Annual and diurnal slant path rain attenuation statistics in Athens obtained with the synthetic storm technique

We present annual/seasonal rainfall rate statistics derived from a 29-month data bank of 1 sample/min, measured by a disdrometer located inside the National Technical University of Athens Campus. We discuss both the inaccuracy and the unavailability of the measuring equipment, as well as the statistical reliability of the rainfall rate sample. Additionally, after applying the Synthetic Storm Technique on the measured rainfall rate time series, we present the first long term simulated annual/seasonal and diurnal rain attenuation statistics, at 12 GHz for a hypothetical downlink from Hellas Sat 2 to Athens. According to the results, we predict that communication downlinks working in the afternoon and early evening hours must be given an extra power margin, to compensate for high rain attenuation.     

Identification of geometrical shapes in paintings and its application to demonstrate the foundations of geometry in 1650 B.C.

In this paper, an original general methodology is introduced to establish whether a handmade shape corresponds to a given geometrical prototype. Using this methodology, one can decide if an artist had the intention of drawing a specific mathematical prototype or not. This analysis is applied to the 1650 B.C. wall paintings from the prehistoric settlement on Thera, and inferences of great archaeological and historical importance are made. In particular, strong evidence is obtained suggesting that the spirals depicted on the wall paintings correspond to linear (Archimedes) spirals, certain shapes correspond to canonical 48-gon and 32-gon, while other shapes correspond to parts of ellipses. It seems that the presented wall paintings constitute the earliest archaeological findings on which these geometrical patterns appear with such remarkable accuracy.     

Cathodic hydrogen charging and mechanical properties of copper

The influence of cathodic hydrogen charging on some mechanical properties of copper was studied. Hydrogen charging was found to decrease both the ultimate tensile strength and the failure strain of copper. The surface layers of copper were hardened, but they recovered to their prior hardness with time passed after hydrogen charging.     

The effect of heat treatment on the corrosion behaviour of 319 aluminium alloy

The effect of various heat treatments on the corrosion behaviour of 319 T1 cast aluminium alloy was investigated. From this alloy, specimens were heat treated in T5, T6 and two steps solution heat treatment T6 conditions and afterwards were subjected to electrochemical corrosion in a 0.1 M NaCl solution (pH = 12). From the above treatments, T5 heat treatment did not improve the corrosion resistance of the as‐received alloy in contrast to T6 heat treatment which improved the corrosion resistance of the same alloy. However, two steps solutionizing T6 treatment showed the best corrosion resistance of the aluminium alloy.     

Controlling Intercell Interference in CDMA-based Fixed Wireless Networks Through Multirate Techniques

A number of research works have suggested a possible capacity advantage when employing direct sequence-code division multiple access (DS-CDMA) instead of the more common time division multiple access option for fixed broadband wireless access networks operating at millimeter-waves. The choice of this multiple access scheme, which, up to now, has not been foreseen by recent WiMax (worldwide interoperability for microwave access) standards, exhibits an improved performance in terms of cochannel interference in the upstream, even though it experiences higher interference in the downstream. The present paper focuses on enhancing the bottleneck downstream performance of fixed wireless networks by proposing a methodology for controlling the signal-to-interference ratio throughout the cell, so that all subscribers meet the given interference quality-of-service specifications. This is accomplished by exploiting the properties of a combination of multirate CDMA techniques. The paper describes a simple policy for code assignment, which is proved to be optimal, incorporating the effect of rain fading over multiple paths. The performance of the algorithm is tested against single code DS-CDMA both under long term channel statistics (static code assignment) and under dynamic channel effects (dynamic code assignment). The first test verifies the compliance of all sector subscribers with more demanding SIR specifications, while the second yields a significant improvement in terms of resource utilization.     

Multi-Hop Wireless Backhaul Networks above 10 GHz: Connectivity and Critical Density Evaluation

Microwave links are the obvious mobile backhauling solution for many mobile operators. Multi-hops are likely to be necessary in order to ensure connectivity for mobile backhaul solutions. The subject of the present paper is the evaluation of the connectivity of wireless multi-hop backhaul networks assuming high frequency transmissions among the relays. A novel analytical physical propagation and engineering model is presented for the calculation of the connectivity of wireless multi-hop networks that appropriate for operating frequencies above 10 GHz. Assuming equal power transmissions from every node and a random spatial node distribution following the homogeneous Poisson process, we calculate the node isolation probability. Furthermore, we calculate the minimum required node density in order to keep the backhaul network almost connected. The sensitivity of the isolation probability and of the minimum node density on frequency of operation, transmission power and climatic conditions is also investigated. Some useful conclusions are drawn.     

On the rain attenuation dynamics: spatial‐temporal analysis of rainfall rate and fade duration statistics

Knowledge about the dynamic characteristics of rain attenuation is of utmost importance for many applications in terrestrial and satellite communication systems operating at frequencies above 10 GHz. Long‐term rain rate statistics and rain rate duration statistics are usually available from meteorological data. In this paper, a spatial–temporal analysis is employed in order to evaluate the rain attenuation power spectrum of a terrestrial/satellite path. The predicted power spectrum is compared with experimental data. Based on the spectral analysis of rainfall rate a method for converting rain rate duration statistics to link fade duration statistics is also proposed. Fade duration statistics are presented for terrestrial and satellite links and compared with available experimental data. The agreement between the predicted results and the experimental data has been found to be quite encouraging. Finally, numerical results are presented for various climatic zones, elevation angles and frequencies. Some very useful conclusions concerning the dynamic properties of rain attenuation for a microwave path are deduced. Copyright © 2003 John Wiley & Sons, Ltd.     

The effect of hydrogen charging on the mechanical behaviour of 5083 wrought aluminum alloy

The effect of hydrogen cathodic charging on the mechanical behaviour of 5083 wrought aluminum alloy has been studied. Hardening of the surface layers of the examined alloy, due to hydrogen absorption, was observed. The tensile tests revealed that the ductility of 5083 wrought aluminum alloy decreased with increasing hydrogen charging time, for a constant value of charging current density, and with increasing charging current density, for a constant value of charging time. However, the ultimate tensile strength of the examined alloy was slightly affected by the hydrogen charging procedure. The cathodically charged 5083 wrought aluminum alloy exhibited brittle transgranular fracture at the surface layers and ductile intergranular fracture at the deeper layers of the alloy.     

A survey of architectures and scenarios in satellite‐based wireless sensor networks: system design aspects

This paper is not a survey related to generic wireless sensor networks (WSNs), which have been largely treated in a number of survey papers addressing more focused issues; rather, it specifically addresses architectural aspects related to WSNs in some way connected with a satellite link, a topic that presents challenging interworking aspects. The main objective is to provide an overview of the potential role of a satellite segment in future WSNs. In this perspective, requirements of the most meaningful WSN applications have been drawn and matched to characteristics of various satellite/space systems in order to identify suitable integrated configurations. Copyright © 2012 John Wiley & Sons, Ltd.