265 research outputs found
Adaptive optical feedforward linearization of optical transceiver for radio over fiber communication link
With the tremendous growth in numbers of mobile data subscribers and explosive demand for mobile data, the current wireless access network need to be augmented in order to keep up with the data speed promised by the future generation mobile network standards. Radio over fiber technology (RoF) is a cost effective solution because of its ability to support numerous numbers of simple structured base stations by consolidating the signal processing functions at the central station. RoF systems are analog systems where noise figure and spurious free dynamic range (SFDR) are important parameters in an RoF link. The nonlinearity of a laser transmitter is a major limiting factor to the performance of an RoF link, as it generates spurious spectral components, leading to intermodulation distortions (IMD), which limit the achievable SFDR of the analog RF wave transmissions. The device nonlinearity can be mitigated through various linearization schemes. The feedforward linearization technique offers a number of advantages compared to other techniques, as it offers good suppression of distortion products over a large bandwidth and supports high operating frequencies. On the other hand, feedforward linearization is a relatively sensitive scheme, where its performance is highly influenced by changing operating conditions such as laser aging, temperature effect, and input signal variations. Therefore, for practical implementations the feedforward system has to be real-time adaptive. This thesis aims to develop an adaptive optical feedforward linearization system for radio over fiber links. Mathematical analyses and computer simulations are performed to determine the most efficient algorithm for the adaptive controller for laser transmitter feedforward linearization system. Experimental setup and practical measurement are performed for an adaptive feedforward linearized laser transmitter and its performance is optimized. The adaptive optical feedforward linearization system has been modeled and simulated in MATLAB Simulink. The performances of two adaptive algorithms, which are related to the gradient signal method, such as least mean square (LMS) and recursive least square (RLS) have been compared. The LMS algorithm has been selected because of its robustness and simplicity. Finally, the adaptive optical feedforward linearization system has been set up with digital signal processor (DSP) as the control device, and practical measurement has been performed. The system has achieved a suppression of 14 dB in the third order IMD products over a bandwidth of 30 MHz, in a two-tone measurement at 1.7 GHz
Experimental Demonstration and Performance Enhancement of 5G NR Multiband Radio over Fiber System Using Optimized Digital Predistortion
This paper presents an experimental realization of multiband 5G new radio (NR) optical front haul (OFH) based radio over fiber (RoF) system using digital predistortion (DPD). A novel magnitude-selective affine (MSA) based DPD method is proposed for the complexity reduction and performance enhancement of RoF link followed by its comparison with the canonical piece wise linearization (CPWL), decomposed vector rotation method (DVR) and generalized memory polynomial (GMP) methods. Similarly, a detailed study is shown followed by the implementation proposal of novel neural network (NN) for DPD followed by its comparison with MSA, CPWL, DVR and GMP methods. In the experimental testbed, 5G NR standard at 20 GHz with 50 MHz bandwidth and flexible-waveform signal at 3 GHz with 20 MHz bandwidth is used to cover enhanced mobile broad band and small cells scenarios. A dual drive Mach Zehnder Modulator having two distinct radio frequency signals modulates a 1310 nm optical carrier using distributed feedback laser for 22 km of standard single mode fiber. The experimental results are presented in terms of adjacent channel power ratio (ACPR), error vector magnitude (EVM), number of estimated coefficients and multiplications. The study aims to identify those novel methods such as MSA DPD are a good candidate to deploy in real time scenarios for DPD in comparison to NN based DPD which have a slightly better performance as compared to the proposed MSA method but has a higher complexity levels. Both, proposed methods, MSA and NN are meeting the 3GPP Release 17 requirements
Neural Network DPD for Aggrandizing SM-VCSEL-SSMF-Based Radio over Fiber Link Performance
This paper demonstrates an unprecedented novel neural network (NN)-based digital predistortion (DPD) solution to overcome the signal impairments and nonlinearities in Analog Optical fronthauls using radio over fiber (RoF) systems. DPD is realized with Volterra-based procedures that utilize indirect learning architecture (ILA) and direct learning architecture (DLA) that becomes quite complex. The proposed method using NNs evades issues associated with ILA and utilizes an NN to first model the RoF link and then trains an NN-based predistorter by backpropagating through the RoF NN model. Furthermore, the experimental evaluation is carried out for Long Term Evolution 20 MHz 256 quadraturre amplitude modulation (QAM) modulation signal using an 850 nm Single Mode VCSEL and Standard Single Mode Fiber to establish a comparison between the NN-based RoF link and Volterra-based Memory Polynomial and Generalized Memory Polynomial using ILA. The efficacy of the DPD is examined by reporting the Adjacent Channel Power Ratio and Error Vector Magnitude. The experimental findings imply that NN-DPD convincingly learns the RoF nonlinearities which may not suit a Volterra-based model, and hence may offer a favorable trade-off in terms of computational overhead and DPD performance
Digital Signal Processing Techniques Applied to Radio over Fiber Systems
The dissertation aims to analyze different Radio over Fiber systems for the front-haul applications. Particularly, analog radio over fiber (A-RoF) are simplest and suffer from nonlinearities, therefore, mitigating such nonlinearities through digital predistortion are studied. In particular for the long haul A-RoF links, direct digital predistortion technique (DPDT) is proposed which can be applied to reduce the impairments of A-RoF systems due to the combined effects of frequency chirp of the laser source and chromatic dispersion of the optical channel. Then, indirect learning architecture (ILA) based structures namely memory polynomial (MP), generalized memory polynomial (GMP) and decomposed vector rotation (DVR) models are employed to perform adaptive digital predistortion with low complexities. Distributed feedback (DFB) laser and vertical capacity surface emitting lasers (VCSELs) in combination with single mode/multi-mode fibers have been linearized with different quadrature amplitude modulation (QAM) formats for single and multichannel cases. Finally, a feedback adaptive DPD compensation is proposed. Then, there is still a possibility to exploit the other realizations of RoF namely digital radio over fiber (D-RoF) system where signal is digitized and transmits the digitized bit streams via digital optical communication links. The proposed solution is robust and immune to nonlinearities up-to 70 km of link length. Lastly, in light of disadvantages coming from A-RoF and D-RoF, it is still possible to take only the advantages from both methods and implement a more recent form knows as Sigma Delta Radio over Fiber (S-DRoF) system. Second Order Sigma Delta Modulator and Multi-stAge-noise-SHaping (MASH) based Sigma Delta Modulator are proposed. The workbench has been evaluated for 20 MHz LTE signal with 256 QAM modulation. Finally, The 6x2 GSa/s sigma delta modulators are realized on FPGA to show a real time demonstration of S-DRoF system. The demonstration shows that S-DRoF is a competitive competitor for 5G sub-6GHz band applications
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
Nonlinear impairments and mitigation technologies for the next generation fiber-wireless mobile fronthaul networks
The proliferation of Internet-connected mobile devices and video-intensive services are driving the growth of mobile data traffic in an explosive way. The last mile of access networks, mobile fronthaul (MFH) networks, have become the data rate bottleneck of user experience. The objective of this research are two-fold. For analog MFH, nonlinear interferences among multiple bands of mobile signals in a multi-RAT multi-service radio-over-fiber (RoF)-based MFH system are investigated for the first time. The nonlinear impairments of both single-carrier and multi-carrier signals are investigated, and it is experimentally demonstrated that inter-channel interferences play a more important role in the performance degradation of analog MFH than the nonlinear distortions of each individual signal. A digital predistortion technique was also presented to linearize the analog MFH links. On the other hand, for digital MFH, we experimentally demonstrate a novel digitization interface based on delta-sigma modulation to replace the state-of-the-art common public radio interface (CPRI). Compared with CPRI, it provides improved spectral efficiency and enhanced fronthaul capacity, and can accommodate both 4G-LTE and 5G mobile services.Ph.D
Digital Signal Processing for Optical Communications and Coherent LiDAR
Internet data traffic within data centre, access and metro networks is experiencing
unprecedented growth driven by many data-intensive applications. Significant
efforts have been devoted to the design and implementation of low-complexity
digital signal processing (DSP) algorithms that are suitable for these short-reach
optical links. In this thesis, a novel low-complexity frequency-domain (FD)
multiple-input multiple-output (MIMO) equaliser with momentum-based gradient
descent algorithm is proposed, capable of mitigating both static and dynamic
impairments arising from the optical fibre. The proposed frequency-domain
equaliser (FDE) also improves the robustness of the adaptive equaliser against
feedback latencies which is the main disadvantage of FD adaptive equalisers under
rapid channel variations.
The development and maturity of optical fibre communication techniques over
the past few decades have also been beneficial to many other fields, especially
coherent light detection and ranging (LiDAR) techniques. Many applications
of coherent LiDAR are also cost-sensitive, e.g., autonomous vehicles (AVs).
Therefore, in this thesis, a low-cost and low-complexity single-photodiode-based
coherent LiDAR system is investigated. The receiver sensitivity performance of this
receiver architecture is assessed through both simulations and experiments, using
two ranging waveforms known as double-sideband (DSB) amplitude-modulated
chirp signal and single-sideband (SSB) frequency-modulated continuous-wave
(FMCW) signals. Besides, the impact of laser phase noise on the ranging precision
when operating within and beyond the laser coherence length is studied. Achievable
ranging precision beyond the laser coherence length is quantified
ALL-OPTICAL METHOD FOR DISTORTION ELIMINATION IN PHASE-MODULATED MICROWAVE PHOTONIC LINKS
Microwave photonic systems arose from the need to accomplish engineering goals that were not possible in the electrical domain alone. One of the first applications of photonic systems was in long haul microwave signal transport, leveraging the wide bandwidth and low propagation loss afforded by optical fiber. As these microwave photonic links became more widespread, so too did the efforts to improve their performance, most importantly the desire to minimize noise and distortion within the link in order to maximize dynamic range and linear performance. In this thesis, we demonstrate a new class of methods for microwave photonic link linearization that connects insights from a seemingly unrelated field- nonlinear optics- with microwave photonics to enhance the distortion-free operation of these links. Finally, we show applications for these methods in other areas of microwave photonics, particularly microwave signal generation
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