FOCUS TECHNOLOGY


that rely on fixed-rate transmission is they lack flexibility’ said Dr Kyle Guan, a research scientist at Nokia Bell Labs. ‘At shorter distances, it is possible to transmit data at much higher rates, but fixed-rate systems lack the capability to take advantage of that opportunity.’ Using the capabilities of modern


distance-adaptive transmission systems, Guan set about building a mathematical model to determine the optimal layout of network infrastructure for data transfer between cloud data centres and the end users. ‘Te question I wanted to answer


was how to design a network that would allow for the most efficient flow of data traffic,’ said Guan. “‘In a continent-wide system, what would be the most effective [set of] locations for data centres and how should bandwidth be apportioned? It quickly became apparent that my model would have to reflect not just the flow of traffic between data centres and end users, but also the flow of traffic between data centres.’ External industry research


suggests traffic between the data centres represents about a third of total cloud traffic. It includes activities such as data backup and


load balancing, whereby tasks are completed by multiple servers to maximise application performance. ‘My preliminary results showed


that in a continental-scale network with optimised data centre placement and bandwidth allocation, distance-adaptive transmission can use 50 per cent less wavelength resources, or light transmission and reception equipment, compared to fixed-rate transmission,’ said Guan. ‘On a functional level, this could allow cloud service providers to significantly increase the volume of traffic supported on the existing


Transceiver noise limits optical system capacity, researchers find


Lidia Galdino from University College London’s department of Electronic and Electrical Engineering. She was surprised to discover that the transceivers used to transmit and receive optical signals have more of an impact on system performance than previously thought. Working as part of the UNLOC research


W


project, Dr Galdino’s research involves running experiments to find out how to maximise the capacity of optical fibre communications systems. Tese experiments test new techniques for compensating for nonlinearities, which ultimately limit the capacity of data transmission over standard optical fibre. A particular focus of the UCL team is a technique known as digital back-propagation (DBP). DBP detects the important properties of the light – amplitude and phase – and then digitally reverses the light’s journey to cancel out known distortions. In labs around the world, researchers have


consistently found that theoretical results greatly overestimate the gains they can achieve compared to practical experiments. Dr Galdino wanted to determine the cause of these discrepancies; essentially, what was everyone doing wrong? Until now, researchers thought the random


rotation of light polarisation in fibre known as polarisation mode dispersion (PMD), which cannot be compensated by DBP, was the main


12 FIBRE SYSTEMS Issue 15 • Spring 2017


hy do lab results never live up to simulations when determining the capacity of optical fibre? Tat’s the question considered by Dr


Galdino demonstrates for the first time that transceiver noise interferes nonlinearly with the signal, meaning it doesn’t increase proportionately as signal power is increased. Before now, researchers underestimated the impact of transceiver noise by thinking that the interference it caused increased at a steady rate as signal power is increased. ‘We’ve proposed a new approach that correctly


Transceiver noise is a fundamental limit to optical system performance, says Dr Lidia Galdino of UCL Department of Electronic and Electrical Engineering (pictured)


reason lab results didn’t return the gains they had hoped for. However, despite this observed degradation in nonlinearity compensation due to PMD, theory still substantially overestimated the gains measured in experiments. Tis is because simulations assume ideal


transceivers and overestimate the performance of nonlinearity compensation techniques. Realising this, Dr Galdino and her colleagues


entered the parameters for their transceiver into a simulation. When the results from the lab matched the theory, they confirmed the issue must be with transceiver noise. ‘Tis is hugely significant for the design of fibre infrastructure. It is now possible to identify all sources of error in system performance and, therefore, to find techniques to mitigate them,’ she said. In a paper published in Optics Express, Dr


accounts for the nonlinear interference between the transceiver noise and signal in an optical fibre link. For the first time, every system designer can easily predict their transmission system performance in seconds,’ said PhD student Daniel Semrau, co-author of the study. Te noise produced by the transceiver comes


from digital-to-analogue and analogue-to-digital converters in the transmitter and receiver respectively. Tis fundamental noise is present even in our state-of-the-art equipment, but in time manufacturers will be able to improve on this. Te UNLOC research will help systems designers to correctly predict and design next-generation, high-performance optical transmission systems, says Galdino. ‘We should consider transceiver noise as a more


fundamental limit to performance than PMD,’ said Dr Galdino. ‘We can now precisely estimate achievable gains by applying DBP in realistic optical systems. We’ve been doing all the right things and DBP is still one of the most powerful ways to maximise capacity or increase transmission distances.’l


@fibresystemsmag | www.fibre-systems.com


fibre-optic network with the same wavelength resources. ‘Other important factors that have


to be considered include the proximity of data centres to renewable sources of energy that can power them, and latency – the interval of time that passes from when an end user or data centre initiates an action and when they receive a response,’ he said. Future research will involve


integrating these types of factors into his model so that he can run simulations that even more closely mirror the complexity of real-world conditions.l


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