A new approach to amplifier linearization by the generalized baseband signal injection method

The authors present a new technique for the reduction of third-order intermodulation distortion (IMD) in microwave amplifiers. A baseband predistortion signal is injected into a diode circuitry and the main amplifier to mix with the fundamental to generate a canceling signal for the suppression of the inherent IMD component. The proposed method can achieve higher linearity performance, in comparison to the conventional difference-frequency approach, and unlike many other techniques, no RF circuitry, such as variable gain amplifiers and phase-shifters, is needed other than baseband amplifiers. Both two-tone and vector signal measurement results are included.     

Theoretical and experimental study of amplifier linearization basedon harmonic and baseband signal injection technique

This paper presents a novel linearization scheme for RF amplifiers based on simultaneous harmonic and baseband signal injection. In this method, second-order frequency components generated by predistortion circuits are fed to the input of the main amplifier to mix with the fundamental signal for third-order intermodulation distortion (IMD) cancellation. A general and rigorous analytical formulation of baseband, harmonic, and the proposed injection techniques is presented, and from these derived expressions, the optimum conditions for IMD suppression are developed. The result also reveals the practical limitation of the proposed method subject to gain and phase error associated with the RF and baseband circuitry. For comparison purposes, an amplifying system is constructed for the experimental investigation of second-order signal injection approach. Both two-tone and digitally modulated waveforms are employed in these measurements     

Wiener功率放大器的一种简化预失真方法

根据Hammerstein模型的一种最优两步辨识算法的思路,针对预失真技术的特点对Hammerstein模型进行了两次转换,建立了一种等效的但形式简单的记忆非线性预失真器模型——非线性抽头延时多项式模型,从而简化了Wiener功放的预失真。仿真结果表明,利用所提出的记忆预失真器模型能使用较少的参数,快速、简便地实现记忆放大器的预失真,而且具有满意的线性化性能。     

Frequency-offset Cartesian feedback for MRI power amplifier linearization

High-quality magnetic resonance imaging (MRI) requires precise control of the transmit radio-frequency (RF) field. In parallel excitation applications such as transmit SENSE, high RF power linearity is essential to cancel aliased excitations. In widely-employed class AB power amplifiers, gain compression, cross-over distortion, memory effects, and thermal drift all distort the RF field modulation and can degrade image quality. Cartesian feedback (CF) linearization can mitigate these effects in MRI, if the quadrature mismatch and dc offset imperfections inherent in the architecture can be minimized. In this paper, we present a modified Cartesian feedback technique called "frequency-offset Cartesian feedback" (FOCF) that significantly reduces these problems. In the FOCF architecture, the feedback control is performed at a low intermediate frequency rather than dc, so that quadrature ghosts and dc errors are shifted outside the control bandwidth. FOCF linearization is demonstrated with a variety of typical MRI pulses. Simulation of the magnetization obtained with the Bloch equation demonstrates that high-fidelity RF reproduction can be obtained even with inexpensive class AB amplifiers. Finally, the enhanced RF fidelity of FOCF over CF is demonstrated with actual images obtained in a 1.5 T MRI system.     

一种高效的用于RF功率放大器线性化的自适应预失真结构

分析了当前文献中主要的几种自适应预失真结构,发现这些自适应预失真结构均不利于高效最小二乘算法的直接应用,从而限制了预失真技术的自适应性能.提出了一种新的自适应预失真结构,可直接使用高效的最小二乘算法对预失真器进行自适应更新.仿真结果表明利用此结构可快速、高效地实现非线性RF功率放大器的线性化.     

Dynamically optimum lookup-table spacing for power amplifier predistortion linearization

The lookup-table-based digital adaptive predistortion (DAPD-LUT) approaches are low cost and effective for power amplifier (PA) linearization in wireless applications. However, most existing DAPD-LUT schemes are sub-optimum because they adopt uniformly spaced LUTs regardless of the system state information (SSI), i.e., the PA characteristics and the input signal statistics. Other existing DAPD-LUT schemes assume either full or partial knowledge of the SSI to optimize and then to freeze the LUT spacing. Without prior knowledge of the SSI, we propose an SSI-learning low-complexity procedure to optimize the LUT spacing for a DAPD-LUT scheme. The proposed procedure is capable of online adapting the LUT spacing for PAs with various nonlinear characteristics, for input signals with various statistics, and for wireless environments with various time-varying properties.     

一种适用于间接学习结构的功率放大器线性化算法

针对目前基于间接学习结构(indirect learning architecture, ILA)的线性化算法对于功率放大器的非线性补偿效果较差、频谱失真改善效果不明显的缺点,提出了一种基于功率检测模块的交替迭代算法。该算法利用数据窗截取ILA中后失真器反馈回来的数据流并通过功率检测模块计算其功率大小,筛选出功率最大的信号数据流,根据迭代次数,使得其输出在大功率信号流与随机信号流之间相互切换并送至后失真模块进行训练. 预失真模块和后失真模块均采用记忆多项式模型. 采用峰均比为9 dB的LTE信号通过在线测试平台RF WebLab对真实GaN功放进行仿真,结果表明,新提出算法的带外抑制效果相对于传统的顺序数据流处理算法与大功率数据流处理算法分别优化了5 dB与2.5 dB左右,其平均归一化均方误差也分别优化了1 dB与0.6 dB左右.     

RF power amplifier linearization through amplitude and phasepredistortion

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity     

Piecewise-polynomial and cascade models of predistorter for linearization of power amplifier

To combat non-linear signal distortions in a power amplifier we suggest using predistorter with cascade structure in which first and second nodes have piecewise-polynomial and polynomial models. On example of linearizing the Winner-Hammerstein amplifier model we demonstrate that cascade structure of predistorter improves precision of amplifier??s linearization. To simplify predistorter??s synthesis the degree of polynomial model used in first node should be moderate, while precision should be improved by higher degree of second node??s polynomial.     

一种基带预失真RF功率放大器线性化技术的模型仿真与实验

提出了一种基于相关函数计算的自适应环延迟补偿法,用于功率放大器的的数字式基带预失真线性化技术。对采用这种软件延时补偿方法的预失真系统特性进行了仿真,并采用通用的数字信号处理器在硬件上实现了一个功放预失真系统性化模型进行了仿真,并采用通用的数字信号处理器在硬件上实现了一个功放预失真线性化模型。仿真和实验结果表明,在功率放大器的基带预失真中采用方法可以获得至少２０ｄＢ的线性度改善。     

A modified model and linearization method for solid state power amplifier

For successful implementation of a multi-standard receiver, a reconfigurable receiver architecture and a reconfigurable component design technique are essential. In this paper, an adaptable receiver architecture and a reconfigurable RF design technique using a switchable passive network are proposed. To verify the proposed design technique, the reconfigurable mixer and QVCO are designed using a flexible matching network and flexible LC tank, respectively. The measurement results of each component well prove the usefulness of the switchable passive network. Y.-K. Jang is now working for Samsung Electronics, Kiheung, Korea.     

Ka频段10 W自适应射频预失真线性化固态功放研制

射频功率放大器的特性会随信道切换、环境温度、工作状态等多种因素发生变化,为了保证功率放大器的优良工作特性,具有自适应性能的预失真系统就显得非常重要。提出了一种自适应反馈检测方法,以减小放大器输出信号的幅度失真和相位失真作为系统自适应的优化目标,采用多方向搜索优化算法对预失真系统进行优化调整,使系统始终处于最优工作状态。研制了工作于Ka频段10 W自适应射频预失真线性化固态功放原理样机,当工作温度为-40℃~+60℃时,在3 GHz的工作带宽内,三阶交调指标优于-32 dBc。测试结果表明该功放具有工作频带宽、温度适应性广等特点。     

An accurate digital baseband predistorter design for linearization of RF power amplifiers by a genetic algorithm based Hammerstein structure

In this paper, a novel digital predistorter design based on the Hammerstein structure is proposed in order to linearize radio frequency power amplifiers. A genetic algorithm optimization method has been proposed to accurately identify the coefficients of a Wiener model for the power amplifier. Digital predistorter design based on the proposed Hammerstein model has been carried out according to the accurate Wiener model. The validation of the suggested model is carried out using the simulation of the power amplifier and the digital predistortion excited by 64QAM signals in the advanced design system software. According to the simulation results, the criterion of an adjacent channel power ratio decreased by about 16 dB. The simulation results show the adjacent channel power ratio of almost ??46 dBc. In order to assess the feasibility of the proposed predistorter, it is completely implemented in the Kintex FPGA using Vivado HLS. This proposed model enables a more accurate modeling of nonlinear distortion and memory effects compared to the previous linearization methods. This paper presents the new linearization method using the genetic algorithm based Hammerstein structure.     

射频功放单元ADEF线性化技术的理论分析及模型仿真

本文从调制的角度，对由调幅-调幅转换、调幅-调相转换产生射频功放单元互调干拓的机理进行了分析，推导了调幅-调幅转换系数与信号对三阶互调干扰抑制比（C/I3）的函数关系式。并在这个理论基础上，采用ADEF，即自适应双包络反馈线性化技术对射频功放单元的非线性特性进行补偿。最后，建立了自适应双包络反馈线性电路的模型并进行了仿真，结果表明，由于此种技术的采用，C/I3的值达到了60dB左右。     

Effects of even-order nonlinear terms on power amplifier modeling and predistortion linearization

Power amplifier (PA) is an essential component in communication systems and is nonlinear in nature. Digital baseband predistortion is an emerging cost effective approach to linearize a PA. To study PA nonlinear characteristics and to construct a predistorter, accurate nonlinear models are often necessary. Polynomials have been used extensively for modeling the behavior of the PA or the predistorter. For bandpass communication signals, attention has been paid mainly to odd-order nonlinear terms. In this paper, we reveal the benefit of including even-order nonlinear terms in baseband modeling of the PA and in enhancing predistortion performance.     

A GaAs MCM power amplifier of 3.6 V operation with high efficiencyof 49% for 0.9 GHz digital cellular phone systems

An extremely small GaAs PA (power amplifier) has been implemented using AlN multilayer MCM for 0.9 GHz digital cellular phones. The present PA exhibited high efficiency of 49% with drain supply voltage as low as 3.6 V. This PA was designed to provide matching circuits with the maximum gain at the input side and the minimum intermodulation distortion at the output side. Nonlinear simulation result verifies that this matching condition provides the lowest π/4-shift QPSK distortion and indicates that the phase shift of the amplifier is mainly caused by source-drain resistance     

A low complexity pre‐distortion scheme for power amplifier linearization in wideband applications

In this paper, a nonlinear autoregressive with exogenous inputs (NARX) digital pre‐distortion scheme to linearize power amplifiers is proposed. The proposed NARX digital pre‐distortion gives better accuracy and spectral leakage suppression compared with other commonly used Volterra‐based techniques. The stability criterion of the NARX digital pre‐distortion is derived from the frequency domain analysis. Simulation is carried out with a 20 MHz single carrier long‐term evolution signal and two‐carrier long‐term evolution signal and instantaneous to average ratio of 6.2 dB at 0.01% complementary cumulative distribution function (CCDF). The results of simulation analysis show a slight improvement in adjacent channel leakage ratio performance with 30% reduction in the number of floating point operations compared with conventional pre‐distortion techniques. Copyright © 2016 John Wiley & Sons, Ltd.     

Adaptive neuro-fuzzy inference system (ANFIS) digital predistorter for RF power amplifier linearization

This paper describes an adaptive digital predistorter (ADP) for RF power amplifier (PA) linearization using an adaptive neuro-fuzzy inference system (ANFIS). The ANFIS predistorter (PD) employs the advantage of real-time modeling of the PA's responses in determining the PD's functions. The amplitude and phase corrections for the PD are represented in an easy-to-understand fuzzy if-then rule, while the parameters involved in the fuzzy representation are trained using neural networks algorithms, namely gradient-descent and least squares estimate (LSE). Experimental results show that a 26.3-dB improvement in linearity for a two-tone signal is obtained, while a distorted WCDMA signal is suppressed by at least 12 dB. The adaptability of the ANFIS PD to instantaneous variation in PA responses through time is also demonstrated, and results show that the ANFIS PD is capable of adapting to simulated environmental changes, which is a topic often omitted by researchers in this area. Further testing demonstrated that the tuning parameters involved in the training could be reduced by more than half for a fairly nonlinear PA without significantly degrading the suppression capability.     

Spatial separation of the third-order intermodulation for amplifier linearization

A quadrature balanced structure is presented for linearizing amplifiers. It works by routing the most annoying distortion component (the third-order intermodulation) to a port, while routing the useful information signals to a distinct port. This arrangement is not prone to oscillations as the negative feedback method is. It also has advantages over predistortion and feedforward linearization methods, as it is simpler and needs no adjustments. Theoretically, third-order intermodulation products would be totally eliminated when using the proposed arrangement. In practice, it was observed that, even with non-perfectly balanced amplifiers, the third-order intermodulation level was reduced up to 17?dB. The fifth-order intermodulation components were not reduced.