Modeling and Compensation of Nonlinear Distortion in Multi-Antenna RF Transmitters

Multi-antenna systems are utilized as a way to increase spectral efficiency in wireless communications. In a transmitter, the use of several parallel transmit paths and antennas increases system complexity and cost. Cost-efficient solutions, which employ active antenna arrays and avoid expensive isolators, are therefore preferred. However, such solutions are vulnerable to crosstalk due to mutual coupling between the antennas, and impedance mismatches between amplifiers and antennas. Combined with the nonlinear behavior of the power amplifiers, these effects cause nonlinear distortion, which deteriorates the quality of the transmitted signals and can prevent the transmitter from meeting standard requirements and fulfilling spectrum regulations. Analysis, assessment and, if necessary, compensation of nonlinear distortion are therefore essential for the design of multi-antenna transmitters.In this thesis, a technique for modeling and predicting nonlinear distortion in multi-antenna transmitters is presented. With this technique, the output of every individual transmit path, as well as the radiated far-field of the transmitter can be predicted with low computational effort. The technique connects models of the individually characterized transmitter components. It can be used to investigate and compare the effects of different power amplifier and antenna array designs at early design stages without complicated and expensive measurements.Furthermore, a digital predistortion technique for compensating nonlinear distortion in multi-antenna transmitters is presented. Digital predistortion is commonly used in transmitters to compensate for undesired nonlinear hardware effects. The proposed solution combines a linear function block with dual-input predistorters. The complexity is reduced compared to existing techniques, which require highly complex multivariate predistorter functions. Finally, a technique for identifying multi-antenna transmitter models and predistorters from over-the-air measurements using only a small set of observation receivers is presented. Conventional techniques require a dedicated observation receiver in every transmitter path, or one or more observation receivers that are shared by several paths in a time-interleaved manner. With the proposed technique, each receiver is used to observe several transmitter paths simultaneously. Compared to conventional techniques, hardware cost and complexity can be reduced with this approach. In summary, the signal processing techniques presented in this thesis enable a simplified, low-cost design process of multi-antenna transmitters. The proposed algorithms allow for feasible, low-complexity implementations of both digital and analog hardware even for systems with many antennas, thereby facilitating the development of future generations of wireless communication systems

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.Comment: 7 pages, 9 figures, Accepted for presentation at Globe-Com workshop on Massive MIM

Digital Predistortion for Multi-Antenna Transmitters Affected by Antenna Crosstalk

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, and a dual-input DPD block in every transmit path. By avoiding the use of multi-input DPD blocks in every transmit path, the complexity of the proposed technique is kept low and scales more favorably with the number of antennas than competing techniques. It is shown that all blocks can be identified from measurements of the PA output signals using least-squares estimation. Measurement results of a four-path transmitter are presented and used to evaluate the proposed DPD technique against existing techniques. The results show that the performance of the proposed DPD technique is similar to those of existing techniques, while the complexity is lower

Prediction of Nonlinear Distortion in Wideband Active Antenna Arrays

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 taking into account the joint effects of PA nonlinearity, antenna crosstalk and mismatch for wideband modulated signals. By combining multiple instances of the PA model with linear dynamic antenna simulations we develop the analysis technique. The proposed method allows the prediction of the output signal of every antenna in an arbitrarily sized TX array, as well as the total far-field radiated wave of the TX for any input signal with low computational effort. A 2.12 GHz four-element TX demonstrator based on GaAs PAs is implemented to verify simulation results with measurements. The proposed technique is a powerful tool to study hardware characteristics, as for example the effects of antenna design and element spacing. It can be used in cases where experiments are not feasible, and thus aid the development of next generation wireless systems

Coding efficiency of bandlimited PWM based burst-mode RF transmitters

Abstract—The burst-mode transmitter has been proposed as an efficiency enhancement technique for signals with high peak-to-average-power ratios (PAPRs), where appropriate modulation schemes such as pulse-width modulation (PWM) are used to generate a switching signal to drive the radio frequency (RF) power amplifier (PA). Ideally, a PWM signal is of infinite bandwidth, however, high-frequency spectral components are attenuated, e.g., by the bandlimitation of the PA matching network or the PWM circuit itself. Bandlimitation introduces ripples in the signal amplitude, which might reduce the PA efficiency in burst-mode operation. In this paper, we show that a bandlimited PWM signal does not nec-essarily degrade the overall transmitter efficiency because of the higher coding efficiency. The coding efficiency of bandlimited PWM signals, i.e., PWM signals with a finite number of harmonics, is derived analytically and verified by measurements. Additionally, from the measurements, we exemplarily determine the number of harmonics for bandlimited PWM signals with the best transmitter efficiency-linearity trade-off, and therefore demonstrate the excellence of bandlimited PWM for burst-mode transmitters in terms of transmitter efficiency as well as transmission signal quality. I

Multiplierless implementation of an aliasing-free digital pulsewidth modulator

Digital pulsewidth modulators are used to transform nonconstant amplitude signals into pulsed signals, such that the information lying in the signal amplitude is encoded in the widths of pulses. Because of the inherent aliasing distortion in digital pulsewidth-modulated signals, additional signal processing steps are required to make pulsewidth modulation (PWM) suitable for applications like digital audio amplification or burst-mode radio-frequency transmitters. These processing steps, however, entail an undesirable increase in computational effort. This brief presents a multiplierless implementation of a digital aliasing-free pulsewidth modulator using lookup tables, adders, and arithmetic shifts only. Mathematical equations of asymmetric double-edge PWM are given, as well as a modified aliasing-free version of this PWM technique that directly integrates the distortion-avoiding signal processing steps into the pulsewidth modulator. Based on these equations, a multiplierless implementation of the aliasing-free PWM (AF-PWM) is developed. Simulation results obtained with a Simulink fixed-point model show that the proposed modulator implementation provides a feasible solution for realizing AF-PWM with low computational effort

Analysis of thermal effects in active antenna array transmitters using a combined EM/circuit/thermal simulation technique

This paper presents a new technique for efficient system level analysis of joint nonlinear thermal and RF effects in active antenna array transmitters. The technique is based on the integration of an electro-thermal simulation with comprehensive electro-magnetic (EM) and nonlinear temperature-dependent behavioral models for the active circuits. Simulation results for a 64 (8×8) element GaN-based antenna array MIMO transmitter serving two independent users are used to illustrate the predictive capabilities offered by the proposed analysis. © 2015 IEEE

Modeling and Linearization of Multi-Antenna Transmitters Using Over-the-Air Measurements

In this paper, we present a technique to model and linearize a multi-antenna transmitter using only a small set of observation receivers that perform over-the-air measurements. We assume that the transmitter suffers from distortion due to power amplifier (PA) nonlinearities but not from crosstalk. By avoiding the use of an observation receiver in every transmitter branch, the hardware complexity and cost of multi-antenna transmitters can be reduced. First, equations are developed to extract PA models from observation receivers. Based on the extracted PA models, predistorters can then be identified for every transmit branch. We present simulation results that demonstrate that it is indeed possible to model and linearize a set of PAs using only one single observation receiver