Association Control for Wireless LANs: Pursuing Throughput Maximization and Energy Efficiency

International audienceBecause the access points (APs) and the stations (STAs) of a community access network are deployed at the users' desired places, the APs and STAs tend to concentrate in certain areas. A concentration of STAs often results in the AP(s) and STAs in that particular area suffering from severe congestion. A concentration of APs, on the other hand, may cause energy wastage. While a number of association control schemes are proposed to alleviate congestion in WLANs, the existing schemes do not necessarily maximize throughput and do not consider energy consumption. In this paper, we analytically formulate the network throughput as the multiplication of the success probability, frame transmission rate, and channel air-time ratio. The second and third components can easily be monitored and controlled based on measurements of local link and channel condition using the off-the-shelf WLAN devices. On the other hand, the first component, success probability is a function of the number of contending nodes that is extremely difficult to monitor in overlapping WLANs. Due to this reason, we extend our theoretical study and show that success probability can be indirectly maximized by controlling air-time ratio. Finally, we propose an association control scheme that aims at maximizing throughput and reducing energy consumption by taking account of the multiplication of frame transmission rate and air-time ratio. The proposed scheme is evaluated by computer simulations and testbed experiments conducted under real-world complex scenarios with UDP and TCP traffic. Both the simulations and actual implementations confirm the correctness of the theoretical work and the effectiveness of the proposed scheme

Thinning minimization for forming aluminum beverage can end shells

Forming simulations of the can end shell have been implemented based on both of the axisymmetric model and three-dimensional models, for a better understanding of the forming process. The comparison shows that the simulation results agree reasonably well with the experimental observations of the actual forming process. The influence of the loads applied to tools, the clearance between tools, the shape of the tool profile and the position of tools have been investigated, based on the axisymmetric model to save computational time. The design optimization method based on the numerical simulations have been applied to search the optimum design points, in order to reduce the thinning subjected to the constraints of the geometric shape of the shell and the suppression of wrinkles. The optimization results show that the thinning can be improved up to 4% by optimizing the forming route, adjusting the clearance and the load, and modifying the tool shape. © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved

ソウヤ センジョウ ニオケル 2.5 Mc/s クウデン ザツオン キョウド ノ カンソク ケッカ

Field intensity of atmospheric radio noise on 2.5Mc/s was measured on board the "SOYA" along the course from Tokyo to Antarctica. Results of the analysis of the measurements are summarized as follows: 1) The daytime noise intensity which is predominant in the tropical region and near South Africa corresponds well to the meteorological situation. 2) Atmospheric radio noise intensity measured on board the "SOYA" agrees well with the values predicted on the "Noise Map" prepared by CRPL and with the result of measurement at the fixed stations (Singapore and Pretoria). 3) Characteristics of the atmospheric radio noise were examined in connection with the solar zenith angle and the latitude. Both the mode of diurnal variation and the relation between absorption and solar zenith angle very distinctly with the latitude. 4) Decrease in the intensity at midnight towards the south from about 32°S is greater than that of the intensity calculated only from the distance (the so-called unabsorbed field intensity) and smaller than what was predicted on the CRPL\u27s map. 5) Abnormal enhancement of the radio noise was seen when the ship went under the pouring rain, while the noise fell down to zero level with a flash of lightning, and then it rose exponentially with the time constant of the order of 1 minute. These facts seem to suggest the mechanism how the atmospheric radio noise originates in rain falls. In other words, the precipitation noise due to rainfall occurs in the presence of both raindrops and the static field between the earth and the cell of charged clouds

Extension of the operational regime of the LHD towards a deuterium experiment

As the finalization of a hydrogen experiment towards the deuterium phase, the exploration of the best performance of hydrogen plasma was intensively performed in the large helical device. High ion and electron temperatures, Ti and Te, of more than 6 keV were simultaneously achieved by superimposing high-power electron cyclotron resonance heating onneutral beam injection (NBI) heated plasma. Although flattening of the ion temperature profile in the core region was observed during the discharges, one could avoid degradation by increasing the electron density. Another key parameter to present plasma performance is an averaged beta value $\left\langle \beta \right\rangle$ . The high $\left\langle \beta \right\rangle$ regime around 4% was extended to an order of magnitude lower than the earlier collisional regime. Impurity behaviour in hydrogen discharges with NBI heating was also classified with a wide range of edge plasma parameters. The existence of a no impurity accumulation regime, where the high performance plasma is maintained with high power heating  >10 MW, was identified. Wide parameter scan experiments suggest that the toroidal rotation and the turbulence are the candidates for expelling impurities from the core region

Results of Ionospheric Observation at Syowa Base, Antarctica, during the Solar Eclipse

During the solar eclipse on August 11, 1961, the continuous recording of h'-f observation was made in vertical incidence at Syowa Base in Antarctica, and in spite of the period of very low solar zenith angle, it was possible to find the effect of the eclipse on the ionograms. The recordings at this time were obtained of only the F2 region in the ionosphere, and the variation of electron density during the solar eclipse at a certain height of the F2 region was more remarkable than the variation of the critical frequency. The results of analysis of the observations are summarized as follows: (i) Electron in the F2 region seems to decrease by the attachment process, and the order of magnitude of the attachment coefficient can be estimated at 1.5×10^ to 2.0×10^sec^ at 300km. (ii) On the assumption of the above-mentioned values as the attachment coefficient, the computed values of an electron density during the eclipse show a good agreement with the observational values up to the maximum phase of the eclipse, but beyond the maximum phase, the observational values continue to increase rapidly in electron density until after the end of eclipse. It may be suggested that the phenomena of electron increase are due to an effect such as the ionization of charged particles, (iii) In our observation, the value of scale height obtained, as the attachment process of electron to (O_2) oxygen, is about 30 km at 300 km levels of the F2 layer

Risk of mortality following hip fracture in Japan

Background. Excessive mortality and morbidity are serious problems after hip fracture in the elderly. Methods. Hip fractures in persons aged 50 years or older were prospectively registered in Japan in 2000. Questionnaires regarding both the first onset and the second 120-day period after hip fracture were obtained from 759 patients, 546 of whom were female and 213 male. Results. Their average age at the time of fracture was 80 years. Altogether, 68 people (9%) died within 120 days after fracture; and 25 patients died within 30 days. Those dying within 120 days and those alive after hip fracture were compared. By univariate analysis, risk factors were poor walking ability, need for a walking aid, low body mass index, history of falls, and lack of active exercise; however, none of these factors was identified as a risk factor by multivariate analysis. By multivariate analysis, the five risk factors associated with mortality were male sex, older age, high American Academy of Anesthesiology (ASA) grade, dementia, and residence in an institution. Conclusions. During the treatment and rehabilitation period special attention should be paid to patients with chronic diseases and reduced mental status

CHARACTERISTICS OF THE SHORT WAVE PROPAGATION FROM TOKYO TO SYOWA STATION, ANTARCTICA

Measurements of the field intensity of 6055 KHz (NSB, Nippon Short Wave Broadcasting), 9595 KHz (NSB) and 15300 KHz (NHK, Radio Japan) were made at Syowa Station in Antarctica, from April 1966 to January 1967. Results were compared with the calculated values by the two methods developed by K. MIYA and CRPL (Central Radio Propagation Laboratory), and with the analized result by MIYA'S method on the whole It was found that the propagation of the recieved radio wave was always controlled by MUF (Maximum Usable Frequency) of the ionosphere at Syowa Station, and that the propagation was also made by the Es (Sporadic E) mode at near midnight in winter there Also, we found that the propagation of 9595 KHz had the best condition of all the frequencies measured thiough all seasons