Christian Fager

Bio: Christian Fager is an academic researcher from Chalmers University of Technology. The author has contributed to research in topics: Amplifier & Power-added efficiency. The author has an hindex of 37, co-authored 215 publications receiving 4466 citations. Previous affiliations of Christian Fager include Ericsson.

A Comparative Analysis of the Complexity/Accuracy Tradeoff in Power Amplifier Behavioral Models

Abstract: A comparative study of state-of-the-art behavioral models for microwave power amplifiers (PAs) is presented in this paper. After establishing a proper definition for accuracy and complexity for PA behavioral models, a short description on various behavioral models is presented. The main focus of this paper is on the modeling accuracy as a function of computational complexity. Data is collected from measurements on two PAs-a general-purpose amplifier and a Doherty PA designed for WiMAX-for different output power levels. The models are characterized in terms of accuracy and complexity for both in-band and out-of-band error. The results show that, among the models studied, the generalized memory polynomial behavioral model has the best tradeoff for accuracy versus complexity for both PAs, and can obtain high performance at half of the computational cost of all other models analyzed.

Design of Varactor-Based Tunable Matching Networks for Dynamic Load Modulation of High Power Amplifiers

Abstract: In this paper, the design of varactor-based tunable matching networks for dynamic load modulation of high power amplifiers (PAs) is presented. Design guidelines to overcome the common breakdown, and tunability problems of the varactors for high power applications are proposed. Based on the guidelines, using commercially available abrupt junction silicon varactors, a tunable matching network is built and measured. The matching network is then used for load modulation of a 1-GHz 7-W class-E LDMOS PA. Static measurements of the load modulated PA show that the power-added efficiency of the PA is improved by an absolute value of 10% at 10-dB backoff. This promising result proves, for the first time, the feasibility of load modulation techniques for high-power applications in the gigahertz frequency range.

A comprehensive analysis of IMD behavior in RF CMOS power amplifiers

Abstract: This paper presents a comprehensive analysis of nonlinear intermodulation distortion (IMD) behavior in RF CMOS power amplifiers (PAs). Separate analyses are presented for small- and large-signal operation regimes. Especially, a new, simple, large-signal behavioral IMD analysis method is presented that allows the mechanisms dominant for IMD generation to be identified and their individual contributions to be studied. By combining these analyses, typical IMD versus input power characteristics of MOSFET PAs can be predicted and understood for different classes of operation. Various measurements made on a 950-MHz RF CMOS PA are used to demonstrate typical behavior and validate the proposed theory. Prediction of IMD using a standard CMOS transistor model is also evaluated and is shown to give good agreement with the measurements.

A Modified Doherty Power Amplifier With Extended Bandwidth and Reconfigurable Efficiency

Abstract: This paper derives the theory and presents measurements of a new power amplifier based on the Doherty power amplifier topology. It is theoretically shown that the proposed amplifier can simultaneously provide high efficiency at both full output power and at output power back-off, over a much improved bandwidth compared to the conventional Doherty power amplifier. It is also shown that the proposed amplifier allows reconfiguration of the efficiency in power back-off without the need of tunable elements.

Design of a Highly Efficient 2–4-GHz Octave Bandwidth GaN-HEMT Power Amplifier

Abstract: In this paper, the design, implementation, and experimental results of a high-efficiency wideband GaN-HEMT power amplifier are presented. A method based on source-pull/load-pull simulation has been used to find optimum source and load impedances across the bandwidth and then used with a systematic approach to design wideband matching networks. Large-signal measurement results show that, across 1.9-4.3 GHz, 9-11-dB power gain and 57%-72% drain efficiency are obtained while the corresponding power-added efficiency (PAE) is 50%-62%. Moreover, an output power higher than 10 W is maintained over the band. Linearized modulated measurements using a 20-MHz long-term evolution signal with 11.2-dB peak-to-average ratio show an average PAE of 27% and 25%, an adjacent channel leakage ratio of -44 and -42 dBc at 2.5 and 3.5 GHz, respectively.

Massive MIMO Networks: Spectral, Energy, and Hardware Efficiency

Abstract: Massive multiple-input multiple-output MIMO is one of themost promising technologies for the next generation of wirelesscommunication networks because it has the potential to providegame-changing improvements in spectral efficiency SE and energyefficiency EE. This monograph summarizes many years ofresearch insights in a clear and self-contained way and providesthe reader with the necessary knowledge and mathematical toolsto carry out independent research in this area. Starting froma rigorous definition of Massive MIMO, the monograph coversthe important aspects of channel estimation, SE, EE, hardwareefficiency HE, and various practical deployment considerations.From the beginning, a very general, yet tractable, canonical systemmodel with spatial channel correlation is introduced. This modelis used to realistically assess the SE and EE, and is later extendedto also include the impact of hardware impairments. Owing tothis rigorous modeling approach, a lot of classic "wisdom" aboutMassive MIMO, based on too simplistic system models, is shownto be questionable.

Massive MIMO: ten myths and one critical question

Abstract: Wireless communications is one of the most successful technologies in modern years, given that an exponential growth rate in wireless traffic has been sustained for over a century (known as Cooper’s law). This trend will certainly continue, driven by new innovative applications; for example, augmented reality and the Internet of Things. Massive MIMO has been identified as a key technology to handle orders of magnitude more data traffic. Despite the attention it is receiving from the communication community, we have personally witnessed that Massive MIMO is subject to several widespread misunderstandings, as epitomized by following (fictional) abstract: “The Massive MIMO technology uses a nearly infinite number of high-quality antennas at the base stations. By having at least an order of magnitude more antennas than active terminals, one can exploit asymptotic behaviors that some special kinds of wireless channels have. This technology looks great at first sight, but unfortunately the signal processing complexity is off the charts and the antenna arrays would be so huge that it can only be implemented in millimeter-wave bands.” These statements are, in fact, completely false. In this overview article, we identify 10 myths and explain why they are not true. We also ask a question that is critical for the practical adoption of the technology and which will require intense future research activities to answer properly. We provide references to key technical papers that support our claims, while a further list of related overview and technical papers can be found at the Massive MIMO Info Point: http://massivemimo. eu

State of the Art in 60-GHz Integrated Circuits and Systems for Wireless Communications

Abstract: This tutorial presents an overview of the technological advances in millimeter-wave (mm-wave) circuit components, antennas, and propagation that will soon allow 60-GHz transceivers to provide multigigabit per second (multi-Gb/s) wireless communication data transfers in the consumer marketplace. Our goal is to help engineers understand the convergence of communications, circuits, and antennas, as the emerging world of subterahertz and terahertz wireless communications will require understanding at the intersections of these areas. This paper covers trends and recent accomplishments in a wide range of circuits and systems topics that must be understood to create massively broadband wireless communication systems of the future. In this paper, we present some evolving applications of massively broadband wireless communications, and use tables and graphs to show research progress from the literature on various radio system components, including on-chip and in-package antennas, radio-frequency (RF) power amplifiers (PAs), low-noise amplifiers (LNAs), voltage-controlled oscillators (VCOs), mixers, and analog-to-digital converters (ADCs). We focus primarily on silicon-based technologies, as these provide the best means of implementing very low-cost, highly integrated 60-GHz mm-wave circuits. In addition, the paper illuminates characterization techniques that are required to competently design and fabricate mm-wave devices in silicon, and illustrates effects of the 60-GHz RF propagation channel for both in-building and outdoor use. The paper concludes with an overview of the standardization and commercialization efforts for 60-GHz multi-Gb/s devices, and presents a novel way to compare the data rate versus power efficiency for future broadband devices.