both for EDFAs and for MC-EYDFAs). The model was applied to two new submarine backbone networks of 29 and 10 nodes specially designed for the Mediterranean Sea and the Aegean Sea (Greece), respectively. A special characteristic of these submarine networks is that they can act as supplemental and backup networks to adjacent terrestrial ones. The results of the cost analysis showed that fraction joint switching turns out to be more economic than independent switching, (5% cost savings for all spatial dimensions S = 4, 8, 12, 16). Joint switching is also more cost effective than independent switching, also outperforming fraction joint switching in all spatial dimensions (for S > 4). Beyond the S = 4 threshold, joint switching achieves significant costs savings, in the order of 8% for S = 12.