Digital sinusoidal oscillators with high frequency resolution and low harmonic distortion

In this paper, we propose two new techniques to significantly improve the frequency resolution of a digital look-up-table (LUT) tunable sinusoidal oscillator. The proposed techniques are based on storing and reading the sine and cosine components of two sinusoidal functions that are not harmonically related and then evaluating the sine or cosine of the phase difference. The proposed techniques are shown to be very efficient in increasing the effective table length and, hence, the frequency resolution of the oscillator. Further performance improvement can be achieved by using interpolation methods as demonstrated. Simulation results verify the analytical results and show that the total harmonic distortion of the generated sinusoid is of the same order as that of the conventional digital LUT tunable sinusoidal oscillator.     

基于非迭代算法和非直接学习结构的查询表TWTA预失真器

提出了一种基于查询表和非直接学习结构的适用于行波管放大器（TWTA）的基带预失真器。通过引入线性增长的周期性训练序列和失真检测算法，实现了预失真器非迭代方法的参数捕获。与普通查询表预失真器相比，该预失真器不但能离线处理失真数据，而且不需要使用基于最小代价函数的迭代算法更新预失真器查询表中的内容，从而能在降低对系统处理速度要求的同时，回避了算法收敛性问题。仿真结果表明，所提预失真器可大大降低由TWTA非线性所引入的带内失真和频谱再生。     

基于线性神经网络预测的实时谐波检测

随着电力电子装置的应用日益广泛，电网中的谐波污染也日趋严重。准确、快速地检测系统电流中的谐波成分，是保证有源电力滤波器具备良好工作性能的关键。文中详细地阐述了针对电网中谐波变化的特点，给出一种谐波电流实时检测方法，能够正确预测出未来时刻的谐波电流值。本文给出的基于线性神经网络的实时谐波检测系统，可以利用前5个采样点的值来预测下一个采样点的值，也就是可以在第五个采样周期后准确地跟踪上基波的变化。实现了谐波检测的实时性。     

Tree-structured method for LUT inverse halftoning and for imagehalftoning

The authors previously proposed a look up table (LUT) based method for inverse halftoning of images. The LUT for inverse halftoning is obtained from the histogram gathered from a few sample halftone images and corresponding original images. Many of the entries in the LUT are unused because the corresponding binary patterns hardly occur in commonly encountered halftones. These are called nonexistent patterns. In this paper, we propose a tree structure which will reduce the storage requirements of an LUT by avoiding nonexistent patterns. We demonstrate the performance on error diffused images and ordered dither images. Then, we introduce LUT based halftoning and tree-structured LUT (TLUT) halftoning. Even though the TLUT method is more complex than LUT halftoning, it produces better halftones and requires much less storage than LUT halftoning. We demonstrate how the error diffusion characteristics can be achieved with this method. Afterwards, our algorithm is trained on halftones obtained by direct binary search (DBS). The complexity of TLUT halftoning is higher than the error diffusion algorithm but much lower than the DBS algorithm. Also, the halftone quality of TLUT halftoning increases if the size of the TLUT gets bigger. Thus, the halftone image quality between error diffusion and DBS will be achieved depending on the size of the tree-structure in the TLUT algorithm     

基于PLI算法的AAC反量化和PNS解码模块设计

针对AAC(Advanced Audio Coding)解码器中具有复杂运算因而难以硬件实现的反量化模块和知觉噪音替代(Perceptual Noise Substitution,PNS)模块,提出用分段线性插值(Partition and Linear Interpolation,PLI)进行近似计算的方法.其基本原理是将复杂的曲线分割成多个区间,在每个区间内用线性插值的方法进行近似.利用该方法设计的反量化模块和PNS解码模块获得了较高的运算精度,最大运算误差率分别为2e-4和5e-3,全部运算可分解为简单的查表、乘法、加法和移位运算,易于硬件实现.FPGA验证结果表明,反量化模块仅耗用了45个逻辑单元和2 304ROM bits,PNS解码模块仅耗用了21个逻辑单元和144ROM bits,且解码音质良好,没有产生可感知的噪音.     

用神经网络恢复残差的图像超分辨率算法

本文提出了一种基于残差线性恢复的图像超分辨率算法框架,以已有的图像插值算法为基础,利用人工神经网络恢复图像残差,相迭加得到高分辨率图像的估计.在数学上论证了基于神经网络的图像插值算法的依据,并证明了本算法比以往的算法具有更好的性能.理论上,任何一种单帧的图像插值算法都可以被引入本算法框架.同时给出了算法实例和测试结果.     

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。     

On Attaining the Multiple Solutions of Selective Harmonic Elimination PWM Three-Level Waveforms Through Function Minimization

Selective harmonic elimination pulsewidth modulation techniques are some of the control methods used in voltage/current source converters. However, challenges such as the task of finding all the multiple sets of solutions of the switching angles for a given problem may be difficult to deal with. In this paper, a direct minimization of the nonlinear transcendental trigonometric Fourier functions in combination with a random search is discussed. The unipolar (three-level) waveform is used to illustrate the proposed method confirming its ability to find multiple sets of solutions, including a case where 51 angles are sought for single- and three-phase applications. A simple harmonic distortion factor is studied for each set of solutions to assess their performance against the noneliminated harmonics. The results presented both at theoretical and experimental level are in close agreement and confirm the robustness of the proposed approach.     

Look-up table (LUT) method for inverse halftoning

In this paper we propose look-up table (LUT) based methods for inverse halftoning of images. The LUT for inverse halftoning is obtained from the histogram gathered from a few sample halftone images and corresponding original images. The method is extremely fast (no filtering is required) and the PSNR and visual image quality achieved is comparable to the best methods known for inverse halftoning. The LUT inverse halftoning method does not depend on the specific properties of the halftoning method, and can be applied to any halftoning method. Then, an algorithm for template selection for LUT inverse halftoning is introduced. We demonstrate the performance of the LUT inverse halftoning algorithm on error diffused images and ordered dithered images. We also extend LUT inverse halftoning to color halftones.     

OFDM系统中非线性放大器的改进预失真技术

针对OFDM系统中非线性放大器对信号造成的非线性失真影响,采用基于查询表的自适应预失真技术对其进行矫正.在基于最小均方误差算法的经典自适应预失真算法的基础上,采用线性内插算法来更新查询表的内容,从而提高自适应预失真算法的矫正效果;结合OFDM符号的概率分布特性,提出一种增加较大幅度OFDM信号迭代次数的方法,来提高预失...     

一种用于数字D类功放的伪自然采样算法

该文针对均匀采样脉冲宽度调制(UPWM)型数字D类功放,提出了一种新的伪自然采样算法以校正其在开关信号调制时产生的谐波失真。该算法融合了三阶和一阶拉格朗日插值法,并结合伪自然采样点位置判断法而形成,可在计算复杂度较低的同时,达到较好的谐波失真校正效果。该文使用FPGA搭建了一个基于该算法以及其它同类算法的开关信号调制器测试系统。测试结果表明,相比同类算法,该算法基本消除了谐波失真且硬件消耗适中,显示了更大的优越性。     

抑制光纤检波器系统光强波动影响的解调方法

基于海上油藏勘探领域的光纤检波器光电复合空分阵列系统,通过两路PGC 干涉检波信号相除以构造非线性函数,提出了一种采用查找表实现该非线性函数解调的新方法。相比传统的两路PGC 干涉检波信号线性组合的反正切解调方法,构造非线性函数解调新方案不仅可以补偿生成载波时直接调制DFB 光源带来的高频光强波动干扰,也可以抑制其他因素造成的光强随机波动给采用定常系统参数解调带来的影响。实验结果表明:内调制DFB 激光器,采用非线性函数解调方案比线性组合方案在谐波抑制方面优势明显,两者HSR 平均值相差达26.37 dB;当系统光强降低50%,非线性函数解调方案的结果HSR 平均值优于线性组合方案得到的结果,两者相差达17.53 dB。     

Inverse halftoning algorithm using edge-based lookup table approach

The inverse halftoning algorithm is used to reconstruct a gray image from an input halftone image. Based on the recently published lookup table (LUT) technique, this paper presents a novel edge-based LUT method for inverse halftoning which improves the quality of the reconstructed gray image. The proposed method first uses the LUT-based inverse halftoning method as a preprocessing step to transform the given halftone image to a base gray image, and then the edges are extracted and classified from the base gray image. According to these classified edges, a novel edge-based LUT is built up to reconstruct the gray image. Based on a set of 30 real training images with both low- and high-frequency contents, experimental results demonstrated that the proposed method achieves a better image quality when compared to the currently published two methods, by Chang et al. and Mes$80e and Vaidyanathan.     

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     

1:4 interpolation FIR filter

A new 1:4 interpolation FIR filter is presented. Using the symmetries of the filter coefficients and the contents of the lookup table (LUT), the two filters for in-phase (I-phase) and quadrature-phase (Q-phase) share the LUT and activate only the selected part of the LUT. Experimental results show that the proposed filter reduces the power consumption and the gate area by 28 and 5% compared to conventional filters.     

色域扩展中的快速查找表结构

在立方体插值算法的基础上,提出了一种适合于实时应用的快速查找表结构.其中将涉及颜色空间转换和色域扩展的复杂算法使用硬件上的综合查找表完成,使用了计算速度较快的四面体插值算法.同传统的立方体插值算法相比,有效降低了硬件资源消耗并提高了运算速度.     

Filter Lookup Table Method for Power Amplifier Linearization

This paper presents a new method of digital adaptive predistortion for linearization of power amplifiers (PAs) exhibiting memory effects. The predistorter (PD) device consists of a lookup table (LUT) gain followed by a codebook of filters addressed by the index of the LUT. The adaptation is derived from direct learning for the LUT gains and indirect learning for the filter coefficients. We compared our results with those of two reference methods: a simple LUT system (with direct learning) and a memory polynomial system (with indirect learning). The performances of the new approach lie between those of the two reference methods in terms of adjacent channel power regrowth and error vector magnitude. The LUT is the less complex of the three methods, but it is a memoryless system, and it cannot correct the memory effects in the PA. The memory polynomial PD is more powerful, but its complexity is very high. The new technique, due to the addition of filters to the LUT, has possibilities to compensate not only for the nonlinearity but also for the memory effects in the PA, and it is one order of magnitude less complex than the memory polynomial system     

A time-domain estimation method of rapidly time-varying channels for OFDM-based LTE-R systems

This paper addresses the performance of fast doubly selective fading channel estimation combined with Inter-Carrier Interference (ICI) cancellation for Long Term Evolution (LTE) communication platform in the High Speed Railway (HSR) environment. We consider the Channel Impulse Response (CIR) coefficients with a critical Doppler frequency shift and multi-path fading that were taken from the WINNER II channel model and the D2a propagation scenario, where the conditions of HSR are analyzed. As multi-path fading increases and the channel varies in the order of the symbol period, we first propose a novel approach for designing a pilot symbol structure in the time domain. Then, we describe the deployment of the proposed pilot symbol structure to estimate the channel in the time domain. Channel information corresponding to the data positions is obtained by linear interpolation. In each OFDM symbol, the slope and the initial value for establishing an interpolation function are estimated to adapt to the time variation of the channel. An accurate estimate of channel state information is used for the purpose of ICI cancellation. The simulation results show that the channel estimated by our proposed method can follow the real channel well, even in a very high Doppler frequency. The estimation method in terms of Mean Squared Error (MSE) significantly outperforms the state-of-the-art methods. The combination of our channel estimator with several interference cancelers provides a considerably better system performance than that achieved when frequency channel estimation is used.     

A globally convergent and consistent method for estimating the shape parameter of a generalized Gaussian distribution

We propose a novel methodology for estimating the shape parameter of a generalized Gaussian distribution (GGD). This new method is based on a simple estimating equation derived from a convex shape equation. The estimating equation is completely independent of gamma and polygamma functions. Thus, no lookup table, interpolation, or additional subroutine to evaluate these functions are required for real-time implementations of the proposed method, which is in contrast to all existing methods. Furthermore, we establish that the shape equation has a unique global root on the positive real line and the Newton-Raphson root-finding algorithm converges to the unique global root from any starting point in a semi-infinite interval Theta_min. More importantly, we show that the sample-based shape estimating equation has a unique global root with probability tending to one and the root is consistent for the true shape parameter. Finally, we prove via fixed point arguments that, with probability tending to one, the Newton-Raphson algorithm converges to the unique global root of the sample shape estimating equation from any starting point in Theta_min. Some numerical experiments are also provided to demonstrate the global convergence and the excellent finite sample performance of the proposed method     

一种基于双查找表自适应预失真结构的射频功率放大器线性化方法

同其它使用简单函数实现数据查找表索引的自适应预失真结构不同，本文采用专门的地址查找表来实现数据查找表索引，组成了双查找表结构的自适应预失真器。仿真结果表明，采用本文方法可以使得射频功率放大器对相邻信道的干扰得到25dB以上的性能改善。