In a recent blog entitled “The Submarine Network Seascape in 2020”, I wrote about what I believe are key areas for focused submarine network innovation in 2020. One key area is Spatial Division Multiplexing (SDM) cables. This new wet plant design allows submarine cable operators to “side-step” the Shannon Limit by expanding Channel Bandwidth (B) in the equation below, which is the usable optical bandwidth in the submarine cable. In other words, the more bandwidth available in the cable, the more capacity is enabled. It’s as simple as that. 


Figure 1: The Shannon Limit Equation

Once a submarine cable (wet plant) is laid upon the seabed, the Channel Bandwidth (B) is fixed and is dictated by the number of fiber pairs and the total usable optical spectrum of the optical repeaters (a historical industry misnomer of what are today, optical amplifiers). This means that once a submarine cable is deployed, one must improve the Signal-to-Noise Ratio, on the right side of the equation above, to increase the Channel Capacity (C). This is exactly what the industry has been doing for years with constant technology innovation taking place in the Submarine Line Terminating Equipment (SLTE) and the coherent modems they house.

However, as we get ever-closer to the Shannon Limit of a submarine optical fiber, we start to experience diminishing returns in terms of the upgrade leaps in total information-carrying capacity of the optical fiber. This means that the industry focus must shift back to the wet plant interconnecting the SLTE coherent modems.

Side-stepping the Shannon Limit

Compared to rapid, ongoing SLTE coherent modem innovation over the past decade, the wet plants they connect to have witnessed comparatively less innovation – until recently. One way to expand the Channel Bandwidth (B) in the equation above is to add many more fiber pairs to the submarine cable to provide a higher aggregate of usable optical spectrum in the submarine cable. This is referred to as Spatial Division Multiplexing (SDM). Modern submarine cables have 4 to 8 Fiber Pairs (FP), while SDM offers 12 to 16 FPs, and potentially more in the future. 

As an industry proof point, the first SDM submarine cable will be Google’s transatlantic 6,400km  Dunant cable, which supports up to 250Tb/s of overall capacity provided by an aggregate of 12 fiber pairs – very impressive!

With great fiber count cables comes great responsibility

SDM cables offer massive increases in submarine cable information-carrying capacity. However, like any new technology, new opportunities come alongside new challenges. In the SDM case, challenges and opportunities are directly related to the greater number of fiber pairs. The primary opportunity is straightforward – much more information-carrying capacity. New challenges include how to responsibly label so many fiber pairs to distinguish between them, how to rapidly and effectively characterize higher fiber-count SDM cables, particularly in an Open Cable model, and how to quickly turn-up many more fiber pairs, wavelengths, and Digital Line Segments (DLS). 

To better understand the new challenges and opportunities associated with SDM wet plant designs, check out the webinars below where insights on the submarine network industry in general and SDM cables in particular are provided by Ciena, OFS, and TeleGeography.