Generalization of the Finite-Difference-Based Time-Domain Methods Using the Method of Moments

Many finite-difference-based time-domain methods have been developed in the past three decades. Typical among them are the finite-difference time-domain method of Yee's scheme, the transmission-line-matrix method, the multiresolution time-domain method, the pseudospectral time-domain method, and the unconditionally stable finite-difference time-domain methods. All these methods have become powerful tools in solving electromagnetic structure problems, yet their formulations appear to be unrelated. The concept of the method of moments (MoM) is applied in this paper to generalize all these different finite-difference-based methods. It is shown theoretically that differences among various finite-difference-based methods lie in different choices of basis functions used in solution expansions and weighting functions in error testing, in both time and space. The significance of this paper is twofold: 1) all the finite-difference time-domain-based methods can now be unified under the framework of MoM and 2) any other new finite-difference-based time-domain methods, including hybrid techniques, may now be developed with MoM procedure.     

Hyperion高光谱影像波段选择方法比较研究

Hyperion影像的光谱分辨率高,数据体积庞大,而且相邻波段之间的相关性强,信息冗余度较高, 给数据处理与解译带来了很多问题。鉴于此,提出了通过将分段主成分分析和波段指数相结合来开展波段选择与降维研究的思想。 同时采用自适应波段选择法、波段指数法和主成分分析累计贡献率方法进行了波段选择方法的对比研究;对4种波段选择方法所得到的结 果进行了最佳波段组合、地物可分性和图像变换比较分析。实验结果表明,分段主成分分析与波段指数综合方法可以有效抑制由于全局变换造成局部重要光谱被滤除的现象 ,同时还可兼顾自适应分区后各子区间及区间内波段之间的相关性,有效降低高光谱数据的维度。由此可见,该方法的波段选择效 果优于传统的自适应波段选择方法、波段指数法以及主成分分析累计贡献率方法。     

光波在纯水中的色散特性理论计算结果比较

Sellmeier色散公式和扩展的Lorentz-Lorenz色散函数表达式是两种常用的描述不同温度条件下物质折射率与入射光波长的变化关系式。为了探讨两者在计算光在水中色散问题的区别,运用两种不同的计算模型,在400 ~1100 nm波长范围内,四种不同温度下,分别模拟了蒸馏水中折射率、群折射率、群速度色散及三阶色散随光波长的变化曲线,同时分析了由此两种不同理论计算模型给出的主要计算结果之间的差异。结果显示,尽管由两种关系式计算得出的蒸馏水的折射率和群折射率基本相同,但群速度色散和三阶色散的结果却存在很大的差别。     

军事通信网络系统可靠性的分层指标体系研究

根据军事通信网络的体系结构,结合ISO协议的分层方式,给出了一种通信网络可靠性指标的层次分析模型与方法。定义了通信网络可靠性的内涵,描述了可靠性的分层定义、指标、及其关键测度。该分析方法可应用于基于通信网络系统的分析、规划、评价和质量管理等领域,也可应用于系统的时效特性、协同性、及时性等性能参数的工程分析。     

基于决策树分类算法异构数据的索引优化

海量数据的索引是提高分布式环境下海量数据的查询重要手段。为了构建高效的索引结构,人们提出了多种异构数据索引优化方法。文中给出了基于决策树分类算法下的索引优化方法。基于决策树分类算法构建索引决策树,利用该索引决策树对各个子空间表的属性列进行决策,建立索引表,根据索引表数据建立索引,再根据各子空间上的索引构建全局索引。该二级索引结构为快速定位索引信息提供了技术支持。实验结果表明,索引决策树是一个对优化异构数据索引合适的方法。     

Crosstalk identification in xDSL systems

Crosstalk among telephone lines in the same or neighboring bundles is a major impairment in current xDSL systems. This paper proposes a novel idea of an impartial third party that identifies the crosstalk coupling functions among the twisted pairs in these xDSL systems. The crosstalk identification technique includes the following four major procedures: (1) the transmitted and received signals from each DSL modem for a predefined time period are collected and sent to the third party; (2) the signals are resampled according to the clock rate of the receiver of interest; (3) the signals' timing differences are estimated by cross correlation; and (4) the crosstalk coupling functions are estimated using the least-squares method. The performance of the cross correlation and least-squares methods is analyzed to determine the amount of data needed for identification. Simulation results show that the proposed methods can identify the crosstalk functions accurately and are consistent with theoretical analysis. These identified crosstalk functions can be used to significantly improve the data rate (e.g., multiuser detection) and to facilitate provisioning, maintenance, and diagnosis of the xDSL systems     

光子晶体光纤的矢量有效折射率分析方法

由于具有复杂的折射率分布，光子晶体光纤一般需要采用数值方法进行研究。有效折射率方法最早用来研究光子晶体光纤的模式特征及其色散特性，但是一般采用标量近似的方法。建立了研究光子晶体光纤的矢量有效折射率方法，并用于计算光子晶体光纤的色散常数。类比于阶跃型折射率光纤，只需将电磁场所满足的相应的贝塞尔函数代入阶跃型折射率光纤所满足的矢量特征方程即可求解光子晶体光纤包层的有效折射率。采用矢量有效折射率方法的计算结果与基于有限元方法求解的数值结果和测量结果吻合得更好。     

Refractive index profiling-state of the art

Problems associated with different calibration techniques and some instrumental effects that can determine instrumental accuracy are discussed. It is shown how transmission effects can be observed and used to generate a correction curve for refracted near-field instruments. High-quality fiber slices needed for axial interferometry were used to obtain refractive index differences on suitable fibers to ±1% with the measurements traceable to national standards. Interferometry was used to calibrate a multiple-step fiber which is available in 10-m lengths with a certificate of calibration. The limitations of the technique are discussed, and preliminary results of a study on the effects of residual stress and stress relief on measurements are given. Comparisons are made between measurements made by axial interferometry and refracted near field on a range of fibers. It is shown that commercial profilers can produce reliable measurements of refractive index difference, absolute refractive index, profile shape, and numerical aperture that agree well with measurements by other techniques     

光源的显色性与显色指数

本文从色度学及此法计算过程，指出此方法由于种种原因，它的精确度十分有限，应该是一种数值范围表明一类显色质量，有关文献把５０左右以下分为“劣”、５０左右到７５左右为“一般”、７５左右以上为“优”。两光源的显色指数相差二、三个百分点说明不了哪个显色性较优。     

通信网络宏观可靠性指标的全代数化算法

为了更全面和客观地评价宽带大容量通信网络的可靠性能,引入宏观可靠性指标--总容量归一化加权可靠性指标概念,它是把通信容量和链路可靠性参数有机地综合在一起的指标.解决该指标全代数化计算的关键问题是寻找能够算出网络各节点之间全部路由的代数化路由算法.研究出一种逻辑代数化网络路由算法,n个节点的网络只需n次矩阵变换运算,就能得到任意节点之间的全部路由.基于新路由算法研究出网络可靠性指标的全代数化算法,它易于编写程序,利用计算机可以很方便地算出通信网络的可靠性指标,解决了总容量归一化加权可靠性指标计算困难问题,且达到实用化程度.利用算例验证了算法的正确性,并对算法的各计算过程进行了详细说明.     

PTC电极浆料用超细银粉制备过程中的消泡方法探究

研究了PTC(正温度系数陶瓷热敏电阻)电极浆料用超细银粉制备过程中温度和分散剂对泡沫高度的影响,探讨了几种消泡方法的优劣。结果表明,高温时,反应热聚集会造成液体局部沸腾,使泡沫高度增加;添加分散剂可增加泡沫液膜厚度和泡沫高度,高温高分散剂浓度使得银粉制备过程中必须进行消泡。消泡剂对生成银粉的粒径、灼烧失重等的影响较大,冷水降温消泡法用水量较大,负压消泡法效果显著。酒精、冷水消泡法可用于小批量生产,负压消泡法是提高生产量的好方法。     

Boolean Difference Techniques for Time-Sequence and Common-Cause Analysis of Fault-Trees

Fault trees are a major model for the analysis of system reliability. In particular, Boolean difference methods applied to fault trees provide a widely used measure of subsystem criticality. This paper generalizes the fault-tree model to time-varying systems and uses timedependent Boolean differences to analyze such systems. In particular, suitable partial Boolean differences provide maximal and minimal solution sets for sensitization conditions. A method of common-cause failure analysis based on partial time-dependent Boolean differences allows the study of failures due to repeated occurrences, at different times, of the same phenomenon. Such methods generalize to systems with repair, and under certain assumptions of independence, steady-state distributions can be used for the analysis of system faults. These methods are generally useful in reliability and sensitivity analysis.     

光纤聚合物电光调制器电极对波导特性的影响

有机聚合物高速电光调制器是全光纤器件,插入损耗低,集成度高,能极大地降低模式失配和调制电压,提高调制效率,是未来超长距离超大容量高速光通信系统中的关键器件之一。将一种基于长周期光纤光栅的光纤聚合物电光调制器看作5层均匀圆光波导,考虑两层电极及其复折射率的实际情况,利用紧致的超格子方法,在复数域分析这种光纤聚合物电光调制器的波导特性。从模式折射率的实部和虚部、模场分布等角度对数值计算结果进行分析表明,只忽略外电极对调制器的波导特性影响很小,考虑电极时长周期光栅的谐振波长比不考虑电极时大,从而影响调制特性。     

Metal-Clad Multilayer Waveguide: Circle-Chain Convergent Method and Two Perturbation Methods

Computer modeling studies for the calculation of the propagation constants and attenuation coefficients of TE mode in an asymmetric metal/ dielectric/dielectric layers structure are presented. Two different perturbation methods, circle chain convergent method and Downhill method are used. The last two methods can be used easily for the waveguides consist of any number of layers with complex refractive index due to gain and loss. The developed programs were run on a personal computer and numerical results are presented.     

Accuracy of approximate methods for the evaluation of chromaticdispersion in dispersion-flattened fibers

The accuracy of three approximate methods of estimating chromatic dispersion in dispersion-flattened fibers is examined. In these methods, the material dispersion of fiber is approximated by (a) the dispersion of pure fused silica, (b) the dispersion of the core material, and (c) the weighted average of material dispersion of various layers of the fiber. The actual values and three approximate values of the dispersion are calculated and compared for fibers with two, three, and four cladding regions. Examples of satisfactory and poor accuracy are given. The accuracy of approximate methods depends on the index differences and their first- and second-order derivatives with respect to wavelength. The smaller the index differences and their derivatives, the smaller the error in the approximate dispersion. However, small index differences along do not guarantee small errors. For GeO₂-doped fibers, the accuracy of approximate methods b and c is much better than that of method a     

Head and trunk segment moments of inertia estimation using angular momentum technique: validity and sensitivity analysis

Classical models to estimate the head and trunk (HT) moments of inertia (I) are limited to populations from which the anthropometric measures were obtained. The purposes of this study were to determine if the angular momentum technique can be used to estimate subject-specific HT's I values and test its validity and sensitivity. Twenty-three adults who participated in this study were divided into three morphological groups according to their body mass index (BMI). Using the proposed technique, the HT's I values were estimated for the whole sample and compared to three well-known methods to test its validity. The sensitivity of the proposed method was verified while applied to individuals with different BMI (i.e., lean, normal, and obese). The angular momentum technique gave I values within the range of those of the three methods for the entire sample. Statistical differences were identified between the lean and obese groups in relative radii of gyration for the anteroposterior and mediolateral axes ( P<0.05). Since the proposed technique makes no assumption on the mass distribution and segments' geometry, it appeared to be more sensitive to body morphology changes in estimating the HT's I values in lean and obese subjects compared to the classical methods.     

Dynamic gain equalization in two-stage fiber amplifiers

Relative gain differences among channels in WDM-amplified lightwave systems can be corrected in two-stage fiber amplifiers having complementary gain spectra in each stage. With two channels spaced by 2.5 nm, relative gain corrections of 1 dB were demonstrated. Simulations show that this method can be used to dynamically equalize the channel gain in long-distance transmission systems     

一种集成的字符集检测方法研究

有些文本缺少字符集信息,因而阻碍了字符信息的正确解析。为了获得字符集信息可以根据文本中字符信息的统计特征以及各种字符集之间的差异来识别出文本的字符集。集成的字符集检测方法通过综合应用编码模式检测方法、字符分布方法和双字符序列检测方法来检测字符集。它根据算法复杂度和每种算法的准确率来调度算法,使得在用一种方法不能确定字符集的情况下利用其它方法来检测出字符集实验结果表明：集成的字符检测技术具有较高的准确率。集成的字符检测方法在某些情况下通过适度增加一些时间来获得较高的精度。     

基于热电耦合激励的TSV三维封装内部缺陷识别方法研究

由于硅通孔互连(Through Silicon Via,TSV)三维封装内部缺陷深藏于器件及封装内部,采用常规方法很难检测.然而TSV三维封装缺陷在热-电激励的情况下可表现出规则性的外在特征,因此可以通过识别这些外在特征达到对TSV三维封装内部缺陷进行检测的目的 .文章利用理论与有限元仿真相结合,对比了正常TSV与典型缺陷TSV的温度分布,发现了可供缺陷识别的显著差异.分析结果表明,在三种典型缺陷中,含缝隙TSV与正常TSV温度分布差异最小;其次为底部空洞TSV,差异最大的为填充缺失TSV.由此可知,通过检测热-电耦合激励下的TSV封装外部温度特征,可实现TSV三维封装互连结构内部缺陷诊断与定位.