全文获取类型
收费全文 | 3259篇 |
免费 | 268篇 |
国内免费 | 53篇 |
专业分类
化学 | 223篇 |
晶体学 | 5篇 |
力学 | 77篇 |
综合类 | 3篇 |
数学 | 111篇 |
物理学 | 767篇 |
无线电 | 2394篇 |
出版年
2024年 | 12篇 |
2023年 | 46篇 |
2022年 | 68篇 |
2021年 | 64篇 |
2020年 | 69篇 |
2019年 | 69篇 |
2018年 | 51篇 |
2017年 | 82篇 |
2016年 | 99篇 |
2015年 | 111篇 |
2014年 | 216篇 |
2013年 | 214篇 |
2012年 | 160篇 |
2011年 | 209篇 |
2010年 | 171篇 |
2009年 | 177篇 |
2008年 | 206篇 |
2007年 | 231篇 |
2006年 | 192篇 |
2005年 | 155篇 |
2004年 | 127篇 |
2003年 | 94篇 |
2002年 | 89篇 |
2001年 | 68篇 |
2000年 | 63篇 |
1999年 | 65篇 |
1998年 | 60篇 |
1997年 | 54篇 |
1996年 | 52篇 |
1995年 | 29篇 |
1994年 | 37篇 |
1993年 | 34篇 |
1992年 | 35篇 |
1991年 | 14篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 16篇 |
1987年 | 9篇 |
1985年 | 18篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 18篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 3篇 |
排序方式: 共有3580条查询结果,搜索用时 93 毫秒
131.
132.
133.
The IEEE 802.22 standard based on wireless Cognitive Radio (CR) is an optimal solution to resolve the inefficient spectrum utility problem. In this paper, we focus on the spectrum allocation in IEEE 802.22 mesh networks and propose a new graph-theory algorithm. The algorithm aims at two objectives: one is the sum of the allocated channel bandwidth is maximum, and the other is the number of users can be active simultaneity is maximum. In this proposed algorithm, the topology of network was modeled as a general graph and could be transformed into a weighted complete bipartite-graph by three processes. The simulations show that the presented algorithm can improve the performance of spectrum allocation. 相似文献
134.
The secondary usage of spectrum has been investigated in Cognitive Radio (CR) network to resolving the spectrum scarcity issue in wireless communication. When Primary Users (PU) who own the spectrum appear, spectrum handoff is needed to maintain the communications of Secondary Users. But the decision making of spectrum handoff is a challenge issue for CR network, because the input of decision making, which obtain through spectrum sensing, is heterogeneous and inexact. In this paper we will use fuzzy logic control theory to solve this issue and make use of new information for handoff operation: the probability of PU’s occupancy at a certain channel. Our new algorithm can make more intelligent decision compared to simple traditional spectrum handoff decision making and reduce the probability of spectrum handoff, also the performance of SU’s communication can be enhanced. 相似文献
135.
RFID技术近年来取得快速发展,成为物联网的关键技术之一.利用试验设计方法对RFID应用部署测试进行了优化,借助正交试验设计方法在多因子多水平的条件下迅速制定测试计划,了解组合测试中多因子对测试结果影响的主次因素和规律,有效地解决组合爆炸问题.还通过多元线性回归分析和最小二乘估计建立响应面方程拟合多个自变量与因变量之间的关系,分别通过正交试验、零水平点拟合试验和最速上升试验快速获得优化方案,从而为在复杂环境条件下提高RFID系统性能,预测部署效果,优化部署条件提供可靠的保证. 相似文献
136.
本文分析和比较了南京市自来水公司下属五个水厂一、二级泵房内装设的水泵变频调速装置前后的运行数据,得出了变频调速技术在给水工程应用中的实际效果及适用性。 相似文献
137.
Most mobile radio networks have been planned based on the classical cellular concept. However, alternative planning strategies
that lead to more efficient network configurations are necessary due to the fact that the traffic density is generally far
from constant throughout the service area, making necessary the relocation of base stations inside the traffic hotspots. If
the traffic is characterized in a discrete way, the optimization of base stations location resembles vector quantization,
a well-known problem in signal processing. In this paper, we use this analogy to propose a mobile radio network planning algorithm.
Simulation results show that higher trunking efficiency as well as improved frequency assignment can be obtained if an existing
mobile radio network is redesigned using the presented strategy.
Raúl Chávez-Santiago was born in Oaxaca City, Mexico. He obtained the B.Sc. degree in communications and electronics engineering in 1997 from
the School of Mechanical and Electrical Engineering (ESIME-IPN), and the M.Sc. degree in electrical engineering in 2001 from
the Center of Research and Advanced Studies of the National Polytechnic Institute (CINVESTAV-IPN) in Mexico City.
He has professional experience as Computer Networking Engineer, and Research and Teaching Assistant. Currently, he is completing
his studies toward the Ph.D. degree at Ben-Gurion University (BGU) of the Negev, Israel. In 2002, he received the URSI Young
Scientist Award. His main research interests are the optimal planning of radio communication networks, the electromagnetic
compatibility of terrestrial and satellite radio systems, and the efficient use of the radio spectrum. He is an IEEE and IEICE
student member, and a URSI radio scientist.
Avi Raymond was born in Yavne, Israel. He received the B.Sc. and M.Sc. degrees in electrical engineering from Ben-Gurion University (BGU)
of the Negev in 2000 and 2005, respectively.
He worked for two years in a communication company in the field of Telephony and Broadband Services. He also worked as a Research
Assistant in the communications laboratory at BGU. He currently works at Elta Electronic Industries Ltd. as System Engineer
and pursues studies toward a second M.Sc. degree in systems engineering at the Technion Institute of Technology, Haifa, Israel.
His research interests are the optimization algorithms for frequency assignment in cellular networks.
Vladimir Lyandres was born in 1944, in Vologda, Russia. He received the M.Sc. degree in communications engineering in 1966 and the Ph.D. degree
in communications theory in 1972 from the State University of Telecommunications (SUT), Saint Petersburg, Russia. He was with
SUT until 1990 holding a position of Senior Scientific Associate and working on research and development of digital transmission
systems, modeling of radio communication channels and algorithms of frequency planning for broadcasting and cellular systems.
Since 1991 he holds a position of Researcher at the communications laboratory and Professor at the Department of Electrical
and Computer Engineering, Ben-Gurion University (BGU) of the Negev, Israel. His research interests include synthesis of Markov
models, combinatorial optimization, and adaptive power loading. He is a senior member of IEEE and member of IEICE. 相似文献
138.
采用光传输的DWDM技术扩展运营商的网络带宽,不仅极大地利用了有限而且昂贵的光缆资源,而且增加了光纤带宽的利用率,还降低了整个系统的维护和建设成本.首先对DWDM技术进行了介绍;然后通过对达州广电城域网的业务网络带宽需求量的分析,再结合实际情况量身定制了达州广电网络的建设方案;还对达州广电整个OTN网络中DWDM设备的实际应用情况进行了分析,从网络规划、设备确定、设备安装和调试、承载业务等方面详细阐述了DWDM设备在承载网络中的实际应用. 相似文献
139.
OFDM与FM带内数模混叠同播是中国发展数字调频的一项重要候选技术,评估OFDM对FM解调的影响是选择OFDM与发射功率比的重要参考依据.在给出了三种常见调频解调算法公式的基础上,通过仿真评估了OFDM对FM解调失真度和信噪比的影响.仿真结果表明,在OFDM与FM发射功率比高于-11 dB时发生明显的捕获效应. 相似文献
140.
The state-of-the-art multi-core computer systems are based on Very Large Scale three Dimensional (3D) Integrated circuits (VLSI). In order to provide high-speed vertical data transmission in such 3D systems, efficient Through-Silicon Via (TSV) technology is critically important. In this paper, various Radio Frequency (RF) TSV designs and models are proposed. Specifically, the Cu-plug TSV with surrounding ground TSVs is used as the baseline structure. For further improvement, the dielectric coaxial and novel air-gap coaxial TSVs are introduced. Using the empirical parameters of these coaxial TSVs, the simulation results are obtained demonstrating that these coaxial RF-TSVs can provide two-order higher of cut-off frequencies than the Cu-plug TSVs. Based on these new RF-TSV technologies, we propose a novel 3D multi-core computer system as well as new architectures for manipulating the interfaces between RF and baseband circuit. Taking into consideration the scaling down of IC manufacture technologies, predictions for the performance of future generations of circuits are made. With simulation results indicating energy per bit and area per bit being reduced by 7% and 11% respectively, we can conclude that the proposed method is a worthwhile guideline for the design of future multi-core computer ICs. 相似文献