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排序方式: 共有137条查询结果,搜索用时 203 毫秒
1.
Prasad N. Varanasi M. K. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2006,52(12):5284-5296
The connection between the average codeword or frame error probability (FEP) of space-time codes and the outage probability over general block-fading multiple-input multiple-output (MIMO) channels is established. Three archetypal problems are considered under general fading distributions in a single framework wherein the receiver has channel state information whereas the transmitter knows a) the fading distribution but not the channel realization b) the channel realization but must follow a short term (per codeword) average power constraint, and c) the channel realization but is constrained only by a long-term average power constraint. Three telescoping sets of space-time codes are defined for a given rate and it is shown that average FEPs arbitrarily close to the respective outage probabilities for each of the three cases a)-c) can be achieved by codes in each set for sufficiently large frame lengths. For the smallest set among the three which contains codes with a spectral norm constraint that is stricter than the average or maximum energy constraints commonly assumed, firm sphere-packing lower bounds on the FEP are obtained, and, consequently, strong converse theorems are proved which assert that the respective outage probabilities also represent the best achievable FEP in the large frame-length limit. Moreover, the set of spectral norm constrained codes are also shown to be large enough to contain universal codes that can communicate reliably over any channel realization for which the mutual information exceeds the information rate of the code 相似文献
2.
We study multiuser receiver design and analysis for synchronous code-division multiple-access (CDMA) channels with time-varying Rayleigh fading. Starting from an error probability criterion, we first derive a near-optimum receiver for this channel that admits a detector-estimator decomposition, has certain asymptotic optimality properties and a complexity which is independent of the length of the observation interval. The performance of this detector is analytically characterized and contrasted with that of the optimal multiuser detector for the time-invariant (or static) CDMA Rayleigh-fading channel when it is implemented over the time-varying channel. Notable among our conclusions is the fact that, unlike the static channel multiuser detector, the time-varying channel detector is able to withstand not only the estimated interference from the other system users, but also, the residual interference (that cannot be estimated) arising out of imperfect estimation of the interferer fading parameters. Using estimation error covariance information, this detector shows flexibility in accommodating a wide range of interferer fading conditions 相似文献
3.
4.
We consider a structural approach to the consensus building problem in multi‐group multi‐layer (MGML) distributed sensor networks (DSNs) common in many natural and engineering applications. From among the possible network structures, we focus on bipartite graph structure as it represents a typical MGML structure and has a wide applicability in the real world. We establish exact conditions for consensus and derive a precise relationship between the consensus value and the degree distribution of nodes in a bipartite MGML DSN. We also demonstrate that for subclasses of connectivity patterns, convergence time and simple characteristics of network topology can be captured by explicit algebra. Direct inference of the convergence behavior of consensus strategies from MGML DSN structure is the main contribution of this paper. The insights gained from our analysis facilitate the design and development of large‐scale DSNs that meet specific performance criteria. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
5.
Varanasi S. R. Murthy Koichi Kawahara Yasuko Saito Takashi Matsuzaki Tadao Watanabe 《Journal of the American Ceramic Society》2005,88(10):2902-2907
The orientation and grain boundary microstructure of alumina in reactive metal penetration Al/Al2 O3 composites are studied using orientation imaging microscopy and the results are compared with those of sintered polycrystalline Al2 O3 . The interconnected Al2 O3 in the composite material is separated by Σ3 boundaries (twins) with a 60° rotation around the [0001] direction. A high frequency (∼100%) of Σ3 coincidence boundaries in composite alumina is remarkable since only ∼12% of boundaries in a sintered polycrystalline Al2 O3 are of special nature. The coincidence boundaries in the in situ alumina grow in a coherent and faceted manner. 相似文献
6.
Vidhya Chakrapani Jyothish Thangala Mahendra K. Sunkara 《International Journal of Hydrogen Energy》2009,34(22):9050-9059
Tungsten oxide (WO3) nanowire array samples were nitrided in a NH3 atmosphere to get complete conversion to tungsten nitride (W2N) nanowires. UV–vis absorption spectroscopy shows that the band gap of WO3 reduced from 2.9 eV to 2.2 eV after nitridation to W2N. Photoelectrochemical properties of both WO3 and W2N nanowire array electrodes were investigated. WO3 nanowire arrays show maximum incident photon-to-current conversion efficiency of 85% at 370 nm at 1.2 V vs. SCE. The high quantum efficiency is attributed to the nanowire architecture which ensures efficient light absorption and charge transport. The nanowire arrays were stable even up to 8 h of continuous gas evolution. W2N nanowire arrays showed good photoactivity even at moderate bias. However, the pure W2N electrodes were unstable with respect to photocorrosion. The mixed phase W2N–WO3 nanowires showed improvement in stability compared to pure W2N nanowire arrays. 相似文献
7.
M. Lakshmi Kantam T. Ramani L. Chakrapani B.M. Choudary 《Catalysis communications》2009,10(4):370-372
Nanocrystalline copper(II) oxide catalyzed three-component coupling of aromatic amines, β-ketoesters and cinnamaldehyde to afford the corresponding 1,4-dihydropyridines in moderate to good yields under mild conditions is described. The catalyst can be recovered and reused for four cycles with almost consistent activity. 相似文献
8.
Carbon nanotubes (CNTs) were grown on diamond-coated Si substrates and free-standing diamond wafers to develop efficient thermal interface materials for thermal management applications. High-quality, translucent, free-standing diamond substrates were processed in a 5 kW microwave plasma chemical vapor deposition (CVD) system using CH4 as precursor. Ni and Ni-9%W-1.5%Fe catalyst islands were deposited to nucleate CNTs directly onto the diamond substrates. Randomly-oriented multi-walled CNTs forming a mat of ∼5 μm thickness and consisting of ∼20 nm diameter tubes were observed to grow in a thermal CVD system using C2H2 as precursor. Transmission electron microscopy and Raman analyses confirmed the presence of high-quality CNTs on diamond showing a D/G peak ratio of 0.2-0.3 in Raman spectra. 相似文献
9.
Venkatasubbaiah Krishnan Ravindranathan Bindu Vadapalli Chandrasekhar Varanasi S. R. Murthy 《Journal of the American Ceramic Society》2002,85(2):504-506
A new method for chemically cross-linking polysilastyrene using divinylbenzene as the cross-linking agent is reported. The procedure involves a single-step synthesis using the alkali-metal sodium to promote the polymerization of dimethyldichlorsilane in the presence of the comonomers phenylmethyldichlorosilane and divinylbenzene. The cross-linked polymer can be readily converted to β-SiC on pyrolysis at 1500°C. The β-SiC obtained by this procedure is nanocrystalline and has a grain-size distribution of 8–20 nm. 相似文献
10.
Guess T. Varanasi M.K. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2005,51(1):173-187
A framework is presented that allows a number of known results relating feedback equalization, linear prediction, and mutual information to be easily understood. A lossless, additive decomposition of mutual information in a general class of Gaussian channels is introduced and shown to produce an information-preserving canonical decision-feedback receiver. The approach is applied to intersymbol interference (ISI) channels to derive the well-known minimum mean-square error (MMSE) decision-feedback equalizer (DFE). When applied to the synchronous code-division multiple-access (CDMA) channel, the result is the MMSE (or signal-to-interference ratio (SIR) maximizing) decision-feedback detector, which is shown to achieve the channel sum-capacity at the vertices of the capacity region. Finally, in the case of the asynchronous CDMA channel we are able to give new connections between information theory, decision-feedback receivers, and structured factorizations of multivariate spectra. 相似文献