When self-consistency makes a difference

Compound semiconductor power RF and microwave device modeling requires, in many cases, the use of selfconsistent electrothermal equivalent circuits. The slow thermal dynamics and the thermal nonlinearity should be accurately included in the model; otherwise, some response features subtly related to the detailed frequency behavior of the slow thermal dynamics would be inaccurately reproduced or completely distorted. In this contribution we show two examples, concerning current collapse in HBTs and modeling of IMPs in GaN HEMTs. Accurate thermal modeling is proved to be be made compatible with circuit-oriented CAD tools through a proper choice of system-level approximations; in the discussion we exploit a Wiener approach, but of course the strategy should be tailored to the specific problem under consideratio

Cancellation of Power Amplifier Induced Nonlinear Self-Interference in Full-Duplex Transceivers

Recently, full-duplex (FD) communications with simultaneous transmission and reception on the same channel has been proposed. The FD receiver, however, suffers from inevitable self-interference (SI) from the much more powerful transmit signal. Analogue radio-frequency (RF) and baseband, as well as digital baseband, cancellation techniques have been proposed for suppressing the SI, but so far most of the studies have failed to take into account the inherent nonlinearities of the transmitter and receiver front-ends. To fill this gap, this article proposes a novel digital nonlinear interference cancellation technique to mitigate the power amplifier (PA) induced nonlinear SI in a FD transceiver. The technique is based on modeling the nonlinear SI channel, which is comprised of the nonlinear PA, the linear multipath SI channel, and the RF SI canceller, with a parallel Hammerstein nonlinearity. Stemming from the modeling, and appropriate parameter estimation, the known transmit data is then processed with the developed nonlinear parallel Hammerstein structure and suppressed from the receiver path at digital baseband. The results illustrate that with a given IIP3 figure for the PA, the proposed technique enables higher transmit power to be used compared to existing linear SI cancellation methods. Alternatively, for a given maximum transmit power level, a lower-quality PA (i.e., lower IIP3) can be used.Comment: To appear in proceedings of the 2013 Asilomar Conference on Signals, Systems & Computer

Modeling Approaches for Active Antenna Transmitters

The rapid growth of data traffic in mobile communications has attracted interest to Multiple-Input-Multiple-Output (MIMO) communication systems at millimeter-wave (mmWave) frequencies. MIMO systems exploit active antenna arrays transmitter configurations to obtain higher energy efficiency and beamforming flexibility. The analysis of transmitters in MIMO systems becomes complex due to the close integration of several antennas and power amplifiers (PAs) and the problems associated with heat dissipation. Therefore, the transmitter analysis requires efficient joint EM, circuit, and thermal simulations of its building blocks, i.e., the antenna array and PAs. Due to small physical spacing at mmWave, bulky isolators cannot be used to eliminate unwanted interactions between PA and antenna array. Therefore, the mismatch and mutual coupling in the antenna array directly affect PA output load and PA and transmitter performance. On the other hand, PAs are the primary source of nonlinearity, power consumption, and heat dissipation in transmitters. Therefore, it is crucial to include joint thermal and electrical behavior of PAs in analyzing active antenna transmitters. In this thesis, efficient techniques for modeling active antenna transmitters are presented. First, we propose a hardware-oriented transmitter model that considers PA load-dependent nonlinearity and the coupling, mismatch, and radiated field of the antenna array. The proposed model is equally accurate for any mismatch level that can happen at the PA output. This model can predict the transmitter radiation pattern and nonlinear signal distortions in the far-field. The model\u27s functionality is verified using a mmWave active subarray antenna module for a beam steering scenario and by performing the over-the-air measurements. The load-pull modeling idea was also applied to investigate the performance of a mmWave spatial power combiner module in the presence of critical coupling effects on combining performance. The second part of the thesis deals with thermal challenges in active antenna transmitters and PAs as the main source of heat dissipation. An efficient electrothermal modeling approach that considers the thermal behavior of PAs, including self-heating and thermal coupling between the IC hot spots, coupled with the electrical behavior of PA, is proposed. The thermal model has been employed to evaluate a PA DUT\u27s static and dynamic temperature-dependent performance in terms of linearity, gain, and efficiency. In summary, the proposed modeling approaches presented in this thesis provide efficient yet powerful tools for joint analysis of complex active antenna transmitters in MIMO systems, including sub-systems\u27 behavior and their interactions

Transmitter Linearization for mm-Wave Communications Systems

There is an ever increasing need for enabling higher data rates in modern communication systems which brings new challenges in terms of the power consumption and nonlinearity of hardware components. These problems become prominent in power amplifiers (PAs) and can significantly degrade the performance of transmitters, and hence the overall communication system. Hence, it is of central importance to design efficient PAs with a linear operation region. This thesis proposes a methodology and a comprehensive framework to address this challenge. This is accomplished by application of predistortion to a mm-wave PA and an E-band IQ transmitter while investigating the trade-offs between linearity, efficiency and predistorter complexity using the proposed framework.In the first line of work, we have focused on a mm-wave PA. A PA has high efficiency at high input power at the expense of linearity, whereas it operates linearly for lower input power levels while sacrificing efficiency. To attain both linearity and efficiency, predistortion is often used to compensate for the PA nonlinearity. Yet, the trade-offs related to predistortion complexities are not fully understood. To address this challenge, we have used our proposed framework for evaluation of predistorters using modulated test signals and implemented it using digital predistortion and a mm-wave PA. This set-up enabled us to investigate the trade-offs between linearity, efficiency and predistorter complexity in a systematic manner. We have shown that to achieve similar linearity levels for different PA classes, predistorters with different complexities are needed and provided guidelines on the achievable limits in term linearity for a given predistorter complexity for different PA classes.In the second line of work, we have focused on linearization of an E-band transmitter using a baseband analog predistorter (APD) and under constraints given by a spectrum emission standard. In order to use the above proposed framework with these components, characterizations of the E-band transmitter and the APD are performed. In contrast to typical approaches in the literature, here joint mitigation of the PA and I/Q modulator impairments is used to model the transmitter. Using the developed models, optimal model parameters in terms of output power at the mask limit are determined. Using these as a starting point, we have iteratively optimized operating point of the APD and linearized the E-band transmitter. The experiments demonstrated that the analog predistorter can successfully increase the output power by 35% (1.3 dB) improvement while satisfying the spectrum emission mask

Nonlinear mechanisms in passive microwave devices

Premi extraordinari doctorat curs 2010-2011, àmbit d’Enginyeria de les TICThe telecommunications industry follows a tendency towards smaller devices, higher power and higher frequency, which imply an increase on the complexity of the electronics involved. Moreover, there is a need for extended capabilities like frequency tunable devices, ultra-low losses or high power handling, which make use of advanced materials for these purposes. In addition, increasingly demanding communication standards and regulations push the limits of the acceptable performance degrading indicators. This is the case of nonlinearities, whose effects, like increased Adjacent Channel Power Ratio (ACPR), harmonics, or intermodulation distortion among others, are being included in the performance requirements, as maximum tolerable levels. In this context, proper modeling of the devices at the design stage is of crucial importance in predicting not only the device performance but also the global system indicators and to make sure that the requirements are fulfilled. In accordance with that, this work proposes the necessary steps for circuit models implementation of different passive microwave devices, from the linear and nonlinear measurements to the simulations to validate them. Bulk acoustic wave resonators and transmission lines made of high temperature superconductors, ferroelectrics or regular metals and dielectrics are the subject of this work. Both phenomenological and physical approaches are considered and circuit models are proposed and compared with measurements. The nonlinear observables, being harmonics, intermodulation distortion, and saturation or detuning, are properly related to the material properties that originate them. The obtained models can be used in circuit simulators to predict the performance of these microwave devices under complex modulated signals, or even be used to predict their performance when integrated into more complex systems. A key step to achieve this goal is an accurate characterization of materials and devices, which is faced by making use of advanced measurement techniques. Therefore, considerations on special measurement setups are being made along this thesis.Award-winningPostprint (published version

Behavioral modelling of GaN RF-power amplifier

Abstract. In this thesis memory effects and nonlinearities of Gallium Nitride (GaN) Doherty power amplifier (PA) were studied for measurement based behavioral modelling purposes. In SoC simulations a PA model is needed to simulate the performance of different linearization algorithms and to optimize the digital pre-distortion (DPD) design to cancel the memory effects of the PA, thus the model needs to be capable of modelling the memory effects sufficiently. Aim was to study if there were any differences in power amplifiers behavior and memory effects between time division duplexing (TDD) and frequency division duplexing (FDD) and what kind of model topologies are needed to model the PA sufficiently. In this thesis, two PAs were measured in different operation modes. Characterization setup was built, and an equalizer was characterized to remove the frequency selectivity of the test setup to obtain more accurate measurement results. Two signal bandwidths of 20MHz and 100MHz were used to extract data from power amplifier output with FDD and TDD operation. A generalized memory polynomial was fitted to model the PAs and found to be sufficient to model FDD operation. However, with TDD operation generalized memory polynomial model was not as accurate due to complex memory effects such as thermal and trapping memory. Models were also validated by using a digital pre-distorter and compared with measurement results and the models seem to work well and provide adjacent channel power ratio (ACPR) of -53.5dBc on lower channel and -53.3dBc on upper channel with 100MHz signal.GaN RF-tehovahvistimen käyttäytymistason mallinnus. Tiivistelmä. Tässä työssä tutkittiin Galliumnitraatti (GaN) Doherty-tehovahvistimen (PA) muistiilmiöitä ja epälineaarisuutta mittauksiin perustuvaa käyttäytymistason mallinnusta varten. SoC-simuloinneissa tarvitaan PA-mallia erilaisten linearisointialgoritmien suorituskyvyn simuloimiseksi. Erityisesti digitaalisen esisäröttimen (DPD) suunnittelun optimoimiseksi tehovahvistimessa esiintyvän muistin kumoamiseksi mallin on pystyttävä mallintamaan muistia riittävällä tarkkuudella. Työn tavoitteena oli selvittää, onko tehovahvistimien käyttäytymisessä ja muisti-ilmiöissä eroja aika- ja taajuusdupleksoinnin (TDD, FDD) välillä ja millaisia mallitopologioita tarvitaan, jotta tehovahvistinta voidaan mallintaa riittävällä tarkkuudella. Tässä työssä käytettiin kahta tehovahvistinta eri toimintatilojen mittaamiseen. Työssä rakennettiin mittausympäristö ja lisättiin taajuuskorjain kumoamaan mittausympäristön taajuusselektiivisyyttä. Kahta signaalinkaistanleveyttä 20 MHz:a ja 100 MHz:ä käytettiin datan keräämiseen tehovahvistimen ulostulosta aika- ja taajusjakoista dupleksointia käyttäen. Tehovahvistimen mallintamiseen sovitettiin muistipolynomi, jonka todettiin olevan riittävän tarkka FDD-toiminnan mallintamiseen, mutta TDD-toiminnassa malli ei ollut yhtä tarkka monimutkaisten muisti-ilmiöiden, kuten lämpö- ja elektronien ansoitusmuistin, vuoksi. Mallit validoitiin myös käyttämällä digitaalista esisärötystä ja niitä verrattiin mittaustuloksiin. Mallit näyttävät toimivan hyvin ja tuottavan vierekkäisen kanavan tehosuhteen (ACPR) -53,5dBc alemmalla kanavalla ja -53,3dBc ylemmällä kanavalla 100MHz signaalilla

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

Improved Nonlinear Model Implementation for VCSEL Behavioral Modeling in Radio-Over-Fiber links

Open Access provided by `Alma Mater Studiorum - Università di Bologna' within the CRUI CARE AgreementInternational audienc

Energy-Efficient Distributed Estimation by Utilizing a Nonlinear Amplifier

abstract: Distributed estimation uses many inexpensive sensors to compose an accurate estimate of a given parameter. It is frequently implemented using wireless sensor networks. There have been several studies on optimizing power allocation in wireless sensor networks used for distributed estimation, the vast majority of which assume linear radio-frequency amplifiers. Linear amplifiers are inherently inefficient, so in this dissertation nonlinear amplifiers are examined to gain efficiency while operating distributed sensor networks. This research presents a method to boost efficiency by operating the amplifiers in the nonlinear region of operation. Operating amplifiers nonlinearly presents new challenges. First, nonlinear amplifier characteristics change across manufacturing process variation, temperature, operating voltage, and aging. Secondly, the equations conventionally used for estimators and performance expectations in linear amplify-and-forward systems fail. To compensate for the first challenge, predistortion is utilized not to linearize amplifiers but rather to force them to fit a common nonlinear limiting amplifier model close to the inherent amplifier performance. This minimizes the power impact and the training requirements for predistortion. Second, new estimators are required that account for transmitter nonlinearity. This research derives analytically and confirms via simulation new estimators and performance expectation equations for use in nonlinear distributed estimation. An additional complication when operating nonlinear amplifiers in a wireless environment is the influence of varied and potentially unknown channel gains. The impact of these varied gains and both measurement and channel noise sources on estimation performance are analyzed in this paper. Techniques for minimizing the estimate variance are developed. It is shown that optimizing transmitter power allocation to minimize estimate variance for the most-compressed parameter measurement is equivalent to the problem for linear sensors. Finally, a method for operating distributed estimation in a multipath environment is presented that is capable of developing robust estimates for a wide range of Rician K-factors. This dissertation demonstrates that implementing distributed estimation using nonlinear sensors can boost system efficiency and is compatible with existing techniques from the literature for boosting efficiency at the system level via sensor power allocation. Nonlinear transmitters work best when channel gains are known and channel noise and receiver noise levels are low.Dissertation/ThesisPh.D. Electrical Engineering 201