A P-matrix based model for SAW grating waveguides taking into account modes conversion at the reflection

Several models exist for analyzing the wave-guiding effect of a reflective grating. On the one hand, there are models based on scalar waveguide theory. These models consider that a device can be described as being made of several regions having different velocities. On the other hand, an extension of the coupling of modes (COM) model taking into account the transverse dimension has been developed. This paraxial COM model predicts that guidance is possible even when there is no velocity difference between the interior and the exterior of the grating region. Guidance, under such circumstances, is due only to differences in reflectivity between regions. Following from this insight, a new approach has been developed: guided modes and the continuum of radiating modes are first determined. At each period, reflections then are considered as occurring only in the reflective regions, so that the modes are truncated. Thus, at each reflection (and transmission), each mode is converted into a distribution of all modes. Dispersion curves very similar to those shown by other researchers are obtained by this method. They show, in particular, the existence of guided modes even when the wave velocity in all regions is identical. This model can be used to more easily analyze practical devices and exhibits a good agreement with experimental results.     

Measuring and Improving Image Resolution by Adaptation of the Reciprocal Cell

Traditionally, discrete images are assumed to be sampled on a square grid and from a special kind of band-limited continuous image, namely one whose Fourier spectrum is contained within the rectangular reciprocal cell associated with the sampling grid. With such a simplistic model, resolution is just given by the distance between sample points.Whereas this model matches to some extent the characteristics of traditional acquisition systems, it doesn't explain aliasing problems, and it is no longer valid for certain modern ones, where the sensors may show a heavily anisotropic transfer function, and may be located on a non-square (in most cases hexagonal) grid.In this work we first summarize the generalizations of Fourier theory and of Shannon's sampling theorem, that are needed for such acquisition devices. Then we explore its consequences: (i) A new way of measuring the effective resolution of an image acquisition system; (ii) A more accurate way of restoring the original image which is represented by the samples. We show on a series of synthetic and real images, how the proposed methods make a better use of the information present in the samples, since they may drastically reduce the amount of aliasing with respect to traditional methods. Finally we show how in combination with Total Variation minimization, the proposed methods can be used to extrapolate the Fourier spectrum in a reasonable manner, visually increasing image resolution.     

The effect of microstructure on the permeability of metallic foams

Pressure drop was measured across complex and simple structure metallic foams at different velocity ranges using air as working fluid. Darcian and non-Darcian permeability parameters, K and C, were determined by fitting experimental data with widely accepted quadratic model of Hazen-Dupuit-Darcy. Generally, the experimental results are in good agreement with the model. The differences in K and C values between the two types of metallic foams are due to the different microstructure. For the simple structure specimens, permeability K increased whereas non-Darcian permeability C decreased with increasing pore diameter. The effect of pore size on the permeability of complex structure metallic foams seems to be opposite to that observed with the simple structure specimens and to results reported by other researchers on other porous medium. This discrepancy mainly stems from the differences in window concentration in addition to some heterogeneity in the foam that impeded the gas flow on one side of the specimens. The difference in pressure drop observed in the different metallic foams is due to combined effect of K and C. However, for simple structure foams, K and C could be predicted by Ergun-like model using appropriate values for the empirical constants. The permeability K is significantly affected by pore size and porosity. The quadratic term of Hazen-Dupuit-Darcy equation is mainly due to the inertia of the flow and partially to the drag exerted by the microstructure of the metallic foam. For both foams, as the porosity increases, pressure drop decreases and permeability, K, increases. The introduction of the open cross sectional area term enabled better understanding of the permeability of metallic foams with intricate morphologies.     

CMOS inverters based positive type second generation current conveyor

A new versatile class AB low-voltage second generation current conveyor based on CMOS inverters operating in transconductance mode is presented in this letter. Against traditional design based on CCII+, the circuit is able to operate at low supply voltages and offers numerous advantages like class AB operation, large voltage and current swing, synthesis from digital inverters. Simulation results from a typical 0.35 μm CMOS process had demonstrated the circuit capability to operate at high frequency over wide voltage and wide current swings. The proposed circuit operation has been acted from measurements with the HEF4069UBP from Philips semiconductors [1].     

On the (Non-)Existence of Polynomial Kernels for P l -Free Edge Modification Problems

Given a graph G=(V,E) and a positive integer k, an edge modification problem for a graph property Π consists in deciding whether there exists a set F of pairs of V of size at most k such that the graph $H=(V,E\vartriangle F)$ satisfies the property Π. In the Π edge-completion problem, the set F is constrained to be disjoint from E; in the Π edge-deletion problem, F is a subset of E; no constraint is imposed on F in the Π edge-editing problem. A number of optimization problems can be expressed in terms of graph modification problems which have been extensively studied in the context of parameterized complexity (Cai in Inf. Process. Lett. 58:171–176, 1996; Fellows et al. in FCT, pp. 312–321, 2007; Heggernes et al. in STOC, pp. 374–381, 2007). When parameterized by the size k of the set F, it has been proved that if Π is a hereditary property characterized by a finite set of forbidden induced subgraphs, then the three Π edge-modification problems are FPT (Cai in Inf. Process. Lett. 58:171–176, 1996). It was then natural to ask (Bodlaender et al. in IWPEC, 2006) whether these problems also admit a polynomial kernel. in polynomial time to an equivalent instance (G′,k′) with size bounded by a polynomial in k). Using recent lower bound techniques, Kratsch and Wahlström answered this question negatively (Kratsch and Wahlström in IWPEC, pp. 264–275, 2009). However, the problem remains open on many natural graph classes characterized by forbidden induced subgraphs. question to characterize for which type of graph properties, the parameterized edge-modification problems have polynomial kernels. Kratsch and Wahlström asked whether the result holds when the forbidden subgraphs are paths or cycles and pointed out that the problem is already open in the case of P ₄-free graphs (i.e. cographs). This paper provides positive and negative results in that line of research. We prove that Parameterized cograph edge-modification problems have cubic vertex kernels whereas polynomial kernels are unlikely to exist for the P _l -free edge-deletion and the C _l -free edge-deletion problems for l?7 and l≥4 respectively. Indeed, if they exist, then NP?coNP/poly.     

Finding and Counting Vertex-Colored Subtrees

The problems studied in this article originate from the Graph Motif problem introduced by Lacroix et al. (IEEE/ACM Trans. Comput. Biol. Bioinform. 3(4):360–368, 2006) in the context of biological networks. The problem is to decide if a vertex-colored graph has a connected subgraph whose colors equal a given multiset of colors M. It is a graph pattern-matching problem variant, where the structure of the occurrence of the pattern is not of interest but the only requirement is the connectedness. Using an algebraic framework recently introduced by Koutis (Proceedings of the 35th International Colloquium on Automata, Languages and Programming (ICALP), Lecture Notes in Computer Science, vol. 5125, pp. 575–586, 2008) and Koutis and Williams (Proceedings of the 36th International Colloquium on Automata, Languages and Programming (ICALP), Lecture Notes in Computer Science, vol. 5555, pp. 653–664, 2009), we obtain new FPT algorithms for Graph Motif and variants, with improved running times. We also obtain results on the counting versions of this problem, proving that the counting problem is FPT if M is a set, but becomes #W[1]-hard if M is a multiset with two colors. Finally, we present an experimental evaluation of this approach on real datasets, showing that its performance compares favorably with existing software.     

Energy-aware feedback control for a H.264 video decoder

Embedded devices using highly integrated chips must cope with conflicting constraints, while executing computationally demanding applications under limited energy storage. Automatic control and feedback loops appear to be an effective solution to simultaneously accommodate for performance uncertainties due to the tiny scale gates variability, varying and poorly predictable computing demands and limited energy storage constraints. This paper presents the example of an embedded video decoder controlled by several feedback loops to carry out the trade-off between decoding quality and energy consumption, exploiting the frequency and voltage scaling capabilities of the chip. The inner loop controls the dynamic voltage and frequency scaling through a fast predictive control strategy. The outer loop computes the scheduling set-points needed by the inner loop to process frames decoding. The feedback loops have been implemented on a stock PC and experimental results are provided.     

Leaf area index mapping in northern Canada

Leaf area index (LAI) is an important structural vegetation parameter that is commonly derived from remotely sensed data. It has been used as a reliable indicator for vegetation's cover, status, health and productivity. In the past two decades, various Canada-wide LAI maps have been generated by the Canada Centre for Remote Sensing (CCRS). These products have been produced using a variety of very coarse satellite data such as those from SPOT VGT and NOAA AVHRR satellite data. However, in these LAI products, the mapping of the Canadian northern vegetation has not been performed with field LAI measurements due in large part to scarce in situ measurements over northern biomes. The coarse resolution maps have been extensively used in Canada, but finer resolution LAI maps are needed over the northern Canadian ecozones, in particular for studying caribou habitats and feeding grounds.

In this study, a new LAI algorithm was developed with particular emphasis over northern Canada using a much finer resolution of remotely sensed data and in situ measurements collected over a wide range of northern arctic vegetation. A statistical relationship was developed between the in situ LAI measurements collected over vegetation plots in northern Canada and their corresponding pixel spectral information from Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) data. Furthermore, all Landsat TM and ETM+ data have been pre-normalized to NOAA AVHRR and SPOT VGT data from the growing season of 2005 to reduce any seasonal or temporal variations. Various spectral vegetation indices developed from the Landsat TM and ETM?+?data were analysed in this study. The reduced simple ratio index (RSR) was found to be the most robust and an accurate estimator of LAI for northern arctic vegetation. An exponential relationship developed using the Theil–Sen regression technique showed an R ² of 0.51 between field LAI measurement and the RSR. The developed statistical relationship was applied to a pre-existing Landsat TM 250 m resolution mosaic for northern Canada to produce the final LAI map for northern Canada ecological zones. Furthermore, the 250 m resolution LAI estimates, per ecological zone, were almost generally lower than those of the CCRS Canada-wide VGT LAI maps for the same ecozones. Validation of the map with LAI field data from the 2008 season, not used in the derivation of the algorithm, shows strong agreement between the in situ LAI measurement values and the map-estimated LAI values.     

Feasibility of piecewise-constant control sequences for timed continuous Petri nets

In this study, the determination of control actions for timed continuous Petri nets is investigated by the characterisation of attractive regions in marking space. In particular, attraction in finite time, which is important for practical issues, is considered. Based on the characterisation of attractive regions, the domain of admissible piecewise constant control actions is computed, and sufficient conditions to verify the feasibility of the control objectives are proposed. As a consequence, an iterative procedure is presented to compute piecewise constant control actions that correspond to local minimum time control for timed continuous Petri nets.