全文获取类型
收费全文 | 388836篇 |
免费 | 29790篇 |
国内免费 | 16365篇 |
学科分类
工业技术 | 434991篇 |
出版年
2024年 | 1615篇 |
2023年 | 6077篇 |
2022年 | 10743篇 |
2021年 | 15124篇 |
2020年 | 11752篇 |
2019年 | 9454篇 |
2018年 | 10661篇 |
2017年 | 12102篇 |
2016年 | 10904篇 |
2015年 | 14593篇 |
2014年 | 18788篇 |
2013年 | 22645篇 |
2012年 | 24379篇 |
2011年 | 26878篇 |
2010年 | 23253篇 |
2009年 | 22315篇 |
2008年 | 22034篇 |
2007年 | 20996篇 |
2006年 | 21654篇 |
2005年 | 18834篇 |
2004年 | 12325篇 |
2003年 | 10850篇 |
2002年 | 10176篇 |
2001年 | 9273篇 |
2000年 | 9325篇 |
1999年 | 10543篇 |
1998年 | 8706篇 |
1997年 | 7209篇 |
1996年 | 6735篇 |
1995年 | 5635篇 |
1994年 | 4610篇 |
1993年 | 3253篇 |
1992年 | 2641篇 |
1991年 | 2065篇 |
1990年 | 1579篇 |
1989年 | 1302篇 |
1988年 | 1068篇 |
1987年 | 718篇 |
1986年 | 553篇 |
1985年 | 361篇 |
1984年 | 256篇 |
1983年 | 209篇 |
1982年 | 207篇 |
1981年 | 137篇 |
1980年 | 142篇 |
1979年 | 73篇 |
1978年 | 33篇 |
1977年 | 38篇 |
1976年 | 56篇 |
1975年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Biao Chen Hao Wang 《Signal Processing, IEEE Transactions on》2004,52(7):2047-2057
Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed. 相似文献
42.
43.
44.
By exploiting a general cyclostationary (CS) statistics-based framework, this letter develops a rigorous and unified asymptotic (large sample) performance analysis setup for a class of blind feedforward timing epoch estimators for linear modulations transmitted through time nonselective flat-fading channels. Within the proposed CS framework, it is shown that several estimators proposed in the literature can be asymptotically interpreted as maximum likelihood (ML) estimators applied on a (sub)set of the second- (and/or higher) order statistics of the received signal. The asymptotic variance of these ML estimators is established in closed-form expression and compared with the modified Crame/spl acute/r-Rao bound. It is shown that the timing estimator proposed by Oerder and Meyr achieves asymptotically the best performance in the class of estimators which exploit all the second-order statistics of the received signal, and its performance is insensitive to oversampling rates P as long as P/spl ges/3. Further, an asymptotically best consistent estimator, which achieves the lowest asymptotic variance among all the possible estimators that can be derived by exploiting jointly the second- and fourth-order statistics of the received signal, is also proposed. 相似文献
45.
Data mining extracts implicit, previously unknown, and potentially useful information from databases. Many approaches have been proposed to extract information, and one of the most important ones is finding association rules. Although a large amount of research has been devoted to this subject, none of it finds association rules from directed acyclic graph (DAG) data. Without such a mining method, the hidden knowledge, if any, cannot be discovered from the databases storing DAG data such as family genealogy profiles, product structures, XML documents, task precedence relations, and course structures. In this article, we define a new kind of association rule in DAG databases called the predecessor–successor rule, where a node x is a predecessor of another node y if we can find a path in DAG where x appears before y. The predecessor–successor rules enable us to observe how the characteristics of the predecessors influence the successors. An approach containing four stages is proposed to discover the predecessor–successor rules. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 621–637, 2006. 相似文献
46.
47.
论文提出了一种混沌通信系统的噪音衰减算法,该算法利用混沌同步现象在接收端获得正确的噪音估计值,从而从接收信号中滤除噪音恢复出正确的信号。通过数值仿真试验表明该方法是可行的。 相似文献
48.
49.
基于模式理论光栅椭偏参数反演的数值模拟 总被引:5,自引:2,他引:3
将一种广泛用于求解系统优化问题的方法——正单纯形法,求解光栅的椭偏方程。首先,利用求解光栅的傅立叶模式理论对TE和TM波的复反射系数进行求解。然后计算出其相应的椭偏参数(△,Ψ),并在该值的基础上加入不同偏差的随机高斯噪声,将加入噪声后的值(△m,Ψm)作为模拟测量值。最后使用优化算法进行反演。通过对几种常用面形光栅椭偏参数的数值模拟,一方面表明傅立叶模式理论计算光栅的椭偏参数不仅精度高。而且速度快;另一方面表明利用正单纯形法得到的光栅参数值很接近于正演时假设的参数值,从而从理论上证明了利用椭偏法测量光栅各种光学参数的可行性。 相似文献
50.