With great fiber count comes great responsibility
Spatial Division Multiplexing (SDM) cables are a key focus area for submarine network innovation in 2020. Ciena’s Brian Lavallée explains how SDM cables offer massive increases in submarine cable carrying capacity and the challenges associated with these new wet plant designs.
By Brian Lavallée – 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, 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.
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.
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! more>