全文获取类型
收费全文 | 60681篇 |
免费 | 9534篇 |
国内免费 | 7530篇 |
专业分类
化学 | 34087篇 |
晶体学 | 659篇 |
力学 | 2507篇 |
综合类 | 480篇 |
数学 | 4970篇 |
物理学 | 17115篇 |
无线电 | 17927篇 |
出版年
2024年 | 241篇 |
2023年 | 1427篇 |
2022年 | 1866篇 |
2021年 | 2377篇 |
2020年 | 2234篇 |
2019年 | 2215篇 |
2018年 | 1938篇 |
2017年 | 2024篇 |
2016年 | 2636篇 |
2015年 | 2909篇 |
2014年 | 3527篇 |
2013年 | 4370篇 |
2012年 | 5332篇 |
2011年 | 5353篇 |
2010年 | 4056篇 |
2009年 | 3964篇 |
2008年 | 4231篇 |
2007年 | 3892篇 |
2006年 | 3705篇 |
2005年 | 3018篇 |
2004年 | 2182篇 |
2003年 | 1881篇 |
2002年 | 1702篇 |
2001年 | 1434篇 |
2000年 | 1270篇 |
1999年 | 1237篇 |
1998年 | 939篇 |
1997年 | 866篇 |
1996年 | 760篇 |
1995年 | 719篇 |
1994年 | 680篇 |
1993年 | 509篇 |
1992年 | 438篇 |
1991年 | 403篇 |
1990年 | 344篇 |
1989年 | 233篇 |
1988年 | 183篇 |
1987年 | 153篇 |
1986年 | 133篇 |
1985年 | 103篇 |
1984年 | 70篇 |
1983年 | 47篇 |
1982年 | 48篇 |
1981年 | 27篇 |
1980年 | 22篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1971年 | 4篇 |
1957年 | 8篇 |
1936年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Optimal diagonal precoder for multiantenna communication systems 总被引:3,自引:0,他引:3
In this paper, we examine a multiantenna single-user wireless communication system fitted with a QR-based successive cancellation receiver (QR receiver). Initially, our consideration is confined to uncoded binary phase shift keying (BPSK) signals transmitted through independent and identically distributed (IID) Rayleigh fading channels and to the design of an optimum precoder for the transmitter. For minimum feeding back of the channel state information (CSI) to the transmitter from the receiver, we stipulate the precoder to be in the form of a power loading square diagonal matrix. We proceed to develop the theory for the design of this diagonal matrix based on the minimization of the lower bound of the average bit error rate (BER) of transmission. The design obtained provides substantially lower error rates than most of other existing schemes under the same environment. The corresponding gain in signal-to-noise ratio (SNR) can be several decibels. To further improve the performance, we extend the design to include an optimal detection order of the received bits using an iterative approach. This iterative process proves to have fast convergence and results in a design providing significant SNR gain. We also propose a subchannel dropping scheme for cases in which SNR is low, and when the minimum BER precoder is equipped with this scheme, its average performance can be substantially superior to the Vertical Bell Laboratories Layered Space-Time (V-BLAST) detection. We extend our design of the optimum precoder to quadrature amplitude modulation (QAM) modulation scheme and similar performance gain has been observed. 相似文献
43.
Yan WANG Rong Quan FENG 《数学学报(英文版)》2005,21(4):773-778
A Cayley map is a Cayley graph embedded in an orientable surface such that. the local rotations at every vertex are identical. In this paper, balanced regular Cayley maps for cyclic groups, dihedral groups, and generalized quaternion groups are classified. 相似文献
44.
根据某雷达系统对测量信号的快速传输要求,提出采用滑动帧的处理方法并在FPGA中实现,克服了传统帧处理难以满足快速传输要求的不足。阐述了滑动帧结构的构造思想,详述了滑动帧结构的特点及具体功能模块的实现。根据实际应用需求构造了滑动帧结构处理器,试验和应用结果表明设计可行有效,能充分满足实时数据传输的要求。 相似文献
45.
Wang Qing-Lun Zhao Bin Liao Dai-Zheng Yan Shi-Ping Cheng Peng Jiang Zong-Hui 《Transition Metal Chemistry》2003,28(3):326-330
The copper(II) complex Na2[Cu(pba)] · 6H2O has been synthesized and its structure determined. It consists of a novel two-dimensional network of [Cu(pba)]2– anions connected to each other through the sodium ion as a bridge. Spectroscopic and magnetic properties have been measured and a spin distribution calculation has been carried out with the GAUSSIAN-94. Theoretical calculations indicate the rather larger spin density on nitrogen atoms should be responsible for the satellite lines appearing beside the main hyperfine coupling signs of copper(II). The antiferromagnetic interaction may be due to the intermolecular interaction and/or different copper(II) ions through the Na+ bridge in the same layer. 相似文献
46.
47.
Briand et al. (Electron. Comm. Probab. 5 (2000) 101–117) gave a counterexample and proposition to show that given g,g-expectations usually do not satisfy Jensen's inequality for most of convex functions. This yields a natural question, under which conditions on g, do g-expectations satisfy Jensen's inequality for convex functions? In this paper, we shall deal with this question in the case that g is convex and give a necessary and sufficient condition on g under which Jensen's inequality holds for convex functions. To cite this article: Z. Chen et al., C. R. Acad. Sci. Paris, Ser. I 337 (2003). 相似文献
48.
This paper presents an investigation on a single-mode-multimode-single-mode fiber structure. A one-way guided-mode propagation analysis for the circular symmetry waveguide is employed to model the light propagation and the approximated formulations are derived and evaluated concerning the accuracy. Phase conjunction of the multimode interference within the fiber structure is revealed. A simple way to predict and analyze the spectral response of the structure is presented through the space to wavelength mapping with the derived approximated formulations. The prediction of spectral response is verified numerically and experimentally. 相似文献
49.
50.