Epälineaarinen vääristymä laajakaistaisissa analogia-digitaalimuuntimissa

This thesis discusses nonlinearities of analog-to-digital converters (ADCs) and their mitigation using digital signal processing (DSP). Particularly wideband radio receivers are considered here including, e.g., the emerging cognitive radio applications. In this kind of receivers, a single ADC converts a mixture of signals at different frequency bands to digital domain simultaneously. Different signals may have considerably different power levels and hence the overall dynamic range can be very large (even 50–60 dB). Therefore, even the smallest ADC nonlinearities can produce considerable amount of nonlinear distortion, which may cause a strong signal to block significantly weaker signal bands. One concrete source of nonlinear distortion is waveform clipping due to improper signal conditioning in the input of an ADC. In the thesis, a mathematical model for this phenomenon is derived through Fourier analysis and is then used as a basis for an adaptive interference cancellation (AIC) method. This is a general method for reducing nonlinear distortion and besides clipping it can be used, e.g., to compensate integral nonlinearity (INL) originating from unintentional deviations of the quantization levels. Additionally, an interpolation method is proposed in this thesis to restore clipped waveforms and hence reduce nonlinear distortion. Through several computer simulations and corresponding laboratory radio signal measurements, the performance of the proposed post-processing methods is illustrated. It can be seen from the results that the methods are able to reduce nonlinear distortion from a weak signal band in a considerable manner when there are strong blocking signals in the neighboring channels. According to the results, the AIC method would be a highly recommendable post-processing technique for modern radio receivers due to its general ability to reduce nonlinear distortion regardless of its source. /Kir10Tässä työssä käsitellään analogia-digitaalimuuntimien (AD-muuntimien) epälineaarisuuksia ja niiden lieventämistä digitaalisen signaalinkäsittelyn (DSP) avulla. Tätä on tarkasteltu erityisesti laajakaistaisten radiovastaanottimien näkökulmasta, joka käsittää mm. tulevat kognitiiviseen radioon liittyvät sovellukset. Tällaisissa vastaanottimissa yksittäinen AD-muunnin muuntaa samanaikaisesti useita eri taajuuskaistoilla olevia signaaleita digitaaliseen muotoon, jolloin yhteenlaskettu dynaaminen alue voi olla hyvin suuri (jopa 50–60 dB). Tämän takia AD-muuntimen pienimmätkin epälineaarisuudet voivat aiheuttaa huomattavasti epälineaarista vääristymää, minkä vuoksi voimakas signaali saattaa häiriöllään peittää muilla taajuuskaistoilla olevia selkeästi heikompia signaaleja. Eräs konkreettinen epälineaarisen vääristymän aiheuttaja on aaltomuodon leikkaantuminen AD-muuntimen sisäänmenossa jännitealueen ylittymisen vuoksi. Tässä työssä johdetaan matemaattinen malli kyseiselle ilmiölle Fourier-analyysin avulla ja käytetään sitä lähtökohtana adaptiiviselle häiriönpoistomenetelmälle (AIC-menetelmä). Se on yleisluonteinen menetelmä epälineaarisen vääristymän vähentämiseksi, ja leikkaantumisen lisäksi sitä voidaan käyttää esimerkiksi kompensoimaan integraalista epälineaarisuutta (INL), joka on peräisin kvantisointitasojen tahattomista poikkeamista. Lisäksi tässä työssä esitellään interpolointimenetelmä leikkaantuneen aaltomuodon ehostamiseen siten, että epälineaarinen häiriö vähenee. Esiteltyjen jälkikäsittelymenetelmien suorituskykyä analysoidaan ja havainnollistetaan useilla tietokonesimulaatiolla sekä niitä vastaavilla radiosignaalien laboratoriomittauksilla. Tuloksista voidaan nähdä, että nämä menetelmät kykenevät poistamaan huomattavasti epälineaarista vääristymää heikolta signaalikaistalta silloin, kun naapurikaistoilla on voimakkaita häiriösignaaleja. Tulosten perusteella AIC-menetelmä olisi erittäin suositeltava jälkikäsittelytekniikka moderneihin radiovastaanottimiin, koska se pystyy yleisesti vähentämään epälineaarista vääristymää riippumatta häiriön alkuperästä

Digital signal processing optical receivers for the mitigation of physical layer impairments in dynamic optical networks

IT IS generally believed by the research community that the introduction of complex network functions—such as routing—in the optical domain will allow a better network utilisation, lower cost and footprint, and a more efficiency in energy usage. The new optical components and sub-systems intended for dynamic optical networking introduce new kinds of physical layer impairments in the optical signal, and it is of paramount importance to overcome this problem if dynamic optical networks should become a reality. Thus, the aim of this thesis was to first identify and characterise the physical layer impairments of dynamic optical networks, and then digital signal processing techniques were developed to mitigate them. The initial focus of this work was the design and characterisation of digital optical receivers for dynamic core optical networks. Digital receiver techniques allow for complex algorithms to be implemented in the digital domain, which usually outperform their analogue counterparts in performance and flexibility. An AC-coupled digital receiver for core networks—consisting of a standard PIN photodiode and a digitiser that takes samples at twice the Nyquist rate—was characterised in terms of both bit-error rate and packet-error rate, and it is shown that the packet-error rate can be optimised by appropriately setting the preamble length. Also, a realistic model of a digital receiver that includes the quantisation impairments was developed. Finally, the influence of the network load and the traffic sparsity on the packet-error rate performance of the receiver was investigated. Digital receiver technologies can be equally applied to optical access networks, which share many traits with dynamic core networks. A dual-rate digital receiver, capable of detecting optical packets at 10 and 1.25 Gb/s, was developed and characterised. The receiver dynamic range was extended by means of DC-coupling and non-linear signal clipping, and it is shown that the receiver performance is limited by digitiser noise for low received power and non-linear clipping for high received power

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

Échantillonnage Non Uniforme : Application aux filtrages et aux conversions CAN/CNA (Convertisseurs Analogique-Numérique et Numérique/Analogique) dans les télécommunications par satellite

La théorie de l'échantillonnage uniforme des signaux, développée en particulier par C. Shannon, est à l'origine du traitement numérique du signal. Depuis, de nombreux travaux ont été consacrés à l'échantillonnage non uniforme. Celui-ci permet, d'une part, de modéliser les imperfections des dispositifs d'échantillonnage uniforme. D'autre part, l'échantillonnage peut être effectué de manière délibérément non uniforme afin de bénéficier de propriétés particulières, notamment un assouplissement des conditions portant sur le choix de la fréquence moyenne d'échantillonnage. La plupart de ces travaux reste dans un cadre théorique en adoptant des schémas d'échantillonnage et des modèles de signaux simplifiés. Or, actuellement, dans de nombreux domaines d'application, tels que les communications par satellites, la conversion analogique-numérique s'effectue sous des contraintes fortes pour les largeurs de bande mises en jeu, en raison notamment des fréquences très élevées utilisées. Ces conditions opérationnelles accentuent les imperfections des dispositifs électroniques réalisant l'échantillonnage et induisent le choix de modèles de signaux et de schémas d'échantillonnage spécifiques. Cette thèse a pour objectif général d'identifier des modèles d'échantillonnage adaptés à ce cadre applicatif. Ceux-ci s'appliquent à des signaux aléatoires passe-bande, qui constituent un modèle classique en télécommunications. Ils doivent prendre en compte des facteurs technologiques, économiques ainsi que des contraintes bord de complexité et éventuellement intégrer des fonctionnalités propres aux télécommunications. La première contribution de cette thèse est de développer des formules d'échantillonnage non uniforme qui intègrent dans le domaine numérique des fonctionnalités délicates à implémenter dans le domaine analogique aux fréquences considérées. La deuxième contribution consiste à caractériser et à compenser les erreurs de synchronisation de dispositifs d'échantillonnage non uniforme particuliers, à savoir les convertisseurs analogique-numérique entrelacés temporellement, via des méthodes supervisées ou aveugles