ECOC 2016 – Nokia Bell Labs, working with Deutsche Telekom T-Labs and the Technical University of Munich, has achieved terabit transmission capacity and unprecedented spectral efficiency in a network field trial using a new modulation technique, called probabilistic constellation shaping.

This research will help telecom operators and enterprises to further optimise the distance and capacity of their optical networks – by dynamically adjusting transmission rates to suit the channel conditions.

The transmission rate achieved is close to the theoretical maximum information transfer rate of the channel, and thus approaches the Shannon Limit for the fibre link. (For more on the Shannon Limit, see the feature from our Autumn 2016 issue, Breaking the glass ceiling).

The work was carried out as part of the European-funded research project Safe and Secure European Routing (SASER), and Nokia Bell Labs will present the joint research at ECOC today in session M.1.D. under the title ‘Field Demonstration of 1 Tbit/s Super-Channel Network Using Probabilistically Shaped Constellations’.

Probabilistic constellation shaping (PCS) takes coherent transmission based on quadrature amplitude modulation – where information is encoded in both amplitude and phase – and modifies the probability with which constellation points are used. Constellation points are the ‘alphabet’ of the modulation scheme, corresponding to all the distinct symbols that could be used in that scheme.

Traditionally, all constellation points in the coding scheme are used with the same frequency. In contrast PCS uses constellation points with high amplitude less frequently than those with lesser amplitude, resulting in signals that, on average, are more resilient to noise and other impairments. This allows the transmission rate to be optimised, delivering up to 30 per cent greater reach.

Last year, researchers from TUM won the Nokia Bell Labs Prize for their work on constellation shaping. 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 fibre-optic systems transmit data faster, further, and with unparalleled flexibility.’

Now, in close collaboration with Nokia Bell Labs, the technology has been further developed, and tested under real-world conditions across the network of Deutsche Telekom T-Labs – an important step towards commercialisation of the technology.

The field trial was operated from Nokia Bell Labs facility in Stuttgart using three different loopback locations: in Darmstadt, Nuremberg and Berlin. Four-channel super-channels were successfully transmitted, using a variety of signalling rates combined with probabilistically shaped constellations based on 16QAM, 36QAM and 64QAM. The fastest super-channels reached one terabit per second and the highest spectral efficiency reached 5b/s/Hz.

Marcus Weldon, president Nokia Bell Labs and Nokia CTO, said: ‘Future optical networks not only need to support orders of magnitude higher capacity, but also the ability to dynamically adapt to channel conditions and traffic demand. Probabilistic constellation shaping offers great benefits to service providers and enterprises by enabling optical networks to operate closer to the Shannon Limit to … provide the flexibility and performance required for modern networking in the digital era.’