# Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends
## 3 Recently Announced Submarine Cable Systems
### A Detailed Overview of SDM-Based Technology Cable Systems
From an industrial point of view, some of the biggest submarine services providers, such as Alcatel Submarine Networks and HMN point out the main characteristics of an SDM-based cable system:  
The SDM 1 system powered by ASN [8] is trying to revolutionize the well-known submarine designs by:  
* pushing the limits of theoretical design capacity;
* minimizing nonlinear effects to reduce needed equipment, cost, and complexity;
* designing an efficient optical and electrical network based on repeater pump farming, low \(A_{\text{eff}}\) submarine fibers, and higher fiber counts; and
* working in the optimum spectral efficiency of submarine line terminal equipment (SLTE): 2-3 b/s/Hz and lower chromatic dispersion compensation. * The SDM system powered by HMN [12] Tech involves the high fiber count solution and includes:
* a high fiber count submarine repeater which broke through the fiber count limitation of existing products and can support up to 16 fiber pairs which can double the capacity;
* an industry-leading 39.5 nm ultrawide bandwidth, which covers C-band and extended C-band, to maximize the capacity of one fiber pair; and
* a significantly reduced cost/bit. Table 4 presents a comparison between traditional and SDM submarine cables and summarizes the main features of each cable's technology. \begin{table}
\begin{tabular}{c c c} \hline
**Subsea Component** & **SDM Cable** & **Traditional Cable** \\ \hline
**Submarine Cable** & High Count of FPs & Limited number of FPs \\  & (12, 16 FPs and more in future) & (6 FPs and maximum 8 FPs) \\
**Fiber Effective Area (Aeff)** & Low effective area, & High effective area, \\  & Aeff 110–80 \(\upmu\)m\({}^{2}\), att. 0.155 dB/km & Aeff 150–125 \(\upmu\)m\({}^{2}\), att. 0.15 dB/km \\
**Repeater** & Low power repeaters: (+14 to 20 dBm) & Very high power repeaters: (\(>\)+20 dBm) \\
**Repeater Type** & Repeater pump farming & Each fiber has own laser pumps \\
**Branching Unit ROADMs** & Fiber pair switching in (BUs) & No fiber pair switching in (BUs) \\
**OSNR** & Low OSNR & High OSNR \\
**Modulation Formats** & PCS (probabilistic constellation shaping) & BPSK, QPSK, 8-QAM and 16-QAM \\
**C + L Band Technology** & Currently only C-Band & C + L Band supported up to 144 channels \\  & & per FP \\
**PFE** & Same PFE, capacity can be increased & Same PFE \\ \hline \end{tabular}
\end{table} 
Table 4: A comparison between Traditional and SDM submarine cables [13]. Figure 12 depicts the new FP granularity of SDM (16 FPs) vs. traditional cables (<8 FPs). In this conceptualization, the potential capacity per FP is decreased. For example, in the past, 200 Tb/s could be transmitted by using 8 FPs (25 Tb/s/FP). Now the same 200 Tb/s is achieved by using 16 FPs (12.5 Tb/s/FP), thus bringing technological and economic benefits (easier FP-switching, selling a whole FP to a customer instead of portions of it). Table 5 summarizes the main characteristics of current submarine cable systems: type of topology, cable length, number of landing points, and number of operators of the major cable systems around the world. Note the large number of operators that are involved in many systems. This is expected, as these cables interconnect a number of countries; as a consequence, a number of operators exploit these cables to transmit data from one continent/country to another.