全文获取类型
收费全文 | 909篇 |
免费 | 122篇 |
国内免费 | 23篇 |
专业分类
化学 | 57篇 |
力学 | 7篇 |
综合类 | 8篇 |
数学 | 58篇 |
物理学 | 143篇 |
无线电 | 781篇 |
出版年
2023年 | 8篇 |
2022年 | 6篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 4篇 |
2018年 | 9篇 |
2017年 | 19篇 |
2016年 | 10篇 |
2015年 | 16篇 |
2014年 | 41篇 |
2013年 | 40篇 |
2012年 | 37篇 |
2011年 | 55篇 |
2010年 | 34篇 |
2009年 | 56篇 |
2008年 | 73篇 |
2007年 | 79篇 |
2006年 | 97篇 |
2005年 | 63篇 |
2004年 | 91篇 |
2003年 | 82篇 |
2002年 | 70篇 |
2001年 | 47篇 |
2000年 | 23篇 |
1999年 | 17篇 |
1998年 | 15篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 3篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1982年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有1054条查询结果,搜索用时 875 毫秒
71.
Comparative study of spectroscopic properties of Er^3+/Yb^3+-codoped tellurite glass and fibres under 980nm excitation*
下载免费PDF全文
![点击此处可从《中国物理》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A tellurite fibre of TeO_{2}-ZnO-La_{2}O_{3}-Li_{2}O glass codoped with 20000 ppm ytterbium and 5000 ppm erbium was fabricated by the suction casting and rod-in-tube technologies. The absorption spectrum of Er^{3+}/Yb^{3+} -codoped bulk glass has been measured. From the Judd-Ofelt intensity parameters, the spontaneous emission probability and radiative lifetime τ_{rad} of Er^{3+}:{}^{4}I_{13/2}→{}^{4}I_{15/2} transition for the bulk glass have been calculated. The emission fluorescence spectra and lifetimes around 1.5μm, and subsequent upconversion fluorescence in the range of 500-700nm were measured in fibres and compared with those in bulk glass. The changes in amplified spontaneous emission with fibre length and pumping power was also measured. It was found that the emission spectrum from erbium in fibres is almost twice as broad as the corresponding spectrum in bulk glass when pumped at 980nm. 相似文献
72.
The Spherical Code (SC) problem has many important applications in such fields as physics, molecular biology, signal transmission, chemistry, engineering and mathematics. This paper presents a bilevel optimization formulation of the SC problem. Based on this formulation, the concept of balanced spherical code is introduced and a new approach, the Point Balance Algorithm (PBA), is presented to search for a 1-balanced spherical code. Since an optimal solution of the SC problem (an extremal spherical code) must be a 1-balanced spherical code, PBA can be applied easily to search for an extremal spherical code. In addition, given a certain criterion, PBA can generate efficiently an approximate optimal spherical code on a sphere in the n-dimensional space
n. Some implementation issues of PBA are discussed and putative global optimal solutions of the Fekete problem in 3, 4 and 5-dimensional space are also reported. Finally, an open question about the geometry of Fekete points on the unit sphere in the 3-dimensional space is posed. 相似文献
73.
74.
MSK信号的Turbo同步 总被引:1,自引:0,他引:1
Turbo码的提出使系统的性能对同步参数变得极为敏感。文章利用Turbo同步的方法来对Turbo系统进行迭代参数估计,并用插值的方法来减少译码复杂度,最后针对MSK信号给出同步算法。仿真结果表明,在少量增加译码复杂度的情况下,Turbo同步能够使系统获得与严格同步近似的性能。 相似文献
75.
文章对TD-SCDMA系统中的Turbo编码器结构及编码算法进行了分析,给出了TD-SCDMA系统中Turbo算法在ZSP500 DSP核上的实现,并根据ZSP500 DSP核的特点对实现部分的进行了阐述。 相似文献
76.
K. Vasudevan 《Signal, Image and Video Processing》2007,1(3):239-252
This paper investigates the performance of various “turbo” receivers for serially concatenated turbo codes transmitted through
intersymbol interference (ISI) channels. Both the inner and outer codes are assumed to be recursive systematic convolutional
(RSC) codes. The optimum turbo receiver consists of an (inner) channel maximum a posteriori (MAP) decoder and a MAP decoder
for the outer code. The channel MAP decoder operates on a “supertrellis” which incorporates the channel trellis and the trellis
for the inner error-correcting code. This is referred to as the MAP receiver employing a SuperTrellis (STMAP). Since the complexity
of the supertrellis in the STMAP receiver increases exponentially with the channel length, we propose a simpler but suboptimal
receiver that employs the predictive decision feedback equalizer (PDFE). The key idea in this paper is to have the feedforward
part of the PDFE outside the iterative loop and incorporate only the feedback part inside the loop. We refer to this receiver
as the PDFE-STMAP. The complexity of the supertrellis in the PDFE-STMAP receiver depends on the inner code and the length
of the feedback part. Investigations with Proakis B, Proakis C (both channels have spectral nulls with all zeros on the unit
circle and hence cannot be converted to a minimum phase channel) and a minimum phase channel reveal that at most two feedback
taps are sufficient to get the best performance. A reduced-state STMAP (RS-STMAP) receiver is also derived which employs a
smaller supertrellis at the cost of performance. 相似文献
77.
彭小平 《太赫兹科学与电子信息学报》2007,5(4):267-270
基于Jing-Hassibi的合作分集协议和Rajan等人的普通非正交放大传送(GNAF)协议,提出了具有完全分集和最大似然译码复杂度低的分布空时编码(DSTC)。这些编码是四次群最大似然可译分布空时编码,并且功率在中继之间随时间均匀分布。仿真结果显示这种编码具有较好的性能。 相似文献
78.
基于三种调制模式下的turbo码光无线通信系统分析 总被引:1,自引:0,他引:1
从分析大气对光无线通信系统的影响出发,研究了大气信道特性,建立了大气信道信噪比和能见度的关系.为降低系统误码率,提出了基于OOK、BPSK、BPPM三种调制模式的turbo码的编码方案,推导出三种不同方案对应的turbo码迭代MAP算法.应用建立的信噪比关系,分析和计算了基于三种不同方案的系统的误码率.结果表明,基于BPSK的turbo码系统比OOK和BPPM系统的信噪比降低约3dB,且在高的信噪比情况下,BPSK系统能显著降低系统的误码率.因此,基于BPSK的turbo码方案较适合光无线通信系统. 相似文献
79.
80.
本文研究核电站稳压器抗震分析模型的建立方法。针对某堆型的稳压器,分析内部冷却剂的液面晃动效应及处理方法;对比分析节点个数、筒体壁厚、集中质量等因素对简化模型固有频率的影响,确定了稳压器抗震分析模型的简化原则。针对最终模型进行模态分析,并对计算结果进行评价。 相似文献