On the Geometric Modeling of the Uplink Channel in a Cellular System

To meet the challenges of present and future wireless communications realistic propagation models that consider both spatial and temporal channel characteristics are used. However, the complexity of the complete characterization of the wireless medium has pointed out the importance of approximate but simple approaches. The geometrically based methods are typical examples of low–complexity but adequate solutions. Geometric modeling idealizes the aforementioned wireless propagation environment via a geometric abstraction of the spatial relationships among the transmitter, the receiver, and the scatterers. The paper tries to present an efficient way to simulate mobile channels using geometrical–based stochastic scattering models. In parallel with an overview of the most commonly used propagation models, the basic principles of the method as well the main assumptions made are presented. The study is focused on three well–known proposals used for the description of the Angle–of –Arrival and Time–of–Arrival statistics of the incoming multipaths in the uplink of a cellular communication system. In order to demonstrate the characteristics of these models illustrative examples are given. The physical mechanism and motivations behind them are also included providing us with a better understanding of the physical insight of the propagation medium

Impact of Finger Placement on the Correlation Properties of Rake Combined Signals

3G mobile devices and base stations employ rake receivers. An important issue in the design of such receivers is finger allocation. This paper explores the relationship between finger placement and the correlation properties of rake combined signals. The dependence of correlation coefficients on system parameters such as the multipath characteristics of the propagation channel, the number of users, the processing gain and the thermal noise power is also discussed. Several conclusions useful in the analysis and design of rake receivers are drawn. A low complexity finger placement algorithm is finally suggested. In the proposed receiver, finger allocation is based on the correlation properties of the desired signal component only. The receiver performs close to complex structures in the literature

The FEMM Package: A Simple, Fast, and Accurate Open Source Electromagnetic Tool in Science and Engineering

The finite element method (FEM) is one of the most successful computational techniques for obtaining approximate solutions to the partial differential equations that arise in many scientific and engineering applications. Finite Element Method Magnetics (FEMM) is a software package for solving electromagnetic problems using FEM. The program addresses 2D planar and 3D axisymmetric linear and nonlinear harmonic low frequency magnetic and magnetostatic problems and linear electrostatic problems. It is a simple, accurate, and low computational cost open source product, popular in science, engineering, and education. In this paper the main characteristics and functions of the package are presented. In order to demonstrate its use and exhibit the aid it offers in the study of electromagnetics a series of illustrative examples are given. The aim of the paper is to demonstrate the capability of FEMM to meet as a complementary tool the needs of science and technology especially when factors like the economic cost or the software complexity do not allow the use of commercial products

Closed-form Description of Microwave Signal Attenuation in Cellular Systems

The reliable and accurate description of signal attenuation characteristics is important in the simulation and performance analysis of wireless communications systems. Recent works have provided analytical expres- sions for the path loss statistics in cellular systems consid- ering distance-dependent losses and shadowing. In this paper, we extend this analysis by including small-scale fading. A closed-form expression that gives the path loss density in a cellular network is given. The impact of chan- nel parameters and cell size on signal attenuation is fur- ther investigated. Simulation results and comparisons with measured data in the literature verify the accuracy of the solution. The derived formulation is a useful tool for the modeling and analysis of cellular communications systems

An Efficient Finger Allocation Method for the Maximum Likelihood RAKE Receiver

In wideband wireless communication systems the RAKE receiver is commonly used to collect the resolvable multipath energy and counter the effects of fading through diversity. However, in channels with large delay and energy spread, its high complexity still remains a major issue. This motivates the study and application of computationally efficient finger placement algorithms that significantly reduce the receiver complexity with a reasonable performance loss. In this paper, a low–complexity maximum likelihood RAKE receiver, the Suboptimum – Maximum Power Minimum Correlation (S–MPMC) RAKE is proposed. The allocation of its first two fingers is based on the received signal correlation properties. Their positions determine also the placement of the rest of the fingers. Simulation results are provided to show the operation of the receiver and demonstrate its performance. Comparisons with relevant methods are performed to corroborate the merits of the proposal. The balance on the performance and the complexity of the technique makes it suitable for use in commercial wideband communication systems

On the Effect of Channel Impairments on VANETs Performance

The primary means of studying the performance of vehicular ad hoc networks (VANETs) are computer simulations. Nowadays, the development of analytical models and the use of hybrid simulations that combine analytical modeling with discrete-event simulation are of great interest due to the significant reduction in computational cost. In this paper, we extend previous work in the area by suggesting an analytical model that includes distance-dependent losses, shadowing and small-scale fading. Closed-form expressions for the packet reception probability and the packet forwarding distance in the absence of simultaneous transmissions are presented. Numerical simulations validate the proposed formulation. The impact of path loss and fading on network throughput is explored. Interesting results that shows the efficacy of the approach are provided. The derived formulation is a useful tool for the modeling and analysis of vehicular communication systems

The role of elF2alpha kinases in the resistance to VSV infection and regulation of the stability of the tumor suppressor protein p53 /

PKR has been shown to play an essential role against VSV infection by phosphorylating eIF2alpha leading to the inhibition of protein synthesis. Through this capacity PKR is thought to be a mediator of the anti-viral and anti-proliferative actions of IFNs. In addition to translational control, PKR has been shown to modulate the transcriptional activities of NF-kappaB, Stats and p53. However, experiments with two different PKR-/- mouse models have failed to verify many of the biological functions attributed to PKR. Here, we show that the two PKR-/- MEFs express different PKR truncated proteins suggesting that both PKR-/- models are incomplete knockouts. The expression of the PKR variants may account for the significant signaling differences between cells from the two PKR-/- mice.We also demonstrate that another eIF2alpha kinase, PERK contributes to cellular resistance towards VSV infection. We demonstrate that PERK -/- MEFs are more susceptible to VSV-mediated apoptosis than PERK +/+ MEFs. The higher replication capacity of VSV in PERK-/- MEFs results from their inability to attenuate viral protein synthesis due to an impaired eIF2alpha phosphorylation. We also show that VSV-infected PERK-/- MEFs are unable to fully activate PKR suggesting a cross-talk between the two eIF2alpha kinases in virus infected cells. These findings further implicate PERK in virus infection, and provide evidence that the anti-viral and anti-apoptosic roles of PERK are mediated, at least in part, via the activation of PKR.Despite the translational control function of eIF2alpha kinases, we demonstrate their implications in p53 inactivation. Specifically, we show that PERK activation is responsible for the proteasomal degradation of p53 in a manner that is independent of translational control. This role is not specific for PERK, since the PKR also promotes p53 degradation in response to dsRNA transfection. We established that activation of eIF2alpha kinases leads to the activation of GSK3beta thus promoting the Mdm2-dependent degradation of p53. That is, induction of eIF2alpha kinases leads to the nuclear localization of GSK3beta, and the nuclear export and proteasomal degradation of p53. Our findings establish a novel cross-talk between the eIF2alpha kinases and p53 with significant implications in stress responses that control cell proliferation and tumorigenesis

PROPOSING A MATHEMATICAL SOFTWARE TOOL IN PHYSICS SECONDARY EDUCATION

Abstract: MathCad ® is a very popular software tool for mathematical and statistical analysis in science and engineering. Its low cost, ease of use, extensive function library, and worksheet-like user interface distinguish it among other commercial packages. Its features are also well suited to educational process. The use of natural mathematical notation and built-in measurement units are its two major advantages in teaching and learning. In this paper, its complementary use in the upper secondary physics education in Greece is explored. In order to demonstrate its application in the teaching process, a set of representative examples are presented. The main features and advantages of the software are also pointed out. The paper aims to present the benefits of the application of mathematical information technology tools in secondary physics education. In this effort, MathCad ® is probably the most promising solution