Nokia has lit up a trans-Atlantic fibre for Facebook, in a field trial that showed off 200 Gbps and 250 Gbps wavelengths on a 5,500 km link.
According to Nokia, applying a technique from Bell Labs called probabilistic constellation shaping (PCS) yielded a 2.5x increase in the rated capacity of the New York-Ireland cable used in the test.
PCS was combined with low-linewidth lasers and tricks to compensate for nonlearity in the fibre, to achieve spectral efficiency of 7.46 bits per second per Hertz in the 64-QAM transmissions.
PCS works by changing how QAM (quadrature amplitude modulation) works.
“Constellation” refers to the number of states that can be represented using changes to amplitude and phase. For example, the image below shows a 16-QAM scheme:
QAM schemes use phase (vertical) and amplitude (horizontal) as modulation
IMAGE By Splash, CC BY-SA 3.0, from Wikimedia Commons
In conventional QAM, every point in the constellation has equal weight – that is, all points are used with the same frequency. PCS, on the other hand, tries to choose optimal constellation points for conditions on the channel.
As Nokia explained last year after demonstrating a Terabit channel with Deutsche Telekom:
“PCS … uses constellation points with high amplitude less frequently than those with lesser amplitude to transmit signals that, on average, are more resilient to noise and other impairments. This allows the transmission rate to be tailored to ideally fit the transmission channel, delivering up to 30 percent greater reach.”
Nokia believes the field trial demonstrates that PCS will let the cable handle as much as 32 Tbps per fibre in the future. ®