Adaptive Digital Feedback Predistortion Technique for Linearizing Power Amplifiers

We have developed a new adaptive digital predistortion (DPD) linearization technique based on analog feedback predistortion (FBPD). The lookup-table-based feedback input can remove the bandwidth limitation of the feedback circuit related to the loop delay, and suppress feedback oscillation by accurate digital control of the feedback signal. Moreover, the predistortion (PD) signal can be extracted very efficiently. By combining the feedback linearization and DPD linearization techniques, the performance of the predistorter is enhanced significantly compared to the conventional DPD. To clearly visualize the characteristics of digital FBPD (DFBPD), we have compared it to the conventional DPD based on the recursive least square algorithm using Matlab simulation. The results clearly show that the new method is a good linearization algorithm, better than a conventional DPD. For the demonstration, a Doherty power amplifier with 180-W peak envelope power is linearized using the proposed DFBPD. For a 2.14-GHz forward-link wideband code-division multiple-access signal, the adjacent channel leakage ratio at 2.5-MHz offset is -58 dBc, which is improved by 15 dB at an average output power of 43 dBm     

Analysis of Adaptive Digital Feedback Linearization Techniques

A new lookup-table linearization technique is developed based on the digital feedback and digital feedback/predistortion (DFBPD) concepts. The linearization characteristics are investigated through system simulation of a real power-amplifier model with 90-W peak envelope power. The DFB suppresses forward-path nonlinear distortion as a gain reduction due to the FB effect, and this technique enhances the system tolerance without any bandwidth limitation. As the PD network is added to the FB loop, the linearization performance and system tolerance are further improved because of more accurate PD signal extraction. In addition, the gain is purely determined by the FB path, so the gain fluctuation in the forward path, including amplifier aging and temperature effects, is suppressed. The analysis and simulation allow experimental evaluation of the linearization mechanism and performance of the DFBPD technique for an 802.16e mobile Worldwide Interoperability for Microwave Access signal.      

查找表联合记忆效应补偿技术的宽带自适应预失真算法

为了能够对记忆型功率放大器线性化处理,并能一定程度克服其记忆效应,该文介绍一种自适应数字预失真器。该数字预失真器采用查找表与记忆效应补偿技术相结合的方法,并且利用内插值方法有效减小了查找表幅度量化过程产生的误差。相比记忆多项式预失真器,这种预失真器的计算复杂度较小,却能够得到与其相近的线性化效果。基于功率放大器记忆多项式模型,利用OFDM(Orthogonal Frequency Division Multiplexing)宽带信号验证该文提出的预失真器对记忆型非线性功率放大器的良好线性化效果。     

Linearizability of TWTAs using predistortion techniques

Predistortion linearization is a very effective technique for improving the linearity and efficiency of traveling wave tube (TWT) amplifiers. In this paper, we will study the effectiveness of predistortion techniques for TWT linearization using single-tone, two-tone, and quadrature-amplitude-modulation signals. The results from a series of predistortion linearization experiments for five TWTs covering L-, C-, Ku-, and Ka-bands and including both helix- and coupled-cavity TWTs will be presented. We will demonstrate the additional improvement of fifth-order predistortion linearization over the more commonly used third-order predistortion linearization. To circumvent the complexity and limited availability of a pure fifth-order linearizer, a technique for realizing nonlinear functions of order greater than or equal to five using cascaded third-order nonlinear functions is described. The technique can be used to efficiently generate higher order nonlinearities for predistortion linearization applications. We will demonstrate experimentally that the use of two cascaded third-order functions is comparable to a pure fifth-order implementation in performance.     

基于改进型径向基函数神经网络的功放线性化

提出了一种基于改进型径向基函数神经网络(MRBFNN)的数字预失真线性化模型,用于更为精确地矫正宽带射频功率放大器的动态非线性。该神经网络模型的输入层使用传统的延时抽头以补偿功放的线性记忆效应,同时对每个抽头进行级数展开用于补偿功放的非线性记忆效应,从而更好地抑制功放的动态非线性失真。文中使用WCDMA 三载波信号对一个460MHz 的Doherty 功率放大器进行数字预失真线性化实验。实验结果表明,与传统数字预失真线性化模型相比,基于改进型径向基神经网络的数字预失真线性化模型能更好地抑制宽带功放动态非线性引起的带外频谱再生,其三阶互调(IMD3)失真最多可以抑制23dB,大大提高了功放的线性度,验证了所提出的数字预失真线性化模型的有效性。     

A W-band high-power predistorted direct-conversion digital modulator for transmitter applications

This letter presents a W-band high-power direct-conversion transmitter using digital predistortion techniques for digital modulation applications. The transmitter is a direct-conversion configuration that employs a reflection-type IQ modulator module and a power amplifier module. With the predistortion function in digital signal processing (DSP), this transmitter demonstrated an output channel power of greater than 19 dBm, and the adjacent channel power ratio (ACPR) was improved by 10 and 18 dB for QPSK and /spl pi//4-DQPSK modulation, respectively. To the best of our knowledge, this is the first demonstration of linearization techniques for W-band high-power digital modulation transmitters.     

并发双波段射频功放数字预失真同步性研究

在并发双波段调制信号驱动下,射频功放的非线性更显著和复杂,对其进行线性化将面临新的问题。文中研究了低频波段和高频波段的同步性对线性化的影响,提出同步并发双波段射频信号生成方法,用于解决两频段信号加载时的不同步问题。文中使用1001型(中间两个载波空缺)CDMA2000信号和单载波WCDMA信号分别作为低频段和高频段的测试信号,施加到一个峰值功率为51dBm的Doherty功放上进行实验验证。实验结果表明:两频段不同步将造成无法使用数字预失真方法对功放进行线性化;在同步情况下,采用记忆多项式(MP)可显著提高并发双波段射频功放的线性度。     

A Saturated Doherty Power Amplifier Based On Saturated Amplifier

A saturated Doherty power amplifier (PA) based on the saturated PA tuned by the self-generated harmonic currents is presented. When driven with mobile WiMAX 1FA signal, this Doherty PA demonstrates high efficiency performance (average 57%) over the WiMAX band from 2.5 GHz to 2.7 GHz. Simulated and experimental results allow the evaluation for the load modulation behavior of the saturated PA and bandwidth correlation between the Doherty and saturated unit PAs. The linearity requirement of the WiMAX signal is met using the digital feedback predistortion linearization technique.      

功率放大器数字基带自适应预失真技术研究进展

阐述了目前国内外功率放大器（PA）线性化技术的研究状况，重点研究了其中最有应用前景的数字基带自适应预失真技术及其最新研究进展，分析并指出了相关方法的优缺点，并对其发展前景进行了展望。     

基带预失真线性化技术在DAB系统中的应用

通过典型的功放无记忆模型分析了功放的非线性对DAB信号所造成的失真及基带预失真技术的作用机理。介绍一种基于该机理的硬件实现，通过在数字基带对信号进行预失真处理，补偿后端的射频功放所产生的非线性幅度失真和相位失真，从而有效地改善了DAB射频功放的线性度。     

5G光载无线通信前传系统数字预失真线性化研究

第五代移动通信系统新空口(5G NR)的信号带宽、调制度和峰均比相比4G信号高很多。用于小蜂窝或微蜂窝基站组网的光载无线通信(Radio-over-Fiber, RoF)前传系统的非线性失真也更加严重。为了使5G NR信号线性传输,采用记忆多项式(Memory Polynomial, MP)和广义记忆多项式(Generalized Memory Polynomial, GMP)模型构建数字预失真器,矫正5G RoF前传系统的非线性。在实验中搭建了2 km光纤的RoF前传系统,并采用100 MHz带宽5G NR信号作为测试信号对其进行线性化测试。实验结果表明,采用MP和GMP数字预失真器进行线性化时,5G RoF前传系统的邻信道功率比(Adjacent Channel Power Ratio, ACPR)可分别改善13 dB和17 dB以上。这说明MP和GMP数字预失真器对5G RoF前传系统的非线性具有显著的抑制作用。因此基于MP和GMP两个模型的数字预失真器均可用于5G RoF前传系统的线性化,而且GMP预失真器比MP预失真器对该系统线性化的能力更强。     

5G 功放DPD 的FPGA 宽带实现及自动优化

5G 宽带功放数字预失真器(DPD)的FPGA 实现过程中,常遇到数字处理带宽不够和资源有限问题,对 此,文中提出一种基于双路并行数据流的数字预失真带宽扩展方法和基于Zynq Ultrascale+ MPSoC 的自动化模型优化 验证方法,可快速实现对5G 宽带功放线性化方案的优化。使用该并行处理结构的数字预失真器,克服了数字电路最 大时钟频率造成的对FPGA 线性化带宽的限制,使得数字预失真电路在每个时钟周期内可以处理更多的数据,不仅有 效地增加了数字处理带宽,而且降低了DPD 的功耗。然而,这种带宽增加以消耗更多硬件资源为代价,对此,文中同时 提出了对预失真非线性模型的在线自动优化方法,以简化非线性模型、降低DPD 的硬件资源开销。最后,在Zynq Ultrascale+ FPGA 实验平台上实现了具有两路并行数据处理的I-MSA 自优化数字预失真电路,采用100 MHz 的5G 新无 线电(NR)信号在2. 6 GHz 功率放大器上进行线性化实验验证,获得了满意的预失真性能,验证了所提方法的有效性。     

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     

Dual-band transmitters using digitally predistorted frequency multipliers for reconfigurable radios

Two methods for reconfigurable transmitters using frequency multipliers in conjunction with digital predistortion linearizers are developed. One method utilizes a circuit topology that can be switched between a fundamental-mode in-phase combined amplifier, and a push-push frequency doubler using input phasing. Investigation to maximize output harmonics out of regular power amplifiers (PAs) was performed, and the implementation of the device was successful for the amplifier- and doubler-mode operation. To satisfy optimal load-line conditions for the operation in both modes, a bi-tuned output-combining technique is introduced as well. Measurement results indicate that the circuit is able to transmit 28 dBm of output power at 900 MHz in the amplifier mode, and 22 dBm at 1800 MHz in the doubler mode. In combination with predistortion linearization, the reconfigurable transmitter was shown to be capable of amplifying IS-95B code-division multiple-access (CDMA) signals with an adjacent-channel power ratio (ACPR) up to -58dBc/30kHz. The second suggested method utilizes a fundamental-frequency PA followed by a varactor multiplier that can be bypassed with an RF switch. A varactor-diode doubler with a saturated conversion loss of 1.3 dB was built and tested. Using predistortion linearization techniques on both the PA and doubler, an ACPR of -53dBc/30kHz at 885-kHz offset was achieved for a CDMA signal transmitted at 1850 MHz.     

基于数/模混合预失真技术的功放线性化北大核心CSCD

随着移动通信信号带宽的增加,传统功率放大器数字预失真线性化技术越来越受到采样率的限制。为了使线性化效果更好,文中提出了一种数字预失真和模拟预失真相结合的混合预失真器,利用模拟预失真宽带宽的特点和数字预失真线性化能力强的优势,把模拟预失真和数字预失真融合在一起,共同补偿功放的非线性。由于受实验设备采样率的限制,文中采用了带宽为60 MHz的5 G NR信号对一个中心频率为3.5 GHz的射频功放进行实验验证。实验结果表明:提出的混合预失真器不仅优于单独的数字预失真器和模拟预失真器的非线性矫正性能,而且还能改善数字预失真因采样率限制无法改善的带外互调失真。     

Adaptive predistortion of lasers using multivariable feedback algorithm

An adaptive predistortion technique for direct modulated lasers is proposed and experimentally demonstrated. The predistortion calibration is through multivariable feedback control and does not require digital signal processing or storing data in memories. Therefore, this technique suggests a potential analogue implementation with less circuit complexity. A design example is implemented using standard CMOS technology. Experimental results confirm the validity and the robustness of the feedback algorithm. Measured results show about 5-15-dB reduction of IM3 and HD2 distortion over more than 300-MHz bandwidth.     

Data predistortion techniques using intersymbol interpolation

Two data predistortion techniques are presented that compensate for high-power amplifier (HPA) nonlinearities in digital microwave radio systems by employing quadrature amplitude-modulation (QAM) signal formats. The first one is a T/2-spaced predistortion technique that ensures distortion-free HPA output at two points per symbol interval T. The second is a T/3-spaced predistortion technique which cancels nonlinear distortion at the HPA output at three points per symbol interval. As opposed to conventional data predistortion, which can only compensate for warping of the signal constellation, the new techniques are effective against intersymbol interference. Using the 64- and 256-QAM signal constellations, it is shown that the proposed techniques lead to a very efficient utilization of the available HPA power. It is shown that, of the two techniques, the T/3-spaced data predistortion employs narrower transmit-pulse shaping and achieves higher protection against adjacent-channel interference at the expense of some additional complexity     

马赫—曾德尔外调制器线性化技术

模拟光发射机对驱动源的线性化提出严格要求，因此调制器的线性化技术有着重要的意义。本文对一些马赫－增德尔外调制器线性化技术进行了论述和分析，特别提出一种具有良好应用前景的简单的预失真线性补偿技术。     

An assessment of the efficiency performance of linearization schemes in the Australian mobilesat system

The efficiency of class C type amplifiers with digital predistortion linearizers is discussed. It is shown, by simulation with the AUSSAT MOBILESAT modulation schemes as examples, that the maximum efficiency from such a linearizer amplifier combination is significantly reduced for non-constant envelope modulation schemes on that achievable with constant envelope schemes. This aspect is often neglected when linearizer performance with such amplifers is discussed. The results are generally applicable to Cartesian feedback linearization also.     

Theoretical and Experimental Studies of a Probabilistic-Based Memoryless PA Linearization Technique

This paper studies the performance of a memoryless power amplifier (PA) linearization technique based on a probabilistic approach. This technique employs a nonparametric method to derive a predistorter function, which does not need any parametric modeling and explicit parameter estimation. It only needs to calculate a probabilistic cumulative distribution function (CDF) and a quantile function (an inverse function of the CDF). Histogram and order statistic methods are proposed to perform the calculation. A rigorous analytic formula is derived for the inter-modulation product power (IMPP) of the PA output signal when a finite number of samples as well as a finite number of bins are used to calculate the CDF and the quantile function. The analytic results show that, with the probabilistic-based technique, the IMPP approaches zero as the number of samples approaches infinity and the bin width approaches zero. Computer simulations are utilized to verify the theoretical analysis and to compare the performance of the probabilistic-based linearization technique with those of other memoryless PA linearization techniques, while a prototype experiment is carried out to demonstrate its performance in a practical application. Results show that the technique can accurately determine the predistortion function that effectively compensates for the nonlinearity in the PA, and that it achieves a much better linearization performance compared to other existing methods, especially in the presence of a loop delay in the feedback circuit.