全文获取类型
收费全文 | 57959篇 |
免费 | 7086篇 |
国内免费 | 3792篇 |
专业分类
电工技术 | 5594篇 |
技术理论 | 2篇 |
综合类 | 4455篇 |
化学工业 | 8459篇 |
金属工艺 | 3399篇 |
机械仪表 | 3817篇 |
建筑科学 | 4754篇 |
矿业工程 | 2014篇 |
能源动力 | 1677篇 |
轻工业 | 4232篇 |
水利工程 | 1577篇 |
石油天然气 | 3157篇 |
武器工业 | 720篇 |
无线电 | 7119篇 |
一般工业技术 | 6197篇 |
冶金工业 | 2263篇 |
原子能技术 | 650篇 |
自动化技术 | 8751篇 |
出版年
2024年 | 417篇 |
2023年 | 1408篇 |
2022年 | 2807篇 |
2021年 | 3864篇 |
2020年 | 2636篇 |
2019年 | 1959篇 |
2018年 | 2051篇 |
2017年 | 2302篇 |
2016年 | 2069篇 |
2015年 | 2796篇 |
2014年 | 3323篇 |
2013年 | 3827篇 |
2012年 | 4375篇 |
2011年 | 4495篇 |
2010年 | 3889篇 |
2009年 | 3693篇 |
2008年 | 3575篇 |
2007年 | 3263篇 |
2006年 | 2991篇 |
2005年 | 2633篇 |
2004年 | 1688篇 |
2003年 | 1368篇 |
2002年 | 1234篇 |
2001年 | 1050篇 |
2000年 | 984篇 |
1999年 | 919篇 |
1998年 | 628篇 |
1997年 | 509篇 |
1996年 | 488篇 |
1995年 | 381篇 |
1994年 | 315篇 |
1993年 | 209篇 |
1992年 | 179篇 |
1991年 | 130篇 |
1990年 | 80篇 |
1989年 | 73篇 |
1988年 | 55篇 |
1987年 | 35篇 |
1986年 | 38篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 17篇 |
1981年 | 6篇 |
1980年 | 16篇 |
1979年 | 12篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1959年 | 9篇 |
1951年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Jianhua Liu Jian Li 《Mobile Computing, IEEE Transactions on》2006,5(9):1133-1142
We consider improving the performance of orthogonal frequency-division multiplexing (OFDM)-based mobile broadband wireless communication (MBWC) systems. We show that a recently approved packet-based MBWC standard can cause the resulting systems to suffer from severe performance degradations for time-varying channels due to the lack of a mechanism for tracking the time-varying channels needed for coherent detection. We consider both packet design and signal processing to deal with time-varying channels. For the packet design, we segment an entire packet into multiple subpackets—with each subpacket having zero tail bits to reset the convolutional channel encoder—so that the detection/decoding result of each subpacket can be used to update the channel response for this subpacket. For signal processing, we use weighted polynomial fitting and prediction at the receiver to improve the channel tracking accuracy. Simulation results show that the coherent detection based on our new scheme can significantly outperform the commonly suggested differential modulation/detection methods. 相似文献
62.
容灾系统中存储方案选择的研究 总被引:1,自引:0,他引:1
论文首先阐述了在容灾系统中存储方案的重要性,讨论了存储方案需要考虑的衡量指标,然后介绍了主要的存储方式,根据衡量指标划分了存储方式的特性范围,最后着重研究了容灾系统中存储方案的选择流程,并使用一个实例说明容灾系统中存储方案的选择流程。 相似文献
63.
64.
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
65.
用CMOS工艺实现非接触IC卡天线的集成化设计 总被引:2,自引:0,他引:2
论述了用CMOS工艺实现非接触式IC卡天线的集成化需要考虑的各个方面,建立了集成天线的模型,给出了合理的设计方案,并通过实验验证了模型和设计方案.实验结果表明,采用片上天线完全可以提供非接触式IC卡工作所需要的能量.在频率为2 2 .5 MHz、感应强度为6×10 - 4 T的磁场中,面积为2 m m×2 mm的集成天线可以为10 kΩ的负载提供1.2 2 5 m W的能量. 相似文献
66.
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
67.
CMOS光接收机主放大器设计 总被引:1,自引:0,他引:1
利用CMOS工艺设计一种用于SDH STM 4速率级(622 Mb/s)光纤用户网的光接收机放大电路。此电路由输入/输出缓冲、主放大单元、偏置补偿电路4部分组成。通过直接耦合技术提高增益,降低功耗;利用有源电感负载提高系统带宽。采用商用SmartSpice电路仿真软件和CSMC HJ 0.6μm工艺参数对该电路进行仿真。结果表明,该电路在5 V工作电压下中频增益为81 dB,3 dB带宽为470 MHz。 相似文献
68.
大型往复式压缩机功率大、占地面积大,管道振动问题也更为突出。笔者结合实践经验,对大型往复式压缩机的配管设计进行探讨,说明了平面布置与管道防振设计的密切关系,并介绍了管道防振设计的方法和步骤,以及相对简单的复杂管系气柱固有频率的转移矩阵计算方法。 相似文献
69.
70.
Hao Wang Ying Lin Biao Chen 《Signal Processing, IEEE Transactions on》2003,51(10):2613-2623
We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms. 相似文献