Scientists Successfully Push Fibre Optic Transmissions Close to the Limit

A joint team of researchers working for Nokia Bell Labs, Deutsche Telekom T-Labs and the Technical University of Munich in Germany has successfully used a new technology to push 1Tbps of real-world data transfer over a pure fibre optic cable to hit close to its theoretical maximum (The Shannon Limit).

In the words of Wikipedia, “the noisy-channel coding theorem (sometimes Shannon’s theorem), establishes that for any given degree of noise contamination of a communication channel, it is possible to communicate discrete data (digital information) nearly error-free up to a computable maximum rate through the channel.”

Fibre optic connections don’t have to worry about the same sort of interference (e.g. EM interference) that metallic cables suffer, but optical communications still require sophisticated signal processing to overcome certain problems (e.g. the optical Kerr effect) and you can often only squeeze so much light down such a cable before practical limits like signal power, cost and computer processing get in the way.

In other words, getting the best possible efficiency (i.e. fastest data transfers over a given distance) out of a single optical fibre is still a big challenge. As such the development of a new modulation approach, known as Probabilistic Constellation Shaping (PCS), could offer an important upgrade.

The Technical Explanation

The trial of the novel modulation approach, known as Probabilistic Constellation Shaping (PCS), uses quadrature amplitude modulation (QAM) formats to achieve higher transmission capacity over a given channel to significantly improve the spectral efficiency of optical communications.

PCS modifies the probability with which constellation points – the alphabet of the transmission – are used. Traditionally, all constellation points are used with the same frequency. PCS cleverly 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.

As part of the Safe and Secure European Routing (SASER) project, the experiment was deployed over a real-world optical fibre network belonging to Deutsche Telekom and achieved a net transmission rate of 1Tbps (Terabits per second). The fact that they were able to do this over an existing commercial fibre network is quite impressive.

Gerhard Kramer, Professor at the Technical University of Munich, said:

“Information theory is the mathematics of digital technology, and during the Claude E. Shannon centenary year 2016 it is thrilling to see his ideas continue to transform industries and society. Probabilistic constellation shaping, an idea that won a Bell Labs Prize, directly applies Shannon’s principles and lets fiber optic systems transmit data faster, further, and with unparalleled flexibility.

The success of the close collaboration with Nokia Bell Labs, who further developed the technology, and Deutsche Telekom T-Labs, who tested it under real conditions, is satisfying confirmation that TUM Engineering is a label of outstanding quality, and that TUM teaching gives our students the intellectual tools to compete, succeed and lead globally.”

Annoyingly the team doesn’t say precisely what type or setup of optical fibre network was used or over what distance the test was conducted, which would have been useful in order to compare it with existing achievements. Otherwise the headline speed of 1Tbps is in danger of sounding unremarkable (multi-Terabit announcements over fibre optic comms are now fairly common), even though it’s exactly the opposite.

For example, only a few months ago BT announced that they’d delivered transmission speeds of 2Tbps over a 727km live core network link (London to Dublin) and a separate test pushed 5.6Tbps via a single optical fibre (here). The real-time 5.6Tbps optical superchannel was run over a closed loop network running between the BT Tower and Adastral Park, which comprised 28 x 200Gbps (64GBaud/QPSK) sub-channels, bundled together to provide combined capacity, achieving highest capacity and spectral efficiency.

Hopefully some more detail about the new modulation technology will be released later today when the team presents the results of their joint experiment at the European Conference on Optical Communication (ECOC) in Düsseldorf, Germany.