Christian Fager

Chalmers University of Technology · Department of Microtechnology and Nanoscience

We analyze the uplink performance of a distributed massive multiple-input multiple-output (MIMO) architecture in which the remotely located access points (APs) are connected to a central processing unit via a fiber-optical fronthaul carrying a dithered and 1-bit quantized version of the received radio-frequency (RF) signal. The innovative feature o...

This paper investigates a millimeter-wave sigma-delta-over-fiber distributed multiple-input-multiple-output (D-MIMO) solution, in which a central unit connects two remote radio heads (RRH) with two standardized QSFP28 fiber connections. The four subchannels of the QSFP28 carry three individually controlled intermediate frequency (IF) sigma-delta mo...

The proof of the pudding lies in eating, and that is why 6G testbeds are essential in the progress toward the next generation of wireless networks. Theoretical research toward 6G wireless networks proposes advanced technologies to serve new applications and drastically improve energy performance. Testbeds are indispensable to validate these new tec...

We present the uplink and downlink of a time-division duplex distributed multiple-input multiple-output (D-MIMO) testbed, based on a 1-bit radio-over-fiber architecture, which is low cost and scalable. The proposed architecture involves a central unit (CU) that is equipped with 1-bit digital-to-analog and analog-to-digital converters, operating at...

We analyze the uplink performance of a distributed massive multiple-input multiple-output (MIMO) architecture in which the remotely located access points (APs) are connected to a central processing unit via a fiber-optical fronthaul carrying a dithered and 1-bit quantized version of the received radio-frequency (RF) signal. The innovative feature o...

This article investigates the feasibility of combinerless multilevel outphasing transmitter as a potential architecture for large millimeter-wave (mmWave) phased arrays. We consider two distinct ways of distributing the component signals to the antennas and develop a model for the received signal at each radiated spatial direction from a phased arr...

This article proposes a dual-input dual-output dual-band four-way Doherty power amplifier (DPA) architecture, capable of linearized operation at one frequency band without being disturbed by arbitrary load switching at the other frequency band. The proposed technique is demonstrated through the design, implementation, and measurement of a prototype...

The millimeter-wave (mm-wave) band offers large bandwidth and promises high data rates for wireless communication systems. This work proposes an mm-wave radio-over-fiber (RoF) architecture suitable for wideband mm-wave communication, using a low-pass sigma–delta modulation approach. The central unit (CU) generates baseband sigma–delta bitstreams, i...

Conventional multiband power amplifiers (PAs), when working in time-division duplex (TDD), require the transmit–receive intervals of all bands to be synchronized. This is an undesirable constraint in modern radio base stations. This article proposes a dual-output dual-band Doherty PA (DB-DPA) that enables the loading of one band to be switched on-a...

Millimeter-wave and multiple-input-multiple-output (MIMO) technologies combine broad bandwidth with spatial diversity to offer a greater data rate. This paper investigates a flexible millimeter-wave sigma-delta-over-fiber based transmitter solution with digital beamforming MISO and MIMO functionality. Those functions are controlled by a central uni...

In concurrent multiband settings with large carrier spacings between bands, digital predistorter (DPD) implementations face the issue of how to linearize a sparse signal spectrum spanned over large bandwidths at low computational complexity. Frequency relocation is one possible approach that involves relocating baseband carriers to have reduced fre...

A complete theoretical analysis is presented for a highly efficient power amplifier (PA) architecture based on a non-reciprocal combiner. The architecture consists of two amplifier branches and a microwave circulator acting as the output combiner, referred to as the circulator load-modulated amplifier (CLMA). The continuous-mode (CM) design techniq...

A wideband Cartesian in-phase and quadrature (I/Q) modulator based on dual 10 b RF-digital-to-analog converters (DACs) is presented. Nonoverlapping local oscillator (LO) signals and a segmented nonlinear scaling with scaled unit cells contribute to a high linearity and allow for a low-complexity digital predistortion (DPD). Unit-cell flip-flops (FF...

Analog and/or digital predistortion may be used to linearize power amplifiers (PAs) at the cost of additional hardware. We present a novel scheme which uses segmented non-linear RF-DACs to linearize PAs at no additional hardware cost. An expanding segmented scaling chosen at design time may be adjusted using DAC bias, allowing linearization to be a...

This paper demonstrates a novel power amplifier (PA) architecture; the sequential circulator load modulated amplifier (SCLMA). The sequential configuration resulted in excellent bandwidth and extended back-off efficiency enhancement, making it suitable for modern communication scenarios with large peak-to-average-power-ratio (PAPR) signals. A wideb...

Beyond fifth generation (5G) communication systems aim towards data rates in the tera bits per second range, with improved and flexible coverage options, introducing many new technological challenges in the fields of network architecture, signal processing, and radio frequency front-ends. One option is to move towards cell-free, or distributed mass...

This article introduces the FORMAT array, a reconfigurable millimeter-wave antenna array platform based on antenna tiles. FORMAT stands for Flexible Organization and Reconfiguration of Millimeter-wave Antenna Tiles, which is a unique hardware solution aiming to implement and demonstrate a variety of antenna array concepts, as well as different arra...

This letter presents a power amplifier (PA) design and network synthesis approach to achieve wideband and efficient performance with a very compact circuit size. A design method is presented in detail to convert a canonical filter-based high-order matching network to the proposed matching configuration with transistor parasitic and packaged element...

Distributed MIMO (D-MIMO) with synchronized access points (APs) is a promising architecture for both communications and localization in 5G and beyond systems. In this letter, we develop a time-difference-of-arrival (TDOA)-based indoor localization system using a 2.35-GHz high-speed sigma-delta-over-fiber (SDoF) D-MIMO testbed with 40-MHz bandwidth,...

An active load-pull measurement technique is exploited to test the performance of novel high-efficiency power amplifier (PA) concepts based on non-reciprocity. This technique allows new PA architectures, including its load-modulation, to be evaluated before any construction is necessary. Merely active load -pull measurements of a single representat...

This paper presents a new type of Doherty power amplifiers (DPA) based on non-reciprocal power combiners. We replace the conventional quarter-wave (λ/4) impedance inverter in a DPA by a non-reciprocal gyrator. As proof of concept, a DPA using a commercial circulator has been implemented. The prototype PA exhibits a peak output power of 49.1 ±0.1 dB...

Sigma-Delta-over-Fiber and Analog-Radio-over-Fiber are compared in terms of non-linearity impairments in a transmitter with external optical modulation. The results show that Sigma-Delta-over-Fiber is more robust towards nonlinear characteristics in the modulator.

In this article, we propose an efficient methodology for the electrothermal characterization of power amplifier (PA) integrated circuits. The proposed electrothermal analysis method predicts the effect of temperature variations on the key performances of PAs, such as gain and linearity, under realistic dynamic operating conditions. A comprehensive...

An analytical load-pull-based design methodology for three-stage Doherty power amplifiers (PAs) is presented and demonstrated. A compact output combiner network, together with the input phase delays, is derived directly from transistor load-pull data and the design requirements. The technique opens up a new design space for three-stage Doherty PAs...

A novel behavioral modeling technique for digital predistortion of radio frequency power amplifiers (PAs) is proposed. The proposed basis-propagating selection (BAPS) model has an advantage on complexity since its basis functions are constructed as delay or a combination of existing basis functions. The BAPS model effectively optimizes the running...

We describe a 2x6 bit Cartesian RF IQ-modulator, implemented on 0.15 mm ${}^{2}$ in a 22 nm FDSOI CMOS technology. Measurements show a 3 dB bandwidth of 17–24 GHz and a saturated output power of 10.4 dBm with a peak drain efficiency of 15.6%. The IQ-modulator has been verified up to 8 GS/s. To the best of our knowledge, this is the highest-frequen...

Digital predistortion (DPD) is used to compensate for the distortion of power amplifiers (PA). Inspired by the physical structure of the PA circuit, a feedback structure in the DPD model is favored. In this paper, we proposed an infinite impulse response (IIR) model for PA modeling and linearization, by modifying a generalized memory polynomial (GM...

A novel power amplifier (PA) architecure, the Circulator Load Modulated Amplifier (CLMA), is presented and demonstrated. Based on a 3-port non-reciprocal combiner, the CLMA is able to modulate the load of a class-B amplifier by means of controlling the amplitude and phase of a class-C amplifier. The architecture enables broadband highly-efficient o...

Radio-over-fiber is a popular technique to establish communication links between a central location and many remote antenna units. Many different architectures are available for the downlink, i.e., for the communication link from the central unit to the remote antennas. On the contrary, the low-cost and low-complexity requirement of the remote unit...

A joint design approach for co-integrated antenna and power amplifier (PA), employing a high-efficiency Doherty PA (DPA) architecture and including a band-pass RF filter, is proposed. This design is realized through the optimally distributed and balanced multi-port feeding of the cavity-backed patch antenna element that provides the desired (unique...

The next-generation 5G and beyond-5G wireless systems have stimulated a substantial growth in research, development, and deployment of mm-Wave electronic systems and antenna arrays at various scales. It is also envisioned that large dynamic range modulation signals with high spectral efficiency will be ubiquitously employed in future communication...

As 5G New Radio (NR) is being rolled out, research effort is being focused on the evolution of what is to come in the post-5G era. In order to meet the diverse requirements of future wireless communication in terms of increased capacity and reduced latency, technologies such as distributed massive Multiple-Input Multiple-Output (MIMO), sub-millimet...

Hybrid digital and analog beamforming is an emerging technique for high-data-rate communication at millimeter-wave (mm-wave) frequencies. Experimental evaluation of such techniques is challenging, time-consuming, and costly. This article presents a hardware-oriented modeling method for predicting the performance of an mm-wave hybrid beamforming tra...

A 2x2 integrated antenna array operating at 28 GHz with 4 GHz bandwidth is presented. Conventional PCB technology is used to realize the antenna board, with low fabrication cost by minimizing the number of layers and vias. To overcome limited bandwidth and high mutual coupling, several techniques are used: proximity coupled feedings, slotted patche...

We present a low-loss power combiner, providing a highly integrated interface from an array of mm-wave power amplifiers (PAs) to a single standard rectangular waveguide (WG). The PAs are connected to an array of parallel and strongly coupled microstrip lines that excite a substrate integrated waveguide (SIW) based cavity. The spatially distributed...

A measurement-based approach for the analysis of Doherty power amplifiers (DPAs) is presented. The DPA behavior is emulated using an active load-pull setup, exciting a single device under test (DUT) in either of two states, corresponding to the main and auxiliary branches in a DPA. The dynamic loading condition in one state is determined from the D...

All communication transceivers suffer from nonlinearities in the analog components, such as amplifiers, mixers, ADCs/DACs, etc. Such nonlinearities lead to distortion of the communication signals, and may lead to the transceiver interfering with victims operating at neighbor frequencies. The effects of nonlinearities have been a focus area since th...

This letter presents two highly efficient two-stage power amplifiers (PAs) for 5G applications, implemented in a 22-nm slilicon on insulator (SOI) CMOS technology. High efficiency is achieved by carefully designing the power cells and optimizing the layout. Capacitive neutralization is used to improve the stability and the gain. Both PAs are simila...

A single planar inverted F-antenna is proposed employing two input ports to optimally power-combine the output signals of two nonlinear power amplifiers inside the metal-only low-loss antenna structure. High power added efficiency at back-off power levels is reached through a Doherty combiner architecture, where the optimal power combination is see...

Advances in analog-to-digital converter (ADC) technology have opened up the possibility to directly digitize wideband radio frequency (RF) signals, avoiding the need for analog down-conversion. In this work, we consider an orthogonal frequency-division multiplexing (OFDM)-based massive multi-user (MU) multiple-input multiple-output (MIMO) uplink sy...

Every new generation of mobile systems should provide higher capacity, serve more users, be more energy efficient, and have lower cost. Radio-access hardware constitutes a major bottleneck for reaching these goals, which is a particularly noticeable issue now that 5G communication systems are being developed. A combination of breakthroughs in commu...

Distributed multiple-input-multiple-output (D- MIMO) is a very promising technique to improve capacity and quality of service for emerging 5G systems. In this paper, we propose a flexible and low-cost testbed for D-MIMO systems, where a sigma-delta over fiber (SDoF) solution is proposed to address the fundamental challenge of phase-coherent transmi...

State-of-the-art high-speed multi-mode verticalcavity surface-emitting-lasers (VCSELs) have a 3-dB analog bandwidth of ∼25-GHz and can operate up to 55-Gbps error-free, without need of equalization electronics. This makes them strong candidates as optical sources for ultra-high speed sigma-deltaover- fiber (SDoF) communication links. This paper exa...

This paper presents an analysis and design technique for an RF-input load modulated balanced amplifier (LMBA) with Doherty-like efficiency enhancement characteristics. The LMBA is a recently introduced power amplifier (PA) architecture in which a control signal is injected into the isolation port of the output coupler of a balanced amplifier. This...

Two architectures of concurrent dual‐band six‐port‐based receiver (SPR), which are modeled and calibrated using the augmented Hammerstein model (AHM) are proposed for the first time in this article. The receivers are based on six‐port junctions with one or two local oscillators (LO). The proposed single step calibration algorithms achieve the recov...

This letter investigates the performance of a Doherty power amplifier (DPA) in an $8\times 1$ active antenna transmitter module under an average voltage-standing-wave ratio of 4:1 induced by the array 1-D beam steering and mutual coupling. The GaN-based DPA and a $2\times 1$ segment of the array are verified via measurements at the operating freq...

A generalized 3-dB isolated three-port power divider/combiner is presented. The termination impedances of the two electrically isolated ports are selected as equal complex values of choice. The termination impedance of the third port can be selected as any complex value independent of the other two ports. The presented design equations show a broad...

The normalized mean square error (NMSE) is a performance criteria commonly used in power amplifier (PA) linearization to quantify the amount of in-band distortion encountered at the output of a PA. This letter presents a closed-form expression for the minimum NMSE that could be obtained in a linearization scenario. The derived expression is verifie...

This paper presents a new systematic and efficient method for the design of high-power varactor-based impedance tuners. By incorporating the varactor losses and voltage handling capabilities, the method allows the impedance tuning range to be maximized and losses to be minimized under high-power operating conditions. A 2.2-GHz large-signal double-s...

An analog predistorter and power amplifier (PA) MMIC chipset has been designed to improve the overall linearity for applications in wireless communication at the E-band. The circuits have been implemented in a commercial 0.1 μm InGaAs pHEMT process. The PA delivers an output referred 1-dB gain compression (OP <sub xmlns:mml="http://www.w3.org/1998/...

We report on a record-efficient 30-GHz 21-dBm differential Doherty PA designed in a 130-nm SiGe process. The output network is designed using a recently developed combiner synthesis technique. This technique enables impedance inversion and parasitic compensation to be integrated in one compact network. As a result, the combiner loss is reduced with...

In this paper, a digital predistortion (DPD) technique for wideband multi-antenna transmitters is proposed. The proposed DPD compensates for the combined effects of power amplifier (PA) nonlinearity, antenna crosstalk, and impedance mismatch. The proposed technique consists of a linear crosstalk and mismatch model block shared by all transmit paths...

This paper investigates the back-gating effects due to traps, and presents a new nonlinear trap modeling approach suitable for gallium nitride (GaN) high electron mobility transistors (HEMTs). It is shown that the traps have nonidentical influence on the channel compared with the gate. The potential due to trapped electrons in the buffer and the ga...

In this paper, we propose a technique for comprehensive analysis of nonlinear and dynamic characteristics of multi-antenna transmitters (TXs). The analysis technique is enabled by the development of a Volterra series-based dual-input model for power amplifiers (PAs), which is capable of considering the joint effects of PA nonlinearity, antenna cros...

This paper investigates nonlinear distortion effects in active antenna phased array transmitters. Using a nonlinear modeling technique, the joint interactions between power amplifiers and antennas are investigated in detail. Numerical simulations based on a 28 GHz GaN MMIC PA and a 64-element antenna array are used to exemplify the use of the propo...

This brief presents a novel digital predistortion (DPD) parameter identification technique that requires only the acquisition of either the in-phase (I) or the quadrature (Q) component of the power amplifier (PA) output signal. To this end, an approach that allows us to estimate the parameters of a model using only one of the IQ components of the m...

In this paper, a generic combiner design technique is developed for class-E outphasing transmitters. The design procedure starts with calculation of the combiner network parameters that guarantee high efficiency switch mode operation of the PAs in each branch. Recently developed continuous class-E modes theory is then utilized to create an addition...

The Doherty power amplifier (PA) is now established as one the most widely adopted architectures for enhancement of energy efficiency in wireless communication transmitters. More than 80 years after its invention, most of today's implementations still rely on the original circuit topology. This paper describes a new Doherty solution continuum that...

In this letter, class-B, embedded class-B, and Doherty driver topologies are investigated for RF Doherty Power Amplifiers (PAs). The investigation is firstly conducted theoretically and by simulations and then verified by design and implementation of the different topologies at 2.1 GHz using GaN-HEMT transistors. The results show that the highest l...

A novel Doherty power amplifier (PA) design method enabling high efficiency and high linearity simultaneously is proposed. The output combiner network is treated as a black box, and its parameters, together with the input phase delay, are solved based on given transistor characteristics and design requirements. This opens for new PA solutions with...