Light Communication Channel Research Articles

Secrecy Dimming Capacity in Multi-LED PAM-Based Visible Light Communications

Recently, mobile cloud computing (MCC) has gained a lot of interest for researchers building the next-generation mobile applications. Because unauthorized access may cause serious problems, security and privacy with MCC have become significant issues. This paper addresses the secrecy dimming capacity of secure transmission in MCC over visible light communication (VLC) channels. By obtaining the entropy-maximizing symbol probability of multiple light emitting diode- (LED-) based pulse amplitude modulation (PAM), mathematical analysis of the secrecy dimming capacity of VLC was derived. Simulation results show that the secure transmission ability of multi-LED-based VLC is determined according to the number of activated LEDs and target dimming level. This can be a guideline for practical VLC-based mobile network designers intending to secure wireless transmission and to decide on the number of activated LEDs at target dimming level to operate.

A break to moderate drinkers

Accepted 2 July 2008 Copyright © EEF ISSN 1399-1183 AGORA is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in WEB ECOLOGY and elsewhere. A lighter prose is encouraged and no summary is required. Formal research papers, however short, will not be considered.

Equilibrium of species’ distributions with climate

The degree to which species’ distributions are at equilibrium with current climate is an important issue in the ecological literature (Davis 1986, Gaston 2003). Distinguishing the relative roles of present and past climates in determining species’ distributions is not only of theoretical interest, but is key to understanding responses of species to climate change. Recent work has demonstrated significant departures from equilibrium with current climate for tree distributions in Europe (Svenning and Skov 2004), yet differences in the degree of equilibrium can be expected across organisms whose dispersal abilities (and hence ability to track changing climate conditions) vary greatly. Here we assess differences in equilibrium between present-day climate and vascular plant, breeding bird, amphibian and reptile assemblages in Europe. Our analyses included all know European species of breeding birds (Hagemeijer and Blair 1997), amphibians and reptiles (Gasc et al. 1997), and ca 20% of the European vascular flora (Jalas and Suominen 1972 /1996, Lahti and Lampinen 1999). Assemblages of plants and breeding birds were found to be relatively closer to equilibrium than assemblages of reptiles and amphibians. Results suggest that responses of plant and bird species to climate change are more likely to be accurately forecasted by models correlating present-day distributions with climate, and that reptile and amphibian species will be least capable of shifting distributions, making them most vulnerable to rapid environmental changes. Species are said to be at equilibrium with climate if they occur in all climatically suitable areas whilst being absent from all unsuitable ones (Hutchinson 1957). The observation that species are absent from many suitable areas is trivial. The critical question is how distant from equilibrium are current distributions. Accurate estimates of species’ limits of tolerance to climate can be obtained by means of controlled experiments (Chuine and Beaubien 2001). However, these are expensive and time consuming and attempts to measure departures from equilibrium for large numbers of species have previously used the ‘‘bioclimatic envelope’’ approach, whereby observed species’ distributions are correlated with environmental variables to approximate the ecological requirements of organisms (for review see Pearson and Dawson 2003). With this approach, the ratio between observed and modelled distributions is interpreted as indicating departures of species distributions from equilibrium (Svenning and Skov 2004). By relying on observed distributions (which inherently reflect multiple range determinants, both historical and ecological) to determine ecological requirements, this methodology is likely to underestimate the true range of climate variation that species are able to tolerate. An alternative approach consists of measuring patterns of covariation between species assemblages and climate (Ferrier et al. 2002). It is assumed that if species are at equilibrium with current climate, then patterns of covariation between assemblage composition and climate will be high. A weak covariation between climate and biota may indicate that species tend not to occupy all available climate spaces due to historical and/or ecological factors (notably competition and limited dispersal ability). In some cases it could also imply that the selected climate parameters do not adequately describe the climatic determinants of the distribution of assemblages. However this possibility is inherent to any correlational approach (including bioclimate-envelope modelling) and the choice of climate variables needs to be supported from existing knowledge of species ecologies. FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

‘Latitude’ and geographic patterns in species richness

FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

Parasites and the neutral theory of biodiversity

FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

Extinction in island endemic birds reconsidered

4'. FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

The structure of oribatid mite communities (Acari, Oribatida): patterns, mechanisms and implications for future research

FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

Screening for species potentially sensitive to habitat fragmentation

FORUM is a lighter channel of communication between readers and contributors: it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

Biodiversity and body size: patterns among British birds

FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

Dynamics of small mammals on islands

FORUM is a lighter channel of communication between readers and contributors; it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.