Ulf Gustavsson

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

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.

One-bit massive MIMO: Channel estimation and high-order modulations

We investigate the information-theoretic throughout achievable on a fading communication link when the receiver is equipped with one-bit analog-to-digital converters (ADCs). The analysis is conducted for the setting where neither the transmitter nor the receiver have a priori information on the realization of the fading channels. This means that channel-state information needs to be acquired at the receiver on the basis of the one-bit quantized channel outputs. We show that least-squares (LS) channel estimation combined with joint pilot and data processing is capacity achieving in the single-user, single-receive-antenna case. We also investigate the achievable uplink throughput in a massive multiple-input multiple-output system where each element of the antenna array at the receiver base-station feeds a one-bit ADC. We show that LS channel estimation and maximum-ratio combining are sufficient to support both multiuser operation and the use of high-order constellations. This holds in spite of the severe non-linearity introduced by the one-bit ADCs.

On the impact of hardware impairments on massive MIMO

Massive multi-user (MU) multiple-input multiple-output (MIMO) systems are one possible key technology for next generation wireless communication systems. Claims have been made that massive MU-MIMO will increase both the radiated energy efficiency as well as the sum-rate capacity by orders of magnitude, because of the high transmit directivity. However, due to the very large number of transceivers needed at each base-station (BS), a successful implementation of massive MU-MIMO will be contingent on of the availability of very cheap, compact and power-efficient radio and digital-processing hardware. This may in turn impair the quality of the modulated radio frequency (RF) signal due to an increased amount of power-amplifier distortion, phase-noise, and quantization noise. In this paper, we examine the effects of hardware impairments on a massive MU-MIMO single-cell system by means of theory and simulation. The simulations are performed using simplified, well-established statistical hardware impairment models as well as more sophisticated and realistic models based upon measurements and electromagnetic antenna array simulations.

Throughput Analysis of Massive MIMO Uplink With Low-Resolution ADCs

We investigate the uplink throughput achievable by a multiple-user (MU) massive multiple-input multiple-output (MIMO) system, in which the base station is equipped with a large number of low-resolution analog-to-digital converters (ADCs). Our focus is on the case where neither the transmitter nor the receiver have any a priori channel state information. This implies that the fading realizations have to be learned through pilot transmission followed by channel estimation at the receiver, based on coarsely quantized observations. We propose a novel channel estimator, based on Bussgang’s decomposition, and a novel approximation to the rate achievable with finite-resolution ADCs, both for the case of finite-cardinality constellations and of Gaussian inputs, that is accurate for a broad range of system parameters. Through numerical results, we illustrate that, for the 1-bit quantized case, pilot-based channel estimation together with maximal-ratio combing, or zero-forcing detection enables reliable multi-user communication with high-order constellations, in spite of the severe nonlinearity introduced by the ADCs. Furthermore, we show that the rate achievable in the infinite-resolution (no quantization) case can be approached using ADCs with only a few bits of resolution. We finally investigate the robustness of low-ADC-resolution MU-MIMO uplink against receive power imbalances between the different users, caused for example by imperfect power control.

An RF Carrier Bursting System Using Partial Quantization Noise Cancellation

This paper introduces a method for bandpass cancellation of the quantization noise occurring in high efficiency, envelope pulsed transmitter architectures-or carrier bursting. An equivalent complex baseband model of the proposed system, including the -modulator and cancellation signal generation, is developed. Analysis of the baseband model is performed, leading to analytical expressions of the power amplifier drain efficiency, assuming the use of an ideal class B power amplifier. These expressions are further used to study the impact of key system parameters, i.e. the compensation signal variance and clipping probability, on the class B power amplifier drain efficiency and signal-to-noise ratio. The paper concludes with simulations followed by practical measurements in order to validate the functionality of the method and to evaluate the performance-trend predictions made by the theoretical framework in terms of efficiency and spectral purity.

Characterization of switched mode LDMOS and GaN power amplifiers for optimal use in polar transmitter architectures

In this paper, switched mode power amplifiers classes of operation and device technologies are characterized versus input power and output supply voltage. The results are used to identify optimal control schemes for use in polar transmitter architectures. Then the effects of different control schemes on the requirements for the power amplifier, and the envelope amplifier, as the main building blocks of this architecture, are investigated.

Design of highly efficient, high output power, L-band class D -1 RF power amplifiers using GaN MESFET devices

In this article, two class D-1 amplifiers is presented, both implemented with GaN MESFETs and working around 900 MHz, delivering 20.7/51.1 W output power with 75/78 % peak drain-efficiency. A simple nonlinear model optimized for switched mode operation is developed from basic device data, and is used to predict intrinsic current and voltage waveforms during the design process. In general, very good agreement between modeled and simulated data is obtained considering the limited amount of parasitic parameters covered by the device model.

A general method for passband quantization noise suppression in pulsed transmitter architectures

In this paper we describe a novel method for selective bandpass cancellation of the quantization-noise that occurs in pulsed transmitter architectures, where the signal is partially or completely quantized. A carrier bursting transmitter architecture, where the amplitude part is quantized and then recombined with a phase-modulated RF carrier, is used as a general example to demonstrate the principles of the method proposed. Measurements on a high efficiency 10 W LDMOS PA working at 1 GHz are used to verify the theoretical results.

SiC Varactors for Dynamic Load Modulation of High Power Amplifiers

SiC Schottky diode varactors with a high breakdown voltage, a high tuning ratio, and a low series resistance have been designed and fabricated. These characteristics are particularly necessary for the dynamic load modulation of high power amplifiers (PAs), which is an attractive alternative to other efficiency enhancement techniques. For a SiC Schottky diode varactor with a 50- radius fabricated by using a graded doping profile, a breakdown voltage of 40 V, a tuning range of 5.6, and a series resistance of 0.9 were achieved. The results show the great potential of this type of varactors for the use in the dynamic load modulation of high power amplifiers.

Quantization Noise Minimization in

This paper describes an optimization method for minimization of quantization noise in ΣΔ-based RF transmitters. The aim of the method is to enable the use of reconstruction filters with wider passband, or alternatively, a lower switch-rate. The method uses a general representation of the ΣΔ converters in combination with a differentiable approximation of the quantizer. Based on this, a Monte Carlo-based algorithm is developed around the damped Gauss-Newton iteration. As a result of the suggested algorithm, the residual quantization noise after reconstruction filtering is significantly decreased. Finally, simulations using a bandlimited signal with a Gaussian distribution are used to demonstrate the capabilities of the suggested algorithm when applied with the proposed ΣΔ modulator representation. The resulting performance is compared to several cases of ΣΔ converters designed using traditional methods, demonstrating significant improvements in terms of reduced reconstruction normalized mean square error. This implies that the transmitter efficiency can be improved with minor changes in the modulator implementation.