Parallel Delta-Sigma Modulator-Based Digital Predistortion of Wideband RF Power Amplifiers

In this article, we propose a new robust and highly efficient digital predistortion (DPD) concept for the linearization of wideband RF power amplifiers (PAs). The proposed approach is based on the combination of a parallelized delta-sigma modulator (DSM) and a forward model of the PA. This concept applies multi-rate techniques on a DSM that incorporates the forward PA model in its feedback loop to perform the required signal predistortion. Such a technique eliminates the need of reverse modeling and its associated problems. The multi-rate approach relaxes enormously the clock speed requirement of the DPD, which allows handling high signal bandwidths at feasible sampling rates. Moreover, enhanced performance can be achieved without the need of increasing the order of the modulator which reduces the sensitivity of the system to gain variations and phase distortions caused by the nonlinear PA characteristics. Three time-interleaved parallel DPD (P-DPD) variants are described and introduced, all of them have been shown to offer increased accuracy, and consequently better linearization performance compared to the DSM-based DPD state-of-the-art. The proposed architectures are tested and assessed using extensive real-world RF measurements at the 3.6 GHz band utilizing wideband 100 MHz 5G New Radio (NR) transmit waveforms, evidencing excellent transmit signal quality.publishedVersionPeer reviewe

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

Compensation for Impairments of Frequency Converters in Millimeter Wave Vector Signal Generators

The upcoming fifth generation (5G) of wireless communications aims to utilize millimeter wave (mm-wave) frequencies in its infrastructure to alleviate the crowded spectrum problem below 6 GHz. At higher frequencies, modulation bandwidths of several hundreds of MHz can be utilized to increase system capacities. However, the radio frequency (RF) frontends can exhibit significant amounts of impairments over these wide bandwidths, thereby limiting the achievable output signal quality and capacity. In this work, two signal generation architectures and the accompanying compensation schemes to mitigate the impairments are proposed for the generation of wideband modulated signals at mm-wave frequencies. The frequency dependent IQ imbalance effects in conventional direct conversion signal generation architectures over ultra wide bandwidths are first investigated. For that, a new interleaved multi-tone test signal based identification and compensation scheme is proposed. This scheme was experimentally validated by using an off-the-shelf IQ mixer operating at 30 GHz driven with an interleaved multi-tone signal with 4 GHz of modulation bandwidth and achieving a reduction in the normalized mean squared error (NMSE) from -14 dB to -38 dB. Subsequently, a low-complexity pruned Volterra series based digital predistortion (DPD) scheme was devised to mitigate the nonlinear distortions exhibited by the power amplifier stage and maximize the signal quality of orthogonal frequency division multiplexing (OFDM) signals with modulation bandwidths up to 800 MHz. After compensation of the system with 66 DPD coefficients, the OFDM signal with 800 MHz of modulation bandwidth exhibited an NMSE of -32.4 dB and an adjacent channel power ratio (ACPR) of 45 dBc. However, the challenges associated with the implementation of traditional direct conversion architectures exacerbate as the operating frequency increases. For instance, the performance of high frequency active building blocks, e.g. mixer and amplifiers, deteriorates as the operating frequency approaches the maximum oscillation frequency of the semiconductor technology. To address this challenge, a signal generation system utilizing frequency multipliers to replace the mixer and facilitate frequency upconversion is proposed. A novel Volterra series based behavioural model is also devised to predict the nonlinear behaviour of frequency multipliers and to form the basis for synthesizing a DPD scheme capable of obtaining acceptable signal quality when driven with wideband modulated signals. Various frequency multiplier based signal generation systems were implemented using off-the-shelf frequency doublers, triplers, and quadruplers to serve as proof of concept prototypes. Experiments confirmed the ability to generate modulated signals with competitive error vector magnitudes (EVM) and ACPR levels with low complexity DPD schemes

Characterization and Linearization of 5G mm-Wave Power Amplifiers Using Reduced-Sampling Observation Receiver

In this work, an advanced calibration routine is proposed to determine the frequency response of a feedback receiver over a targeted linearization bandwidth, when only sub-Nyquist (aliased) samples are available. A new approach is then devised to apply a direct learning algorithm along with the proposed receiver calibration routine, and thus linearize a millimeter wave power amplifier (PA), driven by a modulated signal, using digital pre-distortion (DPD) with a reduced feedback sampling rate. The proposed new calibration routine and DPD approach are successfully applied to linearize a PA under test, operating at 24 GHz and driven by single carrier 16-QAM and carrier aggregated LTE signals of 200MHz modulation bandwidth using a feedback receiver with sampling rates of 2 Gsps, 1 Gsps and 500 Msps. Adjacent channel power ratio of about 49 dBc and normalized mean square error of about 2% are obtained at the linearized PA output using the three sampling rates

Receptores de rádio-frequência melhorados e disruptivos

This Ph.D. mainly addresses the reception part of a radio front end, focusing on Radio Frequency (RF) sampling architectures. These are considered to be the most promising future candidates to get better performance in terms of bandwidth and agility, following the well-known Software-Defined Radio (SDR) concept. The study considers the usage of an RF receiver in a standalone operation, i.e., used for receiving unknown data at the antenna, and when used as observation path for Power Amplifier (PA) linearization via Digital Predistortion (DPD), since nowadays this represents a mandatory technique to increase overall system’s performance. Firstly, commercial available RF Analog-Digital-Converters (ADCs) are studied and characterized to understand their limitations when used in DPD scenarios. A method for characterization and digital post-compensation to improve performance is proposed and evaluated. Secondly, an innovative FPGA-based RF single-bit pulsed converter based on Pulse Width Modulation (PWM) is addressed targeting frequency agility, high analog input bandwidth, and system integration, taking profit of an FPGA-based implementation. The latter was optimized based on PWM theoretical behavior maximizing Signal-to-Noise-Ratio (SNR) and bandwidth. The optimized receiver, was afterwards evaluated in a 5G C-RAN architecture and as a feedback loop for DPD. Finally, a brief study regarding DPD feedback loops in the scope of multiantenna transmitters is presented. This Ph.D. contributes with several advances to the state-of-the-art of SDR receiver, and to the so-called SDR DPD concept.Este doutoramento endereça principalmente a componente de receção de um transcetor de rádio-frequência (RF), focando-se em arquiteturas de receção de amostragem em RF. Estas são assim consideradas como sendo as mais promissoras para o futuro, em termos de desempenho, largura de banda e agilidade, de acordo com o conhecido conceito de Rádios Definidos por Software (SDR). O estudo considera o uso dos recetores de RF em modo standalone, i.e., recebendo dados desconhecidos provenientes da antena, e também quando usados como caminho de observação para aplicação de linearização de amplificadores de potência (PAs) via pré-distorção digital (DPD), pois atualmente esta é uma técnica fundamental para aumentar o desempenho geral do sistema. Em primeiro lugar, os conversores analógico-digital de RF são estudados e caracterizados para perceber as suas limitações quando usados em cenários de DPD. Um método de caracterização e pós compensação digital é proposto para obter melhorias de desempenho. Em segundo lugar, um novo recetor pulsado de um bit baseado em Modulação de Largura de Pulso (PWM) e implementado em Agregado de Células Lógicas Programáveis (FPGA) é endereçado, visando agilidade em frequência, largura de banda analógica e integração de sistema, tirando proveito da implementação em FPGA. Este recetor foi otimizado com base no modelo comportamental teórico da modulação PWM, maximizando a relação sinalruído (SNR) e a largura de banda. O recetor otimizado foi posteriormente avaliado num cenário 5G de uma arquitetura C-RAN e também num cenário em que serve de caminho de observação para DPD. Finalmente, um breve estudo relativo a caminhos de observação de DPD no contexto de transmissores multi-antena é também apresentado. Este doutoramento contribui com vários avanços no estado da arte de recetores SDR e no conceito de SDR DPD.Programa Doutoral em Engenharia Eletrotécnic

Techniques for Wideband All Digital Polar Transmission

abstract: Modern Communication systems are progressively moving towards all-digital transmitters (ADTs) due to their high efficiency and potentially large frequency range. While significant work has been done on individual blocks within the ADT, there are few to no full systems designs at this point in time. The goal of this work is to provide a set of multiple novel block architectures which will allow for greater cohesion between the various ADT blocks. Furthermore, the design of these architectures are expected to focus on the practicalities of system design, such as regulatory compliance, which here to date has largely been neglected by the academic community. Amongst these techniques are a novel upconverted phase modulation, polyphase harmonic cancellation, and process voltage and temperature (PVT) invariant Delta Sigma phase interpolation. It will be shown in this work that the implementation of the aforementioned architectures allows ADTs to be designed with state of the art size, power, and accuracy levels, all while maintaining PVT insensitivity. Due to the significant performance enhancement over previously published works, this work presents the first feasible ADT architecture suitable for widespread commercial deployment.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Conversion analogique-numérique Sigma-Delta large bande appliquée à la mesure des non-linéarités des amplificateurs de puissance

Power amplifiers, which are essential elements of any communication system, will play a crucial role in the development of future communication systems. Today improving power amplifiers requires technological advances at the circuit device level, but one also must consider a more global approach. In particular, advances in digital processing can now correct in the early stage of the communication chain some distortions that are generated downstream in the chain. Digital pre-distortion is a correction technique for power amplifiers that has a growing interest because of its completely digital implementation and of its gains in linearity and energy consumption. This technique requires a feedback path where the analog-to-digital converter is a critical element. This component must satisfy the constraints of high resolution , wide bandwidth, and high linearity. In this thesis, we propose a new architecture of analog-to-digital converter based on bandpass Delta-Sigma modulators. This architecture takes advantage of operating bandpass modulators that are designed to work in parallel, each focusing on different frequencies, but also of a particular cascading arrangement to eliminate the useful signal, which has a high power, in order to reduce dynamics constraints. High-level design and simulations were carried out for discrete time and continuous time systems and also required the development of appropriate simulation tools.Les amplificateurs de puissance, éléments constitutifs essentiels de tout système de télécommunication, vont jouer un rôle capital dans le développement des futurs systèmes de communication. Aujourd'hui l'amélioration des amplificateurs de puissance nécessite un progrès technologique au niveau du composant lui même mais doit aussi tenir compte d'une approche plus globale. En particulier, le progrès dans les traitements numériques permet aujourd'hui de corriger en amont certaines distorsions qui seront générées en aval de la chaîne de communication. La pré-distorsion numérique est une technique de correction des amplificateurs de puissance qui connaît un intérêt grandissant de par son intégration complètement numérique et par les gains en linéarité et en consommation. Cette technique nécessite une voie de retour dont un élément critique est le convertisseur analogique-numérique. Ce composant doit répondre à des contraintes de résolution, de bande passante et de linéarité élevées. Dans cette thèse, nous proposons une nouvelle architecture de convertisseur analogique-numérique à base de modulateurs Sigma-Delta passe-bande. Cette architecture tire partie du fonctionnement passe bande des modulateurs que nous faisons travailler en parallèle, chacun centré sur différentes fréquences, mais aussi d'un agencement en cascade particulier pour éliminer le signal utile, qui est de forte puissance, dans le but de diminuer les contraintes de dynamique.La conception haut niveau et les simulations ont été menées pour des systèmes à temps discret et aussi à temps continu et a nécessité le développement d'outils adaptés de simulation se basant sur la boîte à outils Delta Sigma Toolbox de Richard Schreie

Methods for Control, Calibration, and Performance Optimization of Phased Array Systems

Phased array radar systems have proven advantageous in a variety of research applications, offering faster volume scans and unparalleled time-resolution as compared to traditional parabolic dish antenna systems that rely solely on mechanical systems for controlling the direction of radiation. As such, research has accelerated the development of practical phased array systems to realize their full vision. In particular, next generation phased array systems aim to provide additional advantages in the form of re-configurable beam patterns, adaptive digital beamforming, multiple-input multiple-output (MIMO) radar modes, and other software-defined technologies. However, to fully realize a paradigm shift in phased array technology, especially as the ratio of array to sub-array size becomes greater, this requires a corresponding increase in novel digital backend architectures to fully achieve this vision. Therefore, new methods for control, calibration, and performance optimization are required to enable next-generation phased array systems to reach their potential. In this thesis, a variety of practical engineering challenges related to phased array system design are discussed, with system-level implications and relevant theory included where necessary. For instance, for the first time, as explained in this thesis, a GPS disciplined, time-interleaved measurement technique that leveraged real-time control of a beamformer was developed to enable accurate post-processing correction of the phase drift that results from clocking differences between noncoherent physically separated bistatic nodes. In addition, laboratory efficacy of digital predistortion using the memory-polynomial model has been confirmed for the purpose of maximizing an element's usable power while minimizing spectral spreading and achieving desirable output linearity during operation, and a novel method for training predistortion models comprised of a combined software-defined and physical mechanism for measuring transmitter front-end distortion for elements within a digital-at-every element array has been proposed and verified in the lab