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排序方式: 共有311条查询结果,搜索用时 15 毫秒
31.
32.
Enderes Thomas Khoo Swee Chern Somerville Clare A. Samaras Kostas 《Mobile Networks and Applications》2002,7(2):153-161
This paper examines the quality of transmission of voice over cellular, packet-switched networks. The medium access mechanism in the uplink is simulated under various statistical multiplexing scenarios in order to assess the effect of front-end clipping on voice quality. Moreover, the simulation is implemented in a real-time demonstration platform utilized to acquire subjective indicators of voice quality by performing Mean Opinion Score (MOS) tests. Results from the MOS tests are reported, and an analysis of the obtained speech samples is presented. Finally, the results are summarized and potential further directions for the simulation tool and the speech models are discussed. 相似文献
33.
The present period, called the scientific revolution, is characterized by the dominant factors of science and its prodigy child, technology. There is a mismatching of impedance as the power and, consequently, decision-making, are held by people ignorant in science and technology. The human quest for the truth follows three stages: mythological, ideological and scientific. Present power holders base their decisions upon ideology rather than science, with the resulting evils of terror of thermonuclear extinction, poverty, crime, unemployment and other miseries afflicting the human family. A modern economotechnic system should incorporate the amplifiers of production, namely, scientific graduate education and R&D. In this, the scientific graduate education is the derivative of R&D and R&D is the derivative of hard goods and services of production. Analysis of this system suggests that in order to achieve a high standard of living one must have a highly trained work force, a strong scientific graduate education and a strong R&D establishment. To achieve the foregoing the present scientifically blind controllers of the economotechnic system should be replaced by highly trained scientists and engineers. 相似文献
34.
Particles of nonstoichiometric M-type barium hexaferrite with the chemical formula BaFe10.5M0.25O17.05 (M=Mg, Cd, Co, Ni, Nd Zn) have been prepared by ion exchange in Ba containing molten salts from precursor ferrites with the β- or β"-alumina structure. The particles possess a plateletlike shape with diameters as low as 0.2 μm and as diameter-to-thickness ratio between 5 and 10. The Curie temperature is close to 470°C. Saturation magnetization values σs up to 64 emu g and coercive fields jH c ranging from 0.5 to 1.5 kOe were found 相似文献
35.
Anthony Simm Theodoros Theodoulidis Nikolaos Poulakis Gui Yun Tian 《The International Journal of Advanced Manufacturing Technology》2011,54(1-4):223-230
Eddy current testing is one of the most widely used methods in non-destructive testing for the inspection of conductive materials. Numerical modelling of eddy current testing has emerged as an important approach alongside experimental studies. This paper investigates an application of numerical modelling and experimental study as a means of the quantitative non-destructive evaluation (QNDE) of defects in conductive samples. There are two methods of measuring eddy current response, more commonly by measuring the change in impedance of the eddy current probe coil, or as used in this work, by measuring the change in magnetic field directly using magnetic field sensors such as superconducting quantum interference devices, giant magneto resistance, or as in this case Hall sensors. Specifically, measurements made using an eddy current probe containing an excitation coil and a Hall sensor, experimentally obtained using an X?CY scanner table, are compared with a numerical (finite element method) model. The discrepancies between the experimental tests and the numerical models have been analysed and explained, which is an important factor in engineering applications of QNDE. 相似文献
36.
Outer block interleaving and time transmission interval (TTI) are two interrelated parameters for the design of 3G systems.
Particularly, they represent a critical issue for the implementation of UMTS turbo coding in flat Rayleigh fading channels.
Thus, an efficient TTI choice can save computational complexity. In this paper different scenarios are investigated using
the maximum UMTS frame length and simulation results are presented for the four possible outer block interleaver configurations
in the case of flat Rayleigh fading channel. Considering frame error rate (FER) performance we propose a TTI of 80 ms for
rural and urban/suburban outdoor operating environments for the majority of scenarios. For indoor/low range outdoor operating
environment 20 ms is the proposed choice apart from low data rates. 相似文献
37.
Theodoros Perdikis Stelios Psarakis Philippe Castagliola Petros E. Maravelakis 《Quality and Reliability Engineering International》2021,37(3):1266-1284
During the design phase of a control chart, the determination of its exact run length properties plays a vital role for its optimal operation. Markov chain or integral equation methods have been extensively applied in the design phase of conventional control charts. However, for distribution-free schemes, due to the discrete nature of the statistics being used (such as the sign or the Wilcoxon signed rank statistics, for instance), it is impossible to accurately compute their run length properties. In this work, a modified distribution-free phase II exponentially weighted moving average (EWMA)-type chart based on the Wilcoxon signed rank statistic is considered and its exact run length properties are discussed. A continuous transformation of the Wilcoxon signed rank statistic, combined with the classical Markov chain method, is used for the determination of the average run length in the in- and out-of control cases. Moreover, its exact performance is derived without any knowledge of the distribution of sample observations. Finally, an illustrative example is provided showing the practical implementation of our proposed chart. 相似文献
38.
39.
Nikolaos V. Kantartzis Theodoros D. Tsiboukis 《Electrical Engineering (Archiv fur Elektrotechnik)》2005,88(1):55-81
A 3-D curvilinear alternating-direction implicit finite-difference time-domain (ADI-FDTD) algorithm for the broadband analysis and optimised design of anechoic and semi-anechoic chambers with arbitrary shape is presented in this paper. The unconditionally stable and frequency-dependent method develops a new family of spatial/temporal forms which reduce the critical dispersion errors of existing approaches and allow the precise performance evaluation of essential components or dissimilar lossy absorbers before their actual construction. Hence, the proposed technique circumvents prolonged simulations via temporal increments significantly beyond the Courant limit and achieves substantial accuracy even for highly curved details. Numerical results, concerning several realistic structures as well as comparisons with measured data obtained according to specific international EMC standards, demonstrate the preceding advantages and lead to optimal chamber configurations.This work has been supported by the State Scholarships Foundation of Greece (I.K.Y. – Athens, Greece) under Postdoctoral Research Scholarship No. 475/2003–2004. 相似文献
40.
Theodoros Tzouramanis Michael Vassilakopoulos Yannis Manolopoulos 《Data & Knowledge Engineering》2004,49(3):1335-286
In this paper we present a performance comparison of access methods for time-evolving regional data. Initially, we briefly review four temporal extensions of the Linear Region Quadtree: the Time-Split Linear Quadtree, the Multiversion Linear Quadtree, the Multiversion Access Structure for Evolving Raster Images and Overlapping Linear Quadtrees. These methods comprise a family of specialized access methods that can efficiently store and manipulate consecutive raster images. A new simpler implementation solution that provides efficient support for spatio-temporal queries referring to the past through these methods, is suggested. An extensive experimental space and time performance comparison of all the above access methods follows. The comparison is made under a common and flexible benchmarking environment in order to choose the best technique depending on the application and on the image characteristics. These experimental results show that in most cases the Overlapping Linear Quadtrees method is the best choice. 相似文献