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81.
Jeongho Park Jihyung Kim Myonghee Park Kyunbyoung Ko Changeon Kang Daesik Hong 《Wireless Communications, IEEE Transactions on》2006,5(7):1622-1625
The authors present an analysis of the effect of timing offset on channel estimation for comb-type pilot-aided orthogonal frequency division multiplexing (OFDM) systems. Residual timing offset does not negatively affect the channel estimation of the pilot subcarrier, but does corrupt the channel information obtained via interpolation. This paper provides the mean square error (MSE) channel estimation performance when a linear interpolation technique is used in a comb-type pilot-aided OFDM system. Analysis shows that the performance degradation of the channel estimator due to imperfect frame synchronization is dependent on the frequency correlation of the channels and the amount of timing offset 相似文献
82.
Kyung-Han Seo Bong-Ho Lee Nam-Kyoung Kim Jaejung Ko 《Journal of Materials Science》2005,40(23):6151-6156
Stabilization tendencies of the perovskite structure in a Pb(Zn1/3Ta2/3)O3-BaTiO3 pseudobinary system with/without compositional modification by 20 mol% PbTiO3 introduction were compared. In order to promote perovskite phase formation, the B-site precursor method (which is conceptually
similar to the columbite process) was employed in this study. Dielectric properties of sintered samples were investigated
as functions of composition and measurement frequency. Dielectric constant spectra, in the paraelectric temperature region,
were further analyzed in terms of diffuseness. Microstructures of sintered specimens were also investigated and correlated
with perovskite stabilization. 相似文献
83.
Wear and dynamic properties of piezoelectric ultrasonic motor with frictional materials coated stator 总被引:2,自引:0,他引:2
Hyun-Phill Ko Sangsig Kim Jin-Sang Kim Hyun-Jai Kim Seok-Jin Yoon 《Materials Chemistry and Physics》2005,90(2-3):391-395
Piezoelectric ultrasonic motors have been studied, developed and utilized by researchers and companies all over the world. Ultrasonic motors (USM) produce rotational motion based on traveling waves made by the resonant vibrations of piezoceramic. These motors have been recently developed and utilized in practical applications. The dynamic properties and life of piezoelectric ultrasonic motors are strongly related to the frictional material Fused on the sliding surface. In this study, effects of frictional material properties on the performances of piezoelectric ultrasonic motors are investigated. It was possible to improve the torque of a traveling wave type ultrasonic motor by stator's coating. 相似文献
84.
Hyun-Phill Ko Sangsig Kim Chong-Yun Kang Hyun-Jai Kim Seok-Jin Yoon 《Materials Chemistry and Physics》2005,90(2-3):322-326
The contact kinetics of piezoelectric linear motors determines the operational characteristics like speed and torque or transmitted mechanical power and efficiency. Piezoelectric linear motors are driven by tangential stress in the interface between tip of shaking beam and slider. A good contact between the tip and slider is necessary for a reliable analysis of the motor, which is needed for the optimization of its performance. The piezoelectric linear motor was fabricated and the characteristics of the motor were investigated by external conditions such as tip shape with different curvatures and contact force between the tip and the slider. It was found in this investigation that the optimal curvature of the tip and the contact force are curvature of 1 and 10, respectively, for the high actuating speed, and curvature of 1 and 40 N, respectively, for the high actuating force. Finally, tip shape has an influence on the characteristics of linear motor. 相似文献
85.
M. Kočvara 《Structural and Multidisciplinary Optimization》1997,14(4):256-263
We consider the minimum-compliance formulation of the truss topology problem with additional linear constraints on the displacements: the so-called displacement constraints. We propose a new bilevel programming approach to this problem. Our primal goal (upper-level) is to satisfy the displacement constraint as well as possible — we minimize the gap between the actual and prescribed displacement. Our second goal (lower level) is to minimize the compliance — we still want to find the stiffest structure satisfying the displacement constraints. On the lower level we solve a standard truss topology problem and hence we can solve it in the formulation suitable for the fast interior point alogrithms. The overall bilevel problem is solved by means of the so-called implicit programming approach. This approach leads to a nonsmooth optimization problem which is finally solved by a nonsmooth solver. 相似文献
86.
Jung-Hong Chuang Christoph M. Hoffmann Kun-Ming Ko Wei-Chung Hwang 《The Visual computer》1998,14(10):455-470
n -dimensional space, where n>3. This definition can be used for given surfaces that are implicit or parametric. This paper presents a robust, adaptive
polygonization algorithm for evaluating and visualizing geometrically constrained surfaces. Let be the constrained surface, a 2-surface in n-space, and let π() be its projection into the subspace spanned by the first three coordinates. Our polygonization algorithm computes π(). The method works directly with the n-space representation, but performs all major computations in 3-space. Techniques for triangulation, polygon decimation, and
local refinement are also presented. 相似文献
87.
H. C. Lam W. W. Ki A. L. S. Chung P. Y. Ko A. C. Y. Lai S. M. S. Lai P. W. Y. Chou & E. C. C. Lau 《Journal of Computer Assisted Learning》2004,20(2):114-123
Effective teaching should focus the attention of learners to its essential aspects. It follows that instructional software can be designed in such a way that allows learners to experience the important variations in the critical aspects of the content to be learned. This paper reports on the experience of designing such special kinds of instructional learning objects for the learning of Chinese characters. The design of these learning objects takes into consideration not only what Chinese characters are all about but also how learners commonly make errors while they learn to write the characters. Out of the analysis of these learners' errors, variations in the structural features of Chinese characters were pulled out and embodied in the design of the learning objects. Learners tinkering with the learning objects can thus implicitly develop a sense of the structural features or regularity of Chinese characters, which most importantly should prepare the learners to learn more new characters in the future. The main proposal of this paper is the notion of this variation‐affording instructional software that allows learners to attend to the essential aspects of what is to be learned. Furthermore, the idea of the learning object also differs from other instructional software in its small, self‐contained and reusable nature, such that teachers can flexibly embed the learning objects into their own teaching materials. 相似文献
88.
Using regression splines to assess the impact of information technology investments on productivity in the health care industry 总被引:2,自引:0,他引:2
Abstract. This paper explores the impact of information technology (IT) investments on productivity using a new technique, multivariate adaptive regression splines (MARS). We believe that it provides additional insights on the nature of the impact of IT investments on productivity. The results from our study are compared with findings from a previous study that has also used the same data set. While the results of a previous study indicate that IT investments have a positive but uniform impact on productivity, our study suggests that the impact of IT on productivity is not uniform but is contingent on other complementary factors. Our findings describe that the complementary relationship exists between IT and non‐IT related investments. Thus, improved organizational productivity cannot be expected from investment in IT alone but only together with non‐IT investments. Our findings also point out that further investment may not necessarily bring on higher organizational productivity. 相似文献
89.
Single-crystal 3C-silicon carbide (SiC) capacitive pressure sensors are proposed for high-temperature sensing applications. The prototype device consists of an edge-clamped circular 3C-SiC diaphragm with a radius of 400 /spl mu/m and a thickness of 0.5 /spl mu/m suspended over a 2-/spl mu/m sealed cavity on a silicon substrate. The 3C-SiC film is grown epitaxially on a 100-mm diameter <100> silicon substrate by atmospheric pressure chemical vapor deposition. The fabricated sensor demonstrates a high-temperature sensing capability up to 400/spl deg/C, limited by the test setup. At 400/spl deg/C, the device achieves a linear characteristic response between 1100 and 1760 torr with a sensitivity of 7.7 fF/torr, a linearity of 2.1%, and a hysterisis of 3.7% with a sensing repeatability of 39 torr (52 mbar). A wide range of sensor specifications, such as linear ranges, sensitivities, and capacitance values, can be achieved by choosing the proper device geometrical parameters. 相似文献
90.
Tribological behavior of hydrogenated diamond-like carbon (DLC) films and Si incorporated DLC (Si-DLC) films deposited on Si (100) wafer by r.f.-plasma assisted chemical vapor deposition were investigated by ball-on-disk type tribometry in various environments. The friction tests were performed in ambient air of relative humidity ranging from 0% to 90% or dry oxygen environment. We focused on the tribochemical reactions by analyzing chemical composition, chemical bond structure and agglomerated shape of the debris. High and unstable friction behavior was observed in both humid air and dry oxygen environments. In these environments, Auger spectrum analysis showed that the debris contained large amounts of Fe. Significant incorporation of Fe in the debris resulted from the wear of the steel ball, which might be enhanced by the surface oxidation of the ball. These results show that the humidity dependence of friction coefficient is not an inherent tribological property of DLC films but results from the surface reaction of the steel ball in humid environments. Two possible reasons for the Fe rich debris affecting the friction behavior are presented. Reduced dependence of the friction coefficient on the relative humidity observed for Si-DLC films is discussed in terms of the two possible reasons. 相似文献