Geometrical inverse preconditioning for symmetric positive definite matrices

We focus on inverse preconditioners based on minimizing $F(X) = 1-\cos(XA,I)$, where $XA$ is the preconditioned matrix and $A$ is symmetric and positive definite. We present and analyze gradient-type methods to minimize $F(X)$ on a suitable compact set. For that we use the geometrical properties of the non-polyhedral cone of symmetric and positive definite matrices, and also the special properties of $F(X)$ on the feasible set. Preliminary and encouraging numerical results are also presented in which dense and sparse approximations are included

Power Control and Channel Allocation for D2D Underlaid Cellular Networks

Device-to-Device (D2D) communications underlaying cellular networks is a viable network technology that can potentially increase spectral utilization and improve power efficiency for proximitybased wireless applications and services. However, a major challenge in such deployment scenarios is the interference caused by D2D links when sharing the same resources with cellular users. In this work, we propose a channel allocation (CA) scheme together with a set of three power control (PC) schemes to mitigate interference in a D2D underlaid cellular system modeled as a random network using the mathematical tool of stochastic geometry. The novel aspect of the proposed CA scheme is that it enables D2D links to share resources with multiple cellular users as opposed to one as previously considered in the literature. Moreover, the accompanying distributed PC schemes further manage interference during link establishment and maintenance. The first two PC schemes compensate for large-scale path-loss effects and maximize the D2D sum rate by employing distance-dependent pathloss parameters of the D2D link and the base station, including an error estimation margin. The third scheme is an adaptive PC scheme based on a variable target signal-to-interference-plus-noise ratio, which limits the interference caused by D2D users and provides sufficient coverage probability for cellular users. Closed-form expressions for the coverage probability of cellular links, D2D links, and sum rate of D2D links are derived in terms of the allocated power, density of D2D links, and path-loss exponent. The impact of these key system parameters on network performance is analyzed and compared with previous work. Simulation results demonstrate an enhancement in cellular and D2D coverage probabilities, and an increase in spectral and power efficiency.Comment: 35 page

Considerations on the Diffraction Limitations to the Spatial Resolution of Optical Transition Radiation

The interest in using optical transition radiation (OTR) in high energy (multiGeV) beam diagnostics has motivated theoretical and experimental investigations on the limitations brought by diffraction on the attainable resolution. This paper presents calculations of the diffraction effects in an optical set-up using OTR. The OTR diffraction pattern in a telescopic system is calculated taking into account the radial polarization of OTR. The obtained diffraction pattern is compared to the patterns obtained by other authors and the effects of different parameters on the shape and on the size of the OTR diffraction pattern are studied. The major role played by the radial polarization on the shape of the diffraction pattern is outlined. An alternative method to calculate the OTR diffraction pattern is also sketchedComment: Submitted to Particle Accelerator

Chypre: Les territoires fendus d'Aphrodite

Chipre es un espacio emblemático de relaciones y rivalidades entre los hombres, entre el mito y la histoia, entre la causalidad y el esterotipo: cura de Afrodita-Kypris, la diosa qui dio nombre a la isla; antigua potencia económica del conjunto del Mediterráneo gracias a la prosperidad debida a las exportaciones de cobre; actualmente miembro excéntrico de la Unión europea con un territorio dividido por la línea verde, ocupado militarmente en el Norte por Turquía y lleno de bases de soberanía británica en el Sur; más recientemente productor de gas natural, Chipre siempre ha atraido las miradas 'deformantes' que se enfrentan a eta puerta estratégica de un oriente Próximo hoy en ebullición