移动通信系统中宽带功放线性化设计与仿真

自适应数字预失真技术能够解决宽带高峰均比信号经过功放产生的非线性失真问题.主要介绍了自适应数字预失真的结构、预失真器的数学模型及相关的自适应算法.仿真结果表明采用自适应数字预失真技术能够显著地改善功放输出信号的带外频谱失真,是一种有效、灵活的技术.     

一种新的混合预失真器的设计

通信技术的发展对于功率放大器的要求越来越高,主要集中在高线性、高效率的功放.预失真是目前改善功放线性比较好的方法.新的自适应混合预失真系统充分利用模拟预失真,快速简单的和数字预失真算法的精确,输入信号先后经过模拟、数字两级预失真系统,并结合信号包络检测技术进行带外信号调节.通过对仿真结果的比较,IMD性能比单独使用数字预失真系统有了8 dB的改善,同时收敛时间也有所减少.     

毫米波功率放大器的预失真线性化改进研究

针对毫米波功率放大器的非线性失真问题,提出一种记忆多项式预失真优化算法,对功放的非线性失真进行补偿。预失真处理前后功放特性曲线表明该预失真算法的线性化效果明显;预失真处理后输出信号的相邻信道功率比原始输入信号高0.89 dB,比预失真前降低了7.8 dB,降低了相邻信道的干扰,符合理想功放线性化放大原则。     

适用于DLA结构的新型功率放大器数字预失真算法

数字预失真技术通过在功率放大器的前端级联数字预失真器来补偿功放的非线性失真,而直接学习结构(Direct Learning Architecture,DLA)作为一种广泛使用的自适应数字预失真算法结构,因其预失真性能优良,前景最为看好。传统DLA预失真算法需要求得功率放大器高精度模型,其结构复杂且计算量大。针对这一问题,文章引入功放的瞬时复增益(Instantaneous Complex Gain,ICG),提出了两种适用于DLA的新型功放预失真算法:ICG-LMS和ICGRLS,在确保线性化效果的同时,省略功放模型构建过程,大大简化了算法结构。仿真分析和实测结果表明,与传统适用于DLA的自适应预失真算法相比,两种新型预失真算法的运行时间分别减少35.8%和32.1%,而功放输出信号的邻道功率比与原始功放输出相比分别降低了14.8 dB和23.5 dB。     

射频模拟预失真器的研究与设计

利用小信号BJT放大器的非线性特性设计了一个有源模拟预失真器.通过调整小信号放大器的偏置状态和对失真信号的提取控制,使功放的三阶互调失真得到了明显的改善.双音实测数据表明,该预失真器使一个GSM基站功放在5 dB功率回退点附近的三阶互调失真改善了18 dB左右,与传统的射频预失真器相比,该预失真器对功放三阶互调失真的改善度提高了10 dB左右.     

一种用于TETRA系统的数字子带滤波预失真器

本文提出一种适用于TETRA数字集群系统的基于子带滤波技术的数字自适应预失真器。数字子带滤波预失真器将发射信号分为若干子带进行功放线性化处理,降低了预失真过程中运算速率的要求,增加自适应收敛速度。子带滤波技术同时解决了功放宽带化所带来的记忆性问题,扩展了数字预失真器的工作带宽。TETRA多载波发射信号的仿真结果说明,数字子带滤波预失真器对无记忆功放和有记忆功放均具有良好的线性化效果,明显扩大了功放的线性工作范围。      

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.     

一种基于matlab的DPD仿真设计

在WCDMA,LTE等无线通信系统中,高的峰平比(PAPR)对功放线性性和效率提出了更高的要求。自适应预失真技术是一种用于补偿高功率放大器非线性失真的有效技术。文中介绍了数字预失真的原理,模型,并通过在matlab中的仿真,提出一种合适的系数量化方法,最后给出一种基于matlab的数字预失真器设计。仿真结果表明,该设计有效的补偿了功放的非线性,降低了信号的邻近信道泄漏比以及系统功耗,提高系统的性能。     

功放的数字基带预失真技术研究

在射频通信中,大功率放大器是无线通信系统中的必要组成部分,它的非线性失真除了会造成信号带外的频谱扩展,引起临道干扰外,还会造成带内信号失真,提高误码率.因此,如何克服功放的非线性就成了通信领域的一个重要课题和研究热点.本文采用基于查找表的自适应算法,设计了自适应数字基带预失真器并进行了仿真和综合.结果表明自适应数字基带预失真方法可有效地提高功放线性度,从而提高系统性能.     

一种基于Volterra级数的基带数字预失真

提出了一种基于间接学习结构的自适应基带预失真技术来补偿高功率功放的记忆效应。它使用基于Volterra级数的RLS自适应算法。仿真结果表明,该预失真器能够有效地校正由于功率放大器的非线性和记忆效应引起的信号失真。     

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     

An adaptive digital predistortion for compensating nonlinear distortions in RF power amplifier with memory effects

In this paper, an adaptive digital predistortion based on a memory polynomial model is proposed in order to linearize the power amplifier with memory effect. The coefficients of the power amplifier model have been extracted using a least square method and those of predistortion have been identified by applying an indirect learning structure. Finally, the performance of digital predistortion has been demonstrated using the simulation of the power amplifier and the digital predistortion excited by a modulated 16 QAM signal in Matlab software. According to the simulation results, the criterion of adjacent channel power ratio (ACPR) declined by around 15 dB and the input/output power spectrum density of the power amplifier has quite similar curves. The linearized power amplifier output spectrum demonstrates the superiority of the proposed predistorter in eliminating the spectral regrowth which is caused by the memory effect in comparison to the other linearization methods.     

Wiener predistorter for nonlinear satellite downlinks

The nonlinear power amplifier and the analogue output channel filter with nonconstant group delay add nonlinear and linear distortions to the satellite transmitted signal, respectively. Recently, there has been growing interest in using Wiener predistorters, consisting of separate nonlinear and linear components, to compensate for these distortions in the satellite downlinks. The Wiener predistorter has been shown to effectively deal with signal distortions and has lower complexity compared to other state-of-the-art methods. In this paper, we argue that fully compensating the nonconstant group-delay distortion degrades the overall performance of the Wiener predistortion systems. This is primarily due to the increased peak-to-average power ratio of the signal at the output of the linear predistorter. We show that the overall performance of the Wiener predistorter can be improved by undercompensating the group-delay distortion. We propose two optimizations to address the PAPR growth problem and show using simulations that our approach leads to significant improvement in predistortion performance. Using our Wiener predistorter, the total degradation gap to the ideal limiter channel can be reduced to merely 0.34 dB for 64-APSK modulated signals.     

Adaptive digital/RF predistortion using a nonuniform LUT indexing function with built-in dependence on the amplifier nonlinearity

This paper reports on an integrated adaptive digital/RF predistorter using a nonuniform spaced lookup table (LUT) and in-phase/quadrature (I/Q) RF vector multiplier (VM). The LUT contents are directly deduced from the baseband input and output signals of the power amplifier (PA). In addition, a new nonlinear indexing function of the predistortion LUT with built-in dependence on the PA nonlinearity is proposed. This function is made to be robust to the input signal statistics. A comparison of this new indexation method with conventional approaches, namely, power and logarithmic power indexation functions, is carried out. The superiority of the proposed scheme is demonstrated in particular for class-AB amplifiers where the gain of the PA varies over the whole input range of the drive signal. The measured output spectrum of a linearized 90-W peak lateral double-diffused metal-oxide-semiconductor PA reveals a significant reduction of the power emission at the adjacent channels of approximately 15 dB under IS95, single-carrier, and multicarrier wide-band code-division multiple-access signals. The experimental evaluation is carried out using an RF/digital predistorter prototype that mainly includes an envelope detector, a linear I/Q RF VM, field-programmable gate array and digital signal processor, and fast analog/digital and digital/analog converters.     

Two-Tone and Nine-Tone Excitations in Future Adaptive Predistorted Linearised Basestation Amplifiers of Cellular Radio

Linearisation of a radio basestation amplifier using a third order, a fifth order and a seventh order predistortion scheme is studied. Adaptive predistortion using a third order predistorter improved the amplifier intermodulation distortion (IMD) by between 6.4 dB and 33 dB from the third IMD power level (i.e., maximum power amplitude variation of the IMD), provided by the raw amplifier, as measured by a two tone test. The variation in improvement is due to changes in the input power level. Improvement increases as the input power is backed off from the 1 dB compression point. A fifth order predistorter will give an IMD improvement of 8.7 dB and 48.4 dB if properly adjusted. A seventh order predistorter (with the fifth complex coefficient fixed to zero) will show the best performance, particularly throughout a window (approximately from 1.5 dB to 3 dB away from the 1 dB compression point). The IMD improvement throughout this window within the high input range measures between 9.1 dB to 21.7 dB over that achieved with a fifth order predistorter. For a nine-tone test (with uniformly distributed random phase), a third order predistorter will hardly improve the worst IMD power level, provided by the raw amplifier. A fifth order predistorter, if properly adapted will give an IMD improvement of between 8.8 dB and 41.5 dB from the worst IMD level while a seventh order predistorter will give a close performance to that of a fifth order one. Fixing the predistorter coefficients enables a comparative study of the IMD power level improvement between 5th order fixed and adaptive predistorters, for a nine-tone excitation. A 5th order predistorter with its coefficients fixed to those values obtained for optimum IMD level at a backed-off power input value of 8.5 dB from the 1 dB compression point is found to maintain an IMD power variation better than 55 dB from fundamental power level, throughout the input range, up to the backed-off value.     

一种Ku频段新型宽带幅相可调谐模拟预失真器

采用模拟预失真技术设计一款Ku波段预失真器，使用2个MA4E2037肖特基势垒二极管和三段无源传输线产生预失真信号，通过调节偏置电压实现幅度和相位可调；同时结合平衡式结构，改善单支路非线性器件增益扩张曲线斜率不足问题，改善输入驻波比。仿真结果表明，在14 GHz处，增益补偿和相位补偿可分别达到17 dB和60°以上；在频率12~16 GHz，增益扩张和相位扩张可达到15 dB和50°以上，整个频带内预失真器S11小于-17 dB。该预失真器适用频带宽，结构简单，功能实用。     

A nonlinear filtered‐x prediction error method algorithm for digital predistortion in digital subscriber line systems

Digital predistortion of nonlinear systems is an important topic in many practical applications. This paper considers direct predistortion of a Volterra system by connecting in tandem an adaptive Volterra predistorter. The coefficients of the predistorter can be recursively estimated using the nonlinear filtered‐x least mean squares (NFxLMS) algorithm. In this paper, the prediction error method (PEM) is used to derive a novel nonlinear filtered‐x PEM (NFxPEM) algorithm. A simulation study on Volterra systems shows that the NFxPEM algorithm more significantly suppresses spectral regrowth and converges much faster than the NFxLMS algorithm. Also, the NFxPEM algorithm is used in this paper to design more efficient digital predistorter—as compared with the NFxLMS algorithm—for digital subscriber line systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Digital subband filtering predistorter architecture for wireless transmitters

In this paper, a new architecture for baseband digital predistortion dedicated to wide-band transmissions is presented. The proposed predistorter consists of a cascade of a static lookup-table-based predistorter and a subband filtering block. The subband filtering technique is used to improve the correction bandwidth limitation of conventional architectures. The cases of four- and eight-subband decomposition were considered and compared to the case where the full-band signal is processed. This revealed that the subband processing results in a better sensitivity and an independent tuning ability of the predistorter at each subband. In addition, it was shown that, in the proposed predistorter, the computational complexity and the required processing speed are reduced proportionally to the number of subbands. Moreover, in an adaptive digital predistortion context, a quicker convergence can be achieved. Experimental validation of the subband-based predistortion concept was successfully carried on a 100-W peak envelop power lateral double diffused metal-oxide semiconductor power amplifier.     

幅度和相位预失真参数并行估计的间接学习结构预失真方法

为解决传统间接学习结构自适应预失真器中幅度和相位预失真参数之间相互影响的问题,提出了一种对功率放大器的幅度和相位预失真参数进行分开和并行估计的自适应预失真方法,其中幅度和相位预失真器均采用间接学习结构进行训练;将该方法应用在典型的无记忆功率放大器模型中,对DVB-T标准中2k模式OFDM信号的预失真过程进行了实验仿真;仿真结果表明,在预失真多项式阶数相同条件下,改进的间接学习结构预失真方法与传统间接学习结构预失真方法相比较,带外谱抑制增益平均提高了约4dB。      

An adaptive predistorter for a power amplifier based on adjacentchannel emissions [mobile communications]

A slowly adapting predistorter is presented. The approach is to minimize the transmitter output power in spectral regions occupied only by intermodulation (IM) products. In this way, only a spot power measurement is required. This technique relies on the principle that the power amplifier's characteristics vary slowly with time. By monitoring the out-of-band power one can obtain an estimate for the distortion introduced by the power amplifier. Adaptation is accomplished by iterative adjustment of the predistorter parameters to minimize the IM power. For a polynomial predistorter, the authors analytically demonstrate that the IM power is a quadratic function of the coefficients. A variety of algorithms therefore apply. The authors present an analog static predistortion linearization circuit that uses the envelope of the baseband signal to generate the nonlinear functional used in predistorting the input signal. The improvement obtained with an amplitude-modulated input signal was 15 dB in the third- and 5 dB in the fifth-order intermodulation products. The IM improvement could be maintained with the use of a robust direct search algorithm