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51.
YangHongming CaoZhigang 《电子科学学刊(英文版)》2005,22(1):18-24
Rain attenuation is the major problem for Ka-band satellite communications, and the fading due to rain can be well described by a lognormally distributed, first-order autoregressive model. Forward Error-control Coding (FEC) techniques can be used to reduce the effect of the rain attenuation, but the use of FEC causes a reduction in the bandwidth efficiency. In order to increase the bandwidth efficiency as well as maintain high link availability, an Adaptive Forward Error-control Coding (AFEC) scheme with rain fading prediction is proposed and analyzed in this paper. The results show that AFEC offers a good trade-off between link availability and bandwidth efficiency. 相似文献
52.
论述了利用已有地震资料解释成果,根据地质任务建立二维地质模型,在此基础上进行射线追踪、模拟出单炮地震记录和自激自收剖面,从而对观测系统的最大炮检距、道间距等参数进行论证的一些实用方法,这些方法对地震资料采集具有很好的指导作用。在复杂地质构造地震勘探过程中,利用二维地震模型正演技术论证二维观测系统设计能够有效地进行质量技术监控,并能及时指导野外生产,同时验证了以往解释的正确性。 相似文献
53.
文中采用正向递推水量平衡法对重点灌区渠道水利用系数进行了计算,并在运用实测结果和计算结果相结合对各种影响因素进行综合分析的基础上,建立了渠道防渗长度比和渠系水利用系数之间的关系,由此推出全省不同地区、各类灌区及全省的渠系水利用系数,为解决全省灌区渠系渗漏损失的估算提供了新的技术路线。 相似文献
54.
55.
Wireless sensor networks are energy constrained since sensors operate with limited battery capacity. Thus, energy consumption
is one of the most critical issues in the design of routing protocols. In addition, the link quality needs to be taken into
account in the route decision for a wireless multihop network in order to efficiently exploit the inherent spatial diversity.
In this paper we consider energy-aware and link-adaptive routing strategies for UWB (Ultra Wide Band) sensor networks. We
utilize the ranging capabilities offered by UWB and employ adaptive modulation to take advantage of favorable link conditions.
Different routing metrics are proposed based on the availability of sensor node’s location, link quality and next hop battery
capacity information. These routing metrics integrate the measure of next hop remaining battery capacity with the throughput
performance measures, Maximum Forward Progress (MFP) or Maximum Information Progress (MIP). The effectiveness of these metrics
is evaluated in different simulation scenarios in terms of network throughput and lifetime for both random and grid sensor
network topologies.
Jinghao Xuis a D.Sc. candidate of the Department of Electrical and Computer Engineering at the George Washington University. He received
his B.Sc. and M.S. degrees in telecommunication and electrical engineering from the Tianjin University, China, in 1993 and
1996, respectively. From 1996 to 2001, he was with Research Institute of Telecommunication Transmission of Ministry of Information
Industry, Beijing, China, and China Wireless Telecommunication Standards group, where he was involved in the standardization
of IMT-2000 communication systems for China. His research interests include performance evaluation and modeling of wireless
ad hoc networks, Ultra Wide Band systems and multiuser detection techniques.
Bojan M. Peric received the B.S. degree in electrical engineering from the University of Belgrade, Belgrade, Serbia and Montenegro in 2001.
He is currently working toward the D.Sc. degree in electrical engineering at the George Washington University, Washington,
DC. His research interests include wireless communications, with an emphasis on ad hoc networks.
Branimir R. Vojcic is a professor in, and a past Chairman of, the Department of Electrical and Computer Engineering at the George Washington
University. He has received his Dipl. Ing., M.Sc. and D.Sc. degrees from the University of Belgrade in Serbia and Montenegro
in 1980, 1986 and 1989, respectively. His current research interests are in the areas of communication theory, performance
evaluation and modeling mobile and wireless networks, mobile internet, code division multiple access, multiuser detection,
adaptive antenna arrays, space-time coding and ad-hoc networks. He has also been an industry consultant and has published
and lectured extensively in these areas. He co-authored the book: The cdma2000 System for Mobile Communications. Dr Vojcic
received NSF CAREER Award in 1995. He is a Senior Member of IEEE, was an Associate Editor for IEEE Communications Letters
and is presently an Associate Editor for Journal on Communications and Networks. 相似文献
56.
Real-time multimedia applications have to use forward error correction (FEC) anderror concealment techniques to cope with
losses in today’s best-effort Internet. The efficiency of these solutions is known however to depend on the correlation between
losses in the media stream. In this paper we investigate how the packet size distribution affects the packet loss process,
that is, the distribution of the number of lost packets in a block, the related FEC performance and the average loss run length.
We present mathematical models for the loss process of the MMPP+M/D/1/K and the MMPP+M/M/1/K queues; we validate the models
via simulations, and compare the results to simulation results with an MPEG-4 coded video trace. We conclude that the deterministic
packet size distribution (PSD) not only results in lower stationary loss probability than the exponential one, but also gives
a less correlated loss process, both at a particular average link load and at a particular stationary loss probability as
seen by the media stream.Our results show that for applications that can only measure the packet loss probability, the effects
of the PSD on FEC performance are higher in access networks, where a single multimedia stream might affect the multiplexing
behavior. Our results show that the effects of the PSD on FEC performance are higher in access networks, where a single multimedia
stream might affect the multiplexing behavior and thus can improve the queuing performance by decreasing the variance of its
PSD. 相似文献
57.
提出一种基于IOCP实现底层封装基础上的具有高性能的网络转发模块的设计方案。该方案基于Win32平台上并通过c语言开发实现。本文在传统线程池处理并发的基础上,采用多路并发,多线程技术,并通过了严格的压力和性能测试。本文同时提出关联容器映射的方法,保证了数据的准确转发。实验结果表明,该模块在保证准确转发交互的同时,能够支持海量并发连接,且具有较高的数据吞吐量,同时在实际项目应用中也取得了良好的表现。 相似文献
58.
An increasingly widespread interest in developing fully adaptable e-learning systems (e.g., intelligent tutoring systems) has led to the development of a wide range of adaptive processes and techniques. In particular, advances in these systems are based on optimization for each user's learning style and characteristics, to enable a personalized learning experience. Current techniques are aimed at using a learner's personality traits and its effect on learning preferences to improve both the initial learning experience and the information retained (e.g., top-down or bottom-up learning organization). This study empirically tested the relationship between a learner's personality traits, analyzed the effects of these traits on learning preferences, and suggested design guidelines for adaptive learning systems. Two controlled experiments were carried out in a computer-based learning session. Our first experiment showed a significant difference in the learning performance of participants who were identified as introverts vs. those who were identified as being extroverts, according to the MBTI scale. As the distinction between extroverted personality types vs. introverted personality types showed the strongest correlation in terms of different learning styles, we used this criteria in our second experiment to determine whether design guidelines for appropriate content organization could reinforce the aforementioned correlation between personality type and learning experience. 相似文献
59.
60.
S. Muralidharan V. Saraswathy A. Madhavamayandi K. Thangavel N. Palaniswamy 《Electrochimica acta》2008,53(24):7248-7254
Embeddable potential sensor based on MnO2 was assembled and characterised in concrete. The stability, reversibility, polarisability and impedance characteristics have been studied with respect to known reference. The corrosion performance of reinforced steel with respect to MnO2 sensor was monitored by different electrochemical techniques. Reversibility of MnO2 sensor indicated that difference of ±5 mV between the forward and reverse scan indicates the better reversibility characteristics in concrete. The rebar potentials (ER) of steel with respect to MnO2 are −315 and −525 mV for passive and active conditions of rebar in concrete. The corrosion current from potentiodynamic polarisation and Rct from a.c. impedance technique clearly differentiated the behaviour of steel embedded in chloride contaminated concrete (active condition) from uncontaminated concrete (passive condition) with respect to MnO2 sensor. All these studies revealed that corrosion monitoring of steel in concrete using embedded MnO2 as a better potential sensor for steel in concrete. In addition it is easy to fabricate for amenable miniaturisation, varied configuration as demanded for corrosion monitoring in concrete structures. 相似文献