全文获取类型
收费全文 | 72818篇 |
免费 | 10868篇 |
国内免费 | 7545篇 |
专业分类
化学 | 38475篇 |
晶体学 | 685篇 |
力学 | 3136篇 |
综合类 | 352篇 |
数学 | 6244篇 |
物理学 | 19917篇 |
无线电 | 22422篇 |
出版年
2024年 | 174篇 |
2023年 | 1827篇 |
2022年 | 2162篇 |
2021年 | 2986篇 |
2020年 | 2970篇 |
2019年 | 2699篇 |
2018年 | 2289篇 |
2017年 | 2338篇 |
2016年 | 3157篇 |
2015年 | 3438篇 |
2014年 | 4107篇 |
2013年 | 5139篇 |
2012年 | 6183篇 |
2011年 | 6193篇 |
2010年 | 4580篇 |
2009年 | 4459篇 |
2008年 | 4784篇 |
2007年 | 4190篇 |
2006年 | 3866篇 |
2005年 | 3345篇 |
2004年 | 2503篇 |
2003年 | 2029篇 |
2002年 | 1898篇 |
2001年 | 1552篇 |
2000年 | 1380篇 |
1999年 | 1568篇 |
1998年 | 1255篇 |
1997年 | 1144篇 |
1996年 | 1119篇 |
1995年 | 985篇 |
1994年 | 847篇 |
1993年 | 718篇 |
1992年 | 601篇 |
1991年 | 520篇 |
1990年 | 417篇 |
1989年 | 302篇 |
1988年 | 222篇 |
1987年 | 195篇 |
1986年 | 196篇 |
1985年 | 173篇 |
1984年 | 115篇 |
1983年 | 112篇 |
1982年 | 79篇 |
1981年 | 54篇 |
1980年 | 45篇 |
1979年 | 27篇 |
1978年 | 28篇 |
1977年 | 29篇 |
1976年 | 29篇 |
1975年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
101.
Ultrawideband (UWB) transmissions induce pronounced frequency-selective fading effects in their multipath propagation. Multipath diversity gains can be collected to enhance performance, provided that the underlying channel can be estimated at the receiver. To this end, we develop a novel pilot waveform assisted modulation (PWAM) scheme that is tailored for UWB communications. We select our PWAM parameters by jointly optimizing channel estimation performance and information rate. The resulting transmitter design maximizes the average capacity, which is shown to be equivalent to minimizing the mean-square channel estimation error, and thereby achieves the Crame/spl acute/r-Rao lower bound. Application of PWAM to practical UWB systems is promising because it entails simple integrate-and-dump operations at the frame rate. Equally important, it offers a flexible UWB channel estimator, capable of striking desirable rate-performance tradeoffs depending on the channel coherence time. 相似文献
102.
The multihop optical network is the most appropriate solution to satisfy the increasing applications of Internet services. This paper extends the regular Kautz graph to one with multiple layers in order to produce more architectural variations. The connectivity between adjacent layers utilizes the systematic connection patterns of a regular Kautz graph. A routing algorithm based on its property is presented. Optical passive star (OPS) couplers are adopted to implement our new topologies. Three scheduling criteria that can solve the contention problem in the intermediate nodes are evaluated and compared in terms of their capability to improve the accessibility. 相似文献
103.
104.
对微波陶瓷基印制板的制造工艺流程进行了简单的介绍,对所采用的工艺技术的有效性进行了较为详细的论述。 相似文献
105.
106.
107.
Yang C.W. Fang Y.K. Lin C.S. Tsair Y.S. Chen S.M. Wang W.D. Wang M.F. Cheng J.Y. Chen C.H. Yao L.G. Chen S.C. Liang M.S. 《Electronics letters》2003,39(21):1499-1501
A novel technique to form high-K dielectric of HfSiON by doping base oxide with Hf and nitridation with NH/sub 3/, sequentially, is proposed. The HfSiON gate dielectric demonstrates excellent device performances such as only 10% degradation of saturation drain current and almost 45 times of magnitude reduction in gate leakage compared with conventional SiO/sub 2/ gate at the approximately same equivalent oxide thickness. Additionally, negligible flatband voltage shift is achieved with this technique. Time-dependent dielectric breakdown tests indicate that the lifetime of HfSiON is longer than 10 years at V/sub dd/=2 V. 相似文献
108.
109.
Ray-Guang Cheng Cun-Yi Wang Li-Hung Liao Jen-Shun Yang 《Communications Letters, IEEE》2006,10(2):123-125
This work presents a wireless token-passing protocol, named Ripple, for wireless mesh networks (WMNs). In contrast to existing random-access approaches, Ripple uses a decentralized controlled-access approach to protect nodes from unintentional packet collisions and maximize the spatial reuse. The performance of Ripple under an error-free wireless channel was investigated and the accuracy of the analysis was verified by simulation. Simulation results also indicated that Ripple achieved throughput, stability, and QoS enhancement than that of 802.11 DCF under a highly loaded situation. 相似文献
110.
有线网络行业垄断性的改变及竞争形势分析 总被引:2,自引:0,他引:2
有线网络行业既具有自然垄断性,又具有行政垄断性.但随技术进步和国家政策的调整,其自然垄断性被削弱,行政垄断也被打破.在位的有线网络企业面临着两个层面的竞争:一是基于不同视频传输方式的竞争,二是同为有线网络传输方式的竞争,而后者是直接的竞争.对有线网络行业的竞争进行了SWOT分析.最后讨论了竞争形势下在位的有线网络运营商的应对思路. 相似文献