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Optimal Rate Sampling in 802.11 Systems: Theory, Design, and Implementation
Combes, Richard,Ok, Jungseul,Proutiere, Alexandre,Yun, Donggyu,Yi, Yung IEEE 2019 IEEE transactions on mobile computing Vol.18 No.5
<P>Rate Adaptation (RA) is a fundamental mechanism in 802.11 systems. It allows transmitters to adapt the coding and modulation scheme as well as the MIMO transmission mode to the radio channel conditions, to learn and track the (mode, rate) pair providing the highest throughput. The design of RA mechanisms has been mainly driven by heuristics. In contrast, we rigorously formulate RA as an online stochastic optimization problem. We solve this problem and present G-ORS (Graphical Optimal Rate Sampling), a family of provably optimal (mode, rate) pair adaptation algorithms. Our main result is that G-ORS outperforms state-of-the-art algorithms such as MiRA and Minstrel HT, as demonstrated by experiments on a 802.11n network test-bed. The design of G-ORS is supported by a theoretical analysis, where we study its performance in stationary radio environments where the successful packet transmission probabilities at the various (mode, rate) pairs do not vary over time, and in non-stationary environments where these probabilities evolve. We show that under G-ORS, the throughput loss due to the need to explore sub-optimal (mode, rate) pairs does not depend on the number of available pairs. This is a crucial advantage as evolving 802.11 standards offer an increasingly large number of (mode, rate) pairs. We illustrate the superiority of G-ORS over state-of-the-art algorithms, using both trace-driven simulations and test-bed experiments.</P>
Simplified model to study the dynamic behaviour of a bolted joint and its self loosening
Olfa Ksentini,Bertrand Combes,Mohamed Slim Abbes,Alain Daidié,Mohamed Haddar 국제구조공학회 2015 Structural Engineering and Mechanics, An Int'l Jou Vol.55 No.3
Bolted joints are essential elements of mechanical structures and metal constructions. Although their static behaviour is fairly well known, their dynamic behaviour due to shocks and vibrations has been less studied, because of the large size of the finite element models needed for a detailed simulation. This work presents four different simplified models suitable for studying the dynamic behaviour of an elementary bolted joint. Three of them include contact elements to allow sliding of the screw head and the nut on the assembled parts, and the last one allows rotation between screw and nut. A penalty approach based on the Coulomb friction model is used to model contact. The results show that these models effectively represent the dynamic behaviour, with different accuracy depending on the model details. The last model simulates the self loosening of a bolt subjected to transversal vibrations.
Molecular gas and star formation in the red-sequence counter-rotating disc galaxy NGC 4550
Crocker, Alison F.,Jeong, Hyunjin,Komugi, Shinya,Combes, Francoise,Bureau, Martin,Young, Lisa M.,Yi, Sukyoung Blackwell Publishing Ltd 2009 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.393 No.4
<P>ABSTRACT</P><P>We present observations of the CO(1-0) emission in the central 750 pc (10 arcsec) of the counter-rotating disc galaxy NGC 4550, obtained at the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer. Very little molecular gas is detected, only 1×10<SUP>7</SUP> M<SUB>⊙</SUB>, and its distribution is lopsided, with twice as much molecular gas observed at positive relative velocities than at negative relative velocities. The velocity gradient in the CO(1-0) emission shows that the molecular gas rotates like the thicker of the two stellar discs, which is an unexpected alignment of rotations if the thinner disc was formed by a major gas accretion event. However, a simulation shows that the gas rotating like the thicker disc naturally results from the coplanar merger of two counter-rotating disc galaxies, demonstrating the feasibility of this scenario for the formation of NGC 4550. We investigate various star formation tracers to determine whether the molecular gas in NGC 4550 is currently forming stars. Ultraviolet (UV) imaging data and optical absorption line strengths both suggest a recent star formation episode; the best-fitting two-population model to the UV-optical colours yields a mass of young stars of 5.9×10<SUP>7</SUP> M<SUB>⊙</SUB> with an age of 280 Myr. The best information on the current star formation rate is a far-infrared-based upper limit of only 0.02 M<SUB>⊙</SUB> yr<SUP>−1</SUP>. We are thus witnessing NGC 4550 either in a dip within a bursty star formation period or during a more continuous low-level star formation episode.</P>
Long-term effects of maxillary skeletal expander treatment on functional breathing
Andrew Combs,Ney Paredes,Ramon Dominguez-Mompell,Martin Romero-Maroto,Boshi Zhang,Islam Elkenawy,Luca Sfogliano,Layla Fijany,Ozge Colak,Ben Wu,Won Moon 대한치과교정학회 2024 대한치과교정학회지 Vol.54 No.1
Objective: To investigate the long-term effects of maxillary skeletal expander (MSE) treatment on functional breathing. Methods: Objective measures of breathing, the peak nasal inspiratory flow (PNIF), and peak oral inspiratory flow (POIF), and subjective measures of breathing, the visual analog scale (VAS) and nasal obstruction symptom evaluation (NOSE) survey, were used to investigate the long-term effects of MSE in functional breathing. Seventeen patients, mean age 19.4 ± 3.9 years treated at the UCLA Orthodontics Clinic were assessed on their functional breathing at 3 timepoints: pre-expansion (T0), post-expansion (T1), and post-orthodontic treatment (T2). Results: Immediately after expansion (T1), all the objective functional breathing values were significantly increased in comparison to T0 (P < 0.05). The VAS total, VAS right and VAS left were significantly lower at T1 in comparison to T0 (P < 0.05). At 26.8 ± 3.9 months after MSE expansion (T2), PNIF total, PNIF right, PNIF left, and POIF were significantly higher when compared to T0 (P < 0.05). Also, VAS total, VAS right and VAS left were significantly lower at T2 when compared to T0 (P < 0.05). Additionally, there was a positive correlation between PNIF and the magnitude of expansion at anterior nasal spine and zygomaticomaxillary point (ZMA). There was a positive correlation between total VAS and the magnitude of expansion at the ZMA. There were no significant changes for the NOSE subjective breathing measurement at all time comparisons. Conclusions: Overall, MSE treatment produces an increased objective and subjective airway improvement that continues to remain stable in the long-term post expansion.
From Apple to Omega: an analysis of brand placement in 2010 top-earning films
Michael Devlin,Justin Combs 한국마케팅과학회 2015 마케팅과학연구 Vol.25 No.1
This study utilized a content analysis to measure brand placements in the top 25 filmsreleased in 2010 and compared results to a similar population of top-earning filmsreleased in 1991. The content analysis measured the frequency and modality of eachplacement, character interaction, visual placement and visual prominence, andaccounted for the valence of each scene containing a product placement in terms ofpositive or negative hedonic values. Findings revealed there were substantially morebrand placements and more prevalent use of prominent placement and characterinteraction in the 2010 films, compared with those in 1991. The study’s findingsprovide rationale for future studies exploring the effectiveness of product placementsand set a comparable benchmark for future analysis examining product placementtrends.
조인호,송길호,김창식,Azuma Nobuhide A. Combs,박진,서창민,박정현,편영식 대한기계학회 2005 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.19 No.11S
The industrial beneficial effects of nano-structured surface modification of tool steel (SKD 61) have been studying. Ultrasonic cold forging technology(UCFT) is one of severe plastic deformations for the improvement of mechanical properties by making nano-structure on surface. The basic mechanism of UCFT and experimental device for treatment of trimmer knives(SKD 61) are presented. Test results of UCFT treated knives in cold steel milling line and analysis why the improved service life is achieved are explained.
Novel flexible reflective color media with electronic inks
Tim Koch,여종석,Zhang-Lin Zhou,Qin Liu,Jeff Mabeck,Gregg Combs,Vincent Korthuis,Randy Hoffman,Brad Benson,Dick Henze 한국정보디스플레이학회 2011 Journal of information display Vol.12 No.1
A novel architecture and proprietary electronic inks were developed to provide disruptive digital-media solutions based on an electrokinetic technology platform. The flexible reflective electronic media (eMedia) was fabricated by imprinting three-dimensional microscale structures with a roll-to-roll manufacturing platform. The HP technologies enable the required attributes for eMedia, such as low power, transparency, print-quality color, continuous levels of gray, and lowcost scalability. Pixelation was also demonstrated by integrating with the prototype oxide thin-film transistor backplane,and the system architecture was further developed by stacking primary-colorant layers for color reflective-display application. The innovations described in this paper are currently being developed further for the eSkins, eSignage, and ePaper applications.