Lead researcher, Dr Robert Maher, UCL Electronic & Electrical Engineering, said: “While current state-of-the-art commercial optical transmission systems are capable of receiving single channel data rates of up to 100 gigabits per second (Gb/s), we are working with sophisticated equipment in our lab to design the next generation core networking and communications systems that can handle data signals at rates in excess of 1 terabit per second (Tb/s).
“For comparison this is almost 50,000 times greater than the average speed of a UK broadband connection of 24 megabits per second (Mb/s), which is the current speed defining “superfast” broadband. To give an example, the data rate we have achieved would allow the entire HD Games of Thrones series to be downloaded within one second.”
Nature Scientific Reports - Increasing the information rates of optical communications via coded modulation: a study of transceiver performance
The team determined the best way of encoding information in optical signals, taking into account the limitations of the transmitter and receiver. They then applied coding techniques commonly used in wireless communications, but not yet widely used in optical communications, to ensure the transmitted signals are adapted to distortions in the system electronics.
Using UNLOC’s state-of-the-art lab facilities, the researchers built the new optical system and measured its performance. Fifteen channels, each carrying an optical signal of different wavelength were modulated using the 256QAM format typically used in cable modems, combined and sent to a single optical receiver for detection. By grouping the channels together, the team created a ‘super-channel’ which although not yet commercially available, is widely believed to be a way forward for the next generation of high-capacity communication systems.
“Using high-bandwidth super-receivers enables us to receive an entire super-channel in one go. Super-channels are becoming increasingly important for core optical communications systems, which transfer bulk data flows between large cities, countries or even continents. However, using a single receiver varies the levels of performance of each optical sub-channel so we had to finely optimise both the modulation format and code rate for each optical channel individually to maximize the net information data rate. This ultimately resulted in us achieving the greatest information rate ever recorded using a single receiver,” said Dr Maher.
In this study, the researchers connected the transmitter directly to the receiver to achieve the maximum data rate. They will now test the system and measure the achievable data rates in a long distance transmission scenario where optical signals can become distorted as they travel through thousands of kilometers of optical fibers.
Abstract - Increasing the information rates of optical communications via coded modulation: a study of transceiver performance
Optical fibre underpins the global communications infrastructure and has experienced an astonishing evolution over the past four decades, with current commercial systems transmitting data rates in excess of 10 Tb/s over a single fibre core. The continuation of this dramatic growth in throughput has become constrained due to a power dependent nonlinear distortion arising from a phenomenon known as the Kerr effect. The mitigation of fibre nonlinearities is an area of intense research. However, even in the absence of nonlinear distortion, the practical limit on the transmission throughput of a single fibre core is dominated by the finite signal-to-noise ratio (SNR) afforded by current state-of-the-art coherent optical transceivers. Therefore, the key to maximising the number of information bits that can be reliably transmitted over a fibre channel hinges on the simultaneous optimisation of the modulation format and code rate, based on the SNR achieved at the receiver. In this work, we use an information theoretic approach based on the mutual information and the generalised mutual information to characterise a state-of-the-art dual polarisation m-ary quadrature amplitude modulation transceiver and subsequently apply this methodology to a 15-carrier super-channel to achieve the highest throughput (1.125 Tb/s) ever recorded using a single coherent receiver.
SOURCES - University College of London, Nature Scientific Reports