Measurement techniques for the characterization of radio frequency gallium nitride devices and power amplifiers

The rapid growth of mobile telecommunications has fueled the development of the fifth generation (5G) of standards, aiming to achieve high data rates and low latency. These capabilities make use of new regions of spectrum, wider bandwidths and spectrally efficient modulations. The deployment of 5G relies on the development of radio-frequency (RF) technology with increased performance. The broadband operation at high-power and high-frequency conditions is particularly challenging for power amplifiers (PA) in transmission stages, which seek to concurrently maximize linearity and energy efficiency. The properties of Gallium Nitride (GaN) allow the realization of active devices with favorable characteristics in these applications. However, GaN high-electron mobility transistors (HEMTs) suffer from spurious effects such as trapping due to physical defects introduced during the HEMT growth process. Traps dynamically capture and release mobile charges depending on the applied voltages and temperature, negatively affecting the RF PA performance. This work focuses on the development of novel measurement techniques and setups to investigate trapping behavior of GaN HEMTs and PAs. At low-frequency (LF), charge dynamics is analyzed using pulsed current transient characterizations, identifying relevant time constants in state-of-the-art GaN technologies for 5G. Instead, at high-frequency, tailored methods and setups are used in order to measure trapping effects during the operation of HEMTs and PAs in RF modulated conditions. These RF characterizations emulate application-like regimes, possibly involving the control of the device’s output load termination. Therefore, an innovative wideband active load pull (WALP) setup is developed, using the acquisition capabilities of standard vector-network-analyzers. Moreover, the implications of performing error-vector-magnitude characterizations under wideband load pull conditions are studied. Finally, an efficient implementation of a modified-Volterra model for RF PAs is presented, making use of a custom vector-fitting algorithm to simplify the nonlinear memory operators and enable their realization in simulation environments

Supply modulated GaN HEMT power amplifiers - From transistor to system

Power amplifiers (PAs) for mobile communication applications are required to fulfil stringent requirements concerning linearity while keeping a high efficiency over a wide power range and bandwidth. To achieve this, a number of advanced PA topologies have been developed, mostly based on either load modulation, such as Doherty PAs or load modulation balanced PAs, or on supply modulation such as envelope tracking or envelope elimination and restoration. Supply modulation has an advantage over other topologies as the power range of high efficiency can be realised over arbitrary bandwidths, only limited by the bandwidth of the PA. This does, however, come at the cost of a significantly more complicated voltage supply. Instead of a static supply voltage, the PA needs to be provided with one which is rapidly changing, requiring a supply modulator capable of powering the PA while modulating its supply voltage. This thesis investigates a number of challenges in supply modulated power amplifiers, ranging from the transistor itself to circuit design and system level considerations and focusses on power levels up to 10 W and frequencies between 1 GHz and 4 GHz. Transistors, as the centre-piece of a PA, determine how well the PA reacts to a changing supply voltage. In this work, the traits that make GaN HEMTs suitable for supply modulated PAs were investigated, and gain variation with changing supply voltage was established as an important parameter. This gain variation is described in detail and its impacts on PA performance are discussed. By comparing transistors in literature, gain variation has been demonstrated to be a prevalent characteristic in transistors with GaN HEMTs showing a very wide range of gain variation. Using a small-signal model based on measurements, the voltage dependent behaviour of the feedback capacitance CGD is, for the first time, identified as the origin of small-signal gain variation. This is traced down to the gate field plate which is commonly used to combat surface trapping effects in GaN HEMTs. With this in mind, two different ways of changing the transistor geometry to reduce the impact of gain variation and thus optimise the transistor for operation in supply modulated PAs are discussed and demonstrated using a 250 nm GaN HEMT. As a result of the non-linearity of the feedback and gate-source capacitances, the input impedance of GaN HEMTs changes with supply voltage and drive power. This prevents the transistor from being matched at all supply voltages and input powers and introduces phase distortion. Using simulation and measurement, the impact of input impedance on linearity and efficiency of supply modulated power amplifiers is demonstrated on a 2.9 GHz 10 W PA. Careful selection of the input impedance allows improvement of AM/PM distortion of a supply modulated PA with little cost in terms of AM/AM and PAE. I Supply modulators have a significant impact on efficiency and linearity of the ET system. One supply modulator topology with the potential to generate a supply voltage with a high modulation bandwidth is the RF modulator in which the input DC voltage is turned into an RF signal and rectified, resulting in an output voltage which depends on the excitation of the PA. While PAs are well understood in every detail, there are gaps in the understanding of RF rectifiers. Using active load-pull/source-pull measurements, intrinsic gate and drain waveforms of a GaN HEMT operated as a rectifier are demonstrated for the first time. This allows in-detail evaluation of the impact of the gate termination in self-synchronous rectifiers. It also allows detailed analysis of the loss mechanisms in rectifiers and formulation of the required impedances to realise efficient self-synchronous operation, resulting in efficiencies exceeding 90% over wide power ranges. Using waveform engineering, a new type of RF modulator, with potentially very high bandwidths, based on even harmonic generation/injection is proposed. The necessary operating conditions of the rectifier part of the modulator are emulated using an active load-pull/source-pull system to successfully demonstrate that the rectifier behaves as predicted. Using a simple demonstrator, preliminary measurements were conducted and the RF modulator was shown to work, reaching efficiencies up to 78%. As PA and supply modulator are combined, they present impedances to each other. These impedances have a significant impact on the behaviour of both sub-systems. A simple way to characterise both the impedance presented to the PA by the modulator and the impedance presented to the modulator by the PA is described. Using a state-of-the-art modulator, these impedances are measured, the modulator impedance is demonstrated to be close to the simulated value. These measurements also demonstrate that the impedances change significantly with the operating conditions

Array Architectures and Physical Layer Design for Millimeter-Wave Communications Beyond 5G

Ever increasing demands in mobile data rates have resulted in exploration of millimeter-wave (mmW) frequencies for the next generation (5G) wireless networks. Communications at mmW frequencies is presented with two keys challenges. Firstly, high propagation loss requires base stations (BSs) and user equipment (UEs) to use a large number of antennas and narrow beams to close the link with sufficient received signal power. Consequently, communications using narrow beams create a new challenge in channel estimation and link establishment based on fine angular probing. Current mmW system use analog phased arrays that can probe only one angle at the time which results in high latency during link establishment and channel tracking. It is desirable to design low latency beam training by exploring both physical layer designs and array architectures that could replace current 5G approaches and pave the way to the communications for frequency bands in higher mmW band and sub-THz region where larger antenna arrays and communications bandwidth can be exploited. To this end, we propose a novel signal processing techniques exploiting unique properties of mmW channel, and show both theoretically, in simulation and experiments its advantages over conventional approaches. Secondly, we explore different array architecture design and analyze their trade-offs between spectral efficiency and power consumption and area. For comprehensive comparison, we have developed a methodology for optimal design of system parameters for different array architecture candidates based on the spectral efficiency target, and use these parameters to estimate the array area and power consumption based on the circuits reported in the literature. We show that the hybrid analog and digital architectures have severe scalability concerns in radio frequency signal distribution with increased array size and spatial multiplexing levels, while the fully-digital array architectures have the best performance and power/area trade-offs.The developed approaches are based on a cross-disciplinary research that combines innovation in model based signal processing, machine learning, and radio hardware. This work is the first to apply compressive sensing (CS), a signal processing tool that exploits sparsity of mmW channel model, to accelerate beam training of mmW cellular system. The algorithm is designed to address practical issues including the requirement of cell discovery and synchronization that involves estimation of angular channel together with carrier frequency offset and timing offsets. We have analyzed the algorithm performance in the 5G compliant simulation and showed that an order of magnitude saving is achieved in initial access latency for the desired channel estimation accuracy. Moreover, we are the first to develop and implement a neural network assisted compressive beam alignment to deal with hardware impairments in mmW radios. We have used 60GHz mmW testbed to perform experiments and show that neural networks approach enhances alignment rate compared to CS. To further accelerate beam training, we proposed a novel frequency selective probing beams using the true-time-delay (TTD) analog array architecture. Our approach utilizes different subcarriers to scan different directions, and achieves a single-shot beam alignment, the fastest approach reported to date. Our comprehensive analysis of different array architectures and exploration of emerging architectures enabled us to develop an order of magnitude faster and energy efficient approaches for initial access and channel estimation in mmW systems

Energy-Sustainable IoT Connectivity: Vision, Technological Enablers, Challenges, and Future Directions

Technology solutions must effectively balance economic growth, social equity, and environmental integrity to achieve a sustainable society. Notably, although the Internet of Things (IoT) paradigm constitutes a key sustainability enabler, critical issues such as the increasing maintenance operations, energy consumption, and manufacturing/disposal of IoT devices have long-term negative economic, societal, and environmental impacts and must be efficiently addressed. This calls for self-sustainable IoT ecosystems requiring minimal external resources and intervention, effectively utilizing renewable energy sources, and recycling materials whenever possible, thus encompassing energy sustainability. In this work, we focus on energy-sustainable IoT during the operation phase, although our discussions sometimes extend to other sustainability aspects and IoT lifecycle phases. Specifically, we provide a fresh look at energy-sustainable IoT and identify energy provision, transfer, and energy efficiency as the three main energy-related processes whose harmonious coexistence pushes toward realizing self-sustainable IoT systems. Their main related technologies, recent advances, challenges, and research directions are also discussed. Moreover, we overview relevant performance metrics to assess the energy-sustainability potential of a certain technique, technology, device, or network and list some target values for the next generation of wireless systems. Overall, this paper offers insights that are valuable for advancing sustainability goals for present and future generations.Comment: 25 figures, 12 tables, submitted to IEEE Open Journal of the Communications Societ

Semi-automatic liquid filling system using NodeMCU as an integrated Iot Learning tool

Computer programming and IoT are the key skills required in Industrial Revolution 4.0 (IR4.0). The industry demand is very high and therefore related students in this field should grasp adequate knowledge and skill in college or university prior to employment. However, learning technology related subject without applying it to an actual hardware can pose difficulty to relate the theoretical knowledge to problems in real application. It is proven that learning through hands-on activities is more effective and promotes deeper understanding of the subject matter (He et al. in Integrating Internet of Things (IoT) into STEM undergraduate education: Case study of a modern technology infused courseware for embedded system course. Erie, PA, USA, pp 1–9 (2016)). Thus, to fulfill the learning requirement, an integrated learning tool that combines learning of computer programming and IoT control for an industrial liquid filling system model is developed and tested. The integrated learning tool uses NodeMCU, Blynk app and smartphone to enable the IoT application. The system set-up is pre-designed for semi-automation liquid filling process to enhance hands-on learning experience but can be easily programmed for full automation. Overall, it is a user and cost friendly learning tool that can be developed by academic staff to aid learning of IoT and computer programming in related education levels and field

Applications

Volume 3 describes how resource-aware machine learning methods and techniques are used to successfully solve real-world problems. The book provides numerous specific application examples: in health and medicine for risk modelling, diagnosis, and treatment selection for diseases in electronics, steel production and milling for quality control during manufacturing processes in traffic, logistics for smart cities and for mobile communications

Why Space? : The Opportunity for Materials Science and Innovation

Advanced materials (and their manufacturing) are one of the 7 transformational ‘technology families’ identified by UK Government, where there is both a key opportunity for growth and existing globally competitive research and development (R&D) expertise tied with industrial strength. Similarly, at both a local and global scale, the space sector continues to grow in both size and ambition. With the development of a robust, space launch and provider ecosystem, the ability to access space is accelerating, bringing with it the key opportunity to harness the space environment to augment this technology family’s development heralding solutions to terrestrial challenges. Coupled with this sizable opportunity, are the significant plans for space infrastructure and exploration, that require novel material and manufacturing processes, enhanced properties, and solutions to achieve these.  Therefore, there exists a strong foundation for a cohesive case that could bring these communities together and demonstrate the case to key actors (from funders and policy makers to scientists and entrepreneurs) on the opportunity for materials with space. From fundamental research to applied industry solutions, this paper harnesses perspectives from across these communities to better understand the possibilities for research, innovation, and growth. To build this foundation, it is important to contextualise that materials science is an extremely broad field where scientists seek to understand the formation, structure, and properties of materials on various scales, ranging from the atomic to the microscopic and to the macroscopic (large enough to be visible). The properties a material has (such as strength or electrical conductivity) are determined by its structure. Hence, establishing quantitative and predictive relationships between the way a material is processed, its structure (how atoms or larger inclusions are arranged), and its properties is of paramount importance. Gravity is a major contributing factor to this understanding. Materials processing in general, and metals in particular, are often influenced by gravity-driven mechanisms such as solidification. In this case, the liquid-to-solid transition of pure metals is affected by both convection and sedimentation which will ultimately determine the structure of the material. To better understand the complex relationship of processing to a material’s structure, scientists are exploring the use of microgravity facilities to conduct materials-science experiments, where the aforementioned undesired effects are reduced. This includes the use of both in-orbit based facilities (such as the international space station) as well ground- based facilities (such as drop towers). Studying the effects of the space environment on the properties and behaviour of many fluid and solid “terrestrial systems”, could lead to the development of novel manipulation strategies and materials in space, with properties or functionalities that cannot be obtained in normal gravity conditions. For example, this can inform the development of the next generation of advanced materials with superior physicochemical properties to support human space exploration as well as revolutionising established processes on Earth, including design and manufacturing. R&D in this area, could also help address key roadmap points for space exploration (such as in-situ resource utilisation), as well as those cited in terrestrial R&D roadmaps (such as increasing the efficiency and capacity for novel semi-conductor manufacturing). This in turn can drive global leadership, foster international collaboration and development of novel solutions to terrestrial challenges. Fundamental to enabling this is recognising, championing, and stimulating this opportunity. This paper, its authored contributions, and the derived recommendations within, aim to provide a ‘small step’ on the journey