30 research outputs found
Propagation mechanism modelling in the near region of circular tunnels
ArtĂculo sobre comunicaciones ferroviarias.
Abstract: Along with the increase in operating frequencies in advanced radio communication systems utilised inside tunnels, the location of the break point is further and further away from the transmitter. This means that the near region lengthens considerably and even occupies the whole propagation cell or the entire length of some short tunnels. To begin with, this study analyses the
propagation loss resulting from the free-space mechanism and the multi-mode waveguide mechanism in the near region of circular tunnels, respectively. Then, by conjunctive employing the propagation theory and the three-dimensional solid geometry, a general analytical model of the dividing point between two propagation mechanisms is presented for the first time. Moreover, the model is validated by a wide range of measurement campaigns in different tunnels at different frequencies. Finally, discussions on the
simplified formulae of the dividing point in some application situations are made. The results in this study can be helpful to grasp the essence of the propagation mechanism inside tunnels
Effects of five carbamide peroxide bleaching gels on composite resin microhardness.
The purpose of this study was to evaluate the effects of five home bleaching products containing 15-16% carbamide peroxide on the microhardness of microhybrid composite resin Z-250 (3M/Espe). A total of 72 specimens were fabricated in cylindrical acrylic matrices (4 x 2 mm), filled with composite resin and photo-activated for 40 seconds. They were divided in 6 study groups (n = 12), according to the bleaching product: Review (SS White), Magic Bleaching (Vigodent), Opalescence (Ultradent), Whiteness Perfect (FGM), Claridex (BiodinĂąmica), and a control group (not bleached). Specimens were exposed to 1 cc of bleaching gel for 6 hours daily for 2 weeks. The control group specimens were kept in artificial saliva throughout this time. All the specimens were then analyzed in a microhardness tester. Knoop hardness measurements were performed, and the results were submitted to parametric statistical analysis (analysis of variance and Tukey's test). Mean Knoop values and standard deviation were: baseline, 68.52a (4.28); control, 63.42b (7.16); Whiteness Perfect, 57.57c (1.81); Magic Bleaching, 57.22c (3.84); Opalescence, 57.03cd (4.00); Claridex, 53.64de (3.33); Review 51.45e (2.82). Identical letters mean statistical equality according to Tukey's test at the 5% significance level. The products significantly decreased Z-250 (3M/Espe) microhardness
Communication systems of highâspeed railway: A survey
Modern railway services are required to deliver good quality services to the passenger throughout the whole journey. These include improved performances, safety, and reduce delays. There is also the requirement for inâtrain customer experience such as infotainment and access to reliable communication systems. The railway industry has employed different types and generations of communication systems in recent times. Signaling systems were used at the early stage of the railway services. Today, communication systems such as the secondâgeneration communication system, also known as the global system for mobile communications (GSM), the Thirdâgeneration (3G) and the Fourthâgeneration (4G) are utilized in the railway industry. In this paper, we present a brief history of railway communication systems, their features and applications. In addition, we discuss the technical challenges and potential solutions of inâtrain communication systems and how data can be distributed onâboard and between the train coaches using state of the art and hybrid communication technologies