Author:

Keywords:

Science & Technology, Technology, Engineering, Electrical & Electronic, Engineering

Abstract:

A software prediction tool called EPICS (Enhanced Propagation for Indoor Communications Systems) was developed at the ESAT-TELEMIC division of the K. U. Leuven in two versions: a Geometric Optics (GO) version and a Physical Optics (PO) version. However, like many other three-dimensional package, this can only determine the signal in an environment that can be decomposed into (ir)regular hexahedral obstacles (with 6 sides like rectangular blocks, cubes, etc.) or (complex) combinations of them. Although most of the real life environment can be approximated by these hexahedral obstacles, this might lead to some artefacts like periodic radar cross section variations, the need for multiple diffractions to calculate the signal behind a cylindrical obstacle, or reflections that are ignored (e. g., because the approximated side plane is positioned so that a reflection on that plane can not reach the receiver) is existing. To calculate the signal more accurately for those cases, we need to implement curved obstacles into EPICS. In a first step to achieve this goal, the introduction of cylindrical obstacles is investigated. In this paper, the general strategy is discussed. The first step is to determine the different intermediate (i. e., penetration, reflection and diffraction) points on the ray between transmitter and receiver. Efficient computational routines have been written and tested for this purpose, mostly solving the problem first in two dimensions (projected in a plane perpendicular to the axis of the cylinder) and then transforming this solution to the three-dimensional problem. Once these intermediate points have been found, one can start with the computation of the electromagnetic field. In the case of a penetration, the intermediate point(s) can be found very easily (crossing point(s) of a line and a circle) and the electromagnetic computations don't differ from the computations with hexahedral obstacles. For the reflection by a non perfectly conducting surface, the plane wave Fresnel reflection coefficients can be used. Also the finite thickness of the cylindrical walls can be taken into account, using internal (multiple) reflections, if the losses are high or the reflection coefficient of the wall is not to large. For the diffractions, the two-dimensional geometric problem that needs to be solved to find the diffraction points is the determination of the tangent line to a circle (both from transmitter and receiver). Note that both can have two tangent lines, and one might have to match the two corresponding diffraction points. In this case, the electromagnetic computations for the vertical (i. e., field component parallel with the axis of the cylinder) and horizontal polarisation are done separately. An important issue in these computations is the convergence of the series used for the calculation of the field. The reflection points on a cylindrical wall can not be found as easily as in the previous two cases. In general, an iterative process is required. This implies that the search for a good starting value is an important issue. Therefore some efficient computer programs were written to find firstly a good starting value of the Newton-Raphson iteration. As for the electromagnetic computations, one has to take into account that the caustics are transformed after the reflections and thus another amplitude factor has to be taken into account. Although the described routines are not (yet) a part of the EPICS software, new routines based on Geometric Optics (GO) have been written and tested (in matlab) to predict penetration, reflection and diffraction of electromagnetic fields around cylindrical obstacles. This will be used to compute the effects of a curved airport terminal on an Instrument Landing System (ILS).