Adaptive Predistorters for Linearization of High-Power Amplifiers in OFDM Wireless Communications

Orthogonal frequency division multiplexing (OFDM) has several desirable attributes which make it a prime candidate for a number of emerging wireless communication standards. However, one of the major problems posed by OFDM is its high peak-to-average power ratio (PAPR), which seriously limits the power efficiency of the high-power amplifier (HPA) because of the nonlinear distortion resulting from the high PAPR. We provide a new mixed computational/analytical approach for adaptive compensation of this nonlinear distortion for cases in which the HPA is a traveling wave tube amplifier (TWTA) and solid state power amplifier (SSPA). TWTAs are used in wireless communication systems when high transmission power is required as in the case of the digital satellite channel, and SSPAs are generally used in mobile communication systems. Compared to previous predistorter techniques based on LUT (look-up table) or adaptive schemes, our approach relies on the analytical inversion of Saleh's TWTA model and Rapp's SSPA model in combination with a nonlinear parameter estimation algorithm. This leads to a sparse and yet accurate representation of the predistorter, with the capability of tracking efficiently any rapidly time-varying behavior of the HPA. Computer simulations results illustrate and validate the approach presented.     

UFMC系统中基于Volterra滤波器的非线性失真补偿方案

为实现通用滤波多载波(UFMC)通信系统高效、可靠的通信性能,需在最大程度上补偿由记忆型高功率放大器(HPA)引起的非线性失真.为解决HPA造成的失真问题,本文提出了一种基于Volterra滤波器的非线性失真补偿(V-NLDC)技术.该技术利用了Volterra级数的稀疏特性和能够模拟任意精度非线性系统的性质以逐次逼近的方式对信号进行预失真.将预失真后的信号传送至HPA,然后采用噪声消除器做进一步噪声消除处理,以达到更小失真度的目的.同时,本研究采用收敛速度快、性能稳定的自适应最小二乘法(RLS),可根据环境变化自适应地计算Volterra滤波器和噪声消除器的系数.通过大量蒙特卡罗仿真实验证实了所提出的非线性失真补偿技术可以很好的补偿由记忆型HPA非线性失真所造成的影响,从而优化系统性能.     

OFDM系统中基于限幅和压缩感知的非线性失真补偿算法

正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）信号具有较高的峰均功率比（Peak to Average Power Ratio,PAPR）,不仅影响功率放大器（High Power Amplifier,HPA）的工作效率,而且HPA使得OFDM信号产生严重的非线性失真,导致系统的误比特率（Bite Error Rate,BER）增大.本文基于限幅和压缩感知（Compressive Sensing,CS）提出了改进的补偿算法,发送端采用限幅降低信号的PAPR,接收端首先采用改进的逆模型方式减小HPA引入的非线性失真,再采用CS抵消由限幅引入的信号失真.仿真表明,所提方法不仅明显降低了OFDM信号的PAPR,而且有效提高了系统的BER性能.     

一种自适应预失真器的实现及算法的改进

自适应数字预失真是补偿高功率放大器非线性的一种有效技术。本文基于RASCAL算法,给出了数字预失真器在OFDM系统中的实现结构,并对调整方法作了一些改进。通过仿真表明,改进的方法明显加快了收敛速度。     

OFDM系统中基于压缩感知恢复由限幅和HPA产生的非线性失真研究

正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）系统的主要缺点之一就是有较高的峰均功率比（Peak to Average Power Ratio,PAPR）,降低了功率放大器（High Power Amplifier,HPA）的工作效率,同时HPA引入的非线性失真,恶化了系统的误比特率（Bite Error Rate,BER）性能.本文所提算法将限幅和HPA引入的非线性失真视为一个整体来考虑,利用与限幅噪声在时域上的近似稀疏性,对整个非线性过程进行建模.发送端通过限幅降低了OFDM信号的PAPR,在接收端,选取受噪声干扰小的可靠性观测向量,最小化信道噪声的影响,基于非线性模型计算得到的参数,利用压缩感知（Compressive Sensing,CS）算法能有效地恢复总的非线性失真信号,提升了系统的BER性能.     

一种有效的自适应预失真算法

自适应预失真技术是一种用于补偿功率放大器引起的非线性失真的技术 ,本文提出了一种改进算法用来加快预失真算法的收敛速度     

Nonlinear Compensation of High Power Amplifier Distortion for Communication Using a Histogram-Based Method

A probabilistic approach to compensate the nonlinear distortion caused by a high power amplifier (HPA) in a communication system is proposed here. This is a nonparametric method that involves estimating two probabilistic cumulative distribution functions without any explicit parameter estimation as in the conventional compensation techniques. It is shown analytically that the maximum compensation error of the proposed method is bounded and small. An adaptive implementation is developed. Experiments are setup to collect the HPA data. Real data analysis shows that the proposed predistorter has an improved performance compared to the conventional polynomial and neural network predistortion techniques.     

Threshold IBO of HPA in the predistorted OFDM communication system

Orthogonal frequency division modulation (OFDM) system is very useful for the multi-path channel and highly bandwidth-efficient system. However, OFDM signal shows the high peak to average power ratio (PAPR) so that nonlinear distortion can happen in the high power amplifier (HPA). This paper studies BER performance variation according to input back off (IBO) values when the predistorter is used for the compensation for the HPA nonlinearity. In the case of the solid state power amplifier (SSPA) of nonlinearity parameter p=0.5 and IBO=0 dB, the system with predistorter is poorer than the one without predistorter. But this situation is inverted, if IBO is increased from 0 dB to 3 dB. So, we can find that there is a threshold value of IBO for the predistorter to effectively compensate for the nonlinear SSPA when the sub-carrier number and p value are changed. This threshold IBO value is proportional to the nonlinearity parameter p value but it is nearly independent of the sub-carrier number because amplitude and distribution are normalized. The OFDM system with predistorter can be improved only when thee IBO value is higher than the threshold value.     

MQAM 功放器件非线性失真均衡器

本文给出了一种具有记忆能力的自适应非线性失真预均衡器,用于补偿MQAM高功率放大器在非线性区域工作时引入的具有记忆的3阶非线性失真,采用Volterra级数模型分析并补偿这种非线性失真,系统模拟证明这种结构简单的预均衡器对系统的非线性失真具有较好的校正作用。     

An analytical framework for CDMA systems with a nonlinear amplifier and AWGN

The design of code-division multiple-access (CDMA) transmission systems for satellite communications requires an appropriate consideration of the distortion effects due to on-board nonlinear amplification. The aim of this paper is to provide an analytical framework for the evaluation of the in-band nonlinear distortion effects on the performance of CDMA systems. Both synchronous systems with orthogonal codes and asynchronous systems are considered. It is first shown that, when the users accessing the channel have the same power and their number is sufficiently large, the nonlinear distortion in the decision variables at the receiver can be simply described by a complex scale factor, which depends on the high-power-amplifier (HPA) characteristics only, and an additive noise, which is uncorrelated to the useful signal. Moreover, an analytical formulation of the bit error probability and the total degradation as a function of the output back-off and number of users is given. In the results, which are obtained for three classes of HPA models (i.e., the traveling wave tube amplifier, the solid-state power amplifier, and the amplifier with ideal predistortion), the performance and the capacity of power-limited systems is also discussed.     

影响预失真OFDM系统的关键参数IBO阈值的研究

OFDM系统的一个主要缺陷就是峰平比过高引起的高功率放大器(HPA)的非线性失真.该文研究了当使用预失真器进行补偿时,系统的接收误码率BER随HPA的输入回退IBO值变化而产生的性能改变,从而给出系统性能有效改善时,HPA的IBO阈值.     

An adaptive pre-distorter for the compensation of HPA nonlinearity

A high power amplifier (HPA) is used for the amplification of transmitting communication signals. However, it produces distortions by creating AM/AM and AM/PM modulations in the transmitting signal, Accordingly, this nonlinearity results in bandwidth expansion and nonlinear distortion in the in-band signal. This paper proposes an algorithm for the operation of a pre-distorter, which is composed of a look-up table (LUT), that can compensate for the distortion produced by an HPA. For the fast initialization of the LUT, an estimation algorithm is also proposed for the HPA characteristics. Furthermore, an adaptive algorithm based on the minimization of the mean square error is proposed to compensate for the time-varying property of an HPA. The performance of the proposed algorithm is analyzed by applying the algorithm to an 8-level vestigial sideband (VSB) modulation to be used in the ATSC terrestrial digital TV system     

一种基于模糊辨识的OFDM系统预失真器设计

将有记忆的高功率放大器(HPA)描述成一个线性动态系统级联一个无记忆非线性系统的Wiener系统模型,由Sugeno模糊模型自适应跟踪HPA的无记忆非线性曲线,通过离线过程设计OFDM系统数字预失真器。为了消除HPA记忆效应,首先在频域得到信号的均衡补偿,然后通过傅里叶逆变换后得到时域记忆性补偿。该预失真器具有收敛速度快、系统稳定、补偿效果好的特点。     

Intersymbol interference due to linear and nonlinear distortion

The paper provides a recursion model for the calculation of the probability density function (pdf) of intersymbol interference (ISI) which is caused by the combined effect of linear channel dispersion and of nonlinear distortion. The nonlinearity introduces statistical interdependencies between the interfering symbols and these dependencies are implicitly taken into account in a trellis-structured recursion rule. The results were verified by time consuming Monte Carlo simulation and show, e.g., that the nonlinear characteristic of a high power amplifier (HPA) reduces in some cases the error probability caused by the linear dispersion. Surprisingly, the ISI due to the nonlinear amplifier is increased in the case of offset modulation     

无线OFDM系统自适应预失真器实现

本文深入研究了无线OFDM系统的预失真技术，针对OFDM信号的高峰平比特性及高功率放大器带来的非线性失真影响，提出了基于查表法的基带自适应预失真器解决方案，给出其自适应递归算法及查询表内容的更新策略：并利用计算机仿真对本方案的性能进行了分析，仿真结果表明：本方案可以有效改善放大器的非线性特性。     

微波功率放大器非线性特性分析

为判断微波功率放大器非线性失真的主要影响因素,首先,在传统幂级数模型的基础上对功率放大器的非线性幅度失真和相位失真进行拟合,基于包络分析法给出了功率放大器非线性失真与幅度和相位失真间的解析关系;其次,对幅度失真和相位失真引起的非线性失真进行了分析,给出了两者之间的等效失真关系式,据此可对任意给定的功率放大器进行分析,以确定非线性失真的主要影响因素,并用于指导模拟预失真线性化器的设计与调试;最后,通过对一Ka频段行波管放大器的非线性测试及模拟预失真线性化,验证了所提出的功率放大器非线性分析的正确性及幅相等效失真关系式的有效性。     

BER Performance of OFDM Signals in Presence of Nonlinear Distortion due to SSPA

Orthogonal frequency division multiplexed signals typically exhibit large dynamic ranges due to superposition of a large number of subcarrier signals. As a result, such signals are highly vulnerable to distortion caused by high power amplifier (HPA) nonlinearity, leading to degradation in bit error rate (BER). In turn, the HPA operating point should be backed off to compensate for such degradation at the expense of the HPA efficiency. In this paper, the considered amplifier chain is a solid state power amplifier of Rapp’s model. We develop the factors contributing to BER degradation in terms of the input back-off (IBO) level assuming quadrature phase shift keying modulation. Consequently, the BER performance for such an amplifier model is developed as a function in IBO. Then, we show how this developed performance can be used to determine the required HPA operating point to achieve a certain required BER. Accordingly, an improvement in the amplifier efficiency is achieved. All developed analytic expressions are compared against simulation measurements, which show that good agreement exists.     

SSPA高功率放大器的预失真设计与实现

仿真分析了由于高功率放大器(High Power Amplifier,HPA)的非线性失真对OFDM信号造成的影响,设计了基于清华大学DMB-T高清晰度数字电视标准的SSPA(Solid State Power Amplifier)3 W功放,利用了多项式拟合及FPGA技术,考虑到曲线拟合误差和HPA的特性,采用了分段拟合,硬件实现了SSPA的预失真,可将带外功率失真降低约10 dB左右,在很大程度上抑制了带外及带内失真.     

强干扰下非线性放大器对伪码测距性能的影响研究

高功率放大器(HPA)作为高精度伪码测距系统中发射链路的重要组成部分,会将非线性失真引入到信号中,影响导航信号的测距精度。在干扰信号存在的条件下,放大器引入的幅频失真和相频失真也更加复杂。通过理论推导和仿真分析,研究干扰条件下的非线性特性对伪码测距的影响,给出了放大器工作在不同工作点下的伪码测距误差结果。研究结果对实际工程有一定的参考价值。     

一种单入双出式神经网络的自适应预失真技术研究

放大器存在固有的非线性失真特性,会对通信系统性能造成严重影响。为了克服或降低放大器非线性特性,需要对其进行线性化处理。首先对功率放大器的非线性失真进行了数学分析,简单地介绍了预失真技术的基本原理和神经网络的基本知识,接着根据放大器幅度和相位的失真特性提出了一种简单的单入双出式神经网络自适应数字基带预失真技术,该技术能同时对放大器幅度和相位失真做出改善。最后以双音信号和16QAM信号为例进行了Matlab仿真。仿真结果印证了该技术具有较好的性能。