共查询到20条相似文献，搜索用时 640 毫秒
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超宽带无线电通信越来越引人注目，超宽带无线电通信技术是未来通信发展方向之一，UWB发展的趋势是要求终端短距离、低发射功率、低费用。在实际信道中，由于脉冲形状的失真性能大大下降，例如在室内环境中，由于墙壁和障碍物的存在，电磁波通常会发生反射、衍射、散射等，接收机到的信号是许多信号的叠加，信号估计较复杂。主要是对超宽带信道进行估计和接收机的性能检测，并用仿真加以证实。 相似文献
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对喜马拉雅和横断山地区特有的水龙骨科的宽带蕨属Platygyria Ching&S.K.Wu植物进行了形态学、分类学、生态和植物地理学的观察与分析，探讨了该属植物的系统位置、亲缘关系和生态适应性，推测宽带蕨属是随着喜马拉雅山脉的隆升，由瓦韦属Lepisorus（J.Sm.）Ching网眼瓦韦群植物适应高寒生态环境演化而来的新生类群。确定宽带蕨属有5种，即川西宽带蕨P.soulieana（Christ）X.C.Zhang&Q.R.Liu、多变宽带蕨P.variabilis Ching&S.K.Wu、耳基宽带 相似文献
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设计了一种基于模拟器件运算放大器实现的宽带可控增益放大器.该装置的信号放大由两片可控增益运算放大器VCA822完成.放大器工作频带宽,最宽可至130MHZ以上,实现宽带放大;放大器噪声小.动态范围宽.采用MGC和AGC电路实现增益的手动控制和自动控制,手动增益调节范围大于80dB,自动增益控制太子40dB.若为小信号状态,则控制调节范围更宽.放大器输入输出阻抗均为50Ω.可方便和前后级电路匹配. 相似文献
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近几年来,在长距离超宽带(UWB)无线通信方面受到了高度重视,但由于传输距离变长后,出现一些难以克服的困难。现在2000m以上的长距离超宽带无线通信系统称为了超宽带通信中的热点和难点。本文系统地构建检测色噪声下超宽带微弱信号的方案。用多线性调频信号作为超宽带无线通信的导频信号,并假定通信过程中收到色噪声的干扰,首先对其进行多尺度离散小波(包)变换,将每个尺度的接收信号用隐马尔可夫模型(HMM)进行描述,并结合分数阶傅立叶变换(FrFT)处理多分量线调频信号的思想和原理,从而实现色噪声干扰下的时间频率双弥散超宽带弱接收信号的检测与参数估计。 相似文献
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X波段宽带低噪声放大器设计 总被引：1，自引：0，他引：1
介绍与实现了一种X波段宽带低噪声放大器（LNA）。该放大器选用NEC公司的低噪声放大管NE3210S01（HJFET）,采用微带分支线匹配结构和两级级联的方式。利用ADS软件进行设计、优化和仿真。最后研制的放大器在10-13GHz范围内增益为（21.5±1）dB,噪声系数小于2 dB,驻波比小于1.8。 相似文献
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直序列扩频与跳频扩频在实际中最常用的两种扩频信号。文章介绍了基于跳频（FH）信号通信系统抗干扰的基本原理。借助MATLAB仿真软件,采用MFSK调制方式,在宽带干扰、不同比例的部分频带干扰条件下,对不同跳频速率的系统性能进行仿真。通过分析仿真结果,较好的说明了跳频信号通信系统的优点,最后给出了提高跳频系统抗干扰能力的相应措施。 相似文献
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MB-OFDM联盟提出了一种有关超宽带（UWB）无线个域网IEEE802．15．3a的标准提议,该提议采用了正交频分（0FDM）技术。与窄带OFDM不同的是,UWB-OFDM信号频谱中各予载波间有一定的间隙,正是基于这一特点,文章采用一种称为是N音∑-Δ调制技术,在量化噪声频谱中插入N个零点,使得UWB-OFDM信号谱中能量大的部分落在量化噪声谱中零点的位置,这样在接收端用一个合适的梳状陷波滤波器将噪声虑除,能大大提高系统性能。 相似文献
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A research group led by Prof. ZHAI Qiwei from the Institute for Nutritional Sciences under the CAS Shanghai Institutes for Biological Sciences has discovered that even relatively low doses of resveratrol-a chemical found in the skins of red grapes in red wine-can improve the sensitivity of mice to the hormone insulin according to a report in the October issue of Cell Metabolism.…… 《中国科学院院刊(英文版)》2008,22(1):14-15
Aresearch group led by Prof. ZHAI Qiwei from the Institute for Nutritional Sciences under the CAS Shanghai Institutes for Biological Sciences has discovered that even relatively low doses of resveratrol--a chemical found in the skins of red grapes and in red wine--can improve the sensitivity of mice to the hormone insulin, according to a report in the October,2007 issue of Cell Metabolism. As insulin resistance is often characterized as the most critical factor contributing to the development of Type 2 diabetes, the findings“provide a potential new therapeutic approach for preventing or treating” both conditions, the researchers said. 相似文献
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Marjorie Armstrong-Stassen Margaret Landstrom Ramona Lumpkin 《The Information Society》1998,14(2):153-164
This study examined how students who had no prior experience with videoconferencing would react to the use of videoconferencing as an instructional medium. Students enrolled in seven different courses completed a questionnaire at the beginning of the semester and again at the end of the semester. Students at the origination and remote sites did not differ in their reactions toward videoconferencing but there was a significant difference for gender. Women reacted less favorably to videoconferencing. Compared to the beginning of the semester, students reported significantly less positive attitudes toward taking a course through videoconferencing at the end of the semester. There were no significant differences in students' attitudes toward videoconferencing across courses at the beginning of the semester but there were significant differences across the courses at the end of the semester. The results suggest the need for better preparation for both students and instructors. 相似文献
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A joint study by Prof. ZHANG Zhibin from the CAS Institute of Zoology his co-workers from Norway US Swiss have indicated that historical outbreaks of migratory locusts in China were associated with cold spells suggesting that China's projected climate warming could decrease the pest's numbers. The study was published in Proceedings of the National Academy of Sciences on September .…… 《中国科学院院刊(英文版)》2008,22(1):6-7
Ajoint study by Prof. ZHANG Zhibin from the CAS Institute of Zoology and his co-workers from Norway, US and Swiss have indicated that historical outbreaks of migratory locusts in China were associated with cold spells, suggesting that China＇s projected climate warming could decrease the pest＇s numbers. The study was published in Proceedings of theNational Academy of Sciences on 17 September, 2007. 相似文献
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Active biological molecules functional structures can be fabricated into a bio-mimetic system by using molecular assembly method. Such materials can be used for the drug delivery disease diagnosis therapy new nanodevice construction.…… 《中国科学院院刊(英文版)》2008,22(1):22-22
Active biological molecules and functional structures can be fabricated into a bio-mimetic system by using molecular assembly method. Such materials can be used for the drug delivery, disease diagnosis and therapy, and new nanodevice construction. 相似文献
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Caroline Haythornthwaite 《The Information Society》2001,17(3):211-226
A computer-mediated group is a complex entity whose members exchange many types of information via multiple means of communication in pursuit of goals specific to their environment. Over time, they coordinate technical features of media with locally enacted use to achieve a viable working arrangement. To explore this complex interaction, a case study is presented of the social networks of interactions and media use among members of a class of computer-supported distance learners. Results show how group structures associated with project teams dominated who communicated with whom, about what, and via which media over the term, and how media came to occupy their own communication niches: Webboard for diffuse class-wide communication; Internet Relay Chat more to named others but still for general communication across the class; and e-mail primarily for intrateam communication. Face-to-face interaction, occurring only during a short on-campus session, appears to have had a catalytic effect on social and emotional exchanges. Results suggest the need to structure exchanges to balance class-wide sharing of ideas with subgroup interactions that facilitate project completion, and to provide media that support these two modes of interaction. 相似文献
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《中国科学院院刊(英文版)》2008,22(1):2-3
CAS should stick to the principle of rendering service to, and giving impetus for, the development of China＇s science enterprise by making S＆T innovations, said CAS President LU Yongxiang. The CAS president made the remarks in a recent talk to communicate the gist of the winter session of the Party＇s Leading-member Group at CAS, which was held from 7 to 11 January in Beijing. 相似文献
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Electronic data interchange (EDI) provides means for interorganizational communication, creates network externalities, requires an advanced information technology (IT) infrastructure, and relies on standards. In the diffusion of such innovations, institutional involvement is imperative. Such institutions contain governmental agencies, national and global standardization organizations, local government, and nonprofit private organizations like industry associations. The last type of organizations we call intermediating institutions. They intermediate or coordinate ("inscribe") the activities of a group of would-be adopters. Unfortunately, little is known of how these organizations shape the EDI diffusion trajectory. In this article we examine one specific type of intermediating organizations?industry associations?and how they advanced the EDI diffusion process in the grocery sectors of Hong Kong, Denmark and Finland. We identify six institutional measures, placed into a matrix formed by the mode of involvement (influence vs. regulation) and the type of diffusion force (supply push vs. demand pull), that can be mobilized to further the EDI diffusion. Industry associations were found to be active users of all these measures to varying degrees. Their role was critical especially in knowledge building, knowledge deployment, and standard setting. Furthermore, institutional involvement varied due to policy and cultural contingencies and power dependencies. 相似文献
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《中国科学院院刊(英文版)》2008,22(1)
With great care, Dr. ZHOU Zhonghe takes out a package wrapped by cotton tissue from a drawer and says: "This is the gem of our collections: the fossil of a bird that lived 125 million years ago!" Then, pointing at a tiny mound, he explains: "Look, this is the claw and that is the head. It was in the egg shell and ready to hatch ... The species fell into a family of waterside inhabitants." 相似文献
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Leah A. Lievrouw 《The Information Society》2000,16(2):155-159
This essay focuses on universal service and the Internet as means to support social and political participation. The emphasis on access to telecommunications systems in conventional approaches to universal service is contrasted with access to content. A model of the information environment is described that accounts for the roles of content and conduit, both of which are necessary conditions to achieve true access. A method is outlined for employing information indicators to observe or measure the information environment. 相似文献
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Puay Tang 《The Information Society》1998,14(1):19-31
The increasing prospects of digital piracy has prompted the perceived need by electronic publishers to adopt technical systems of protection, and governments to reform their copyright laws. This article is a preliminary study of the management of intellectual property by electronic publishers, defined as those involved in the production of online databases, and CD-ROMs. It focuses on three main issues: (1) how electronic publishers view the increasing threat of piracy; (2) the methods of protection employed to protect intellectual property in digital format; and (3) the importance of technological protection of intellectual property in electronic publications. The analysis is based on a sample of 23 UK electronic publishers. The interviews revealed an interesting assortment of protection methods and did not show that technological protection was a preferred way. Instead, the means of protection, in addition to copyright law, comprised niche markets, pricing, trust, bad publicity, and nontechnical and technical means. 相似文献