Constructing efficient substructure-based preconditioners for BEM systems of equations

In this work, a generic substructuring algorithm is employed to construct global block-diagonal preconditioners for BEM systems of equations. In this strategy, the allowable fill-in positions are those on-diagonal block matrices corresponding to each BE subregion. As these subsystems are independently assembled, the preconditioner for a particular BE model, after the LU decomposition of all subsystem matrices, is easily formed. So as to highlight the efficiency of the preconditioning proposed, the Bi-CG solver, which presents a quite erratic convergence behavior, is considered. In the particular applications of this paper, 3D representative volume elements (RVEs) of carbon-nanotube (CNT) composites are analyzed. The models contain up to several tens of thousands of degrees of freedom. The efficiency and relevance of the preconditioning technique is also discussed in the context of developing general (parallel) BE codes.     

Transverse stress profiles reconstruction for finite element analysis of laminated plates

The present work focuses on a posteriori, equilibrium based, reconstruction of transverse stress profiles in the finite element analysis of FSDT laminated plates. The accuracy of this reconstruction depends on accuracy of the first and second-order derivatives of the plate stress resultants, which is not guaranteed by most available low-order plate finite elements. To cure this trouble, two different strategies, based on the Recovery by Compatibility in Patches procedure, are here proposed and compared. Numerical results of typical reconstructed transverse stress profiles are presented showing the effectiveness of the proposed approach.     

The influence of lateral carrier diffusion and surface recombination on the behavior of semiconductor optical amplifier (SOA)-based MMIs

The influence of lateral carrier diffusion and surface recombination on the self-imaging properties of semiconductor-optical-amplifier-based multimode interference couplers has been verified by simulations using a beam propagation method. It shows a significant degradation of the self-imaging properties of these devices. Buried heterostructures or deeply etched waveguide structures can decrease the impact when the degree of surface recombination is sufficiently low.     

B-spline explicit active surfaces: an efficient framework for real-time 3-D region-based segmentation

A new formulation of active contours based on explicit functions has been recently suggested. This novel framework allows real-time 3-D segmentation since it reduces the dimensionality of the segmentation problem. In this paper, we propose a B-spline formulation of this approach, which further improves the computational efficiency of the algorithm. We also show that this framework allows evolving the active contour using local region-based terms, thereby overcoming the limitations of the original method while preserving computational speed. The feasibility of real-time 3-D segmentation is demonstrated using simulated and medical data such as liver computer tomography and cardiac ultrasound images.     

OFDMA femtocells: a roadmap on interference avoidance - [topics in radio communications]

OFDMA femtocells have been pointed out by the industry as a good solution not only to overcome the indoor coverage problem but also to deal with the growth of traffic within macrocells. However, the deployment of a new femtocell layer may have an undesired impact on the performance of the macrocell layer. The allocation of spectrum resources and the avoidance of electromagnetic interference are some of the more urgent challenges that operators face before femtocells become widely deployed. In this article a coverage and interference analysis based on a realistic OFDMA macro/femtocell scenario is provided, as well as some guidelines on how the spectrum allocation and interference mitigation problems can be approached in these networks. Special attention is paid to the use of self-configuration and self-optimization techniques for the avoidance of interference.     

Accuracy of the zero crossings of the second directional derivative as an edge detector

In this paper the accuracy of the second directional derivative edge detector is analyzed, based on a number of idealized edge models. The results are compared with those for the Laplacian edge detector. Errors are shown to be small under a number of conditions. These conditions are less severe for the second directional derivative than for the Laplacian edge detector. Spurious or phantom edges can be removed by checking the sign of the third directional derivative, though this is not enough to remove all large errors. Indeed, it is also shown that large errors will be obtained if no threshold is set on the magnitude of a third order derivative.This work was supported by the Belgian National Fund for Scientific Research (NFWO).