基于指数时延的无线传感器网络时钟同步估计

无线传感器网络的时间同步是无线传感器网络的一个重要的研究方向.本文利用时钟偏移量误差的规律性,提出基于指数时延的无线传感器网络时钟同步估计算法.本算法是一种基于发送者同步模型,在DMTS算法的基础上引入指数时延得到的.理论与仿真实验都证明本算法对时钟偏移量的估计是相合的,同时实验还证明本算法比DMTS算法具有更高的精度和稳定性.     

WSN中虚拟时戳与父亲责任链时钟同步算法

通过对无线传感器网络时钟同步算法的研究,提出了适用于单跳网的虚拟时戳时钟同步算法与适用于多跳网的父亲责任链时钟同步算法。为确保整个时钟同步的健壮性与同步过程低的能量消耗,进而提出了携子寻父算法,当利用邻居表构造的层次链路树发生断链时,其以较小的代价快速恢复父亲责任链。实验结果显示其具有较高的精度,适合于大规模无线传感器网络的数据采集与监测等应用。     

WSN中的时钟漂移与偏移补偿机制研究

在简要阐述无线传感器网络时间同步的发展情况后,基于经典的发送者-接收者双向同步原理,提出一种基于时钟漂移与偏移的同步补偿机制(CDCO算法)。实验仿真结果表明,相对于TPSN算法,CDCO算法可以减少同步误差。     

基于分簇的无线传感器网络时间同步算法

时间同步是无线传感器网络中一个重要支撑技术,为了提高时间同步精度,提出了一种基于分簇的无线传感器网络时间同步算法;在部署无线传感器网络的初期阶段,建立簇状拓扑结构,首先是基站与簇首节点实现同步,然后簇内实现同步,最终建立一个全网统一的时钟,在同步过程中,采用了成对节点间的同步算法,很好利用了多信道广播方式;该算法能很好地满足无线传感器网络低能耗的要求;性能分析和实验结果表明,该算法减少了同步层次,提高了同步精度。     

无线传感器网络时间同步综述

无线传感器在网络应用时要求传感器节点保持时间同步,但传统的时间同步方法并不适用于无线传感器网络。为此,指出设计时间同步协议所面临的问题,对现有无线传感器网络时间同步算法进行总结,分析典型算法对时钟偏差和时钟漂移的处理,并给出进一步的研究方向。     

无线传感器网络的时间同步算法研究

时间同步是研究多跳ad hoc无线网络的重要问题,例如无线传感器网络,许多具体应用需要传感器节点本地时钟的同步,要求各种程度的同步精度.无线传感器网络设备的一些固有属性,例如能量的限制、存贮、计算和带宽,与节点分布的高密度结合,使传统的时间同步算法不适合于这些网络.因此,越来越多的研究集中在设计适合于无线传感器网络的时间同步算法.首先回顾了时间同步的问题和无线传感器网络时间同步的需要,然后详细介绍了针对无线传感器网络设计的基本的时间同步算法,最后对各种算法进行了比较分析,并指出了下一步的研究方向.     

无线传感器网络TPSN的优化算法

通过对无线传感器网络时钟同步算法TPSN(传感器网络时间同步协议)的研究,提出一种TPSN的优化算法;在TPSN算法时钟同步的过程中,由于节点时钟的不稳定性以及节点间消息交换延迟会引起同步误差,针对引起同步误差的这两个因素,基于已经存在的TPSN时钟同步算法,利用贝叶斯估计的先验和后验分布对TPSN算法进行优化,来达到减小同步误差的目的;使用NS2仿真软件进行的仿真实验显示优化后的算法比原算法的同步误差明显减小,同步精度显著提高.     

基于Bootstrap采样粒子滤波的WirelessHART时间同步算法﹡

针对无线HART传感器网络时间同步精度较低、能耗过大等问题,提出了一种基于Bootstrap采样的粒子滤波时间同步算法。在未知网络延迟分布的情况下,为了减少观测次数,对发送端和接收端的时间戳观测值进行Bootstrap采样,采用混合粒子滤波算法,获得精确的时钟偏移,从而不仅降低了无线HART传感网络时间同步误差,而且使能耗减小。最后,实验表明,对于无线HART网状分层网络,当观测量达到10以上时,粒子滤波算法获得的时间偏差的均方误差约是最大似然估计算法的50%,而基于Bootstrap采样的粒子滤波算法获得的时间偏差的均方误差约是最大似然估计算法的35%,仿真的结果验证了该方法的可行性和有效性。     

网络测量中高精度时间戳研究与实现

在网络测量中,时间戳记录数据包接收和发送的时间,是时延、带宽以及抖动等网络参数测量的基础.受缓存延迟、中断响应时间不确定等因素影响,软件时间戳只能达到毫秒级精度;基于GPS(global positioning system)时钟的硬件时间戳可以达到纳秒级精度,但其成本高且安装部署不方便.基于对网络测量系统中时间戳误差来源的详细分析,设计了精确时间戳测量系统,系统利用硬件产生时间戳,消除各种软件延迟影响,通过基于预测的时钟同步算法PCS(prediction-based clock synchronization)同步各测量节点时钟,达到了与GPS相当的精度.基于自行设计的千兆网卡实现了测量系统原型,测试表明各测量节点时间戳误差不超过100ns.     

基于CC2530的改进TPSN算法

针对无线传感器网络(WSNs)振动监测同步精度低的问题,提出了一种基于帧首定界符(SFD)硬件捕获机制的改进传感器网络时间同步协议(TPSN).采用硬件捕获时间戳的方式消除部分软件延迟因素,如发送延迟、访问延迟和接收延迟,在补偿时钟偏差的同时采用最小二乘法估计补偿时钟漂移.为验证算法可行性,选用链状拓扑结构,在CC2530硬件平台上进行了单跳及多跳同步测试.试验结果表明:单跳误差0.727μs,2跳误差2.65 μs,3跳误差6.833μs.相比典型TPSN,提出的算法具有较好的同步效果,可满足振动监测应用需求.     

European Community policy and the market

Abstract This paper starts with some reflections on the policy considerations and priorities which are shaping European Commission (EC) research programmes. Then it attempts to position the current projects which seek to capitalise on information and communications technologies for learning in relation to these priorities and the apparent realities of the marketplace. It concludes that while there are grounds to be optimistic about the contribution EC programmes can make to the efficiency and standard of education and training, they are still too technology driven.     

一种自适应子融合集成多分类器方法

融合集成方法已经广泛应用在模式识别领域,然而一些基分类器实时性能稳定性较差,导致多分类器融合性能差,针对上述问题本文提出了一种新的基于多分类器的子融合集成分类器系统。该方法考虑在度量层融合层次之上通过对各类基多分类器进行动态选择,票数最多的类别作为融合系统中对特征向量识别的类别,构成一种新的自适应子融合集成分类器方法。实验表明,该方法比传统的分类器以及分类融合方法识别准确率明显更高,具有更好的鲁棒性。     

Explanation and prediction: an architecture for default and abductive reasoning

Although there are many arguments that logic is an appropriate tool for artificial intelligence, there has been a perceived problem with the monotonicity of classical logic. This paper elaborates on the idea that reasoning should be viewed as theory formation where logic tells us the consequences of our assumptions. The two activities of predicting what is expected to be true and explaining observations are considered in a simple theory formation framework. Properties of each activity are discussed, along with a number of proposals as to what should be predicted or accepted as reasonable explanations. An architecture is proposed to combine explanation and prediction into one coherent framework. Algorithms used to implement the system as well as examples from a running implementation are given.     

Three Process Perspectives: Organizations, Teams, and People

This paper provides the author's personal views and perspectives on software process improvement. Starting with his first work on technology assessment in IBM over 20 years ago, Watts Humphrey describes the process improvement work he has been directly involved in. This includes the development of the early process assessment methods, the original design of the CMM, and the introduction of the Personal Software Process (PSP)^SM and Team Software Process (TSP){^SM}. In addition to describing the original motivation for this work, the author also reviews many of the problems he and his associates encountered and why they solved them the way they did. He also comments on the outstanding issues and likely directions for future work. Finally, this work has built on the experiences and contributions of many people. Mr. Humphrey only describes work that he was personally involved in and he names many of the key contributors. However, so many people have been involved in this work that a full list of the important participants would be impractical.     

基于复小波噪声方差显著修正的SAR图像去噪

提出了一种基于复小波域统计建模与噪声方差估计显著性修正相结合的合成孔径雷达(Synthetic Aperture Radar,SAR)图像斑点噪声滤波方法。该方法首先通过对数变换将乘性噪声模型转化为加性噪声模型,然后对变换后的图像进行双树复小波变换(Dualtree Complex Wavelet Transform,DCWT),并对复数小波系数的统计分布进行建模。在此先验分布的基础上,通过运用贝叶斯估计方法从含噪系数中恢复原始系数,达到滤除噪声的目的。实验结果表明该方法在去除噪声的同时保留了图像的细节信息,取得了很好的降噪效果。     

How do children do mathematics with LOGO?

Abstract  This paper considers some results of a study designed to investigate the kinds of mathematical activity undertaken by children (aged between 8 and 11) as they learned to program in LOGO. A model of learning modes is proposed, which attempts to describe the ways in which children used and acquired understanding of the programming/mathematical concepts involved. The remainder of the paper is concerned with discussing the validity and limitations of the model, and its implications for further research and curriculum development.     

Editorial: Special issue on recent advances in hybrid control theory and applications

正The demands of a rapidly advancing technology for faster and more accurate controllers have always had a strong influence on the progress of automatic control theory.In recent years control problems have been arising with increasing frequency in widely different areas,which cannot be addressed using conventional control techniques.The principal reason for this is the fact that a highly competitive economy is forcing systems to operate in regimes where     

Information for Authors

正Aim The Journals of Zhejiang University-SCIENCE(A/B/C)areedited by the international board of distinguished Chinese andforeign scientists,and are aimed to present the latest devel-opments and achievements in scientific research in China andoverseas to the world’s scientific circles,especially to stimulateand promote academic exchange between Chinese and for-eign scientists everywhere.     

Enhancing a leaf radiative transfer model to estimate concentrations and in vivo specific absorption coefficients of total carotenoids and chlorophylls a and b from single-needle reflectance and transmittance

The relative concentrations of different pigments within a leaf have significant physiological and spectral consequences. Photosynthesis, light use efficiency, mass and energy exchange, and stress response are dependent on relationships among an ensemble of pigments. This ensemble also determines the visible characteristics of a leaf, which can be measured remotely and used to quantify leaf biochemistry and structure. But current remote sensing approaches are limited in their ability to resolve individual pigments. This paper focuses on the incorporation of three pigments—chlorophyll a, chlorophyll b, and total carotenoids—into the LIBERTY leaf radiative transfer model to better understand relationships between leaf biochemical, biophysical, and spectral properties.Pinus ponderosa and Pinus jeffreyi needles were collected from three sites in the California Sierra Nevada. Hemispheric single-leaf visible reflectance and transmittance and concentrations of chlorophylls a and b and total carotenoids of fresh needles were measured. These data were input to the enhanced LIBERTY model to estimate optical and biochemical properties of pine needles. The enhanced model successfully estimated reflectance (RMSE = 0.0255, BIAS = 0.00477, RMS%E = 16.7%), had variable success estimating transmittance (RMSE = 0.0442, BIAS = 0.0294, RMS%E = 181%), and generated very good estimates of carotenoid concentrations (RMSE = 2.48 µg/cm², BIAS = 0.143 µg/cm², RMS%E = 20.4%), good estimates of chlorophyll a concentrations (RMSE = 10.7 µg/cm², BIAS = − 0.992 µg/cm², RMS%E = 21.1%), and fair estimates of chlorophyll b concentrations (RMSE = 7.49 µg/cm², BIAS = − 2.12 µg/cm², RMS%E = 43.7%). Overall root mean squared errors of reflectance, transmittance, and pigment concentration estimates were lower for the three-pigment model than for the single-pigment model. The algorithm to estimate three in vivo specific absorption coefficients is robust, although estimated values are distorted by inconsistencies in model biophysics. The capacity to invert the model from single-leaf reflectance and transmittance was added to the model so it could be coupled with vegetation canopy models to estimate canopy biochemistry from remotely sensed data.     

Mr. Twinn’s bombes

This article discusses the history and design of the special versions of the bombe key-finding machines used by Britain’s Government Code & Cypher School (GC&CS) during World War II to attack the Enigma traffic of the Abwehr (the German military intelligence service). These special bombes were based on the design of their more numerous counterparts used against the traffic of the German armed services, but differed from them in important ways that highlight the adaptability of the British bombe design, and the power and flexibility of the diagonal board. Also discussed are the changes in the Abwehr indicating system that drove the development of these machines, the ingenious ways in which they were used, and some related developments involving the bombes used by the U.S. Navy’s cryptanalytic unit (OP-20-G).