When Can a Narrowband Power Amplifier Be Considered to Be Memoryless and when Not?

This paper tries to get a response to the following question: When can a narrowband power amplifier (PA) be considered to be memoryless and when can it not be considered memoryless? To this end, a thorough and consistent analysis of the notions and definitions related with the above topic is carried out. In the considerations presented, two models of the narrowband PA are exploited interchangeably: the black box model widely used in the literature and a model developed here, which is based on the Volterra series. These two models complement each other. In this paper, the conditions for a linear or nonlinear narrowband PA to be memoryless or approximately memoryless or possessing memory are derived and illustrated. They are formulated in terms of the signal delay as well as in terms of the  amplitude-to-phase (AM/PM) conversion of the amplifier. Furthermore, the two possible interpretations of the amplitude-to-amplitude (AM/AM) and AM/PM conversions are given a mathematical framework. That is these conversions are presented through some operations. One set of these operations allows to treat the AM/AM and AM/PM conversions as distortions of the modulating signals. Or equivalently as distortions of a given signal constellation when it passes through the PA. Finally, it is proved that the Saleh’s and Ghorbani’s models of the AM/AM and AM/PM conversions occurring in the PAs, which were published in the literature, are not memoryless ones

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

On Correct Understanding and Classification of Saleh’s and Related Models of AM/AM and AM/PM Conversions

In this paper, some of the existing classifications of the Saleh’s and related models of the AM/AM and AM/PM conversions occurring in communication power amplifiers (PA) are reviewed. It is shown that these classifications are inconsistent and must be refined. Obviously, carrying out such a refinement properly needs a good knowledge and correct understanding of the mechanisms leading to the AM/AM and AM/PM conversions. This was achieved in this paper by performing a thorough analysis of the PA behavior using an analytical tool, the Volterra series. The main points of this analysis are presented here in great detail. Among others, it is shown that the influence of the PA memory on the AM/PM conversion is two-fold: direct and indirect. The former can be however fully neglected. On the other hand, the indirect influence caused by “the interaction of the carrier with the PA memory” cannot be neglected when the PA has not enough wideband frequency characteristics. The latter effect mentioned causes changes in the carrier phase that are received as the phase changes of the baseband modulating signal

A Digital Predistortion Scheme Exploiting Degrees-of-Freedom for Massive MIMO Systems

The primary source of nonlinear distortion in wireless transmitters is the power amplifier (PA). Conventional digital predistortion (DPD) schemes use high-order polynomials to accurately approximate and compensate for the nonlinearity of the PA. This is not practical for scaling to tens or hundreds of PAs in massive multiple-input multiple-output (MIMO) systems. There is more than one candidate precoding matrix in a massive MIMO system because of the excess degrees-of-freedom (DoFs), and each precoding matrix requires a different DPD polynomial order to compensate for the PA nonlinearity. This paper proposes a low-order DPD method achieved by exploiting massive DoFs of next-generation front ends. We propose a novel indirect learning structure which adapts the channel and PA distortion iteratively by cascading adaptive zero forcing precoding and DPD. Our solution uses a 3rd order polynomial to achieve the same performance as the conventional DPD using an 11th order polynomial for a 100x10 massive MIMO configuration. Experimental results show a 70% reduction in computational complexity, enabling ultra-low latency communications.Comment: IEEE International Conference on Communications 201

Design and implementation of adaptive baseband predistorter for OFDM nonlinear transmitter. Simulation and measurement of OFDM transmitter in presence of RF high power amplifier nonlinear distortion and the development of adaptive digital predistorters based on Hammerstein approach.

The objective of this research work is to investigate, design and measurement of a digital predistortion linearizer that is able to compensate the dynamic nonlinear distortion of a High Power Amplifier (PA). The effectiveness of the proposed baseband predistorter (PD) on the performance of a WLAN OFDM transmitter utilizing a nonlinear PA with memory effect is observed and discussed. For this purpose, a 10W Class-A/B power amplifier with a gain of 22 dB, operated over the 3.5 GHz frequency band was designed and implemented. The proposed baseband PD is independent of the operating RF frequency and can be used in multiband applications. Its operation is based on the Hammerstein system, taking into account PA memory effect compensation, and demonstrates a noticeable improvement compared to memoryless predistorters. Different types of modelling procedures and linearizers were introduced and investigated, in which accurate behavioural models of Radio Frequency (RF) PAs exhibiting linear and nonlinear memory effects were presented and considered, based on the Wiener approach employing a linear parametric estimation technique. Three new linear methods of parameter estimation were investigated, with the aim of reducing the complexity of the required filtering process in linear memory compensation. Moreover, an improved wiener model is represented to include the nonlinear memory effect in the system. The validity of the PA modelling approaches and predistortion techniques for compensation of nonlinearities of a PA were verified by several tests and measurements. The approaches presented, based on the Wiener system, have the capacity to deal with the existing trade-off between accuracy and convergence speed compared to more computationally complex behavioural modelling algorithms considering memory effects, such as those based on Volterra series and Neural Networks. In addition, nonlinear and linear crosstalks introduced by the power amplifier nonlinear behaviour and antennas mutual coupling due to the compact size of a MIMO OFDM transmitter have been investigated

Analysis and Compensation of Power Amplifier Distortions in Wireless Communication Systems

Wireless communication devices transmit message signals which should possess desirable power levels for quality transmission. Power amplifiers are devices in the wireless transmitters which increase the power of signals to the desired levels, but produce nonlinear distortions due to their saturation property, resulting in degradation of the quality of the transmitted signal. This thesis talks about the analysis and performance of communication systems in presence of power amplifier nonlinear distortions. First, the thesis studies the effects of power amplifier nonlinear distortions on communication signals and proposes a simplified design for identification and compensation of the distortions at the receiver end of a wireless communication system using a two-step pilot signal approach. Step one involves the estimation of the channel state information of the wireless channel and step two estimates the power amplifier parameters. Then, the estimated power amplifier parameters are used for transmitter identification with the help of a testing procedure proposed in this thesis. With the evolution of millimeter wave wireless communication systems today, study and analysis of these systems is the need of the hour. Thus, the second part of this thesis is extended to study the performance of millimeter wave wireless communication systems in presence of power amplifier nonlinear distortions and derives an analytical expression for evaluation of the symbol error probability for this system. The proposed analysis evaluates the performance of millimeter wave systems theoretically without the need of simulations, and is helpful in studying systems in the absence of actual hardware

Design and Linearization of Energy Efficiency Power Amplifier in Nonlinear OFDM Transmitter for LTE-5G Applications. Simulation and measurements of energy efficiency power amplifier in the presence of nonlinear OFDM transmitter system and digital predistortion based on Hammerstein-Wiener method

This research work has made an effort to understand a novel line of radio frequency power amplifiers (RFPAs) that address initiatives for efficiency enhancement and linearity compensation to harmonize the fifth generation (5G) campaign. The objective is to enhance the performance of an orthogonal frequency division multiplexing-long term evolution (OFDM-LTE) transmitter by reducing the nonlinear distortion of the RFPA. The first part of this work explores the design and implementation of 15.5 W class AB RF power amplifier, adopting a balanced technique to stimulate efficiency enhancement and redeeming exhibition of excessive power in the transmitter. Consequently, this work goes beyond improving efficiency over a linear RF power amplifier design; in which a comprehensive investigation on the fundamental and harmonic components of class F RF power amplifier using a load-pull approach to realise an optimum load impedance and the matching network is presented. The frequency bandwidth for both amplifiers was allocated to operate in the 2.620-2.690 GHz of mobile LTE applications. The second part explores the development of the behavioural model for the class AB power amplifier. A particular novel, Hammerstein-Wiener based model is proposed to describe the dynamic nonlinear behaviour of the power amplifier. The RF power amplifier nonlinear distortion is approximated using a new linear parameter approximation approach. The first and second-order Hammerstein-Wiener using the Normalised Least Mean Square Error (NLMSE) algorithm is used with the aim of easing the complexity of filtering process during linear memory cancellation. Moreover, an enhanced adaptive Wiener model is proposed to explore the nonlinear memory effect in the system. The proposed approach is able to balance between convergence speed and high-level accuracy when compared with behavioural modelling algorithms that are more complex in computation. Finally, the adaptive predistorter technique is implemented and verified in the OFDM transceiver test-bed. The results were compared against the computed one from MATLAB simulation for OFDM and 5G modulation transmitters. The results have confirmed the reliability of the model and the effectiveness of the proposed predistorter.Fundacão para a Ciência e a Tecnologia, Portugal, under European Union’s Horizon 2020 research and innovation programme ... grant agreement H2020-MSCA-ITN- 2016 SECRET-722424 I also acknowledge the role of the National Space Research and Development Agency (NASRDA) Sokoto State Government Petroleum Technology Trust Fund (PTDF

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

Contribuição ao estudo do impacto das não linearidades nos sistemas de telecomunicações

Doutoramento em Engenharia ElectrotécnicaEsta tese insere-se na área de Electrónica de Rádio Frequência e Microondas e visa o desenvolvimento de ferramentas que permitam a melhor compreensão e análise do impacto da distorção não linear produzida em amplificadores de potência no desempenho de um sistema de telecomunicações sem fios. Devido à crescente complexidade dos amplificadores a simulação baseada em representações de circuito equivalente tornou-se extremamente pesada do ponto de vista computacional. Assim têm surgido várias técnicas de simulação de sistemas baseadas em modelos comportamentais, ou seja, que tentam aproximar a resposta do sistema a um sinal de entrada, independentemente dos elementos físicos que implementam o circuito. Neste trabalho foram estudadas as principais técnicas de modelação comportamental existentes assim como as principais características de um amplificador de potência que o modelo comportamental deve ser capaz de prever. Uma nova formulação de um modelo comportamental baseado na série de Volterra é apresentada em conjunto com o método de extracção ortogonal dos seus coeficientes. A principal vantagem deste novo método de extracção é permitir a determinação independente de cada valor coeficiente na série, garantindo-se deste modo um modelo com uma capacidade de aproximação óptima. A determinação dos coeficientes na série de modo independente é conseguida com base na reorganização dos termos da série e na identificação ortogonal de cada componente de saída. Adicionalmente, a identificação das componentes de saída de uma não linearidade é ainda utilizada na definição de uma métrica que permite avaliar de modo simples qual é a degradação imposta à qualidade do sinal ao ser passado num amplificador não linear. Esta métrica contabiliza simultaneamente a degradação imposta pelo ruído e pela distorção.This thesis is related to the RF and Microwave Electronics field and the main goal of this thesis is to develop tools that can contribute to understand and analyse the impact of nonlinear distortion generated by power amplifiers on wireless communication systems. Due to the growing complexity of amplifiers, equivalent circuit based simulations become a heavy computational task due to the large number of nonlinear elements to account for. So, several system simulation techniques have been proposed based on behavioural modelling, that is, models that can approximate the system’s response to a given input signal regardless of the physical circuit implementation description. In this thesis, the most important behavioural modelling techniques have been studied as well as the main power amplifier characteristics that the behavioural model should account for. A new formulation of a Volterra series based behavioural model is presented as well as the corresponding coefficient orthogonal extraction procedure. The main advantage of this new extraction method is to allow the independent determination of the exact value of each coefficient, guaranteeing this way an optimum approximation condition. The exact coefficient determination is achieved by reorganizing the series terms to reach independent subsets and by identifying separately each of systems’ output components. In addition, nonlinearity output component separation is also used to define a Figure of Merit that allows the simple evaluation of signal quality degradation when passed through a nonlinear amplifier. This Figure takes into account simultaneously the impact of noise and distortion.FCTFS