Advanced digital predistortion of power amplifiers for mobile and wireless communications

This research work focuses on improving the performances of digital predistorters while maintaining low computational complexity for mobile and wireless communication systems. Initially, the thesis presents the fundamental theory of power amplifiers, overview of existing linearisation and memory-effects compensation techniques and reveals the current issues in the field. Further, the thesis depicts the proposed solutions to the problems, including the developed in-band distortion modelling technique, model extraction methods, memoryless digital predistortion technique based on distortion components iterative injection, baseband equalisation technique for minimising memory effects, Matlab-ADS co-simulation system and adaptation circuit with an offline training scheme. The thesis presents the following contributions of the research work. A generalized in-band distortion modelling technique for predicting the nonlinear behaviour of power amplifiers is developed and verified experimentally. Analytical formulae are derived for calculating predistorter parameters. Two model extraction techniques based on the least-squares regression method and frequency-response analysis are developed and verified experimentally. The area of implementation and the trade-off between the methods are discussed. Adjustable memoryless digital predistortion technique based on the distortion components iterative injection method is proposed in order to overcome the distortion compensation limit peculiar to the conventional injection techniques. A baseband equalisation method is developed in order to provide compensation of memory effects for increasing the linearising performance of the proposed predistorter. A combined Matlab-ADS co-simulation system is designed for providing powerful simulation tools. An adaptation circuit is developed for the proposed predistorter for enabling its adaptation to environmental conditions. The feasibility, performances and computational complexity of the proposed digital predistortion are examined by simulations and experimentally. The proposed method is tuneable for achieving the best ratio of linearisation degree to computational complexity for any particular application

Advanced digital predistortion of power amplifiers for mobile and wireless communications

This research work focuses on improving the performances of digital predistorters while maintaining low computational complexity for mobile and wireless communication systems. Initially, the thesis presents the fundamental theory of power amplifiers, overview of existing linearisation and memory-effects compensation techniques and reveals the current issues in the field. Further, the thesis depicts the proposed solutions to the problems, including the developed in-band distortion modelling technique, model extraction methods, memoryless digital predistortion technique based on distortion components iterative injection, baseband equalisation technique for minimising memory effects, Matlab-ADS co-simulation system and adaptation circuit with an offline training scheme. The thesis presents the following contributions of the research work. A generalized in-band distortion modelling technique for predicting the nonlinear behaviour of power amplifiers is developed and verified experimentally. Analytical formulae are derived for calculating predistorter parameters. Two model extraction techniques based on the least-squares regression method and frequency-response analysis are developed and verified experimentally. The area of implementation and the trade-off between the methods are discussed. Adjustable memoryless digital predistortion technique based on the distortion components iterative injection method is proposed in order to overcome the distortion compensation limit peculiar to the conventional injection techniques. A baseband equalisation method is developed in order to provide compensation of memory effects for increasing the linearising performance of the proposed predistorter. A combined Matlab-ADS co-simulation system is designed for providing powerful simulation tools. An adaptation circuit is developed for the proposed predistorter for enabling its adaptation to environmental conditions. The feasibility, performances and computational complexity of the proposed digital predistortion are examined by simulations and experimentally. The proposed method is tuneable for achieving the best ratio of linearisation degree to computational complexity for any particular application.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Novel power amplifier design using non-linear microwave characterisation and measurement techniques

This thesis, addresses some aspects of the well-known, problem, experienced by designer of radio frequency power amplifiers (RFPA): the efficiency/linearity trade-off. The thesis is focused on finding and documenting solution to linearity problem than can be used to advance the performance of radio frequency (RF) and microwave systems used by the wireless communication industry. The research work, this was undertaken by performing a detailed investigation of the behaviour of transistors, under complex modulation, when subjected to time varying baseband signals at their output terminal: This is what in this thesis will be referred to as “baseband injection”. To undertake this study a new approach to the characterisation of non-linear devices (NLD) in the radio frequency (RF) region, such as transistors, designated as device-under-test (DUT), subjected to time varying baseband signals at its output terminal, was implemented. The study was focused on transistors that are used in implementing RF power amplifiers (RFPA) for base station applications. The nonlinear device under test (NL-DUT) is a generalisation to include transistors and other nonlinear devices under test. Throughout this thesis, transistors will be referred to as ‘device’ or ‘radio frequency power amplifier (RFPA) device’. During baseband injection investigations the device is perturbed by multi-tone modulated RF signals of different complexities. The wireless communication industry is very familiar with these kinds of devices and signals. Also familiar to the industry are the effects that arise when these kind of signal perturb these devices, such as inter-modulation distortion and linearity, power consumption/dissipation and efficiency, spectral re-growth and spectral efficiency, memory effects and trapping effects. While the concept of using baseband injection to linearize RFPAs is not new the mathematical framework introduced and applied in this work is novel. This novel approach NOVEL POWER AMPLIFIER DESIGN USING NON-LINEAR MICROWAVE CHARACTERISATION AND MEASUREMENT TECHNIQUES CARDIFF UNIVERISTY - UK ABSTRACT vi has provided new insight to this very complex problem and highlighted solutions to how it could be a usable technique in practical amplifiers. In this thesis a very rigorous and complex investigative mathematical and measurement analysis on RFPA response to applied complex stimulus in a special domain called the envelope domain was conducted. A novel generic formulation that can ‘engineer’ signal waveforms by using special control keys with which to provide solution to some of the problems highlighted above is presented. The formulation is based on specific background principles, identified from the result of both mathematical theoretical analysis and detailed experimental device characterisation

Compensation of nonlinear distortion in RF amplifiers for mobile communications

Compensation of nonlinear distortion of power amplifiers in mobile communications is an important requirement for improving power consumption performance while maintaining efficiency, since mobile phone became an essential accessory for everyone nowadays. This problem demands a good power amplifier model, in order to develop an effective predistortion system. Current researches are focused on modelling and predistortion of power amplifiers with memory, as well as memoryless ones. Different methods for modelling are used, as the Volterra series, polynomial models, look-up tables, the Hammerstein models, the Wiener models, and artificial intelligence systems. For predistortion feedback, feedforward and digital predistortion techniques are used. Among digital predistortion methods there are artificial intelligence systems, used in this thesis for linearization of power amplifier. This thesis presents developed robust method for modelling power amplifiers without memory effects and gives a comparison of proposed method with least squares method. Also, this research presents two novel techniques based on artificial intelligence systems for modelling and predistortion of highly nonlinear power amplifier with memory. The first approach is based on artificial neural networks, while the second one uses adaptive fuzzy logic systems. Forward and inverse models of power amplifier are created with both proposed methods. Superiority of artificial intelligence systems over partial least squares method is presented. Developed models are employed in a cascade to make a linearized system. Verification of proposed methods is carried out through the signal performance parameters and spectra of measured signal and signal from predistortion system. The feasibility and performances of the proposed digital predistortions are examined by simulations and experiments. The comparison of proposed methods is given to present advantages/disadvantages of both methods. The achieved distortion suppression from 72.2% to 93.6% and spectral regrowth improvement from 11.4 dB to 16.2 dB prove that the proposed methods have great ability to compensate the nonlinear distortion in power amplifier

Development of a high-efficiency power amplifier for envelope tracking applications

Complex and spectrally efficient modulation schemes present a power-efficiency challenge to base station power amplifiers due to the time-varying envelope and high peak-to-average power ratios involved. The envelope tracking architecture is one way to address this issue, where an envelope amplifier provides a dynamic, modulated supply to the power amplifier to reduce power consumption. While most research into envelope tracking focuses on the envelope amplifier, this work focuses on optimising the power amplifier design for envelope tracking using the waveform engineering approach. It studies the behaviour of a highly-efficient power amplifier mode of operation (class-F) using a relatively low-cost high voltage laterally diffused metal oxide semiconductor (HVLDMOS) technology in an envelope tracking environment. A systematic design process is formulated based on identifying the optimum amplifier load and the envelope shaping function, and then applied in the development of an actual class-F power amplifier. The fabricated power amplifier is integrated into an envelope tracking system and is able to produce one of the highest recorded efficiencies compared to current state-of-the-art envelope tracking amplifiers, which are mostly based on Gallium Nitride technology. The limitation of this design is its linearity performance, and the efficiency-linearity trade-off is analysed in detail in this work. The use of continuous mode power amplifiers in envelope tracking is also explored for high-bandwidth operation. The limitation of such a technique is posed by the device nonlinear output capacitance, and this is analysed through the use of a novel characterisation approach called voltage-pull, which is derived from an active load-pull system but uses voltage waveforms as the target instead of loads. This method is also used to investigate the possibility of exploiting the device nonlinear output capacitance as a 2nd harmonic injection source to improve power amplifier efficiency, as predicted in a novel mathematical analysis presented in this work

Dirty RF Signal Processing for Mitigation of Receiver Front-end Non-linearity

Moderne drahtlose Kommunikationssysteme stellen hohe und teilweise gegensätzliche Anforderungen an die Hardware der Funkmodule, wie z.B. niedriger Energieverbrauch, große Bandbreite und hohe Linearität. Die Gewährleistung einer ausreichenden Linearität ist, neben anderen analogen Parametern, eine Herausforderung im praktischen Design der Funkmodule. Der Fokus der Dissertation liegt auf breitbandigen HF-Frontends für Software-konfigurierbare Funkmodule, die seit einigen Jahren kommerziell verfügbar sind. Die praktischen Herausforderungen und Grenzen solcher flexiblen Funkmodule offenbaren sich vor allem im realen Experiment. Eines der Hauptprobleme ist die Sicherstellung einer ausreichenden analogen Performanz über einen weiten Frequenzbereich. Aus einer Vielzahl an analogen Störeffekten behandelt die Arbeit die Analyse und Minderung von Nichtlinearitäten in Empfängern mit direkt-umsetzender Architektur. Im Vordergrund stehen dabei Signalverarbeitungsstrategien zur Minderung nichtlinear verursachter Interferenz - ein Algorithmus, der besser unter "Dirty RF"-Techniken bekannt ist. Ein digitales Verfahren nach der Vorwärtskopplung wird durch intensive Simulationen, Messungen und Implementierung in realer Hardware verifiziert. Um die Lücken zwischen Theorie und praktischer Anwendbarkeit zu schließen und das Verfahren in reale Funkmodule zu integrieren, werden verschiedene Untersuchungen durchgeführt. Hierzu wird ein erweitertes Verhaltensmodell entwickelt, das die Struktur direkt-umsetzender Empfänger am besten nachbildet und damit alle Verzerrungen im HF- und Basisband erfasst. Darüber hinaus wird die Leistungsfähigkeit des Algorithmus unter realen Funkkanal-Bedingungen untersucht. Zusätzlich folgt die Vorstellung einer ressourceneffizienten Echtzeit-Implementierung des Verfahrens auf einem FPGA. Abschließend diskutiert die Arbeit verschiedene Anwendungsfelder, darunter spektrales Sensing, robuster GSM-Empfang und GSM-basiertes Passivradar. Es wird gezeigt, dass nichtlineare Verzerrungen erfolgreich in der digitalen Domäne gemindert werden können, wodurch die Bitfehlerrate gestörter modulierter Signale sinkt und der Anteil nichtlinear verursachter Interferenz minimiert wird. Schließlich kann durch das Verfahren die effektive Linearität des HF-Frontends stark erhöht werden. Damit wird der zuverlässige Betrieb eines einfachen Funkmoduls unter dem Einfluss der Empfängernichtlinearität möglich. Aufgrund des flexiblen Designs ist der Algorithmus für breitbandige Empfänger universal einsetzbar und ist nicht auf Software-konfigurierbare Funkmodule beschränkt.Today's wireless communication systems place high requirements on the radio's hardware that are largely mutually exclusive, such as low power consumption, wide bandwidth, and high linearity. Achieving a sufficient linearity, among other analogue characteristics, is a challenging issue in practical transceiver design. The focus of this thesis is on wideband receiver RF front-ends for software defined radio technology, which became commercially available in the recent years. Practical challenges and limitations are being revealed in real-world experiments with these radios. One of the main problems is to ensure a sufficient RF performance of the front-end over a wide bandwidth. The thesis covers the analysis and mitigation of receiver non-linearity of typical direct-conversion receiver architectures, among other RF impairments. The main focus is on DSP-based algorithms for mitigating non-linearly induced interference, an approach also known as "Dirty RF" signal processing techniques. The conceived digital feedforward mitigation algorithm is verified through extensive simulations, RF measurements, and implementation in real hardware. Various studies are carried out that bridge the gap between theory and practical applicability of this approach, especially with the aim of integrating that technique into real devices. To this end, an advanced baseband behavioural model is developed that matches to direct-conversion receiver architectures as close as possible, and thus considers all generated distortions at RF and baseband. In addition, the algorithm's performance is verified under challenging fading conditions. Moreover, the thesis presents a resource-efficient real-time implementation of the proposed solution on an FPGA. Finally, different use cases are covered in the thesis that includes spectrum monitoring or sensing, GSM downlink reception, and GSM-based passive radar. It is shown that non-linear distortions can be successfully mitigated at system level in the digital domain, thereby decreasing the bit error rate of distorted modulated signals and reducing the amount of non-linearly induced interference. Finally, the effective linearity of the front-end is increased substantially. Thus, the proper operation of a low-cost radio under presence of receiver non-linearity is possible. Due to the flexible design, the algorithm is generally applicable for wideband receivers and is not restricted to software defined radios

RF techniques for IEEE 802.15.4: circuit design and device modelling

The RF circuitry in the physical layer of any wireless communication node is arguably its most important part. The front-end radio is the hardware that enables communication by transmitting and receiving information. Without a robust and high performance front-end, all other higher layers of signal processing and data handling in a wireless network are irrelevant. This thesis investigates the radio circuitry of wireless-networked nodes, and introduces several proposals for improvement. As an emerging market, analysis starts by examining available and ratified network standards suitable for low power applications. After identifying the IEEE 802.15.4 standard (commercially known as ZigBee) as the one of choice, and analysing several front-end architectures on which its transceiver circuitry can be based, an application, the Tyre Pressure Monitoring System (TPMS) is selected to examine the capabilities of the standard and its most suitable architecture in satisfying the application’s requirements. From this compatibility analysis, the most significant shortcomings are identified as interference and power consumption. The work presented in this thesis focuses on the power consumption issues. A comparison of available high frequency transistor technologies concludes Silicon CMOS to be the most appropriate solution for the implementation of low cost and low power ZigBee transceivers. Since the output power requirement of ZigBee is relatively modest, it is possible to consider the design of a single amplifier block which can act as both a Low Noise Amplifier (LNA) in the receiver chain and a Power Amplifier (PA) on the transmitter side. This work shows that by employing a suitable design methodology, a single dual-function amplifier can be realised which meets the required performance specification. In this way, power consumption and chip area can both be reduced, leading to cost savings so vital to the widespread utilisation of the ZigBee standard. Given the importance of device nonlinearity in such a design, a new transistor model based on independent representation of each of the transistor’s nonlinear elements is developed with the aim of quantifying the individual contribution of each of the transistors nonlinear elements, to the total distortion. The methodology to the design of the dual functionality (LNA/PA) amplifier starts by considering various low noise amplifier architectures and comparing them in terms of the trade-off between noise (required for LNA operation) and linearity (important for PA operation), and then examining the behaviour of the selected architecture (the common-source common-gate cascode) at higher than usual input powers. Due to the need to meet the far apart performance requirements of both the LNA and PA, a unique amplifier design methodology is developed The design methodology is based on simultaneous graphical visualisation of the relationship between all relevant performance parameters and corresponding design parameters. A design example is then presented to demonstrate the effectiveness of the methodology and the quality of trade-offs it allows the designer to make. The simulated performance of the final amplifier satisfies both the requirements of ZigBee’s low noise and power amplification. At 2.4GHz, the amplifier is predicted to have 1.6dB Noise Figure (NF), 6dBm Input-referred 3rd-order Intercept Point (IIP3), and 1dB compression point of -3.5dBm. In low power operation, it is predicted to have 10dB gain, consuming only 8mW. At the higher input power of 0dBm, it is predicted to achieve 24% Power-Added Efficiency (PAE) with 8dB gain and 22mW power consumption. Finally, this thesis presents a set of future research proposals based on problems identified throughout its development

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