UWB:个人无线局域网络的高速公路多频带OFDM VS.直接序列DS--两大技术特性的综合剖析

大约3年前,美国联邦通信委员会发布一份历史性的Report&Order报告,将3.1GHz～10.6GHz的7,500MHz频谱配置给超宽带(UltraWideBand,UWB)装置使用。这份报告为产业和学术界带来许多创新与技术进步的机会,业界也纷纷成立各种组织,例如多频带OFDM联盟(Multi-band OFDM Alliance)、UWB论坛以及IEEE802.15.3a特别工作小组,希望为UWB技术建立共同的标准。     

Normovolaemic haemodilution and hyperoxia have no effect on fractal dimension of regional myocardial perfusion in dogs

The neurotoxicity of dibucaine was compared with that of commercially available local anesthetics in studies using rabbit desheathed cervical vagus nerve preparation. Dibucaine dose-dependently suppressed the evoked action potential of myelinated A beta nerve component and nonmyelinated C nerve component. The potential of A beta nerve component was more strongly suppressed, compared with that of C nerve component. At low concentrations of 0.0001-0.001%, the suppression was reversible and recovery with C nerve component was faster and more complete. At higher concentrations, the suppression was irreversible. The minimum concentrations of irreversible blockade were 0.003% for A beta nerve component and 0.03% for C nerve component. Electron microscopically, marked damages in the myelin layer and intraaxonal structure were observed in nerves treated with 0.03% dibucaine. When the neurotoxic effect of dibucaine was compared, in terms of safety margins (minimum concentration of irreversible blockade/clinically used concentrations), with those of commercially available local anesthetics, the rank order was dibucaine, tetracaine and bupivacaine; dibucaine showing the lowest safety margin.     

Molecular statics simulations of buckling and yielding of gold nanowires deformed in axial compression

We use molecular statics simulations with the embedded atom method potential to delineate yielding (material instability) and buckling (structural instability) in gold nanowires deformed axially in compression. It is found that both local (stacking faults) and global instabilities occur when the gold nanowire yields but only global instabilities occur when the nanowire buckles. Furthermore strong surface effects reorient the lattice structure which significantly increases Young’s modulus in the axial direction and cause a nanowire of relatively small slenderness ratio (e.g., 14) to buckle. Upon complete unloading of the nanowires, the average axial stress and the total potential energy revert to their values in the reference configuration for the nanowires that buckled but not for the one that yielded.     

Body weight changes in lactating purebred and crossbred dairy cattle.

Body weights of 396 first and 205 second lactation cows in the first, second, third, and fourth generations of the dairy cattle crossbreeding project at the Illinois Agricultural Experiment Station were studied. Body weights were taken regularly on each animal during the first 9 mo of each lactation with the first weight being taken 3 to 4 days after parturition and subsequent weights at intervals of 30 days thereafter. Among least-squares means there was a decrease in body weight during the first 1 or 2 mo of lactation and a steady increase thereafter in all breed groups for both first and second lactations. There were differences between cows as well as between months for all breed groups in each generation for both lactations. Sums of squares for months were partitioned into linear, quadratic, and cubic components. A third degree polynomial was fitted for each breed group within each generation and separately for each lactation showing the changes in body weights.     

Steady state penetration of thermoviscoplastic targets

Steady state thermomechanical deformations of a target hit by a rigid cylindrical penetrator with an ellipsoidal nose are studied. The material of the target is assumed to be thermally softening but strain and strain-rate hardening. Results computed and presented graphically include the pressure distribution on the nose of the penetrator, dependence of the axial resisting force upon the speed of the penetrator, and the variation of field quantities such as the temperature and strain-rate in the target. Computed results show that the ratio of the major to minor axes of the ellipsoidal nose has a significant effect on the deformations of the target particles in the vicinity of the penetrator nose.     

Effect of frictional force and nose shape on axisymmetric deformations of a thick thermoviscoplastic target

Summary We study thermomechanical deformations of a thermally softening viscoplastic thick target impacted at normal incidence by a cylindrical rod made of a material considerably harder than the target material. Thus we regard the penetrator to be rigid and analyze the effect of the penetrator nose shape and the frictional force at the target/penetrator interface on target's deformations. In the postulated expression for the frictional force, the coefficient of friction, defined as the ratio of the tangential force at a point to the normal force there, is a function of the relative speed of sliding between the two bodies. The computed depth of penetration is found to match very well with that observed in experiments by Forrestal et al. For each nose shape studied, the consideration of frictional forces reduces significantly the computed penetration depth. For the same kinetic energy of the penetrator, the penetrator with a sharp nose gives higher values of the penetration depth as compared to that obtained with a blunt nose.     

The yields of extracted leaf protein from lucerne (Medicago sativa L.)

The effects of fertilisers, frequency of cutting, row spacing and simazine on the yields of dry matter and extractable protein from lucerne (Medicago sativa L.) were studied using the laboratory-scale pulper and press. Between 40 and 65% protein nitrogen was extractable; extractability was not affected by the various treatments but altered with season. Lucerne responded with increased yields of dry matter and extractable protein to fertilisers, simazine (35 g/ha) and rows spaced at 30.5 cm. Six or 8 harvests were found to give greater yields than 5 harvests in 180 days. The annual yields of extractable protein reached 3100 kg/ha and it is felt that better control of pests could lead to even greater yields.     

Effect of electromechanical coupling on static deformations and natural frequencies

A two-way coupled electromechanical theory is used to study static deformations and free vibrations of a laminated hybrid rectangular plate comprised of either piezoceramic (PZT) layers or patches embedded at arbitrary locations in graphite/epoxy layers. A first-order shear deformation theory is used to develop equations for the plate which are solved by the finite-element method (FEM) using eight-node isoparametric elements. Static deflections and natural frequencies computed with open-circuited PZT layers are found to differ significantly from those of grounded PZT layers.