57 research outputs found
A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems
Optical communication systems represent the backbone of modern communication
networks. Since their deployment, different fiber technologies have been used
to deal with optical fiber impairments such as dispersion-shifted fibers and
dispersion-compensation fibers. In recent years, thanks to the introduction of
coherent detection based systems, fiber impairments can be mitigated using
digital signal processing (DSP) algorithms. Coherent systems are used in the
current 100 Gbps wavelength-division multiplexing (WDM) standard technology.
They allow the increase of spectral efficiency by using multi-level modulation
formats, and are combined with DSP techniques to combat the linear fiber
distortions. In addition to linear impairments, the next generation 400 Gbps/1
Tbps WDM systems are also more affected by the fiber nonlinearity due to the
Kerr effect. At high input power, the fiber nonlinear effects become more
important and their compensation is required to improve the transmission
performance. Several approaches have been proposed to deal with the fiber
nonlinearity. In this paper, after a brief description of the Kerr-induced
nonlinear effects, a survey on the fiber nonlinearity compensation (NLC)
techniques is provided. We focus on the well-known NLC techniques and discuss
their performance, as well as their implementation and complexity. An extension
of the inter-subcarrier nonlinear interference canceler approach is also
proposed. A performance evaluation of the well-known NLC techniques and the
proposed approach is provided in the context of Nyquist and super-Nyquist
superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial
Maximizing the optical network capacity
Most of the digital data transmitted are carried by optical fibres, forming the great part of the national and international communication infrastructure. The information-carrying capacity of these networks has increased vastly over the past decades through the introduction of wavelength division multiplexing, advanced modulation formats, digital signal processing and improved optical fibre and amplifier technology. These developments sparked the communication revolution and the growth of the Internet, and have created an illusion of infinite capacity being available. But as the volume of data continues to increase, is there a limit to the capacity of an optical fibre communication channel? The optical fibre channel is nonlinear, and the intensity-dependent Kerr nonlinearity limit has been suggested as a fundamental limit to optical fibre capacity. Current research is focused on whether this is the case, and on linear and nonlinear techniques, both optical and electronic, to understand, unlock and maximize the capacity of optical communications in the nonlinear regime. This paper describes some of them and discusses future prospects for success in the quest for capacity
Digital signal processing for coherent optical fibre communications
In this thesis investigations were performed into digital signal processing (DSP)
algorithms for coherent optical fibre transmission systems, which provide improved
performance with respect to conventional systems and algorithms. Firstly, an
overview of coherent detection and coherent transmission systems is given.
Experimental investigations were then performed into the performance of digital
backpropagation for mitigating fibre nonlinearities in a dual-polarization quadrature
phase shift keying (DP-QPSK) system over 7780 km and a dual-polarization 16-
level quadrature amplitude modulation (DP-QAM16) system over 1600 km. It is
noted that significant improvements in performance may be achieved for a nonlinear
step-size greater than one span. An approximately exponential relationship was
found between performance improvement in Q-factor and the number for required
complex multipliers.
DSP algorithms for polarization-switched quadrature phase shift keying (PS-QPSK)
are then investigated. A novel two-part equalisation algorithm is proposed which
provides singularity-free convergence and blind equalisation of PS-QPSK. This
algorithm is characterised and its application to wavelength division multiplexed
(WDM) transmission systems is discussed.
The thesis concludes with an experimental comparison between a PS-QPSK
transmission system and a conventional DP-QPSK system. For a 42.9 Gb/s WDM
system, the use of PS-QPSK enabled an increase of reach of more than 30%. The
resultant reach of 13,640 km was, at the time of publication, the longest transmission
distance reported for 40 Gb/s transmission over an uncompensated link with standard
fibre and optical amplification
Mitigation of nonlinear impairments for advanced optical modulation formats
Optical fibre networks form the backbone of the global communication infrastructure but are currently experiencing an unprecedented level of stress due to more and more bandwidth-hungry applications. In an effort to address this and avoid a so-called capacity crunch, research groups around the world have focused their attention on more spectrally-efficient modulation formats, to increase available capacity at a competitive cost. However, the drive towards higher- order modulation formats leads to greater transmission impairments, reducing the maximum distance over which increased capacity can be provided. The thesis describes the research work carried out to investigate the achievable transmission distances when using higher order modulation formats together with digital backpropagation (DBP). DBP is a digital signal processing (DSP) algorithm, capable of compensating for deterministic nonlinear impairments by inverting the fibre channel. Single-channel and wavelength-division-multiplexed (WDM) transmission has been investigated in experiment and simulation for a variety of polarisation-division-multiplexed (PDM) modulation formats: binary-phase-shift-keying (PDM-BPSK), quadrature-phase-shift-keying (PDM-QPSK), 8-phase-shift-keying (PDM-8PSK), 8-quadrature amplitude modulation (PDM-8QAM), 16-quadrature amplitude modulation (PDM-16QAM) and polarisation-switched QPSK (PS-QPSK). Record transmission distances were achieved in WDM transmission experiments with PDM-BPSK, PS-QPSK and PDM-QPSK at 42.9Gbit/s as well as for PDM-8PSK and PDM-8QAM at 112Gbit/s, over the most common fibre type: standard single mode fibre (SSMF) and the most common amplification solution: erbium doped fibre amplifiers (EDFA). For the first time, nonlinear compensation has been compared experimentally for different modulation formats and a fixed-complexity DBP algorithm. Its use led to increased benefit for more spectrally efficient modulation formats. Computer simulations were used to explore the upper bounds of achievable performance improvement with DBP, using an algorithm with unconstrained complexity. Furthermore, DBP was investigated for varying symbol rates and channel spacings to investigate trade-offs with respect to the digital receiver bandwidth. It was shown that even though DBP is computationally expensive, it can achieve significant improvements in transmission reach and BER performance. The results presented in this thesis, can be applied to the design of future optical transmission systems
Maximising Achievable Rates of Experimental Nonlinear Optical Fibre Transmission Systems
It is generally expected that the demand for digital data services will continue to grow, placing ever greater requirements on optical fibre networks which carry the bulk of digital data. Research to maximise achievable information rates (AIR) over fibre has led to increasing spectral efficiency, symbol rate and bandwidth use. All of these contribute to transmission impairments due to the nonlinear nature of the optical fibre. This thesis describes research performed to investigate the effects of nonlinear impair- ments on the AIRs of experimental optical fibre transmission. To maximise throughput, the entire available optical bandwidth should be filled with transmission channels. An investigation into large bandwidth transmission through the use of spectrally shaped amplified spontaneous emission noise (SS-ASE) was con- ducted. The enhanced Gaussian noise model is used to analytically describe this tech- nique, and SS-ASE was experimentally shown to provide a lower bound on the AIR. Nonlinear interference (NLI) was modelled from an inter-symbol interference (ISI) model to characterise the noise and was experimentally verified. This new understand- ing helps quantify potential gain available from nonlinearity mitigation. Multicore fibres offer an alternative route to improve AIR, and are susceptible to another noise source known as crosstalk. This inter-core crosstalk can be controlled by suitable design of the fibre, hence in the limiting case, NLI rather than crosstalk will limit AIR. Nonlinearity compensation was, for the first time, experimentally demon- strated in the presence of crosstalk in a homogeneous 7-core fibre and shown to provide an increase in AIR. The results of this thesis can be used to evaluate future transmission systems for maximising information rates. It was shown that experimentally, SS-ASE is a viable transmission tool to evaluate system performance, NLI can be characterised using an ISI model and nonlinearity mitigation is possible in MCF systems limited by crosstalk
Tecnologias coerentes para redes ópticas flexíveis
Next-generation networks enable a broad range of innovative services with
the best delivery by utilizing very dense wired/wireless networks. However,
the development of future networks will require several breakthroughs in
optical networks such as high-performance optical transceivers to support a
very-high capacity optical network as well as optimization of the network
concept, ensuring a dramatic reduction of the cost per bit.
At the same time, all of the optical network segments (metro, access,
long-haul) need new technology options to support high capacity, spectral
efficiency and data-rate flexibility. Coherent detection offers an opportunity
by providing very high sensitivity and supporting high spectral efficiency.
Coherent technology can still be combined with polarization multiplexing.
Despite the increased cost and complexity, the migration to dual-polarization
coherent transceivers must be considered, as it enables to double the spectral
efficiency. These dual-polarization systems require an additional digital signal
processing (DSP) subsystem for polarization demultiplexing. This work seeks
to provide and characterize cost-effective novel coherent transceivers for
the development of new generation practical, flexible and high capacity
transceivers for optical metro-access and data center interconnects. In this
regard, different polarization demultiplexing (PolDemux) algorithms, as well
as adaptive Stokes will be considered.
Furthermore, low complexity and modulation format-agnostic DSP techniques
based on adaptive Stokes PolDemux for flexible and customizable
optical coherent systems will be proposed. On this subject, the performance
of the adaptive Stokes algorithm in an ultra-dense wavelength division multiplexing
(U-DWDM) system will be experimentally evaluated, in offline
and real-time operations over a hybrid optical-wireless link. In addition, the
efficiency of this PolDemux algorithm in a flexible optical metro link based
on Nyquist pulse shaping U-DWDM system and hybrid optical signals will be
assessed. Moreover, it is of great importance to find a transmission technology
that enables to apply the Stokes PolDemux for long-haul transmission
systems and data center interconnects. In this work, it is also proposed
a solution based on the use of digital multi-subcarrier multiplexing, which
improve the performance of long-haul optical systems, without increasing
substantially, their complexity and cost.As redes de telecomunicações futuras permitirão uma ampla gama de serviços
inovadores e com melhor desempenho. No entanto, o desenvolvimento das
futuras redes implicará vários avanços nas redes de fibra ótica, como transcetores
óticos de alto desempenho capazes de suportar ligações de muito
elevada capacidade, e a otimização da estrutura da rede, permitindo uma
redução drástica do custo por bit transportado.
Simultaneamente, todos os segmentos de rede ótica (metropolitanas, acesso
e longo alcance) necessitam de novas opções tecnológicas para suportar
uma maior capacidade, maior eficiência espetral e flexibilidade. Neste contexto,
a deteção coerente surge como uma oportunidade, fornecendo alta
sensibilidade e elevada eficiência espetral. A tecnologia de deteção coerente
pode ainda ser associada à multiplexação na polarização. Apesar de um
potencial aumento ao nível do custo e da complexidade, a migração para
transcetores coerentes de dupla polarização deve ser ponderada, pois permite
duplicar a eficiência espetral. Esses sistemas de dupla polarização requerem
um subsistema de processamento digital de sinal (DSP) adicional para desmultiplexagem
da polarização. Este trabalho procura fornecer e caracterizar
novos transcetores coerentes de baixo custo para o desenvolvimento de uma
nova geração de transcetores mais práticos, flexíveis e de elevada capacidade,
para interconexões óticas ao nível das futuras redes de acesso e metro.
Assim, serão analisados diferentes algoritmos para a desmultiplexagem da
polarização, incluindo uma abordagem adaptativa baseada no espaço de
Stokes.
Além disso, são propostas técnicas de DSP independentes do formato de
modulação e de baixa complexidade baseadas na desmultiplexagem de Stokes
adaptativa para sistemas óticos coerentes flexíveis. Neste contexto, o desempenho
do algoritmo adaptativo de desmultiplexagem na polarização
baseado no espaço de Stokes é avaliado experimentalmente num sistema
U-DWDM, tanto em análises off-line como em tempo real, considerando um
percurso ótico hibrido que combina um sistema de transmissão suportado
por fibra e outro em espaço livre. Foi ainda analisada a eficiência do algoritmo
de desmultiplexagem na polarização numa rede ótica de acesso flexível
U-DWDM com formatação de pulso do tipo Nyquist. Neste trabalho foi
ainda analisada a aplicação da técnica de desmultiplexagem na polarização
baseada no espaço de Stokes para sistemas de longo alcance. Assim, foi
proposta uma solução de aplicação baseada no uso da multiplexagem digital
de múltiplas sub-portadoras, tendo-se demonstrado uma melhoria na eficiência
do desempenho dos sistemas óticos de longo alcance, sem aumentar
significativamente a respetiva complexidade e custo.Programa Doutoral em Engenharia Eletrotécnic
- …