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Nokia and Facebook break subsea spectral efficiency record

OFC 2017, LOS ANGELES – Nokia and Facebook have shown that it should be possible to cram 32Tb/s over a 5,500km submarine cable between New York and Ireland, an increase of two and a half times the cable’s stated operational capacity.

The field trial, which used a Bell Labs invention called “probabilistic constellation shaping” (PCS), also achieved a record spectral efficiency for a submarine cable of 7.46 b/s/Hz.

The results have been accepted as a post-deadline paper – a late-breaking result that’s considered particularly significant – that will be presented at OFC 2017 on Thursday 23 March.

“Facebook approached us last fall and asked if we would like to test our gear on their cable, and of course we jumped at the chance,” said Kyle Hollasch, Nokia’s director of product marketing for optical networking.

Nokia set up tests in two different ways.

The first used Nokia’s commercial PSE2-based coherent transmission technology, which has been shipping in volume for the past year (see Nokia claims capacity leap from new silicon). The design capacity of Facebook’s fibre pair on the America Europe Connect (AEC-1) cable is 13Tb/s, but Nokia showed the potential to reach 17Tb/s using its 1830 PSS optical transport platform running 200G 8QAM wavelengths with real-time digital signal processing (DSP).

“The 200G is notable because this rate had never been tested across an Atlantic system before,” said Hollasch. “It’s important to point out that this is not a lab ‘hero experiment’ using spools of fibre; this is commercial hardware over a real-world deployed fibre.”

The 200G 8-QAM wavelengths supported a spectral efficiency of 4b/s/Hz while exhibiting sufficient performance margin to support reliable, commercial operation, the company reports.

The second field trial used what Nokia describes as a “mature demo” of its PCS technology using benchtop boards and off-line signal processing. PCS concentrates information into the inner symbols in a constellation that are lower power. This allows the overall power of the signal to be turned up, which increases the space between symbols, boosting the signal-to-noise ratio.

Nokia carried out a terrestrial field trial of the technology with DT last year (see Nokia Bell Labs, T-Labs and Technical University of Munich test drive novel modulation scheme).

In the submarine experiment, conceived and planned by Facebook, PCS applied to 64QAM was combined with ultra-narrow linewidth lasers and digital nonlinear compensation (digital back propagation) to achieve a record spectral efficiency of 7.46 b/s/Hz. The experiment used a five-carrier super-channel; a fully populated system would have a capacity of 32Tb/s per fibre.

Hollasch believes PCS will prove to be a powerful technique for optimising the capacity of an optical link. PCS can provide operators with about 1.5dB of ‘shaping gain’ on their transmission system, which translates into capacity upgrades or additional transmission distances for operators. “PCS is like having an infinitely adjustable dial on the modulation scheme. Ultimately, we think this will prove superior to having multiple baud rates,” he said.

Facebook was very happy with the results. Commenting on the trial, Stephen Grubb, global optical network architect at Facebook, said: “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. 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.”

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