mm-Wave Data Transmission and Measurement Techniques: A Holistic Approach

The ever-increasing demand on data services places unprecedented technical requirements on networks capacity. With wireless systems having significant roles in broadband delivery, innovative approaches to their development are imperative. By leveraging new spectral resources available at millimeter-wave (mm-wave) frequencies, future systems can utilize new signal structures and new system architectures in order to achieve long-term sustainable solutions.This thesis proposes the holistic development of efficient and cost-effective techniques and systems which make high-speed data transmission at mm-wave feasible. In this paradigm, system designs, signal processing, and measurement techniques work toward a single goal; to achieve satisfactory system level key performance indicators (KPIs). Two intimately-related objectives are simultaneously addressed: the realization of efficient mm-wave data transmission and the development of measurement techniques to enable and assist the design and evaluation of mm-wave circuits.The standard approach to increase spectral efficiency is to increase the modulation order at the cost of higher transmission power. To improve upon this, a signal structure called spectrally efficient frequency division multiplexing (SEFDM) is utilized. SEFDM adds an additional dimension of continuously tunable spectral efficiency enhancement. Two new variants of SEFDM are implemented and experimentally demonstrated, where both variants are shown to outperform standard signals.A low-cost low-complexity mm-wave transmitter architecture is proposed and experimentally demonstrated. A simple phase retarder predistorter and a frequency multiplier are utilized to successfully generate spectrally efficient mm-wave signals while simultaneously mitigating various issues found in conventional mm-wave systems.A measurement technique to characterize circuits and components under antenna array mutual coupling effects is proposed and demonstrated. With minimal setup requirement, the technique effectively and conveniently maps prescribed transmission scenarios to the measurement environment and offers evaluations of the components in terms of relevant KPIs in addition to conventional metrics.Finally, a technique to estimate transmission and reflection coefficients is proposed and demonstrated. In one variant, the technique enables the coefficients to be estimated using wideband modulated signals, suitable for implementation in measurements performed under real usage scenarios. In another variant, the technique enhances the precision of noisy S-parameter measurements, suitable for characterizations of wideband mm-wave components

Caracterização, modelação e compensação de efeitos de memória lenta em amplificadores de potência baseados em GAN HEMTS

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have emerged as the most compelling technology for the transmission of highpower radio-frequency (RF) signals for cellular mobile communications and radar applications. However, despite their remarkable power capabilities, the deployment of GaN HEMT-based RF power amplifiers (PAs) in the mobile communications infrastructure is often ruled out in favor of alternative siliconbased technologies. One of the main reasons for this is the pervasiveness of nonlinear long-term memory effects in GaN HEMT technology caused by thermal and charge-trapping phenomena. While these effects can be compensated for using sophisticated digital predistortion algorithms, their implementation and model-extraction complexity—as well as the power necessary for their real-time execution—make them unsuitable for modern small cells and large-scale multiple-input multiple-output transceivers, where the power necessary for the linearization of each amplification element is of great concern. In order to address these issues and further the deployment of high-powerdensity high-efficiency GaN HEMT-based RF PAs in next-generation communications and radar applications, in this thesis we propose novel methods for the characterization, modeling, and compensation of long-term memory effects in GaN HEMT-based RF PAs. More specifically, we propose a method for the characterization of the dynamic self-biasing behavior of GaN HEMTbased RF PAs; multiple behavioral models of charge trapping and their implementation as analog electronic circuits for the accurate real-time prediction of the dynamic variation of the threshold voltage of GaN HEMTs; a method for the compensation of the pulse-to-pulse instability of GaN HEMT-based RF PAs for radar applications; and a hybrid analog/digital scheme for the linearization of GaN HEMT-based RF PAs for next-generation communications applications.Os transístores de alta mobilidade eletrónica de nitreto de gálio (GaN HEMTs) são considerados a tecnologia mais atrativa para a transmissão de sinais de radiofrequência de alta potência para comunicações móveis celulares e aplicações de radar. No entanto, apesar das suas notáveis capacidades de transmissão de potência, a utilização de amplificadores de potência (PAs) baseados em GaN HEMTs é frequentemente desconsiderada em favor de tecnologias alternativas baseadas em transístores de silício. Uma das principais razões disto acontecer é a existência pervasiva na tecnologia GaN HEMT de efeitos de memória lenta causados por fenómenos térmicos e de captura eletrónica. Apesar destes efeitos poderem ser compensados através de algoritmos sofisticados de predistorção digital, estes algoritmos não são adequados para transmissores modernos de células pequenas e interfaces massivas de múltipla entrada e múltipla saída devido à sua complexidade de implementação e extração de modelo, assim como a elevada potência necessária para a sua execução em tempo real. De forma a promover a utilização de PAs de alta densidade de potência e elevada eficiência baseados em GaN HEMTs em aplicações de comunicação e radar de nova geração, nesta tese propomos novos métodos de caracterização, modelação, e compensação de efeitos de memória lenta em PAs baseados em GaN HEMTs. Mais especificamente, nesta tese propomos um método de caracterização do comportamento dinâmico de autopolarização de PAs baseados em GaN HEMTs; vários modelos comportamentais de fenómenos de captura eletrónica e a sua implementação como circuitos eletrónicos analógicos para a previsão em tempo real da variação dinâmica da tensão de limiar de condução de GaN HEMTs; um método de compensação da instabilidade entre pulsos de PAs baseados em GaN HEMTs para aplicações de radar; e um esquema híbrido analógico/digital de linearização de PAs baseados em GaN HEMTs para comunicações de nova geração.Programa Doutoral em Telecomunicaçõe

A New Approach to Pruning Volterra Models for Power Amplifiers

The objective of this paper is to present an approach to behavioral modeling that can be applied to predict the nonlinear response of power amplifiers with memory. Starting with the discrete-time, complex-baseband full Volterra model, we define a novel methodology that retains only radial branches that can be implemented with one-dimensional finite impulse response filters. This model is subsequently simplified by selecting a subset of directions using an ad hoc procedure. Both models are evaluated in terms of accuracy in the time and frequency domains and complexity, and are compared with other models described in the literature. The evaluation is conducted using a low-voltage silicon RF driver amplifier and a 5-W PA, which are characterized at different levels with diverse modulation formats, including wideband code-division multiple-access (WCDMA) and orthogonal frequency-division multiplexed (OFDM) signals. In all cases, comparison of the measured and simulated responses confirms the effectiveness of the proposed approach.CICYT TEC2008-06259/TECJunta de Andalucía P07-TIC-0264

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

Single Input Single Output Digital Pre-Distortion at Millimeter Wave Frequencies for Phased Arrays

The limiting fact that is impeding the increase in data rate in the current generation of wireless communication is the limited available spectrum in the sub-6 GHz bands. This has motivated the shift to higher frequencies such as millimeter waves (mm-wave) and terahertz frequencies where modulation bandwidth of several hundreds of MHz can be utilized to increase the communication link capacity. The deployment of high data rate mm-wave base stations will highly depend on the maximum achievable equivalent isotropic radiated power (EIRP) and on the ability to generate reliable and error free wideband signals. High EIRP and high efficiency operation can be achieved by using active phased arrays operated deep into the power amplifiers (PAs) nonlinear region. In this work, a low power and low complexity compensation schemes to mitigate the impairments exhibited by phase arrays driven with wideband signals and high efficient nonlinear PAs at mm-wave frequencies are proposed. Digital pre-distortion (DPD) techniques can provide an attractive solution to linearize high efficiency and high EIRP nonlinear phased arrays at mm-wave frequencies. However, the viable deployment of DPD solutions call for the reduction in the power consumption of the transmitter observation receiver (TOR) feedback path required to train the DPD function. To that end, a low power DPD scheme for linearizing mm-wave hybrid beamforming antenna systems is presented. The proposed DPD scheme exploits the modularity of hybrid beamforming systems. During the training phase, the constituent sub-arrays, are categorized, into (i) the main sub-array that exhibits non-linear distortion and is to be linearized, and (ii) the auxiliary sub-arrays that operate in the backoff region to avoid nonlinearity. To produce the error signal necessary to train the DPD function (and compensate for the distortions exhibited by the main sub-array), the signals transmitted by the main and auxiliary sub-arrays are combined. This error signal is captured using a TOR with low dynamic range and is digitized using a low-bit resolution analog-to-digital converter (ADC). Proof-of-concept validation experiments are conducted by applying the proposed DPD system to linearize an off-the-shelf hybrid-beamforming array comprised of four 64-element sub-arrays, operating at 28 GHz and driven with up to 800 MHz orthogonal frequency-division multiplexing (OFDM) modulated signals. Using the proposed DPD scheme, a TOR with a 4-bit ADC was sufficient to improve the adjacent channel power ratio (ACPR) by 10 dB and the error vector magnitude (EVM) improved from 5.8% to 1.6%. These results are similar to those obtained using a TOR with 16-bit ADCs. Reducing the complexity of the DPD scheme for phased arrays is also of primordial importance to the successful deployment of DPD solutions. For instance, the DPD function needs to be desensitize to the load modulation effects exhibited by large antenna systems and be able to linearize phased arrays at different steering angles. To address the challenges associated with the load modulation for phased arrays, we propose a generalized SISO DPD scheme as solution to minimize the EVM variation at different steering angles. The measurement results of the proposed scheme, using a 400 MHz OFDM signal with subcarriers modulated using 256 QAM and on a commercial 64-elements beamforming array, was able to maintain the EVM below 2% across the full steering range. This solution, however, failed to maintain the ACPR below -45 dBc. The effect of tapering on the load modulation and the array nonlinearity is also analysed. The measurement results using different tapers are used to validate the theory and the simulation results. Using tapering, the ACPR and EVM variation before and after DPD were minimized versus steering angles. For instance, using taper setting 2, the ACPR and EVM are maintained below -46 dBc and 1% from -38° to 45° and below -42.3 dBc and 1.8% from -45° to 45° respectively. Better results are measured when tapering is used in conjunction with the proposed generalized DPD scheme. In that case, the ACPR is improved from -35.5 to at worst -46.4 dBc and at best -50 dBc and the EVM is improved from at worst 4.5% to at worst 1.2% and at best 0.85%. The EVM is also maintained below 0.95% from -39° to 45°

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

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

O impacto dos efeitos da memória de longo termo na linearizabilidade de amplificadores de potência baseados em AlGaN/GaN HEMT

AlGaN/GaN High Electron Mobility Transistor (HEMT)s are among the preferred options for radio-frequency power amplification in cellular base station transmitters and radar applications. However, despite their promising outlook, the pervasiveness of trapping effects makes them resilient to conventional digital predistortion schemes, which not only decrease their current range of applications but could also preclude their integration in future small cells and multiple-input multiple-output architectures where simpler predistortion schemes are mandatory. So, this PhD thesis aims at developing a meaningful link between the device physics and the linearizability of the AlGaN/GaN HEMT-based Power Amplifier (PA). In order to bridge this gap, this thesis begins with a clear explanation for the mechanisms governing the dominant source of trapping effects in standard AlGaN/GaN HEMTs, namely buffer traps. Based on this knowledge, we explain why the best known physically-supported trapping models, used to represent these devices, are insufficient and present a possible improvement to what we consider to be the most accurate model, supported by Technology Computer-Aided Design (TCAD) simulations. This has also been corroborated through a novel double-pulse technique able to describe experimentally both the capture and emission transients in a wide temporal span under guaranteed isothermal conditions. The measured stretched capture transients validated our understanding of the process while the temperature dependence of the emission profiles confirmed buffer traps as the dominant source of trapping effects. Finally, through both simulations and experimental results, we elaborate here the relationship between the emission time constant and the achievable linearity of GaN HEMT-based PAs, showing that the worst-case scenario happens when the emission time constant is on the order of the time between consecutive envelope peaks above a certain amplitude threshold. This is the case in which we observed a more pronounced hysteresis on the gain and phase-shift characteristics, and so, a stronger impact of the memory effects. The main outcome of this thesis suggests that the biggest linearizability concern in standard AlGaN/GaN HEMT-based PAs lies on the large emission time constants of buffer traps.AlGaN/GaN HEMTs estão entre as opções preferidas para amplificação de potência de radiofrequência em transmissores de estacão base celular e aplicações de radar. No entanto, apesar de sua perspetiva promissora, a influência dos efeitos de defeitos com níveis profundos torna-os imunes aos esquemas convencionais de pre-distorção digital. Assim, esta tese de doutoramento visa desenvolver uma ligação significativa entre a física do dispositivo e a linearização de amplificadores de potência baseados em Al- GaN/GaN HEMTs. Por forma a preencher esta lacuna, esta tese começa com uma explicação clara dos mecanismos que governam a fonte dominante de efeitos de defeitos com níveis profundos em AlGaN/GaN HEMTs standard, especificamente defeitos no buffer. Com base neste conhecimento, são aparentadas as falhas dos modelos físicos mais conhecidos de defeitos de nível profundo usados para representar estes dispositivos, assim como uma possível melhoria suportada em simulações de TCAD. Isto é também corroborado por uma nova técnica de duplo-pulso capaz de descrever experimentalmente os transientes de captura e emissão num amplo intervalo temporal sob condições isotérmicas. Os transientes de captura medidos validam a nossa compreensão do processo, enquanto que a dependência da temperatura nos perfis de emissão confirmou os defeitos no buffer como a fonte dominante de efeitos de defeitos com níveis profundos. Por fim, através de simulações e resultados experimentais, elabora-se aqui a relação entre a constante de tempo de emissão e a linearizabilidade dos amplificadores baseados em AlGaN/GaN HEMT, mostrando que o pior cenário acontece quando a constante de tempo de emissão é da mesma ordem do tempo entre picos consecutivos da envolvente acima de um certo limiar de amplitude. Este é o caso para o qual se observa uma histerese mais pronunciada nas características de ganho e fase e, consequentemente, um impacto mais forte dos efeitos de memória. O resultado principal desta tese sugere que a maior preocupação na linearização de amplificadores baseados em AlGaN/GaN HEMTs standard está nas grandes constantes de tempo de emissão dos defeitos no buffer.Programa Doutoral em Engenharia Eletrotécnic

Efficient solid state power amplifiers: power combining and highly accurate AM/AM and AM/PM behavioural models with application to linearisation

Radio Frequency (RF) Power Amplifiers (PAs) are a major contributor to modern communication systems, both in terms of being an enabling technology as well as having the most impact on overall system availability, linearity and power consumption. In order to achieve the most optimum system outcome there needs to be an appropriate method for selecting the most suitable RF PA design approach, as well as being able to select the most appropriate RF PA output device, based on a range of varying requirements, specifications and technologies. The ability to perform these tasks quickly, with improved accuracy, using existing available device data, with minimal or no further device testing and from a range of existing and emerging technologies would provide RF PA designers with significant benefits. The investigations and research provided in this thesis consider a range of existing and emerging RF PA technologies and power combining methods and compares them via a new selection and design methodology developed in this thesis. The new methodology builds on modern design and statistical approaches including manufacturing options that enable an appropriate technology to be selected for Solid State Power Amplifier (SSPA) design. In addition to hard design specifications, the current thesis also considers less tangible specifications, such as graceful degradation, time tomarket and ease of use, as well as alternative design approaches, such as fuzzy logic approaches. With a suitable technology approach determined, a selection of a suitable RF output device(s) is considered. As the demand for new communication services continues to increase, requiring tighter specifications and reduced product delivery time scales, then the ability to accurately and quickly compare available RF PA devices from a range of device technologies or devices from different manufacturers, at both the system and component level, makes such a selection paramount. In this thesis, simplememoryless (AmplitudeModulation/AmplitudeModulation (AM/AM) only) and Quasi-Memoryless (QM) Behavioural Models (BMs) (AM/AM combined with Amplitude Modulation/Phase Modulation (AM/PM)) are reviewed, extended and improved upon, with up to 20 dB Normalised Mean Squared Error (NMSE) modelling improvement achieved over a range of technologies, allowing effective RF PA device selection using these newly developed simple and fast models. This thesis uses recent existing accurate and powerful semi-physical memoryless BMs, suited to RF PA devices, and develops and extends their use for QM modelling. The trade-off from the improvement in the overall accuracy is some further simple processing steps. Furthermore, this thesis also provides a comparison of other models, presented in the literature. The improved simple RF PA device models and extension techniques presented in this thesis show, via simulation and measurement, that the new models are suitable for use over a wide range. Lineariser improvements, linked to the accuracy improvements of the proposed models of this thesis, are also investigated, showing further benefits from this research. Physically based simple QM BMs are also used to model thermal and bias network memory effects, which are becomingmore relevant tomodern communication services that use wider bandwidths, enabling the impacts of RF PA device memory effects to be determined and compared. The feasibility of the developed models and improvements are also utilised in the simulation of a low cost RF PA lineariser. With the trend to smaller localised low cost and power RF mobile wireless repeater cells being away from larger more expensive and complex hardware, used to perform linearisation, this thesis presents a trade-off between complexity and linearisation performance and demonstrates, through modelling and simulation, that 8-10 dB improvement in linearisation performance is achievable with the use of the newly developed models.Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Electronic Engineering, 201