179 research outputs found
Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications
The rapid proliferation of the Internet has been driving communication networks closer and closer to their limits, while available bandwidth is disappearing due to an ever-increasing network load. Over the past decade, optical fiber communication technology has increased per fiber data rate from 10 Tb/s to exceeding 10 Pb/s. The major explosion came after the maturity of coherent detection and advanced digital signal processing (DSP). DSP has played a critical role in accommodating channel impairments mitigation, enabling advanced modulation formats for spectral efficiency transmission and realizing flexible bandwidth. This book aims to explore novel, advanced DSP techniques to enable multi-Tb/s/channel optical transmission to address pressing bandwidth and power-efficiency demands. It provides state-of-the-art advances and future perspectives of DSP as well
Compensation of Laser Phase Noise Using DSP in Multichannel Fiber-Optic Communications
One of the main impairments that limit the throughput of fiber-optic communication systems is laser phase noise, where the phase of the laser output drifts with time. This impairment can be highly correlated across channels that share lasers in multichannel fiber-optic systems based on, e.g., wavelength-division multiplexing using frequency combs or space-division multiplexing. In this thesis, potential improvements in the system tolerance to laser phase noise that are obtained through the use of joint-channel digital signal processing are investigated. To accomplish this, a simple multichannel phase-noise model is proposed, in which the phase noise is arbitrarily correlated across the channels. Using this model, high-performance pilot-aided phase-noise compensation and data-detection algorithms are designed for multichannel fiber-optic systems using Bayesian-inference frameworks. Through Monte Carlo simulations of coded transmission in the presence of moderate laser phase noise, it is shown that joint-channel processing can yield close to a 1 dB improvement in power efficiency. It is further shown that the algorithms are highly dependent on the positions of pilots across time and channels. Hence, the problem of identifying effective pilot distributions is studied.The proposed phase-noise model and algorithms are validated using experimental data based on uncoded space-division multiplexed transmission through a weakly-coupled, homogeneous, single-mode, 3-core fiber. It is found that the performance improvements predicted by simulations based on the model are reasonably close to the experimental results. Moreover, joint-channel processing is found to increase the maximum tolerable transmission distance by up to 10% for practical pilot rates.Various phenomena decorrelate the laser phase noise between channels in multichannel transmission, reducing the potency of schemes that exploit this correlation. One such phenomenon is intercore skew, where the spatial channels experience different propagation velocities. The effect of intercore skew on the performance of joint-core phase-noise compensation is studied. Assuming that the channels are aligned in the receiver, joint-core processing is found to be beneficial in the presence of skew if the linewidth of the local oscillator is lower than the light-source laser linewidth.In the case that the laser phase noise is completely uncorrelated across channels in multichannel transmission, it is shown that the system performance can be improved by applying transmitter-side multidimensional signal rotations. This is found by numerically optimizing rotations of four-dimensional signals that are transmitted through two channels. Structured four-dimensional rotations based on Hadamard matrices are found to be near-optimal. Moreover, in the case of high signal-to-noise ratios and high signal dimensionalities, Hadamard-based rotations are found to increase the achievable information rate by up to 0.25 bits per complex symbol for transmission of higher-order modulations
Machine Learning in Digital Signal Processing for Optical Transmission Systems
The future demand for digital information will exceed the capabilities of current optical communication systems, which are approaching their limits due to component and fiber intrinsic non-linear effects. Machine learning methods are promising to find new ways of leverage the available resources and to explore new solutions. Although, some of the machine learning methods such as adaptive non-linear filtering and probabilistic modeling are not novel in the field of telecommunication, enhanced powerful architecture designs together with increasing computing power make it possible to tackle more complex problems today. The methods presented in this work apply machine learning on optical communication systems with two main contributions. First, an unsupervised learning algorithm with embedded additive white Gaussian noise (AWGN) channel and appropriate power constraint is trained end-to-end, learning a geometric constellation shape for lowest bit-error rates over amplified and unamplified links. Second, supervised machine learning methods, especially deep neural networks with and without internal cyclical connections, are investigated to combat linear and non-linear inter-symbol interference (ISI) as well as colored noise effects introduced by the components and the fiber. On high-bandwidth coherent optical transmission setups their performances and complexities are experimentally evaluated and benchmarked against conventional digital signal processing (DSP) approaches. This thesis shows how machine learning can be applied to optical communication systems. In particular, it is demonstrated that machine learning is a viable designing and DSP tool to increase the capabilities of optical communication systems
Transmissores-recetores de baixa complexidade para redes óticas
Traditional coherent (COH) transceivers allow encoding of information in
both quadratures and the two orthogonal polarizations of the electric field.
Nevertheless, such transceivers used today are based on the intradyne
scheme, which requires two 90o optical hybrids and four pairs of balanced
photodetectors for dual-polarization transmission systems, making its overall
cost unattractive for short-reach applications. Therefore, SSB methods
with DD reception, commonly referred to as self-coherent (SCOH)
transceivers, can be employed as a cost-effective alternative to the traditional
COH transceivers. Nevertheless, the performance of SSB systems
is severely degraded. This work provides a novel SCOH transceiver architecture
with improved performance for short-reach applications. In particular,
the development of phase reconstruction digital signal processing (DSP)
techniques, the development of other DSP subsystems that relax the hardware
requirement, and their performance optimization are the main highlights
of this research.
The fundamental principle of the proposed transceiver is based on the reception
of the signal that satisfies the minimum phase condition upon DD.
To reconstruct the missing phase information imposed by DD, a novel DCValue
method exploring the SSB and the DC-Value properties of the minimum
phase signal is developed in this Ph.D. study. The DC-Value method
facilitates the phase reconstruction process at the Nyquist sampling rate
and requires a low intensity pilot signal. Also, the experimental validation
of the DC-Value method was successfully carried out for short-reach optical
networks. Additionally, an extensive study was performed on the DC-Value
method to optimize the system performance. In the optimization process,
it was found that the estimation of the CCF is an important parameter to
exploit all advantages of the DC-Value method. A novel CCF estimation
technique was proposed. Further, the performance of the DC-Value method
is optimized employing the rate-adaptive probabilistic constellation shaping.Os sistemas de transcetores coerentes tradicionais permitem a codificação
de informação em ambas quadraturas e em duas polarizações ortogonais
do campo elétrico. Contudo, estes transcetores utilizados atualmente são
baseados num esquema intradino, que requer dois híbridos óticos de 90o
e quatro pares de foto detetores para sistemas de transmissão com polarização dupla, fazendo com que o custo destes sistemas seja pouco atrativo
para aplicações de curto alcance. Por isso, métodos de banda lateral única com deteção direta, também referidos como transcetores coerentes simplificados,
podem ser implementados como uma alternativa de baixo custo
aos sistemas coerentes tradicionais. Contudo, o desempenho de sistemas
de banda lateral única tradicionais é gravemente degradado pelo batimento
sinal-sinal. Nesta tese foi desenvolvida uma nova arquitetura de transcetor
coerente simplificada com um melhor desempenho para aplicações de curto
alcance. Em particular, o desenvolvimento de técnicas de processamento
digital de sinal para a reconstrução de fase, bem como de outros subsistemas
de processamento digital de sinal que minimizem os requerimentos
de hardware e a sua otimização de desempenho são o foco principal desta
tese.
O princípio fundamental do transcetor proposto é baseado na receção de
um sinal que satisfaz a condição mínima de fase na deteção direta. Para
reconstruir a informação de fase em falta causada pela deteção direta,
um novo método de valor DC que explora sinais de banda lateral única
e as propriedades DC da condição de fase mínima é desenvolvido nesta
tese. O método de valor DC facilita a reconstrução da fase à frequência
de amostragem de Nyquist e requer um sinal piloto de baixa intensidade.
Além disso, a validação experimental do método de valor DC foi executada
com sucesso em ligações óticas de curto alcance. Adicionalmente,
foi realizado um estudo intensivo do método de valor DC para otimizar o
desempenho do sistema. Neste processo de otimização, verificou-se que o
fator de contribuição da portadora é um parâmetro importante para explorar
todas as vantagens do método de valor DC. Neste contexto, é proposto
um novo método para a sua estimativa. Por último, o desempenho do
método de valor DC é otimizado recorrendo a mapeamento probabilístico
de constelação com taxa adaptativa.Programa Doutoral em Engenharia Eletrotécnic
TRANSMISSION PERFORMANCE OPTIMIZATION IN FIBER-WIRELESS ACCESS NETWORKS USING MACHINE LEARNING TECHNIQUES
The objective of this dissertation is to enhance the transmission performance in the fiber-wireless access network through mitigating the vital system limitations of both analog radio over fiber (A-RoF) and digital radio over fiber (D-RoF), with machine learning techniques being systematically implemented. The first thrust is improving the spectral efficiency for the optical transmission in the D-RoF to support the delivery of the massive number of bits from digitized radio signals. Advanced digital modulation schemes like PAM8, discrete multi-tone (DMT), and probabilistic shaping are investigated and implemented, while they may introduce severe nonlinear impairments on the low-cost optical intensity-modulation-direct-detection (IMDD) based D-RoF link with a limited dynamic range. An efficient deep neural network (DNN) equalizer/decoder to mitigate the nonlinear degradation is therefore designed and experimentally verified. Besides, we design a neural network based digital predistortion (DPD) to mitigate the nonlinear impairments from the whole link, which can be integrated into a transmitter with more processing resources and power than a receiver in an access network. Another thrust is to proactively mitigate the complex interferences in radio access networks (RANs). The composition of signals from different licensed systems and unlicensed transmitters creates an unprecedently complex interference environment that cannot be solved by conventional pre-defined network planning. In response to the challenges, a proactive interference avoidance scheme using reinforcement learning is proposed and experimentally verified in a mmWave-over-fiber platform. Except for the external sources, the interference may arise internally from a local transmitter as the self-interference (SI) that occupies the same time and frequency block as the signal of interest (SOI). Different from the conventional subtraction-based SI cancellation scheme, we design an efficient dual-inputs DNN (DI-DNN) based canceller which simultaneously cancels the SI and recovers the SOI.Ph.D
Equalização digital para sistemas de transmissão ópticos coerentes
This thesis focus on the digital equalization of fiber impairments for coherent optical transmission systems. New efficient and low-complexity equalization and mitigation techniques that counteract fiber nonlinear impairments are proposed and the tradeoff between performance and complexity is numerically assessed and experimentally demonstrated in metro and long-haul 400G superchannels-based transmission systems. Digital backpropagation (DBP) based on low-complexity split-step Fourier method and Volterra series nonlinear equalizers are experimentally assessed in an uniform superchannel system. In contrast with standard DBP methods, these techniques prove to be able to be implemented with larger step-sizes, consequently requiring a reduced number of multiplications, and still achieve a significant reach extension over linear equalization techniques. Moreover, given its structure, the complexity of the proposed Volterra-based DBP approach can be easily adjusted by changing the nonlinear filter dimension according to the system requirements, thus providing a higher flexibility to the nonlinear equalization block. A frequency-hybrid superchannel envisioning near-future flexible networks is then proposed as a way to increase the system bit-rate granularity. The problematic of the power-ratio between superchannel carriers is addressed and optimized for linear and nonlinear operation regimes using three distinct FEC paradigms. Applying a single FEC to the entire superchannel has a simpler implementation and is found to be a more robust approach, tolerating larger uncertainties on the system parameters optimization. We also investigate the performance gain provided by the application of different DBP techniques in frequency-hybrid superchannel systems, and its implications on the optimum power-ratio. It is shown that the application of DBP can be restricted to the carrier transporting the higher cardinality QAM format, since the DBP benefit on the other carriers is negligible, which might bring a substantially complexity reduction of the DBP technique applied to the superchannel.A presente tese foca-se na equalização digital das distorções da fibra para sistemas óticos de transmissão coerente. São propostas novas técnicas eficientes e de baixa complexidade para a equalização e mitigação das distorções não lineares da fibra, e o compromisso entre desempenho e complexidade é testado numericamente e demonstrado experimental em sistemas de transmissão metro e longa distância baseados em supercanais 400G. A propagação digital inversa baseada no método de split-step Fourier e equalizadores não lineares de séries de Volterra de baixa complexidade são testadas experimentalmente num sistema baseado em supercanais uniformes. Ao contrário dos métodos convencionais utilizados, estas técnicas podem ser implementadas utilizando menos interações e ainda extender o alcance do sistema face às técnicas de equalização linear. Para além disso, a complexidade do método baseado em Volterra pode ser facilmente ajustada alterando a dimensão do filtro não linear de acordo com os requisitos do sistema, concedendo assim maior flexibilidade ao bloco de equalização não linear. Tendo em vista as futuras redes flexı́veis, um supercanal hı́brido na frequência é proposto de modo a aumentar a granularidade da taxa de transmissão do sistema. A problemática da relação de potência entre as portadoras do supercanal é abordada e optimizada em regimes de operação linear e não linear utilizando paradigmas diferentes de códigos correctores de erros. A aplicação de um único código corrector de erros à totalidade do supercanal mostra ser a abordagem mais robusta, tolerando maiores incertezas na optimização dos parâmetros do sistema. O ganho de desempenho dado pela aplicação de diferentes técnicas de propagação digital inversa em sistemas de supercanais hı́bridos na frequência é tamém analizado, assim como as suas implicações na relação óptima de potência. Mostra-se que esta pode ser restringida à portadora que transporta o formato de modulação de ordem mais elevada, uma vez que o benefı́cio trazido pelas restantes portadotas é negligenciável, permitindo reduzir significativamente a complexidade do algoritmo aplicado.Programa Doutoral em Telecomunicaçõe
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