Optimal windowing for wideband linear arrays

The problem of constructing a wideband beam pattern that is concentrated in both angle and frequency is discussed. This paper is a direct extension of the work of Slepian and co-workers on time- and frequency-limited functions. It is shown that the singular vectors and singular functions of the mapping relating the set of weights of a linear wideband array to its far-field directivity pattern have both concentration properties and double orthogonality properties and so they can be thought of as the wideband equivalents of the discrete prolate spheroidal sequences and wave functions. These singular functions are used to obtain approximations to a frequency-invariant beam pattern.     

Analysis of Chirp as windowing function through fractional Fourier transform

This article presents a new mathematical model for obtaining the fractional Fourier transform (FrFT) of Chirp function. FrFT is a parameterised transform having an adjustable transform parameter which makes it more flexible and superior over ordinary FT in several applications. It is an important tool used in signal processing for spectral analysis. The closed-form expression derived for FrFT of finite duration Chirp establishes the dependence of FrFT of Chirp on the order of FrFT and the Chirp parameter. The mathematical model obtained shows that the Chirp function can be used as an adjustable window function in the fractional Fourier domain. The main-lobe width, side-lobe level and side-lobe fall-off rate of a Chirp window can be controlled by changing the adjustable transform parameter to different values. For some particular values of fractional angle, Chirp can give better spectral parameters than the existing window functions. By varying the order of FrFT, the variations in spectral parameters of the Chirp window are obtained and studied. The performance of the Chirp window in fractional domain is also compared with some of the existing windows.     

多径信道下OFDM系统小数倍频偏估计算法改进研究

针对多径信道环境下OFDM系统小数倍频偏估计精度不足的问题,提出一种基于循环前缀(CP)估计的改进算法.该算法重点在于准确搜索出受到多径干扰的数据范围,利用未受到多径干扰的CP数据进行ML频偏估计,并对估计值加以修正,使得估计结果更加精确.仿真结果表明,改进算法相对于原算法具有更好的估计性能.     

基于分数时延滤波器的宽带数字信号时延的实现

阐述了基于分数时延滤波器的时延方法设计原理,对其中4种代表性的设计方法的时延性能进行了评估,给出宽带数字线性调频信号的时延仿真结果,该方法可用于宽带雷达数字信号处理。     

Optimal fractional movement-based scheme for PCS location management

In this letter, we propose a fractional movement-based location update scheme for personal communication service networks. Similar to the well-known fractional guard channel scheme for channel assignments, in our proposed scheme, the movement threshold is a real number with a fraction instead of an integer. We prove analytically that there is a unique optimal fractional movement threshold that minimizes the total cost of location updates and paging per call arrival.     

One structure for fractional delay filter with small number ofmultipliers

A wide-bandwidth, high-resolution fractional delay filter (FDF) structure with a small number of multipliers per output sample and a short coefficient computing time is presented. The proposal is based on the use of a frequency FDF design method up to only half of the Nyquist frequency, in a multirate structure     

数字预失真系统中环路小数时延估计

针对数字预失真系统中的环路时延问题,提出一种自适应的小数时延估计算法,可以准确地估计数字预失真系统的环路小数时延,确保数字预失真参数提取的正确性。该算法采用多点平均的方法,消除估算误差对算法性能的影响,从而提高估计算法的精确度和稳定性。仿真结果表明：与以往文献提出的小数时延估计算法相比,提高了小数时延估计的精确度和稳定性,精确度可以达到采样时刻间隔的1.4%。而且,该方法可自适应地更新小数时延,更加容易应用于硬件实现中。     

Joint estimation of carrier frequency offset and power delay profile for MIMO-OFDM systems

This paper develops a Cyclic Prefix（CP）based joint Maximum-Likelihood（ML）estimation algorithm of Carrier Frequency Offset（CFO）and Power Delay Profile（PDP）for Multi-Input Multi-Output Orthogonal Frequency Division Multiplexing（MIMO-OFDM）systems.However,the exact solution of the joint ML estimation is very complex since it needs a search over a multi-dimensional domain.Thus a simplified method is proposed to estimate the CFO and the PDP iteratively via the alternating-projection method which could induce the multidimensional search problem to a sequence of simple one-dimensional searches.Simulations show that the proposed algorithm is more accurate and robust than the existing algorithms.     

Robustness evaluation of fractional order control for varying time delay processes

In this paper, we investigate the robustness of a methodology to design fractional order PI controllers combined with Smith Predictors, for varying time delay processes. To overcome the drawback of possible instability associated with Smith Predictor control structures, mainly due to the changes in the time delay, the design focuses on ensuring robustness of the closed loop system against time delay uncertainties. The proposed method is based on time-domain performance specifications??more accessible to the process engineer, rather than the more abstract notions related to the frequency domain. A second advantage of the proposed method relies on additional robustness to plant uncertainties, achieved by maximizing open-loop gain margin. The convergence problems associated with optimization techniques, previously used in fractional order controller designs, are eliminated by an iterative procedure in computing the gain margin. The simulation example provided demonstrates the efficiency of the proposed method, in comparison to classical integer order PI controller.     

Smith-fuzzy fractional control of systems with time delay

The fractional calculation is over 300 years old, but only in the last decades its application arisen in many areas of science and engineering. The explanation for this expansion in the scientific and tecnologic worlds is due to the good results obtained in solving different problems, which have overcome the classics. On the other hand, in the control area there was an increasing interest in the development of new strategies, which resulted in new algorithms where the use of the fractional calculus revealed an helpfull tool. In this paper we apply the fractional order concepts into two systems with time delay. Several algorithms are investigated and compared, when integrated within a Smith predictor structure and with an optimal fuzzy fractional PD^β+I controller in which the parameters are tuned by a genetic algorithm. Simulations are presented assessing the performance of the proposed fractional schemes.     

Optimal periodic training signal for frequency offset estimation in frequency-selective fading channels

This paper addresses an optimal periodic training signal design for frequency offset estimation in frequency-selective multipath Rayleigh fading channels. For a fixed transmitted training signal energy within a fixed-length block, the optimal periodic training signal structure (the optimal locations of identical training subblocks) and the optimal training subblock signal are presented. The optimality is based on the minimum Cramer-Rao bound (CRB) criterion. Based on the CRB for joint estimation of frequency offset and channel, the optimal periodic training structure (optimality only in frequency offset estimation, not necessarily in joint frequency offset and channel estimation) is derived. The optimal training subblock signal is obtained by using the average CRB (averaged over the channel fading) and the received training signal statistics. A robust training structure design is also presented in order to reduce the occurrence of outliers at low signal-to-noise ratio values. The proposed training structures and subblock signals achieve substantial performance improvement.     

一种基于OFDM信号周期平稳特性的系统时延和频偏的盲估计算法

该文首先提出一种基于信号周期平稳特性的OFDM系统时延和频偏的盲估计算法,然后分析算法,获得一种可以估计非整数倍样点周期的时延和扩大频偏估计范围,实现整个OFDM系统带宽范围内的频偏估计的方法,并指出本算法可以在信道未知的情况下获得好的估计性能,最后给出相应的仿真结果。理论分析和仿真结果表明,本算法有很好的抗噪特性、强的变化信道适应性,与同类方法比较,具有更高的估计精度、估计非整数倍样点周期时延和扩大频偏估计范围的能力。     

宽角扫描侧馈偏置卡塞格伦天线的优化设计

为满足星载天线空间电扫描的要求,研究了宽角扫描侧馈偏置卡塞格伦天线(SFOC)的设计问题.给出了SFOC天线的结构形式,通过射线追踪的方法对SFOC的最佳馈源位置和最佳馈源指向进行了优化设计.二者的确定不仅改善了SFOC天线系统的扫描特性,而且提高了它的工程可实现性.分析结果表明,该SFOC天线在宽角扫描过程中,方向图畸变很小,增益损失和副瓣电平均达到了设计要求,从而为全球扫描星载天线的设计提供了有效的方案.     

多小区OFDMA系统中分数频率复用方法的优化分析

在以OFDMA为基本多址方式的第四代移动通信系统中,频率复用作为提高系统性能的有效方法受到了广泛的关注,特别是分数频率复用和软频率复用。然而,如何进一步优化基于这些频率复用方法的OFDMA系统还有待研究。本文分析了多小区OFDMA系统负载和频率集合分配比例对小区总吞吐量和小区边缘用户数据速率的影响。同时,给出了在保证小区边缘用户服务质量的条件下,使小区总吞吐量最优的小区中心频率分配比例,并用仿真进行验证。仿真结果还表明了,随着分配比例的增大,小区总吞吐量先增大后减小。     

Optimal rational approximations to a delay

The delayed step function is approximated to in an optimum manner using suggestions by Budak and Scanlan.     

基于分数时延的宽带数字波束形成技术

在窄带数字阵列波束形成中,通过补偿各阵元之间的时延带来的相移而合成波束。对于宽带数字阵列,相同的时延不同的频率会带来相移的不同,窄带波束形成方法会导致宽带波束方向图畸变,必须采用宽带数字波束形成技术。通过分析信号带宽对窄带数字波束形成的影响,以及宽带数字波束形成的原理,给出了基于分数时延的宽带数字波束形成方法和仿真的结果,在数字域上实现了宽带波束形成。     

Eigenfilter approach for the design of variable fractional delay FIR and all-pass filters

An eigenfilter approach is presented for designing 1-D and 2-D variable fractional delay FIR and all-pass filters. First, the coefficients of filters are expressed as a polynomial of the fractional delay parameter. Then, the optimal polynomial coefficients are obtained from the elements of the eigenvector corresponding to the minimum eigenvalue of a real, symmetric and positive definite matrix. Finally, several design examples of 1-D and 2-D variable fractional delay FIR and all-pass filters are used to illustrate the effectiveness of the eigenfilter approach.     

Joint estimation of time delay and frequency delay in impulsivenoise using fractional lower order statistics

New methods for time delay estimation and joint estimation of time delay and frequency delay in the presence of impulsive noise are introduced. First, degradation of the conventional approaches based on second-order statistics is shown both theoretically and experimentally. Then, a new class of robust algorithms are developed using the theory of alpha-stable distributions, including the fractional lower order covariance (FLOC) method, which is formulated for the time delay estimation problem and the fractional lower order ambiguity function (FLOAF), which is defined for the joint estimation of time delay and frequency delay. It is shown that these new methods are robust for both Gaussian and non-Gaussian impulsive noise environments. The improved performance is clearly demonstrated through detailed analysis and comprehensive simulations with computer-generated data as well as actual radar and sonar clutter data     

Joint estimation of interference frequency offset and time delay in PCMA system

For cooperative received high-order modulation PCMA (paired carrier multiple access) signals,joint estima-tion of frequency offset and time delay was proposed.The sequence samples stored locally were used as auxiliary data.Estimation of frequency offset and time delay was calculated by optimizing the objective function,which was obtained from the cross-correlation computation of the auxiliary data and mixed signal,and the optimized process was accom-plished by utilizing two-dimensional search.By setting the threshold of joint estimation,the calculated amount was greatly reduced.Modified Cramer-Rao bound (MCRB) of interference frequency offset and time delay was derived,which provided theoretical basis for performance of the proposed algorithm.Simulation results show that the algorithm has similar performance with existing algorithms,but its complexity is reduced by two-thirds.     

New windowing techniques for variable-size block motioncompensation

The conventional overlapped block motion compensation (OBMC) technique, although effective in reducing the blocking effect for fixed-size partitioned frames, cannot be readily used for a frame partitioned region-wise or using variable-size blocks. The generalised OBMC (GOBMC) method, even though generally applicable, is not very effective in reducing the blocking effect and prediction error. Two windowing techniques are presented to reduce the blocking effect for a frame partitioned region-wise or using variable-size blocks. In the first technique, a virtual re-partitioning operation is employed, which maps a partitioned frame into its corresponding fully partitioned frame at the bottom level of the quadtree so that each resulting block has eight neighbouring blocks. In the second technique, the virtual re-partitioning operation for a given block (region) is carried out adaptively and performed locally; however, blocks (regions) need not always be virtually partitioned to the bottom level. Compared to the GOBMC method, the proposed techniques make use of more pixels in the close vicinity of boundaries of the regions in the windowing operation. Simulation results are included of applying the proposed techniques on a number of MPEG video sequences. These results indicate that the proposed techniques are superior to the GOBMC method in terms of reducing the prediction error as well as the blocking effect