Modelação comportamental e pré-distorção digital de transmissores de rádio-frequência

Doutoramento em Engenharia ElectrotécnicaNos atuais sistemas de telecomunicações, os transmissores de rádio-frequência são desenvolvidos tendo maioritariamente em conta a eficiência da conversão da potência fornecida da fonte em potência de rádio-frequência. Este tipo de desenho resulta em amplificadores de potência com características de transmissão não-lineares, que distorcem severamente o envelope de informação no processo de amplificação, gerando distorção fora da banda. Para corrigir este problema utiliza-se um processo de compensação não linear, sendo que a pré-distorção digital se tem favorecido pela sua flexibilidade e precisão. Este método é tipicamente aplicado de uma forma cega, por força bruta até se obter a compensação desejada. No entanto, quando o método se mostra ineficaz, como se verificou em amplificadores de potência baseados em transístores de nitreto de gálio, é difícil saber o que modificar nos sistemas para os tornar de novo úteis. De forma a compreender e desenhar sistemas de pré-distorção digital robustos é necessário, por um lado, perceber o comportamento dos amplificadores de rádio-frequência, por outro, perceber as limitações e relações entre os modelos digitais e o comportamento real do amplificador. Nesse sentido, esta tese explora e descreve estas relações de forma a suportar a escolha de modelos de pré-distorção, desenvolve novos modelos baseados no comportamento dos transístores, e propõe métodos de caracterização para os amplificadores de RF.In current telecommunication systems, the main concern when developing the radio frequency transmitter is power efficiency. This type of design generally leads to a highly nonlinear transmission characteristic, mainly due to the radio frequency power amplifier. This nonlinear transmission severely distorts the information envelope, leading to spectral regrowth, out-of-band distortion. To correct this problem a nonlinear compensation process is employed. For this application, digital predistortion is generally favored for its flexibility and accuracy. Digital predistortion is mostly applied in a blind manner, using brute force until the desired compensation is achieved. Because of this, when the method fails, as it has in gallium nitride based power amplifiers, it is difficult to modify the system to achieve the desired results. To understand and design robust predistortion systems, it is both necessary to have knowledge of the power amplifiers’ behavior, on one hand, and understand the limitations and relations between the digital models and these behaviors, on the other. To do this, this thesis explores and describes these relationships, granting support to the digital predistortion model choice, it further develops new predistortion models based on the physics of the transistors’ behaviors, and it proposes methods for the characterization of radio frequency power amplifiers

Automatic transmit power control for power efficient communications in UAS

Nowadays, unmanned aerial vehicles (UAV) have become one of the most popular tools that can be used in commercial, scientific, agricultural and military applications. As drones become faster, smaller and cheaper, with the ability to add payloads, the usage of the drone can be versatile. In most of the cases, unmanned aerials systems (UAS) are equipped with a wireless communication system to establish a link with the ground control station to transfer the control commands, video stream, and payload data. However, with the limited onboard calculation resources in the UAS, and the growing size and volume of the payload data, computational complex signal processing such as deep learning cannot be easily done on the drone. Hence, in many drone applications, the UAS is just a tool for capturing and storing data, and then the data is post-processed off-line in a more powerful computing device. The other solution is to stream payload data to the ground control station (GCS) and let the powerful computer on the ground station to handle these data in real-time. With the development of communication techniques such as orthogonal frequency-division multiplexing (OFDM) and multiple-input multiple-output (MIMO) transmissions, it is possible to increase the spectral efficiency over large bandwidths and consequently achieve high transmission rates. However, the drone and the communication system are usually being designed separately, which means that regardless of the situation of the drone, the communication system is working independently to provide the data link. Consequently, by taking into account the position of the drone, the communication system has some room to optimize the link budget efficiency. In this master thesis, a power-efficient wireless communication downlink for UAS has been designed. It is achieved by developing an automatic transmit power control system and a custom OFDM communication system. The work has been divided into three parts: research of the drone communication system, an optimized communication system design and finally, FPGA implementation. In the first part, an overview on commercial drone communication schemes is presented and discussed. The advantages and disadvantages shown are the source of inspiration for improvement. With these ideas, an optimized scheme is presented. In the second part, an automatic transmit power control system for UAV wireless communication and a power-efficient OFDM downlink scheme are proposed. The automatic transmit power control system can estimate the required power level by the relative position between the drone and the GCS and then inform the system to adjust the power amplifier (PA) gain and power supply settings. To obtain high power efficiency for different output power levels, a searching strategy has been applied to the PA testbed to find out the best voltage supply and gain configurations. Besides, the OFDM signal generation developed in Python can encode data bytes to the baseband signal for testing purpose. Digital predistortion (DPD) linearization has been included in the transmitter’s design to guarantee the signal linearity. In the third part, two core algorithms: IFFT and LUT-based DPD, have been implemented in the FPGA platform to meet the real-time and high-speed I/O requirements. By using the high-level synthesis design process provided by Xilinx Corp, the algorithms are implemented as reusable IP blocks. The conclusion of the project is given in the end, including the summary of the proposed drone communication system and envisioning possible future lines of research

Digitally-Compensated Wideband 60 GHz Test-Bed for Power Amplifier Predistortion Experiments

Millimeter waves will play an important role in communication systems in the near future. On the one hand, the bandwidths available at millimeter-wave frequencies allow for elevated data rates, but on the other hand, the wide bandwidth accentuates the effects of wireless front-end impairments on transmitted waveforms and makes their compensation more difficult. Research into front-end impairment compensation in millimeter-wave frequency bands is currently being carried out, mainly using expensive laboratory setups consisting of universal signal generators, spectral analyzers and high-speed oscilloscopes. This paper presents a detailed description of an in-house built MATLAB-controlled 60 GHz measurement test-bed developed using relatively inexpensive hardware components that are available on the market and equipped with digital compensation for the most critical front-end impairments, including the digital predistortion of the power amplifier. It also demonstrates the potential of digital predistortion linearization on two distinct 60 GHz power amplifiers: one integrated in a direct-conversion transceiver and an external one with 24 dBm output power

Highly efficient RF power amplifier for wireless LAN applications

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Nonlinear Characterization and Modeling of Radio-Frequency Devices and Power Amplifiers with Memory Effects

Despite the fast development of telecommunications systems experienced during the last two decades, much progress is expected in the coming years with the introduction of new solutions capable of delivering fast data-rates and ubiquitous connectivity. However, this development can only happen through the evolution of radio-frequency systems, which should be capable of working at high-power and high-speed. At the same time, the power dissipation of these systems should be minimized. In this dissertation, methods for the characterization and modeling of transistors and power amplifiers are presented, along with the necessary nonlinear measurements techniques. In particular, dynamic electrical effects, originated by the properties of the semiconductor materials and by the system configurations, are investigated. Consequently, these phenomena, which cannot be ignored to obtain the wanted performance, are empirically identified and included in models for Gallium Nitride (GaN) transistors and power amplifiers driven by a dynamic voltage supply

Mecanismos de geração da distorção não-linear em amplificadores Doherty

Doutoramento em Engenharia ElectrotécnicaNowadays, wireless communications systems demand for greater mobility and higher data rates. Moreover, the need for spectral efficiency requires the use of non-constant envelope modulation schemes. Hence, power amplifier designers have to build highly efficient, broadband and linear amplifiers. In order to fulfil these strict requirements, the practical Doherty amplifier seems to be the most promising technique. However, due to its complex operation, its nonlinear distortion generation mechanisms are not yet fully understood. Currently, only heuristic interpretations are being used to justify the observed phenomena. Therefore, the main objective of this work is to provide a model capable of describing the Doherty power amplifier nonlinear distortion generation mechanisms, allowing the optimization of its design according to linearity and efficiency criteria. Besides that, this approach will allow a bridge between two different worlds: power amplifier design and digital pre-distortion since the knowledge gathered from the Doherty operation will serve to select the most suitable pre-distortion models.Presentemente, os sistemas de comunicações sem fios exigem uma maior mobilidade e elevadas taxas de transferência. Para além disso, a necessidade de eficiência espectral obriga ao uso de esquemas de modulação de envolvente variável. Consequentemente, o desenvolvimento de amplificadores de elevada eficiência, com uma elevada largura de banda e, ao mesmo tempo, lineares, tornou-se num dos maiores desafios para os engenheiros de projeto de amplificadores de potência. Por forma a cumprir estes requisitos muito rigorosos, o amplificador em configuração Doherty parece ser a técnica mais promissora. Contudo, devido à sua complexa operação, os seus mecanismos de geração de distorção não linear não são ainda completamente conhecidos. Atualmente, apenas interpretações heurísticas estão a ser usadas para justificar os fenómenos observados. Nesse sentido, o principal objetivo deste trabalho é desenvolver um modelo capaz de descrever os mecanismos de geração da distorção não linear em amplificadores Doherty, permitindo assim, a optimização do seu projeto, tendo em conta as especificações de linearidade e eficiência. Para além disso, esta abordagem permitirá uma ponte entre dois mundos diferentes: projecto de amplificadores de potência e pré-distorção digital, uma vez que o conhecimento recolhido da operação do Doherty ajudará na escolha de modelos de pré-distorção mais adequados

Modelação comportamental de dispostivos não-lineares (RF-digital) de entrada/saída

Doutoramento em Engenharia ElectrotécnicaEsta tese investiga a caracterização (e modelação) de dispositivos que realizam o interface entre os domínios digital e analógico, tal como os buffers de saída dos circuitos integrados (CI). Os terminais sem fios da atualidade estão a ser desenvolvidos tendo em vista o conceito de rádio-definido-por-software introduzido por Mitola. Idealmente esta arquitetura tira partido de poderosos processadores e estende a operação dos blocos digitais o mais próximo possível da antena. Neste sentido, não é de estranhar que haja uma crescente preocupação, no seio da comunidade científica, relativamente à caracterização dos blocos que fazem o interface entre os domínios analógico e digital, sendo os conversores digital-analógico e analógico-digital dois bons exemplos destes circuitos. Dentro dos circuitos digitais de alta velocidade, tais como as memórias Flash, um papel semelhante é desempenhado pelos buffers de saída. Estes realizam o interface entre o domínio digital (núcleo lógico) e o domínio analógico (encapsulamento dos CI e parasitas associados às linhas de transmissão), determinando a integridade do sinal transmitido. Por forma a acelerar a análise de integridade do sinal, aquando do projeto de um CI, é fundamental ter modelos que são simultaneamente eficientes (em termos computacionais) e precisos. Tipicamente a extração/validação dos modelos para buffers de saída é feita usando dados obtidos da simulação de um modelo detalhado (ao nível do transístor) ou a partir de resultados experimentais. A última abordagem não envolve problemas de propriedade intelectual; contudo é raramente mencionada na literatura referente à caracterização de buffers de saída. Neste sentido, esta tese de Doutoramento foca-se no desenvolvimento de uma nova configuração de medição para a caracterização e modelação de buffers de saída de alta velocidade, com a natural extensão aos dispositivos amplificadores comutados RF-CMOS. Tendo por base um procedimento experimental bem definido, um modelo estado-da-arte é extraído e validado. A configuração de medição desenvolvida aborda não apenas a integridade dos sinais de saída mas também do barramento de alimentação. Por forma a determinar a sensibilidade das quantias estimadas (tensão e corrente) aos erros presentes nas diversas variáveis associadas ao procedimento experimental, uma análise de incerteza é também apresentada.This thesis investigates the characterization (and modeling) of devices that perform the interface between the digital and analog domains, such as the output buffers of the Integrated Circuits (ICs). Modern wireless transceivers are moving towards the software-defined radio (SDR) concept proposed by Mitola. The ideal architecture makes use of powerful digital signal processors (DSP) and extends the digital blocks’s operation as close as possible to the antenna. In this way, it is not surprising that there is a growing concern regarding the characterization of the blocks that perform the interface between the analog and digital domains, being the analog-to-digital and digital-to-analog converters (ADCs/DACs) two good examples of these circuits. Within the high speed digital circuits, such as Flash memories, a similar role is played by the output buffers. They act as an interface between the digital (logic core) and the analog radio frequency domains (package/printed circuit board), determining the signal integrity of the transmitted data. In order to speed up the signal integrity analysis, at the design stage, it is fundamental to have models that are simultaneously computationally efficient and accurate. Typically, the extraction/validation of models for output buffers is performed by using characterization data obtained from either the simulation of the transistor level model (if available) or from measurements. The latter approach is not sensitive to intellectual property issues but is rarely addressed in literature referred to output buffers’ characterization. Therefore, this thesis addresses the development of a novel measurement setup for the characterization and modeling of high speed output buffers, naturally extendable to switched-mode RF-CMOS amplifiers. Based on a welldefined experimental procedure, a state of the art model is extracted and validated. The developed measurement setup addresses not only the integrity of the output signals (signal integrity) but also of the power supply voltages (power integrity). In order to determine the sensitivity of the estimated quantities (voltage and current) to the errors in the different variables of the experimental procedure, an uncertainty analysis is presented

Contributing Towards Improved Communication Systems for Future Cellular Networks

The rapid growth of wireless communications and upcoming requirements of 5G networks are driving interest in the areas from wireless transceivers to sensor nodes. One of the most vital components of the wireless transmitter is the radio frequency power amplifier. A large-signal device model of the transistor is an essential part of the power amplifier design process. Despite the significant developments in large-signal modelling, the models for commercially available devices from the manufacturers are still under continuous development and often lack accuracy. One of the main objectives of this thesis is the validation and extension of an analytic approach as an alternative to conventional large-signal modelling for power amplifier designing. The first contribution is the derivation of new analytical expressions based on the equivalent circuit model, including the extrinsic parasitic elements introduced by the package, to calculate the optimum source and load impedances and to predict the performance of a radio frequency power amplifier. These expressions allow to evaluate the effects of a package on the optimum impedance values and performance. The second contribution is establishing the accuracy of the analytic approach. Harmonic balance simulation is used as the first benchmark to evaluate the method at various bias points and frequencies. The validity of the analytic approach is demonstrated at a frequency of 3.25 GHz for gallium nitride based high power devices with measurement of prototype radio frequency power amplifier designed for the impedance values obtained from the analytic expressions. The third contribution is extending the analytic approach to determine the optimum impedance values for different criteria of maximum gain, linearity and efficiency. The analytic expressions are utilized to gain an understanding of the relationship among the device performance, the elements of devices and package models and I-V characteristics. The wireless sensor networks are essential elements for the realization of the Internet of Things. Sensor nodes, which are the fundamental building blocks of these networks, have to be energy efficient and able to produce energy to reduce the maintenance cost and to prolong their lifetime. The second main aim of the thesis is designing and implementing an ultra-low power autonomous wireless sensor node that harvests the indoor light energy. The forth contribution of this thesis includes a comprehensive comparison of six different solar cell technologies under a controlled light intensity, carried out to determine the best option for indoor light energy harvesting. The power consumption of the node is reduced by selecting the appropriate hardware and implementing a wake-up receiver to reduce the active and idle mode currents. The low power consumption coupled with light energy harvesting significantly prolong the operating lifetime of the node