Fiber link design considerations for cloud-Radio Access Networks

Analog radio over fiber (RoF) links may offer advantages for cloud-Radio Access Networks in terms of component cost, but the behavior of the distortion with large numbers of subcarriers needs to be understood. In this paper, this is presented in terms of the variation between subcarriers. Memory polynomial predistortion is also shown to compensate for RoF and wireless path distortion. Whether for digitized or analog links, it is shown that appropriate framing structure parameters must be used to assure performance, especially of time-division duplex systems

Nonlinear effects in OFDM signal transmission over radio over fibre links

The dynamic range limitations that arise from nonlinearity in low-cost and low complexity directly modulated radio over fibre (RoF) links are examined. Statistical non linear models are presented and applied to the case of a RoF link with a low biased laser diode. The effects of distortion on the Error Vector Magnitude (EVM) performance of Orthogonal Frequency-Division Multiplexing (OFDM) signals with different numbers of subcarriers and the connection to the Peak-ta-Average Power Ratio (PAPR) of the signals are investigated. Statistical distributions of the EVM over a large number of transmitted OFDM frames are gained from experimental measurements and analyses of idealized processes. The measurement results show that as the number of subcarriers is reduced the distribution means are not dependent in the expected way on the statistical PAPR of the transmitted OFDM signals. Instead, it is shown that in regions of moderate distortion the median of the EVM is more closely related to the statistical PAPR and to the required back-off for signals with different numbers of subcarriers. Through the employed statistical analysis, the asymptotic convergence of the EVM result to that expected in the idealized case is observed. The results of this analysis also show, how, including the EVM variance in estimations for back-off might be useful, how, in a measurement, the number of transmitted OFDM frames affects the estimated mean EVM. Differing EVM results for subcarriers at the edge and middle of the signal band show that distortion affects the subcarriers at the middle of the band to a stronger degree and that their behaviour is correlated "to the statistical PAPR of the individual signals. Then, a laser model validated against measured performance is designed arid used for simulating the performance of a subcarrier multiplexed 4th generation mobile/wireless RoF transmission system. Predictions indicate that the architecture provides adequate performance in terms of EVM, for different IFFT sizes and modulation levels of up to 256 QAM (at leastL and a combined raw data rate of up to 3.2 Gbps. Based on a 1.5% EVM transmitter requirement with 256 QAM, a system dynamic range of approximately 5.1 dB is predicted. Finally, the use of companding in a directly modulated RaF link, with the aim of reducing the amplification (and isolation) requirements in the remote antenna unit, is demonstrated, quantified through EVM measurements. The resulting improvements in output power are approximately 4.7 dB at an EVM transmitter requirement of 5.6 %, while in terms of the point of onset of distortion-induced EVM increase, the improvement is approximately 7.6 dB

Orthogonal frequency division multiplexing for next generation optical networks

Next generation optical networks will be required to provide increased data throughput on a greater number of optical channels and will also have to facilitate network flexibility in order to adapt to dynamic traffic patterns. Furthermore, the potentially wide deployment of optical Access and Metropolitan networks in particular require that these challenges are met in a cost effect manner. This thesis examines the use of Orthogonal Frequency Division Multiplexing (OFDM) as a means of helping to meet these requirements for next generation optical systems with a high market volume. OFDM is a multi–carrier modulation technique which exhibits high spectral efficiency and a tolerance to chromatic dispersion making it an excellent candidate for use in next generation optical networks. The work presented in this thesis shows how the use of OFDM in conjunction with novel laser devices and direct detection can be used to construct cost effective, low footprint optical systems. These systems are capable of providing >10Gb/s per optical channel and are suitable for implementation as optical access networks. Furthermore, OFDM is shown to be a realistic candidate for use in an optical switching environment where external modulation is employed and, as such, can be considered for use in next generation metropolitan networks

Visible Light Communication (VLC)

Visible light communication (VLC) using light-emitting diodes (LEDs) or laser diodes (LDs) has been envisioned as one of the key enabling technologies for 6G and Internet of Things (IoT) systems, owing to its appealing advantages, including abundant and unregulated spectrum resources, no electromagnetic interference (EMI) radiation and high security. However, despite its many advantages, VLC faces several technical challenges, such as the limited bandwidth and severe nonlinearity of opto-electronic devices, link blockage and user mobility. Therefore, significant efforts are needed from the global VLC community to develop VLC technology further. This Special Issue, “Visible Light Communication (VLC)”, provides an opportunity for global researchers to share their new ideas and cutting-edge techniques to address the above-mentioned challenges. The 16 papers published in this Special Issue represent the fascinating progress of VLC in various contexts, including general indoor and underwater scenarios, and the emerging application of machine learning/artificial intelligence (ML/AI) techniques in VLC

Enhanced Receivers for OFDM signals with super-QAM constellations

Nowadays, there is a high demand for wireless communication systems with higher through- put. One popular technique widely used in current and developing wireless technologies is Orthogonal Frequency-Division Multiplexing (OFDM) due to its robustness against fre- quency selective fading and high spectral efficiency. To further extend OFDM capacity to meet the near future’s expected demanding needs, OFDM systems with very large Quadrature Amplitude Modulation (QAM) constellations, the so-called super-QAM, are being proposed. However, OFDM signals are prone to nonlinear distortion effects due to their high envelope fluctuations which reduces the system’s performance and this issue is aggravated by the increase in the size of the constellation. For the implementation of effective super-QAM OFDM systems, it is crucial to develop receivers that expect and mitigate the nonlinear distortion on the transmitted signal. In this work, nonlinear distortion on OFDM small QAM and super-QAM constellations signals is studied, along with distortion models and methods to estimate them solely from the transmitted signal, and application of Bussgang noise cancellation receivers and analysis of their performance over a wide range of scenarios.Nos dias de hoje, há uma grande necessidade de criar sistemas de telecomunicação com maior ritmo de dados. Uma técnica popular em tecnologias de telecomunicação atuais e em desenvolvimento é Ortogonal Frequency-Devision Multiplexing (OFDM) devido à sua robustez contra atenuação seletiva na frequência e alta eficiência espectral. Para aumentar ainda mais a capacidade do OFDM de forma a preparar para ritmos ainda mais altos que são expectáveis num futuro próximo, estão a ser propostos sistemas OFDM com enormes constelações de Quadrature Amplitude Modulation (QAM), o chamado super-QAM. O problema é que sinais OFDM são suscetíveis a efeitos de distorção não linear devido às altas flutuações de envolvente e que traz pior desempenho do sistema, sendo esse problema agravado pelo aumento do tamanho da constelação. Para a implementação de sistemas super-QAM OFDM eficazes é crucial desenvolver recetores que mitiguem a distorção não linear no sinal transmitido. Neste trabalho, estuda-se a distorção não linear em sinais OFDM de pequenas cons- telações QAM e super-QAM, modelos de distorção e métodos para estimá-los a partir do sinal transmitido, aplicação de recetores de cancelamento de ruído Bussgang e análise de seu desempenho em diversos cenários

Crest factor reduction techniques for OFDM telecommunication systems

Two popular Crest Factor Reduction techniques applicable to OFDM signals have been studied and implemented within this project. To provide adequate background, a brief description of OFDM communication systems is included in the first sections, and the problem of high Peak to Average Power Ratio in multi-carrier transmission schemes is presented. An overview of the most relevant Crest Factor Reduction techniques is performed, and the Clipping and Filtering and Peak Windowing algorithms are selected as implementation candidates due to their protocol agnostic nature. Several implementation architectures have been discussed, and the most computationally efficient one has been used to implement both algorithms within srsRAN, an open source, full software radio implementation of an LTE network with all of its components. In order to provide a comprehensive evaluation of the developed algorithms, an extensive measurement campaign has been designed and carried out, comprised of synthetic testing, as well as experimental physical signal measurements in a laboratory setting, and an outdoor measurement campaign. To speed up the measurement process, an automated measurement system has been designed in MATLAB, which remotely operates a signal analyser to perform spectrum and statistical power measurements, as well as IQ sample collection of the generated LTE signals. This measurement system has been used to carry out unwanted emissions tests following the LTE conformance testing procedures, which are also discussed within this document. For the outdoor measurements, an LTE single cell network has been deployed using a software radio transceiver and an LTE phone, with the aim of assessing the end to end quality of service improvements provided by the developed algorithms. The gathered results illustrate that the implemented Crest Factor Reduction techniques can provide significant network performance gains in situations with limited available transmission power. Additional gains provided by these techniques include increased efficiency in the operation of power amplification stages, and reduced network deployment costs.Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructur

Techniques en appui des formats de modulation avancés pour les futurs réseaux optiques

Les systèmes de communication optique avec des formats de modulation avancés sont actuellement l’un des sujets de recherche les plus importants dans le domaine de communication optique. Cette recherche est stimulée par les exigences pour des débits de transmission de donnée plus élevés. Dans cette thèse, on examinera les techniques efficaces pour la modulation avancée avec une détection cohérente, et multiplexage par répartition en fréquence orthogonale (OFDM) et multiples tonalités discrètes (DMT) pour la détection directe et la détection cohérente afin d’améliorer la performance de réseaux optiques. Dans la première partie, nous examinons la rétropropagation avec filtre numérique (DFBP) comme une simple technique d’atténuation de nonlinéarité d’amplificateur optique semiconducteur (SOA) dans le système de détection cohérente. Pour la première fois, nous démontrons expérimentalement l’efficacité de DFBP pour compenser les nonlinéarités générées par SOA dans un système de détection cohérente porteur unique 16-QAM. Nous comparons la performance de DFBP avec la méthode de Runge-Kutta quatrième ordre. Nous examinons la sensibilité de performance de DFBP par rapport à ses paramètres. Par la suite, nous proposons une nouvelle méthode d’estimation de paramètre pour DFBP. Finalement, nous démontrons la transmission de signaux de 16-QAM aux taux de 22 Gbaud sur 80km de fibre optique avec la technique d’estimation de paramètre proposée pour DFBP. Dans la deuxième partie, nous nous concentrons sur les techniques afin d’améliorer la performance des systèmes OFDM optiques en examinent OFDM optiques cohérente (CO-OFDM) ainsi que OFDM optiques détection directe (DDO-OFDM). Premièrement, nous proposons une combinaison de coupure et prédistorsion pour compenser les distorsions nonlinéaires d’émetteur de CO-OFDM. Nous utilisons une interpolation linéaire par morceaux (PLI) pour charactériser la nonlinéarité d’émetteur. Dans l’émetteur nous utilisons l’inverse de l’estimation de PLI pour compenser les nonlinéarités induites à l’émetteur de CO-OFDM. Deuxièmement, nous concevons des constellations irrégulières optimisées pour les systèmes DDO-OFDM courte distance en considérant deux modèles de bruit de canal. Nous démontrons expérimentalement 100Gb/s+ OFDM/DMT avec la détection directe en utilisant les constellations QAM optimisées. Dans la troisième partie, nous proposons une architecture réseaux optiques passifs (PON) avec DDO-OFDM pour la liaison descendante et CO-OFDM pour la liaison montante. Nous examinons deux scénarios pour l’allocations de fréquence et le format de modulation des signaux. Nous identifions la détérioration limitante principale du PON bidirectionnelle et offrons des solutions pour minimiser ses effets.Optical communication systems with advanced modulation formats are currently one of the major research focuses of the optical communication community. This research is driven by the ever-increasing demand for higher data transmission rates. In this thesis, we investigate efficient techniques for advanced modulation with coherent detection, and optical orthogonal frequency-division multiplexing (OFDM) and discrete multi-tone (DMT) for both direct detection and coherent detection to improve the performance of optical networks. In the first part, we investigate digital filter back-propagation (DFBP) as a simple semiconductor optical amplifier (SOA) nonlinearity mitigation technique in coherent detection systems. For the first time, we experimentally demonstrate effectiveness of DFBP in compensating for SOA-induced nonlinearities in a 16-ary quadrature amplitude modulation (16-QAM) singlecarrier coherent detection system. We compare performance of DFBP with Runge-Kutta fourth-order method. We examine sensitivity of DFBP performance to its parameters. Afterwards, we propose a novel parameter estimation method for DFBP. Finally, we demonstrate successful transmission of 22 Gbaud 16-QAM signals over 80 km fiber with the proposed parameter estimation technique for DFBP. In the second part, we concentrate on techniques to improve performance of optical OFDM systems, examining both coherent optical OFDM (CO-OFDM) and direct-detection optical OFDM (DDO-OFDM). First, we propose a combination of clipping and predistortion technique to compensate for CO-OFDM transmitter nonlinear distortions. We use piecewise linear interpolation (PLI) for characterizing the transmitter nonlinearity. At the transmitter, we use inverse of the PLI estimate to pre-compensate the nonlinearities induced at the COOFDM transmitter. Second, we design optimized non-square constellations for short-reach DDO-OFDM systems based on two channel noise models. We experimentally demonstrate 100 Gb/s+ OFDM/DMT with direct detection using the optimized QAM constellations. In the third part, we propose and experimentally demonstrate a passive optical network (PON) architecture with DDO-OFDM for the downlink and CO-OFDM for the uplink. We examine two scenarios for the occupied frequency and modulation format of the signals. We identify main limiting impairments of the bidirectional PON and provide solutions to minimize their effects

Advanced modulation for optical communication systems

La demande toujours croissante pour la capacité du réseau conduit au développement de systèmes de communication optique pour couvrir les normes Terabit Ethernet récemment proposées. Les applications de courte distance nécessitent une solution peu coûteuse et peu complexe avec détection directe. Cependant, le coût de la détection cohérente diminue chaque jour et en fait un bon candidat pour les applications à courte distance futures afin d'accroître l'efficacité spectrale et d'utiliser des formats de modulation avancés. Dans cette thèse, nous étudions des solutions pour les applications court-courrier actuelles et futures. Dans la première partie, nous nous concentrons sur des solutions pour les applications de courte distance. Le premier chapitre est la première démonstration de la transmission multi-tonalité discrète (DMT) à plus de 100 Gb/s en utilisant une photonique au silicium en bande O (SiP). Nous comparons expérimentalement le DMT avec la modulation d'amplitude d'impulsion (PAM) sur la bande O. Notre expérience montre qu'en augmentant la longueur des fibres de plus de 10 km, la PAM surpasse le DMT. Pour la bande C, nous utilisons un multi-ton discret à bande latérale unique (SSB-DMT) pour éviter l'effet d'évanouissement de la puissance induit par la dispersion chromatique. Nous étudions l'effet du bruit de phase, de la dispersion chromatique et de la sensibilité du récepteur pour optimiser le signal du DMT et extraire des équations théoriques pour calculer le taux d'erreur binaire (BER) du SSB-DMT. Ensuite, nous comparons la PAM sur bande O avec le SSB-DMT sur bande C et quantifions l'impact des limitations imposées par le matériel sur les deux formats de modulation. Notre étude fournit un outil analytique pour les applications de courte distance afin de sélectionner le format et le matériel de modulation appropriés en fonction de la portée requise, du débit binaire, etc. Dans la deuxième partie, le ciblage des futurs systèmes de détection cohérents justifié l'utilisation d'un format de modulation complexe avec détection cohérente. Nous utilisons un format de modulation avancé dans lequel nous avons combiné la propagation de transformée de Fourier discrète avec le DMT pour augmenter l'efficacité spectrale. Le format de modulation hybride a un rapport de puissance crête à moyenne inférieur (vis-à-vis du DMT) et une efficacité spectrale plus élevée (vis-à-vis de la QAM _a porteuse unique). Dans la première étape, nous comparons expérimentalement les performances des modulations hybrides, DMT standard et monoporteuse en utilisant un modulateur SiP IQ. Ensuite, nous développons une stratégie de contrôle pour le format de modulation hybride en échangeant la non-linéarité de la fonction de transfert du modulateur et le rapport signal / bruit optique. Le format de modulation hybride est ensuite optimisé pour avoir un débit maximum. En utilisant une simulation de Monte Carlo, nous comparons le format de modulation hybride optimisé avec le DMT standard pour différents débits binaires. Enfin, nous avons une comparaison de complexité entre hybride et DMT pour différentes longueurs de fibre.Ever increasing demand for network capacity is driving the development of optical communication systems to cover recently proposed Terabit Ethernet standards. Short haul applications need low cost and low complexity solutions with direct detection, as the cost of coherent detection comes down, it will become a good candidate for future short-haul applications to increase spectral efficiency and exploit advanced modulation formats. In this thesis, we investigate solutions for both current and future short-haul systems. In the first part, we focus on solutions for short haul applications. The first chapter is the first time demonstration of more than 100 Gb/s discrete multi-tone (DMT) transmission using an O-band silicon photonics (SiP). We experimentally compare DMT with pulse amplitude modulation (PAM) on O-band. Our experiment shows that by increasing fiber length more than 10 km, PAM outperforms DMT. For C-band, we use single sideband discrete multi-tone (SSB-DMT) to avoid chromatic dispersion-induced power fading e_ect. We study the effect of phase noise, chromatic dispersion, and receiver sensitivity to optimize DMT signal and extract theoretical equations to calculate bit error rate (BER) of SSB-DMT. Next, we analytically compare PAM on O-band with SSB-DMT on C-band and quantify the impact of hardware-imposed limitations on both modulation formats. Our study provides an analytical tool for short haul applications to select appropriate modulation format and hardware based on required reach, bit rate, etc. In the second part we examine complex modulation formats that will be enabled in the future by low cost, integrated components for coherent detection.. We use an advanced modulation format in which we combined discrete Fourier transform spread with DMT to increase spectral efficiency. Hybrid modulation format has a lower peak to average power ratio (vis-a-vis DMT) and higher spectral efficiency (vis-a-vis single carrier QAM). In the first step, we experimentally compare the performance of the hybrid, standard DMT, and single carrier modulations using a SiP IQ modulator. Next, we develop a driving strategy for hybrid modulation format by trading off the modulator transfer function non-linearity and optical signal to noise ratio. Then hybrid modulation format is optimized to have maximum throughput. Using Monte Carlo simulation we compare optimized hybrid modulation format with standard DMT for different bit rates. Finally, we have a complexity comparison between hybrid and DMT for different fiber lengths to motivate same investigation for long-haul applications where, we should consider fiber non-linearity, attenuation, a polarization multiplexing

Advanced signal processing techniques for the modeling and linearization of wireless communication systems.

Los nuevos estándares de comunicaciones digitales inalámbricas están impulsando el diseño de amplificadores de potencia con unas condiciones límites en términos de linealidad y eficiencia. Si bien estos nuevos sistemas exigen que los dispositivos activos trabajen cerca de la zona de saturación en busca de la eficiencia energética, la no linealidad inherente puede producir que el sistema muestre prestaciones inadecuadas en emisiones fuera de banda y distorsión en banda. La necesidad de técnicas digitales de compensación y la evolución en el diseño de nuevas arquitecturas de procesamiento de señales digitales posicionan a la predistorsión digital (DPD) como un enfoque práctico. Los predistorsionadores digitales se suelen basar en modelos de comportamiento como el memory polynomial (MP), el generalized memory polynomial (GMP) y el dynamic deviation reduction-based (DDR), etc. Los modelos de Volterra sufren la llamada "maldición de la dimensionalidad", ya que su complejidad tiende a crecer de forma exponencial a medida que el orden y la profundidad de memoria crecen. Esta tesis se centra principalmente en contribuir a la rama de conocimiento que enmarca el modelado y linealización de sistemas de comunicación inalámbrica. Los principales temas tratados son el modelo Volterra-Parafac y el modelo general de Volterra para sistemas complejos, los cuales tratan la estructura del DPD y las series de Volterra estructuradas con compressed-sensing y un método para la linealización en un rango de potencias de operación, que se centran en cómo los coeficientes de los modelos deben ser obtenidos.Premio Extraordinario de Doctorado U

Digital Processing for an Analogue Subcarrier Multiplexed Mobile Fronthaul

In order to meet the demands of the fifth generation of mobile communication networks (5G), such as very high bit-rates, very low latency and massive machine connectivity, there is a need for a flexible, dynamic, scalable and versatile mobile fronthaul. Current industry fronthaul standards employing sampled radio waveforms for digital transport suffer from spectral inefficiency, making this type of transport impractical for the wide channel bandwidths and multi-antenna systems required by 5G. On the other hand, analogue transport does not suffer from these limitations. It is, however, prone to noise, non-linearity and poor dynamic range. When combined with analogue domain signal aggregation/multiplexing, it also lacks flexibility and scalability, especially at millimetre wave frequencies. Measurements (matched in simulation) of analogue transport at millimetre wave frequencies demonstrate some of these issues. High data rates are demonstrated employing wide bandwidth channels combined using traditional subcarrier multiplexing techniques. However, only a limited number of channels can be multiplexed in this manner, with poor spectral efficiency, as analogue filter limitations do not allow narrow gaps between channels. To this end, over the last few years, there has been significant investigation of analogue transport schemes combined with digital channel aggregation/ de-aggregation (combining/ separating multiple radio waveforms in the digital domain). This work explores such a technique. Digital processing is used at the transmitter to flexibly multiplex a large number of channels in a subcarrier multiplex, without the use of combiners, mixers/ up-converters or Hilbert transforms. Orthogonal Frequency Division Multiplexing (OFDM) - derived Discrete Multi-Tone (DMT) and Single Sideband (SSB) modulated channels are integrated within a single Inverse Fast Fourier Transform (IFFT) operation. Channels or channel groups are mapped systematically into Nyquist zones by using, for example, a single IFFT (for a single 5G mobile numerology) or multiple IFFTs (for multiple 5G mobile numerologies). The analogue transport signal generated in this manner is digitally filtered and band-pass sampled at the receiver such that each corresponding channel (e.g. channels destined to the same radio frequency (RF)/ millimetre wave (mmW) frequency) in the multiplex is presented at the same intermediate frequency, due to the mapping employed at the transmitter. Analogue or digital domain mixers/ down-converters are not required with this technique. Furthermore, each corresponding channel can be readily up-converted to their respective RF/mmW channels with minimal per-signal processing. Measurement results, matched in simulation, for large signal multiplexes with both generic and 5G mobile numerologies show error-vector magnitude performance well within specifications, validating the proposed system. For even larger multiplexes and/or multiplexes residing on a higher IF exceeding the analogue bandwidth and sampling rate specifications of the ADCs at the receiver, the use of a bandwidth-extension device is proposed to extend the mapping to a mapping hierarchy and relax the analogue bandwidth and sampling rate requirements of the ADCs. This allows the receiver to still use digital processing, with only minimal analogue processing, to band-pass sample smaller blocks of channels from the larger multiplex, down to the same intermediate frequency. This ensures that each block of channels is within the analogue bandwidth specification of the ADCs. Performance predictions via simulation (based on a system model matched to the measurements) show promising results for very large multiplexes and large channel bandwidths. The multiplexing technique presented in this work thus allows reductions in per-channel processing for heterogeneous networking (or multi-radio access technologies) and multi-antenna configurations. It also creates a re-configurable and adaptable system based on available processing resources, irrespective of changes to the number of channels and channel groups, channel bandwidths and modulation formats