IEEE 802.11n MAC frame aggregation mechanisms for next-generation high-throughput WLANs [Medium access control protocols for wireless LANs]

IEEE 802.11n is an ongoing next-generation wireless LAN standard that supports a very highspeed connection with more than 100 Mb/s data throughput measured at the medium access control layer. This article investigates the key MAC enhancements that help 802.11n achieve high throughput and high efficiency. A detailed description is given for various frame aggregation mechanisms proposed in the latest 802.11n draft standard. Our simulation results confirm that A-MSDU, A-MPDU, and a combination of these methods improve extensively the channel efficiency and data throughput. We analyze the performance of each frame aggregation scheme in distinct scenarios, and we conclude that overall, the two-level aggregation is the most efficacious

The intraday determination of liquidity in the NYSE LIFFE equity option markets

We exploit an extensive high-frequency data set of all individual equity options trading at New York Stock Exchange London International Financial Futures and Options Exchange (Amsterdam, London and Paris) in order to study the determination of liquidity during the trading day. In particular, we focus on two main aspects of option liquidity: (i) the intraday behaviour of equity option liquidity and its determinants and (ii) the influence of macroeconomic events and commonality on intraday equity option liquidity. Inventory management models cannot explain the intraday variation in option spreads and depths. Instead, we show that the option liquidity measures are strongly correlated with option volatility. Increases in volatility are associated with decreases in liquidity, a finding that is in line with information asymmetry models and the derivatives hedging theory. However, the relationship between spreads and volume varies across the three markets. Option liquidity reacts strongly to macroeconomic news announcements, especially US events. The average systematic liquidity component is 12% for Amsterdam, 14% for London and 16% for Paris

Diffusion of oxygen in Mg-doped α-Al2O3: the corundum conundrum explained

It has been a puzzle for over two decades that the enhancement of oxygen diffusion in α-Al_{2}O_{3} ,with respect to the amount of Mg doping, is several orders of magnitude less than expected. The standard model, which envisages that transport is mediated by oxygen vacancies induced to compensate the charge of Mg 2+ ions substituting Al 3+ ions, has not been able to explain this anomaly. Here, we report a detailed study of populations of point defects and defect clusters in Mg-doped α-Al_{2}O_{3}. By taking into account calculated defect formation energies from the literature, the condition of charge neutrality, and the environmental parameters (chemical potentials) under which the anomalous trend in oxygen diffusivities were previously observed, we are able to arrive at an explanation. A non-linear relationship between Mg concentration in the system and key native point defects, which serve as mediators of self-diffusion in α-Al_{2}O_{3_ , is predicted: the concentrations of such defects increase much more slowly in the supersaturation regime than in the pre-saturation regime, matching the anomalous result previously observed in α-Al_{2}O_{3} . We identify the reason for this as buffering by positively charged Mg interstitials and Mg–oxygen vacancy clusters, which compensate the negative charges of Mg substitutional defects (Mg^{1−}Al ). This study answers part of the long-standing question about self-diffusion in alumina, referred to by Heuer and Lagerlöf in 1999 as the Corundum Conundrum

Bone angiogenesis and vascular niche remodeling in stress, aging, and diseases

The bone marrow (BM) vascular niche microenvironments harbor stem and progenitor cells of various lineages. Bone angiogenesis is distinct and involves tissue-specific signals. The nurturing vascular niches in the BM are complex and heterogenous consisting of distinct vascular and perivascular cell types that provide crucial signals for the maintenance of stem and progenitor cells. Growing evidence suggests that the BM niche is highly sensitive to stress. Aging, inflammation and other stress factors induce changes in BM niche cells and their crosstalk with tissue cells leading to perturbed hematopoiesis, bone angiogenesis and bone formation. Defining vascular niche remodeling under stress conditions will improve our understanding of the BM vascular niche and its role in homeostasis and disease. Therefore, this review provides an overview of the current understanding of the BM vascular niches for hematopoietic stem cells and their malfunction during aging, bone loss diseases, arthritis and metastasis

Effects of stitching on delamination of satin weave carbon-epoxy laminates under mode I, mode II and mixed-mode I/II loadings

The objective of the present study is to characterize the effect of modified chain stitching on the delamination growth under mixed-mode I/II loading conditions. Delamination toughness under mode I is experimentally determined, for unstitched and stitched laminates, by using untabbed and tabbed double cantilever beam (TDCB) tests. The effect of the reinforcing tabs on mode I toughness is investigated. Stitching improves the energy release rate (ERR) up to 4 times in mode I. Mode II delamination toughness is evaluated in end-notched flexure (ENF) tests. Different geometries of stitched specimens are tested. Crack propagation occurs without any failure of stitching yarns. The final crack length attains the mid-span or it stops before and the specimen breaks in bending. The ERR is initially low and gradually increases with crack length to very high values. The mixedmode delamination behaviour is investigated using a mixed-mode bending (MMB) test. For unstitched specimens, a simple mixed-mode criterion is identified. For stitched specimens, stitching yarns do not break during 25% of mode I ratio tests and the ERR increase is relatively small compared to unstitched values. For 70% and 50% of mode I ratios, failures of yarns are observed during crack propagation and tests are able to capture correctly the effect of the stitching: it clearly improves the ERR for these two mixed modes, as much as threefold

Where do we go from here? An assessment of navigation performance using a compass versus a GPS unit

The Global Positioning System (GPS) looks set to replace the traditional map and compass for navigation tasks in military and civil domains. However, we may ask whether GPS has a real performance advantage over traditional methods. We present an exploratory study using a waypoint plotting task to compare the standard magnetic compass against a military GPS unit, for both expert and non-expert navigators. Whilst performance times were generally longer in setting up the GPS unit, once navigation was underway the GPS was more efficient than the compass. For mediumto long-term missions, this means that GPS could offer significant performance benefits, although the compass remains superior for shorter missions. Notwithstanding the performance times, significantly more errors, and more serious errors, occurred when using the compass. Overall, then, the GPS offers some clear advantages, especially for non-expert users. Nonetheless, concerns over the development of cognitive maps remain when using GPS technologies

How preference change induced by mere action versus inaction persists over time

Understanding how sustainable preference change can be achieved is of both scientific and practical importance. Recent work shows that merely responding or not responding to objects during go/no-go training can influence preferences for these objects right after the training, when people choose with a time limit. Here we examined whether and how such immediate preference change in fast choices can affect choices without time limit one week later. In two preregistered experiments, participants responded to go food items and withheld responses toward no-go food items during a go/no-go training. Immediately after the training, they made consumption choices for half of the items (with a time limit in Experiment 1; without time limit in Experiment 2). One week later, participants chose again (without time limit in both experiments). Half of the choices had been presented immediately after the training (repeated choices), while the other half had not (new choices). Participants preferred go over no-go items both immediately after the training and one week later. Furthermore, the effect was observed for both repeated and new choices after one week, revealing a direct effect of mere (non)responses on preferences one week later. Exploratory analyses revealed that the effect after one week is related to the memory of stimulus-response contingencies immediately after the training, and this memory is impaired by making choices. These findings show mere action versus inaction can directly induce preference change that lasts for at least one week, and memory of stimulus-response contingencies may play a crucial role in this effect

Factors Associated with Unmet Need for Modern Contraception in Post-Conflict Liberia

We examined the association between intimate partner violence and  unmet need for modern contraception in post-conflict Liberia. This is a secondary analysis of data collected using the Priorities for Local AIDS Control Efforts (PLACE) method. Data from 499 sexually experienced young women (aged 14-25) in Montserrado County, Liberia were examined.  Intimate partner violence (55.7%), unintended pregnancy (83.2%), and abortion (45.3%) were pervasive in the study population. An estimated 35.9% of respondents had an unmet need for modern contraception.  However, multivariate logistic regression results did not reveal an  association between intimate partner violence and unmet need (OR 1.11; 95% CI 0.70-1.75). Among covariates examined, only contraceptive use at sexual debut (26.1%) was significantly associated with unmet need (OR 0.27; 95% CI 0.14-0.52). Liberian youth need information about and access to modern contraceptive methods besides condoms. Interventions to identify and treat victims of violence are also needed. Afr J Reprod Health 2014; 18[2]: 58-67).Key words: Intimate partner violence, unintended pregnancy, abortion, unmet need for contraception, adolescents, Liberi

N-player quantum games in an EPR setting

The $N$-player quantum game is analyzed in the context of an Einstein-Podolsky-Rosen (EPR) experiment. In this setting, a player's strategies are not unitary transformations as in alternate quantum game-theoretic frameworks, but a classical choice between two directions along which spin or polarization measurements are made. The players' strategies thus remain identical to their strategies in the mixed-strategy version of the classical game. In the EPR setting the quantum game reduces itself to the corresponding classical game when the shared quantum state reaches zero entanglement. We find the relations for the probability distribution for $N$-qubit GHZ and W-type states, subject to general measurement directions, from which the expressions for the mixed Nash equilibrium and the payoffs are determined. Players' payoffs are then defined with linear functions so that common two-player games can be easily extended to the $N$-player case and permit analytic expressions for the Nash equilibrium. As a specific example, we solve the Prisoners' Dilemma game for general $N \ge 2$. We find a new property for the game that for an even number of players the payoffs at the Nash equilibrium are equal, whereas for an odd number of players the cooperating players receive higher payoffs.Comment: 26 pages, 2 figure