This paper investigates two fundamental characteristics of a wireless multi -hop network: its minimum node degree and its k--connectivity Both topology attributes depend on the spatial distribution of the nodes and their transmission range Using typical modeling assumptions :--- :a random uniform distribution of the nodes and a simple link model :--- :we derive an analytical expression that enables the determination of the required range r0 that creates, for a given node density ρ, an almost surely k--connected network Equivalently, if the maximum r0 of the nodes is given, we can find out how many nodes are needed to cover a certain area with a k--connected network We also investigate these questions by various simulations and thereby verify our analytical expressions Finally, the impact of mobility is discussedThe results of this paper are of practical value for researchers in this area, eg, if they set the parameters in a network--level simulation of a mobile ad hoc network or if they design a wireless sensor network

/pdf/on-the-minimum-node-degree-and-connectivity-of-a-wireless-gd91chp0fp.pdf

On the minimum node degree and connectivity of a wireless multihop network

We consider a large-scale wireless network, but with a low density of nodes per unit area. Interferences are then less critical, contrary to connectivity. This paper studies the latter property for both a purely ad-hoc network and a hybrid network, where fixed base stations can be reached in multiple hops. We assume here that power constraints are modeled by a maximal distance above which two nodes are not (directly) connected. We find that the introduction of a sparse network of base stations does significantly help in increasing the connectivity, but only when the node density is much larger in one dimension than in the other. We explain the results by percolation theory. We obtain analytical expressions of the probability of connectivity in the 1D case. We also show that at a low spatial density of nodes, bottlenecks are unavoidable. Results obtained on actual population data confirm our findings.

/pdf/connectivity-in-ad-hoc-and-hybrid-networks-3erbqzq90n.pdf

Connectivity in ad-hoc and hybrid networks

We study the impact of interferences on the connectivity of large-scale ad hoc networks, using percolation theory. We assume that a bi-directional connection can be set up between two nodes if the signal to noise ratio at the receiver is larger than some threshold. The noise is the sum of the contribution of interferences from all other nodes, weighted by a coefficient /spl gamma/, and of a background noise. We find that there is a critical value of /spl gamma/ above which the network is made of disconnected clusters of nodes. We also prove that if /spl gamma/ is nonzero but small enough, there exist node spatial densities for which the network contains a large (theoretically infinite) cluster of nodes, enabling distant nodes to communicate in multiple hops. Since small values of /spl gamma/ cannot be achieved without efficient CDMA codes, we investigate the use of a very simple TDMA scheme, where nodes can emit only every nth time slot. We show that it achieves connectivity similar to the previous system with a parameter /spl gamma//n.

/pdf/impact-of-interferences-on-connectivity-in-ad-hoc-networks-1mo2at5nri.pdf

Impact of interferences on connectivity in ad hoc networks

We investigate connectivity in the ad hoc network formed between vehicles that move on a typical highway. We use a common model in vehicular traffic theory in which a fixed point on the highway sees cars passing it that are separated by times with an exponentially distributed duration. We obtain the distribution of the distances between the cars, which allows us to use techniques from queuing theory to study connectivity. We obtain the Laplace transform of the probability distribution of the connectivity distance, explicit expressions for the expected connectivity distance, and the probability distribution and expectation of the number of cars in a platoon. Then, we conduct extensive simulation studies to evaluate the obtained results. The analytical model that we present is able to describe the effects of various system parameters, including road traffic parameters (i.e., speed distribution and traffic flow) and the transmission range of vehicles, on the connectivity. To more precisely study the effect of speed on connectivity, we provide bounds obtained using stochastic ordering techniques. Our approach is based on the work of Miorandi and Altman, which transformed the problem of connectivity distance distribution into that of the distribution of the busy period of an equivalent infinite server queue. We use our analytical results, along with common road traffic statistical data, to understand connectivity in vehicular ad hoc networks.

/pdf/analytical-model-for-connectivity-in-vehicular-ad-hoc-4qpd1v59fs.pdf

Analytical Model for Connectivity in Vehicular Ad Hoc Networks

Cette derniere Newsletter avant les Grandes Vacances est, pur concours de circonstances, entierement consacree a l’Education. Mais l’Education sous l’angle de son importance pour la consecration dans l’excellence. La consecration, c’est ce qu’a obtenu Gisele Mophou-Massengo, quand son jury de soutenance l’a declaree Habilitee a Diriger des Recherches. C’est egalement la reconnaissance qui est accordee a Elina Marie Devoue par le jury du prix « Religions et Developpement » pour son article consacre aux Religions et a leurs impacts sur la societe dans la Caraibe.La reconnaissance du savoir et du savoir-faire ne passe pas forcement par sa consecration. Elle passe egalement par sa diffusion la plus large. C’est ce que tente de faire le Ceregmia en organisant depuis 2009 ses « seminaires intra Ceregmia » qui permettent l’expression de la transversalite du laboratoire sur des chantiers en cours ou deja finalises autour de thematiques que nous vous invitons a decouvrir en page 2. De meme, le Ceregmia a lance une nouvelle publication en 2010, ses « Documents de Travail ». Ces Documents d’audience plus confidentielle sont reellement reserves a un public de scientifiques. Diffuser le savoir en l’adaptant aux evolutions de l’environnement et aux exigences du marche. C’est ce qu’a fait le Ceregmia grâce a sa nouvelle offre de formation en Masters qui a ete habilitee par le Ministere et qui ouvrira des la rentree 2010 (p. 4). Par ailleurs, nous avions eu l’occasion de vous entretenir des changements intervenus a l’IFGCar suite au seisme du 12 janvier 2010 en Haiti. Nous sommes heureux aujourd’hui, de vous annoncer que l’IFGCar qui est desormais l’IAC retourne s’installer en Haiti. Les formations en Master reprendront des le 1 er . Octobre. Enfin, Nous vous signalons la parution de deux ouvrages aux Editions Ibis Rouge ils vous invitent a decouvrir la Guyane sous un angle moins connu, celui de l’anthropologie. Charley Granvorka

/pdf/habilitation-a-diriger-des-recherches-1hfqqvwsee.pdf

Habilitation a diriger des recherches

Context. Abundance ratios of the nuclear-spin isomers of H 2 O and NH 3 have been measured in about two dozen comets, with a mean value corresponding to a nuclear-spin temperature of ∼ 30K. The real meaning of these unequilibrated nuclear-spin abundance ratios is still debated. However, an equilibrated water ortho-to-para ratio of 3 is also commonly observed. Aims. The H channel of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-H) on board Rosetta provided high- resolution 2.5–2.9 µ m spectra of H 2 O vapour in the coma of comet 67P/Churyumov-Gerasimenko (67P), which are suitable for the determination of the ortho-to-para ratio (OPR) of water in this comet. Methods. A large dataset of VIRTIS-H spectra obtained in limb-sounding viewing geometry was analysed, covering heliocentric distances from 1.24 to 2.73au and altitudes from a few hundred metres to > 100km. The OPR, together with the H 2 O rotational temperature and column density, were derived for each spectra using a database of ﬂuorescence synthetic spectra that include both fundamental and hot vibrational water bands. The weak lines of the ν 1 , ν 1 + ν 3 − ν 1 and ν 2 + ν 3 − ν 2 bands in the 2.774–2.910 µ m range were used to calculate by how much the strong ν 3 band centred at 2.67 µ m is attenuated due to optical depth effects, expressed by the attenuation factor f atten . Results. Most ortho-to-para ratio determinations are strongly affected by opacity effects, as demonstrated by the observed anti-correlation between the OPR and the column density, and the correlation between the OPR and attenuation factor f atten . Based on both radiative transfer calculations and OPR values obtained in low-opacity conditions, we derive an OPR of 2.94 ± 0.06 for comet 67P. Measured water rotational temperatures show a decrease in gas kinetic temperature with increasing altitude caused by adiabatic cooling. Heliocentric variations are also observed, with warmer temperatures near perihelion. Conclusions. The water ortho-to-para ratio measured in the coma of 67P is consistent with laboratory experiments showing that water vapour that has thermally desorbed from water ice has a statistical value of 3, regardless of the past formation process of water ice.

/pdf/water-ortho-to-para-ratio-in-the-coma-of-comet-67p-churyumov-2iw0lcyb.pdf

Water ortho-to-para ratio in the coma of comet 67P/Churyumov-Gerasimenko

The percolation of a broadcast in a multihop radio network modeled by a spatial Poisson process is studied. The effect of station density and transmission radius on the extent of broadcast percolation is examined. For broadcast percolation in one spatial dimension, analytical expressions for the average extent of percolation are derived. A model for two-dimensional spatial percolation is presented along with related simulation results. The results suggest that in optimizing transmission radius as a function of communication performance measures, the choice of radius may be bounded from below by the need to maintain a desired network connectivity. The connectivity constraint can be relaxed to some extent in certain non-Poisson spatial models. >

Critical connectivity phenomena in multihop radio models

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Water ortho-to-para ratio in the coma of comet 67P/Churyumov-Gerasimenko

Critical connectivity phenomena in multihop radio models