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51.
Uddenfeldt J. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1998,86(7):1319-1324
Cellular radio started in the early 1980s by using analog technologies. Research in voice coding, modulation, and channel coding resulted in second-generation cellular radio based on digital technologies, which were introduced in the early 1990s. These were all based on advanced time division multiple access technology, resulting in better capacity and lower cost. Today, these digital technologies-based on the Global System for Mobility, digital Advanced Mobile Phone System, and personal digital cellular-have more than 100 million subscribers worldwide. The next step is to introduce wide-band packet services for wireless Internet up to 2 Mb/s. These so-called third-generation systems (Universal Mobile Telecommunications Services, IMT2000) are planned to be introduced in the early 2000s 相似文献
52.
Dissolved organic carbon (DOC) dynamics were investigated over a two year period in the Upper Rhǒne River to examine the role of side-arms in providing DOC to the main channel in relation to discharge fluctuations, especially floods. Concentrations of DOC are shown to remain low in space and in time (average 1.5 mg ?1) and to be more related to global hydrological events (precipitation) than to any local flushing for backwaters during floods. The results do not support the general assumption that side-arms are the providers of DOC to the main channel. 相似文献
53.
The present study has been divided into two sets. In the first set, the aim of the experiments was to investigate the dose-response effect of selective serotonin re-uptake inhibitor (SSRI) citalopram on rat exploratory behaviour in the elevated plus-maze. In the second set of experiments, the effect of cholecystokinin (CCK) CCKA and CCKB receptor antagonists, devazepide and L 365260, on citalopram-induced decrease of exploratory behaviour in the elevated plus-maze was studied. Citalopram (5 and 10 mg/kg) decreased the number of open and total arm entries, line crossings on open arms, and percentage of time spent exploring in open arm. Dose 15 mg/kg was without any effect on rat exploratory behaviour. Devazepide (0.01 and 1.0 mg/kg) failed to modify any of the citalopram-induced changes observed. L 365260 (1.0 mg/kg) reversed most of the effects of citalopram: the numbers of open and total arm entries, the number of line crossings, and the percentage of time spent exploring in open arms. L 365260 at dose level 0.01 mg/kg was ineffective. These results support the involvement of the CCKB receptor subtype in SSRI-induced anxiogenic-like effects in rodents. 相似文献
54.
Albrecht Jason E.; O'Brien Edward J.; Mason Robert A.; Myers Jerome L. 《Canadian Metallurgical Quarterly》1995,21(2):364
An implicit assumption of several causal reasoning models is that readers adopt the goals of a narrative's protagonist during text comprehension. In apparent violation of this assumption, readers participating in Experiment 1 of the present study drew inferences relevant to a protagonist's goal even when that goal was already satisfied from the perspective of the protagonist. In Experiments 2 and 3, participants were explicitly asked to view the text situation from the point of view of the protagonist. In this case, the goals of the reader and the protagonist should be the same. In these experiments, participants focused on the goals of the protagonist only when those goals had not been satisfied from the perspective of the protagonist. These results are discussed in terms of reader- and character-based perspectives and in terms of text characteristics that cue perspective taking. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
55.
This investigation focuses on the correlation between the crystallographic orientation of grains with respect to magnetic
properties in Co-Cr films. Based on a distribution measured for the (0002) crystallographic pole densities, modified formulae
describing the texture were used to calculate the coercivity Hc, the squareness ratio Rs (Mr⊥/TMs) and the anisotropy field
Ha for Co-Cr films. In general, calculated values for the coercive force He agreed well with the measured values. The calculated
Rs⊥. values were much larger than the measured ones, however, indicating that the influence of the demagnetizing field and
the magnetostriction on the orientation of the magnetization cannot be neglected.
On leave from Institute of Computing Technology, Academia Sinica, P.O.Box 2704-6, Beijing, P.R. China. 相似文献
56.
Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism 相似文献
57.
Auld B.A. Chimenti D.E. Shull P.J. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1996,43(2):319-325
The transverse resonance approach to guided wave analysis is applied to shear horizontal (SH) wave propagation in periodically layered composites. It is found for SH waves that at high values of the guided wavevector β, the wave energy is trapped in the slower of the two media and propagates accordingly at the slower wavespeed. At low values of β, however, the modes demonstrate a clustering behavior, indicative of the underlying Floquet wave structure. The number of modes in a cluster is observed to correlate with the number of unit cells in the layered plate. New physical insights into the behavior of these systems are obtained by analyzing the partial waves of the guided SH modes in terms of Floquet waves. We show that the fast and slow shear waves in the periodically layered composite play an analogous role to the longitudinal and shear partial waves comprising Lamb waves in a homogeneous plate 相似文献
58.
A 3-wave longitudinal design was used to examine the relationships among emotional restraint, peer drug associations, and gateway drug use in a sample of 1,256 middle school students. Structural equation modeling was used to compare 3 models: (1) One model viewed drug use as a consequence of emotional restraint and peer variables; (2) 1 viewed drug use as a cause of restraint and peer variables; and (3) 1 included reciprocal effects. All 3 models fit the data fairly well. However, the reciprocal model fit the data significantly better than either of the others. Within this model, low emotional restraint was significantly related to subsequent increases in gateway drug use among boys. In contrast, peer drug models and peer pressure were not related to subsequent changes in gateway drug use. Changes in peer drug models were, however, predicted by previous levels of gateway drug use. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
59.
Data from the past few years have shown that as caffeine metabolizes solely in the liver, caffeine elimination can serve as a liver function test. We have collected data by monitoring 40 persons with liver diseases (11 chronic alcoholic hepatitis, 24 liver cirrhosis, 5 non-cirrhotic liver disease). Eight subjects served as controls. The patients with liver cirrhosis were classified according to the Child--Pugh scoring system. To determine caffeine elimination blood samples were collected before and at 3, 6, 9 and 12 hours after oral administration of 0.2 g caffeine. Fasting serum caffeine concentration and concentration 12 hours after administration, serum clearance, half life, peak concentration and volume of distribution have been compared. The respective values measured in patients with non-cirrhotic liver diseases did not differ significantly from the controls. The disappearance of caffeine was significantly decreased in cirrhotics. Our results demonstrated a good correlation between impairment of caffeine elimination and assessment of severity of liver disease by the Child--Pugh classification. Measuring serum levels in samples taken 12 hours after caffeine administration is a simple and useful method in the diagnosis of liver diseases at cirrhotic stage. 相似文献
60.
T P?usa P Górski H Frank-Piskorska M Kowalski J Kruszewski P Kuna M Kurek R Kurzawa W Silny E Zawisza 《Canadian Metallurgical Quarterly》1996,1(4):292-293
A review of isolates of Neisseria meningitidis and other laboratory evidence of meningococcal infection was carried out for the period 1990-1995. Thirty-three patients with evidence of meningococcal disease were identified over the six year period. Group B organisms were the most commonly identified, but an increase in Group C has occurred and is almost equal in number. Data from clinical, laboratory and public health sources should be centralized in a National Surveillance Centre. 相似文献