Opportunistic scheduling of flows with general size distribution in wireless time-varying channels

In this paper we study how to design an opportunistic scheduler when flow sizes have a general service time distribution with the objective of minimizing the expected holding cost. We allow the channel condition to have two states which in particular covers the important special case of ON/OFF channels. We formulate the problem as a multi-armed restless bandit problem, a particular class of Markov decision processes. Since an exact solution is out of reach, we characterize in closed-form the Whittle index, which allows us to define a heuristic scheduling rule for the problem. We then particularize the index to the important subclass of distributions with a decreasing hazard rate. We finally evaluate the performance of the proposed Whittle-index based scheduler by simulation of a wireless network. The numerical results show that the performance of the proposed scheduler is very satisfactory

Compositional analysis of InAs-GaAs-GaSb heterostructures by low-loss electron energy loss spectroscopy

As an alternative to Core-Loss Electron Energy Loss Spectroscopy, Low-Loss EELS is suitable for compositional analysis of complex heterostructures, such as the InAs-GaAs-GaSb system, since in this energy range the edges corresponding to these elements are better defined than in Core-Loss. Furthermore, the analysis of the bulk plasmon peak, which is present in this energy range, also provides information about the composition. In this work, compositional information in an InAs-GaAs-GaSb heterostructure has been obtained from Low-Loss EEL spectra

Factors controlling the geochemical composition of Limnopolar Lake sediments (Byers Peninsula, Livingston Island, South Shetland Island, Antarctica) during the last ca. 1600 years

We sampled a short (57 cm) sediment core in Limnopolar Lake (Byers Peninsula, Livingston Island, South Shetland Islands), which spans the last ca. 1600 years. The core was sectioned at high resolution and analyzed for elemental and mineralogical composition, and scanning electron microscope and energy dispersive X-ray spectrometer (SEM-EDS) analysis of glass mineral particles in selected samples. The chemical record was characterized by a contrasted pattern of layers with high Ca, Ti, Zr, and Sr concentrations and layers with higher concentrations of K and Rb. The former were also enriched in plagioclase and, occasionally, in zeolites, while the latter were relatively enriched in 2 : 1 phyllosilicates and quartz. This was interpreted as reflecting the abundance of volcaniclastic material (Ca rich) versus Jurassic–Lower Cretaceous marine sediments (K rich) – the dominant geological material in the lake catchment. SEM-EDS analysis revealed the presence of abundant volcanic shards in the Ca-rich layers, pointing to tephras most probably related to the activity of Deception Island volcano (located 30 km to the SE). The ages of four main peaks of volcanic-rich material (AD ca. 1840–1860 for L1, AD ca. 1570–1650 for L2, AD ca. 1450–1470 for L3, and AD ca. 1300 for L4) matched reasonably well the age of tephra layers (AP1 to AP3) previously identified in lakes of Byers Peninsula. Some of the analyzed metals (Fe, Mn, Cu, and Cr) showed enrichments in the most recent tephra layer (L1), suggesting relative changes in the composition of the tephras as found in previous investigations. No evidence of significant human impact on the cycles of most trace metals (Cu, Zn, Pb) was found, probably due to the remote location of Livingston Island and the modest research infrastructures; local contamination was found by other researchers in soils, waters and marine sediments on areas with large, permanent research stations. Chromium is the only metal showing a steady enrichment in the last 200 years, but this cannot be directly attributed to anthropogenic pollution since recent research supports the interpretation that climatic variability (reduced moisture content and increased wind intensity) may have resulted in enhanced fluxes of mineral dust and trace elements (Cr among them) to Antarctica. At the same time, some features of the chemical record suggest that climate may have also played a role in the cycling of the elements, but further research is needed to identify the underlying mechanisms.This work was partially supported by projects CGL2010-20672 and REN2000-0345-ANT (Spanish Ministerio de Ciencia e Innovación), POL2006-06635/CGL (Spanish Ministerio de Educación y Cultura), and 10PXIB200182PR (Dirección Xeral de I+D, Xunta de Galicia).Peer reviewe

Estudio de la calidad del árido reciclado y su relación con la resistencia a compresión del hormigón reciclado utilizando una base de datos

This work studies the physical and mechanical properties of recycled concrete aggregate (recycled aggregate from concrete waste) and their influence in structural recycled concrete compressive strength. For said purpose, a database has been developed with the experimental results of 152 works selected from over 250 international references. The processed database results indicate that the most sensitive properties of recycled aggregate quality are density and absorption. Moreover, the study analyses how the recycled aggregate (both percentage and quality) and the mixing procedure (pre-soaking or adding extra water) influence the recycled concrete strength of different categories (high or low water to cement ratios). When recycled aggregate absorption is low (under 5%), pre-soaking or adding extra water to avoid loss in workability will negatively affect concrete strength (due to the bleeding effect), whereas with high water absorption this does not occur and both of the aforementioned correcting methods can be accurately employed.El estudio analiza las propiedades físico-mecánicas de los áridos reciclados de hormigón (procedentes de residuos de hormigón) y su influencia en la resistencia a compresión del hormigón reciclado estructural. Para ello se ha desarrollado una base de datos con resultados de 152 trabajos seleccionados a partir de más de 250 referencias internacionales. Los resultados del tratamiento de la base indican que densidad y absorción son las propiedades más sensibles a la calidad del árido reciclado. Además, este estudio analiza cómo el árido reciclado (porcentaje y calidad) y el procedimiento de mezcla (presaturación o adición de agua extra) influyen en la resistencia del hormigón reciclado de diferentes categorías (alta o baja relación agua-cemento). Cuando la absorción es baja (inferior al 5%) presaturar o añadir agua para evitar pérdidas de trabajabilidad afectan negativamente a la resistencia (debido al bleeding), mientras que cuando es alta esto no sucede y ambos métodos son adecuados

Metales, textilería y cerámica. Tres líneas de análisis para pensar una vinculación entre los habitantes de la llanura santiagueña y el Tawantinsuyu

This paper is based on a previous study (Angiorama y Taboada 2008) which was carried on in order to explain the existence and distribution of metal objects and valliserranas features in the province of Santiago del Estero. Now we incorporate and analyse relevant information about textiles and ceramics. Our research shows that some kind of relationship could have existed between Tawantinsuyu and certain groups from the Salado Medio. We suggest that this connection would have adopted some kind of alliance between the Empire and the localcommunities.Este artículo parte de un trabajo anterior (Angiorama y Taboada 2008) llevado adelante con la finalidad de explicar la presencia y distribución de objetos metálicos con características valliserranas e incaicas en laactual provincia de Santiago de Estero (Argentina). En esta oportunidad avanzamos en nuestra investigación incorporando y analizando información acerca de otras dos tecnofacturas que se muestran como relevantes para el tema abordado: la textilería y la cerámica. Nuestros estudios nos indican que habría existido algún tipo de vínculo entre el Tawantinsuyu y ciertos grupos que habitaban el Salado Medio. En base a nuestros análisis proponemos que esta relación habría adoptado la forma de algún tipo de alianza entre el Imperio y las comunidades locales