Figure 1: The UNLOC programme is structured in four research themes (RT).
Arrows represent feedback and interaction between RTs
The optical channel is nonlinear as a result of the intensity-dependent refractive index in glass. Optical systems have been assumed to be fundamentally limited by fibre nonlinearities. Much research in the UK and beyond is focused on developing new transmission fibres with lower losses and nonlinearity which will allow for higher power levels to be used thus increasing channel capacity. An alternative approach pursued by researchers is to use spatial multiplexing – multi-core fibres or spatial modes in a multimode fibre and apply digital signal processing (DSP) to counteract modal mixing.
However, in order to overcome the 'capacity crunch', one must enter the nonlinear region and develop innovative and radically different approaches to coding, transmission and processing of information. In UNLOC, we aim to show that with the correct application of science and engineering it is possible to break the nonlinear capacity barrier.
The goal of this programme is to devise optical communication systems which employ advanced information theory and DSP to approach the Shannon limit for nonlinear optical channels and overcome the capacity limit of current systems.
We focus on the key areas of high-speed optical fibre transmission. There is a pressing need to:
- quantify the fundamental information theory limits of nonlinear channels carried over existing and future optical fibre infrastructure
- develop and experimentally demonstrate techniques: algorithms, coding and formats to approach these limits in order to maximise the achievable channel capacity while minimising the energy per bit
- apply these techniques on a network-wide scale, achieving the maximum possible overall adaptive network capacity
The programme is divided into four inter-related Research Themes (see Figure 1) that will guide UNLOC researchers in achieving the research goals of the project.
Shannon Capacity for Nonlinear Fibre Channel
Development of the Fundamental Information Theory (IT) for the nonlinear channels.
Team Lead: Prof. Sergei Turitsyn (Aston University)
The main goal of RT1 is to theoretically analyse nonlinear channels and identify what nonlinear elements are required to design channels with capacities which are up to two orders of magnitude above the classical Shannon limit for linear channel. To achieve this goal, UNLOC researchers are:
- developing a theory of nonlinear fibre communication channels with analytical and semi-analytical models of the nonlinear fibre channel
- quantifying fundamental capacity limits of the nonlinear channels carried over optical fibres including multi-core, multi-mode and photonic crystal fibres
- developing information theory techniques to take advantage of the specific nonlinear properties of fibre channels, such as e.g. eigenvalue coding and communications for advanced back propagation
- examining methods of mitigation of nonlinear transmission effects (implementing various nonlinear channels) to maximise capacity
Coding, DSP and Nonlinearity Management
Development of Advanced techniques to maximise Spectral Efficiency.
Team Lead: Robert Killey (UCL)
RT2 explores the practical implementation of techniques and technologies required to realise the capacity and spectral efficiency limits predicted by RT1. An extensive investigation will identify optimal modulation, pulse formats, source coding, spectral arrangement of channels, impairment mitigation and error correction to approach the maximum achievable spectral efficiency in dispersive, nonlinear fibre channels determined in RT1.
Research covers both conventional single-mode and multi-mode fibre and photonic crystal fibre (MMF/FMF/PCF) based links. RT2 will study the effect of random coupling between the modes in a fibre and its impact on approaching the capacity and spectral efficiency limits predicted by the information theory studies in RT1. While RT1 focuses on an ideal information theoretic channel, RT2 considers real transceivers which introduce limitations due to digital quantisation noise, phase noise of the local oscillator laser and distortion of the synchronisation. Thus, RT2 explores real systems taking into account practical non-idealities in order to quantify their impact on the realisable system capacity.
Optical Communications Systems in the Network Context
Team Lead: Benn Thomsen (UCL) and Philip Watts (UCL)
The channel capacity concept is usually applied to point-to-point links. Optical networks, however, are sets of interconnections of nodes by high-capacity optical links. Thus, the capacity in the context of optical networks, and especially nonlinear optical networks, requires the development of techniques for network-wide optimisation.
The main goal of RT3 is the development of a reconfigurable optical network which includes software defined transceivers, adaptive regenerators and electronic and optical routing nodes to meet the offered traffic demand using the least resource and, hence, energy possible. This is a major research challenge requiring the application of distributed control theory, network optimisation and dynamic routing algorithms, in the context of the channels transported being nonlinear. Within RT3, we work on:
- new approaches for network control that are scalable, reliable and make efficient use of network resources in response to temporal-spatial traffic variations
- channel estimation, including both linear nonlinear impairments using the techniques developed within RT1 and RT2
- understanding the capacity and energy savings achievable in a cognitive reconfigurable network
Implementation and System Demostration
Team Lead: Paul Harper (Aston) and Polina Bayvel (UCL)
RT4 focuses on developing a comprehensive programme to experimentally demonstrate and verify the concepts and techniques investigated theoretically in RT1-3 in order to explore their practical and commercial feasibility. The groups at UCL and Aston have been among the pioneers in nonlinear transmission experimental research including distributed amplification and receiver- and transmitter-based digital signal processing techniques.
Although different experimental facilities exist in leading laboratories around the world, including UCL and Aston, there is no single experimental test-bed which would allow the above questions to be addressed with the bit-rates and functionality proposed. In addition to the test-bed, will bring extra benefits in becoming a research facility to the UK academic and industrial partners and collaborators for a range of device, system and network scenarios to be explored. The main objectives within RT4 include:
- experimental verification of theoretical predictions of spectral efficiency × transmission distance, and demonstration of the nonlinear channel capacities beyond the Shannon limit determined in RT1
- experimental verification of the transceiver designs, including real-time DSP algorithms allowing calculation of transceiver, link and network power consumption figures provided by RT2
- experimental verification of the nonlinear capacity of optical networks, including assessment of impairment-aware routing algorithms and the flexible and intelligent use of network resources defined in RT3