Nokia and Facebook claim they have set a new record for spectral efficiency on subsea cables whilst collaborating on field trials of new optical digital signal processing technologies over a 5,500 kilometer transatlantic subsea link.
To increase the capacity capabilities of subsea fiber, Nokia and Facebook tested Nokia Bell Labs’ new probabilistic constellation shaping (PCS) technology. The test showed an increase of almost 2.5x more capacity than the stated optical transmission capacity of the system, demonstrating the feasibility of the technology across a challenging real-world fiber-optic network.
PCS is a technique that uses ‘shaped’ quadrature amplitude modulation (QAM) formats to flexibly adjust transmission capacity to near the physical limits of a given fiber-optic link. In an experiment for an installed submarine link – conceived and planned by Facebook – PCS based on 64 QAM, combined with digital nonlinearity compensation and low-linewidth lasers, achieved a record spectral efficiency of 7.46 b/s/Hz, indicating the potential to upgrade this cable to 32 Tbps per fiber in the future.
Nokia says transmission tests based on its commercially available Photonic Service Engine 2 (PSE-2) validated the successful transmission of 8-QAM wavelengths running at 200 Gbps and 16-QAM wavelengths running at 250 Gbps – which Nokia is billing as a first for transatlantic transmission.
200G 8-QAM wavelengths supported a spectral efficiency of 4 b/s/Hz while exhibiting sufficient performance margin to support reliable, commercial operation.
“Facebook wants to increase the pace of innovation and adoption of next-generation optical technologies. This field trial with Nokia demonstrates that the scalable optical technology of PCS together with narrow linewidth laser sources can achieve capacities extremely close to the Shannon limit,” said Dr. Stephen Grubb, global optical network architect at Facebook. “This ensures that we are both maximizing our investment in submarine cable systems, as well as continuing to drive the cost per bit of submarine transport lower.”
Sam Bucci, head of optical networking at Nokia, said, “By demonstrating promising areas of Nokia Bell Labs research such as PCS, as well as coherent technologies available today, we hope to chart a path forward for the industry towards higher capacities, greater reach, and more network flexibility.”
The results of the trial will be presented in a post-deadline paper at the Optical Fiber Communication Conference and Exhibition (OFC) this week.
Photo by mightyohm 
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