• Nokia and Facebook break subsea spectral efficiency records
• Successfully completed transatlantic submarine trials over a 5,500km cable
• Field trial used Nokia Bell Labs' probabilistic constellation shaping technology
• Increased the stated capacity of the system by almost 2.5 times

Facebook is certainly stepping up its networking activities this year, thanks in no small part to its Telecom Infra Project. It announced this week that it has collaborated with Nokia on field trials of new optical digital signal processing technologies over a 5,500km transatlantic subsea link between Ireland and New York.

The problem being addressed is how to scale telecoms infrastructure, given the rising bandwidth demands of customers. For subsea cable routes, which provide the backbone to data networks, laying new fibre is a difficult and expensive undertaking. The ideal approach would be to improve the efficiency of the existing cable assets.

This is where Nokia Bell Labs comes in. The trial implemented the latest probabilistic constellation shaping (PCS) technology from Bell Labs and resulted in an increase of almost 2.5 times more capacity than the stated optical transmission capacity of the system. It was the first time this approach has been used for installed subsea equipment.

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 optical fibre link. Facebook decided it wanted to see if PCS would be effective on long subsea routes, so it conceived and planned the trial based on 64-QAM combined with digital nonlinearity compensation and low-linewidth lasers. The result was a record spectral efficiency of 7.46 b/s/Hz, indicating the potential to upgrade this cable to 32Tbit/s per fibre in the future.

"Facebook wants to increase the pace of innovation and adoption of next-generation optical technologies,” said Stephen Grubb, Global Optical Network Architect at Facebook. “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."

Transmission tests based on Nokia’s Photonic Service Engine 2 validated the successful transmission of 8-QAM wavelengths running at 200Gbit/s and 16-QAM wavelengths running at 250Gbit/s. In addition, 200Gbit/s 8-QAM wavelengths demonstrated sufficient performance margin to indicate it could support a reliable and commercial operation.

"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,” added Sam Bucci, Head of Optical Networking at Nokia.