Power-efficient modulation formats in coherent transmission systems

Coherent optical transmission systems have a four-dimensional (4-D) signal space (two quadratures in two polarizations). These four dimensions can be used to create modulation formats that have a better power efficiency (higher sensitivity) than the conventional binary phase shift keying/quadrature phase shift keying (BPSK/QPSK) signals. Several examples are given, with some emphasis on a 24-level format and an 8-level format, including descriptions of how they can be realized and expressions for their symbol and bit error probabilities. These formats are, respectively, an extension and a subset of the commonly used 16-level dual-polarization QPSK format. Sphere packing simulations in 2, 3, and 4 dimensions, up to 32 levels, are used to verify their optimality. The numerical results, as the number of levels increases, are shown to agree with lattice-theoretical results. Finally, we point out that the use of these constellations will lead to improved fundamental sensitivity limits for optical communication systems, and they may also be relevant as a way of reducing power demands and/or nonlinear influence. \ua9 2009, IEEE. All rights reserved

Stacked Modulation Formats Enabling Highest-Sensitivity Optical Free-Space Communications

Die vorliegende Arbeit befasst sich mit hochempfindlichen optischen Kommunikationssystemen, wie sie z.B. bei Intersatellitenlinks verwendet werden. Theoretische Überlegungen zur Steigerung der Empfängerempfindlichkeit werden mit Simulations- und Messergebnissen ergänzt und verifiziert. Auf Grund der steigenden Nachfrage nach optischen Links zwischen Satelliten stellt sich die Frage, was sind geeignete Eckparameter, um ein solches System zu beschreiben. Die gigantischen Datenmengen, die von diversen Messgeräten, wie z.B. hochauflösende Kameras auf einem Satelliten generiert werden, bringen die Kapazitäten klassischer HF-Datenlinks an ihre Grenzen. Hier können optische Kommunikationssysteme auf Grund ihrer hohen Trägerfrequenz im Infrarotbereich sehr hohe Datenraten im Terabit/s Bereich ermöglichen. Systeme mit Radiowellen im GHz Bereich als Trägerfrequenz sind hier deutlich limitierter. [7] Linkdistanz, verfügbare Leistung, Pointinggenauigkeit und verfügbare Antennengröße sind einige Parameter, die einen wichtigen Einfluss auf die Leistungsfähigkeit des Systems haben. Je größer die Distanz und desto kleiner die verfügbare Antennengröße sowohl am Sender als auch am Empfänger sind, desto weniger Signalleistung wird den Detektor erreichen. Nimmt man dann noch ungenaues Pointing hinzu, d.h. Sender und Empfänger sind nicht exakt aufeinander ausgerichtet, treten zusätzliche Verluste auf. [7] Ziel dieser Arbeit ist es, ein vereinfachtes System zu implementieren und zu testen, das mit möglichst wenigen Photonen pro Bit bei einer gegebenen Bitfehlerwahrscheinlichkeit bei einer möglichst hohen Datenrate arbeiten kann. Hierfür werden alle Freiheitsgrade einer optischen Welle zur Modulation verwendet, um mit sog. „Stapeln“ von Modulationsformaten eine Empfindlichkeitssteigerung zu erreichen. Die Amplitude des Signals wird durch Pulspositionsmodulation (PPM) moduliert, wobei das zeitlich variable Vorhandensein eines Pulses innerhalb des Symbols die Information enthält. Dieses Modulationsformat weist bis dato die höchste Empfindlichkeit in Literatur und Experimenten auf [4]. Je mehr Möglichkeiten es gibt, einen Puls in einem Symbol zu platzieren, desto höher ist die zu erwartende Empfindlichkeit des Systems. Mit anderen Worten: Steigert man die zeitliche Dauer eines PPM-Symbols, so wächst ebenfalls die Empfängerempfindlichkeit. Da bei diesem Ansatz die Datenrate sinkt, wird in dieser Arbeit eine andere Methode vorgestellt, die Empfindlichkeit eines Übertragungssystems zu steigern, ohne die Symbollänge unnötig in die Länge zu ziehen. Diese Arbeit befasst sich mit dem Stapeln (sog. „Stacking“) von Modulationsformaten, in dem neben der Amplitudenmodulation weitere Freiheitsgrade, wie die Frequenz, Phase und Polarisation geschickt genutzt werden. Bei der Frequenzumtastung (FSK) wird die optische Frequenz je nach Symbol um ein gewisses Maß verschoben. Bei der polarisations-geschalteten Quadratur-Phasenumtastung (PS-QPSK) werden sowohl die Phase, als auch die Polarisation der optischen Welle moduliert [12]. Als Endergebnis erhält man PPM-FSK-PS-QPSK als Modulationsformat mit hoher Empfindlichkeit. Gegenüber dem reinen PPM wird eine theoretische Empfindlichkeitssteigerung von mehr als 1 dB erreicht. Sowohl Simulations- als auch Messergebnisse bestätigen den Empfindlichkeitsgewinn

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

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

Efficient Parallel Carrier Recovery for Ultrahigh Speed Coherent QAM Receivers with Application to Optical Channels

This work presents a new efficient parallel carrier recovery architecture suitable for ultrahigh speed intradyne coherent optical receivers (e.g., ≥100 Gb/s) with quadrature amplitude modulation (QAM). The proposed scheme combines a novel low-latency parallel digital phase locked loop (DPLL) with a feedforward carrier phase recovery (CPR) algorithm. The new low-latency parallel DPLL is designed to compensate not only carrier frequency offset but also frequency fluctuations such as those induced by mechanical vibrations or power supply noise. Such carrier frequency fluctuations must be compensated since they lead to higher phase error variance in traditional feedforward CPR techniques, significantly degrading the receiver performance. In order to enable a parallel-processing implementation in multigigabit per second receivers, a new approximation to the DPLL computation is introduced. The proposed technique reduces the latency within the feedback loop of the DPLL introduced by parallel processing, while at the same time it provides a bandwidth and capture range close to those achieved by a serial DPLL. Simulation results demonstrate that the effects caused by frequency deviations can be eliminated with the proposed low latency parallel carrier recovery architecture.Fil: Gianni, Pablo. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Electrónica. Laboratorio de Comunicaciones Digitales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ferster, Laura. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Electrónica. Laboratorio de Comunicaciones Digitales; ArgentinaFil: Corral Briones, Graciela. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Electrónica. Laboratorio de Comunicaciones Digitales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Hueda, Mario Rafael. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Electrónica. Laboratorio de Comunicaciones Digitales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin