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排序方式: 共有8848条查询结果,搜索用时 15 毫秒
91.
Ekrem Güneş Bernadette Landschreiber David Hartung Matthias T. Elm Christian Rohner Peter J. Klar Sabine Schlecht 《Journal of Electronic Materials》2014,43(6):2127-2133
Bismuth nanotubes have been synthesized and successfully included in Bi1?x Sb x nanoalloys to form composite structures. The nanotubes were synthesized by transformation of a β-BiI precursor with n-BuLi solution leading to tubular bismuth structures. The Bi1?x Sb x nanoalloys were produced by ball milling. Three series of composite structures were synthesized by including different fractions (0 wt.%, 3 wt.%, 5 wt.%) of nanotubes in nanoalloys of different composition x. Investigation of thermoelectric and structural properties revealed a decrease of the thermal conductivity of up to 40% for the composites in comparison with alloys without nanotube inclusions. This effect can be attributed to enhanced phonon scattering. Seebeck coefficients and electrical conductivities were both slightly enhanced in the composite series with 3 wt.% nanotube inclusions, leading to enhancement of $$ ZT \ \left(ZT=\frac {(S^2 \sigma)}{\kappa}\,{ {T}}\right) $$ throughout the series compared with the nanoalloy series without nanotube inclusions. 相似文献
92.
Christian Callegari Rosario G. Garroppo Stefano Giordano Michele Pagano 《International Journal of Communication Systems》2009,22(8):1023-1044
The deployment of multimedia over IP (MoIP), and in particular voice over IP services, requires to solve new security issues they introduce, before completely exploiting the great opportunities they offer to telecommunication market. Furthermore, the implementation of various security measures can cause a marked deterioration in quality of service, which is fundamental to the operation of an MoIP network that meets users' quality expectations. In particular, because of the time‐critical nature of MoIP and its low tolerance for disruption and packet loss, many security measures implemented in traditional data networks are simply not applicable in their current form. This paper presents an analysis of the security options of Session Initiation Protocol‐ (SIP)‐based MoIP architecture aimed at evaluating their impact on delay. In particular, each security option is analyzed in terms of clock cycles needed to perform the related operations. This parameter could be used to estimate the delay introduced by the security mechanisms. Moreover the paper proposes a rigorous definition of five security profiles, which provide different levels of security to a MoIP system. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
93.
Diketopyrrolopyrrole Organic Thin‐Film Transistors: Impact of Alkyl Substituents and Tolerance of Ethylhexyl Stereoisomers 下载免费PDF全文
Matthias Stolte Sabin‐Lucian Suraru Patricia Diemer Tao He Christian Burschka Ute Zschieschang Hagen Klauk Frank Würthner 《Advanced functional materials》2016,26(41):7415-7422
Bis(thiophen‐2‐yl)‐diketopyrrolopyrrole (DPP) dyes bearing various alkyl substituents at the amide positions (n‐butyl, n‐pentyl, n‐hexyl, n‐heptyl, n‐octyl, 2‐ethylhexyl) and chlorine (Cl), bromine (Br), or cyano (CN) substituents at the thiophene positions have been synthesized and investigated with regard to their molecular and semiconducting properties. Intense absorption, strong fluorescence, and reversible oxidation and reduction processes are common to all of these dyes. Their characterization as organic semiconductors in vacuum‐processed thin‐film transistors reveals p‐channel operation with field‐effect mobilities ranging from 0.01 to 0.7 cm2 V?1 s?1. The highest mobility is found for the DPP dyes bearing the 2‐ethylhexyl substituents, which is surprising, considering that as a result of the chiral substituents, this material is a mixture of (R,R), (S,S), and (R,S) stereoisomers. The high carrier mobility in the films of the DPPs bearing stereoisomerically inhomogeneous ethylhexyl groups is rationalized here by single‐crystal X‐ray diffraction (XRD) analysis in combination with XRD and atomic force microscopy studies on thin films, which reveal the presence of slightly different 2D layer arrangements for the n‐alkyl and the 2‐ethylhexyl derivatives. For the cyano‐substituted DPPs possessing the lowest LUMO levels, ambipolar transport characteristics are observed. 相似文献
94.
A multisite, vibrotactile sensory substitution system, that could be used in conjunction with artificial touch sensors in multifingered prostheses, to deliver sensory feedback to upper limb amputees is presented. The system is based on a low cost/power/size smart architecture of off-the-shelf miniaturized vibration motors; the main novelty is that it is able to generate stimuli where both vibration amplitude and frequency as well as beat interference can be modulated. This paper is aimed at evaluating this system by investigating the capability of healthy volunteers to perceive-on their forearms-vibrations with different amplitudes and/or frequencies. In addition, the ability of subjects in spatially discriminating stimulations on three forearm sites and recognizing six different combinations of stimulations was also addressed. Results demonstrate that subjects were able to discriminate different force amplitudes exerted by the device (accuracies greater than 75%); when both amplitude and frequency were simultaneously varied, the pure discrimination of amplitude/frequency variation was affected by the variation of the other. Subjects were also able to discriminate with an accuracy of 93% three different sites and with an accuracy of 78% six different stimulation patterns. 相似文献
95.
Differential Space-Time Block Codes (DSTBC) do not require any radio channel measurement and channel state information neither
on the transmitter nor at the receiver side. Therefore, they are an attractive alternative to coherent Multiple-Input Multiple-output
(MIMO) systems. The classical technical proposal for differential techniques is based on M-ary phase shift keying (M-PSK)
modulation schemes for DSTBC (PSK-DSTBC). One advantage of this scheme is the constant envelop of the transmit signal, but
it is well known that higher-order PSK is less efficient due to the small distance between adjacent points in the constellation
diagram. Therefore, in this paper an alternative modulation technique for DSTBC is discussed, which is based on quadrature
amplitude modulation (QAM-DSTBC). The signal envelope of the transmit signal is not any more constant in this case. Therefore,
the technical challenge of integrating QAM into the DSTBC system design is to control the transmit power in order to avoid
an increase or a decrease in transmit power to some extreme values. The mechanism, which is used in this paper to control
the transmit power is based on an extension of the original QAM constellation diagram. The additional points, which are integrated
into the constellation diagram are used in this case for the mapping procedure to select one of the modulation symbols with
high or alternatively low signal power. This means for each single bit pattern there are almost always two modulation symbols
available in the mapping procedure one with low and one with high signal power, which gives the basis for a signal power control
algorithm. The resulting bit-error-rate (BER) performance of QAM-DSTBC is compared finally to the performance figures of the
original proposal of PSK-DSTBC.
相似文献
Hermann RohlingEmail: |
96.
Rousseau F Hellier P Letteboer MM Niessen WJ Barillot C 《IEEE transactions on medical imaging》2006,25(11):1492-1501
In this paper, three different calibration methods for three-dimensional (3-D) freehand ultrasound (US) are evaluated. Calibration is the process of estimating the rigid transformation from US image coordinates to the coordinate system of the tracking sensor mounted onto the probe. Calibration accuracy has an important impact on quantitative studies. Geometrical precision can also be crucial in many interventions and surgery. The proposed evaluation framework relies on a single point phantom and a 3-D US phantom which mimics the US characteristics of human liver. Four quality measures are used: 3-D point localization criterion, distance and volume measurements, and shape based criterion. Results show that during the acquisition procedure, volumetric measurements and shapes of the reconstructed object depend on probe motion used, particularly fan motions for which errors are larger. It is also shown that accurate calibration is essential to obtain reliable quantitative information. 相似文献
97.
98.
Ian Glendinning Michael Nlle Christian Hausleitner Erwin Greilinger 《International Journal of Satellite Communications and Networking》2019,37(3):269-282
We describe a method for identifying the source of a satellite interferer using a single satellite. The technique relies on the fact that the strength of a carrier signal measured at the downlink station varies with time due to a number of factors, and we use a quantum‐inspired algorithm to compute a “signature” for a signal, which captures part of the pattern of variation that is a characteristic of the uplink antenna. We define a distance measure to numerically quantify the degree of similarity between two signatures, and by computing the distances between the signature for an interfering carrier and the signatures of the known carriers being relayed by the same satellite at the same time, we can identify the antenna that the interferer originated from, if a known carrier is being relayed from it. As a proof of concept, we evaluate the performance of the technique using a simple statistical model applied to measured carrier data. 相似文献
99.
Deployment of RSS-Based Indoor Positioning Systems 总被引:1,自引:0,他引:1
Christian Esposito Massimo Ficco 《International Journal of Wireless Information Networks》2011,18(4):224-242
Location estimation based on Received Signal Strength (RSS) is the prevalent method in indoor positioning. For such positioning systems, a massive collection of training samples is needed for their calibration. The accuracy of these methods is directly related to the placement of the reference points and the radio map used to compute the device location. Traditionally, deploying the reference points and building the radio map require human intervention and are extremely time-consuming. In this paper we present an approach to reduce the manual calibration efforts needed to deploy an RSS-based localization system, both when using only one RF technology or when using a combination of RF technologies. It is an automatic approach both to build a radio map in a given workspace by means of a signal propagation model, and to assess the system calibration that best fits the required accuracy by using a multi-objective genetic algorithm. 相似文献
100.
This work is based on the observation that existing energy management techniques for mobile devices, such as Dynamic Voltage
Scaling (DVS), are non-cooperative in the sense that they reduce the energy consumption of a single device, disregarding potential
consequences for other constraints (e.g., end-to-end deadlines) and/or other devices (e.g., energy consumption on neighboring
devices). This paper argues that energy management in distributed wireless real-time systems has to be end-to-end in nature,
requiring a coordinated approach among communicating devices. A cooperative distributed energy management technique (Co-DVS)
is proposed that (1) adapts and maintains end-to-end latencies within specified timeliness requirements (deadlines) and (2)
enhances energy savings at the devices with the highest pay-off factors that represent the relative benefits or significance of conserving energy at a device. The proposed technique employs a feedback-based
approach to dynamically distribute end-to-end slack among the devices based on their pay-off factors. 相似文献