Radio-Tracer A Tool for Deterministic Simulation of Wave Propagation Fernando Aguado Agelet, Fernando P´erez Font´an ETSI Telecomunicaci´on, Universidad de Vigo Campus Universitario s/n, E–36200 Vigo, Spain phone: +34 986 812122, fax: +34 986 812116 e-mail: ffaguado,
[email protected]
ABSTRACT Ray-tracing and UTD (Uniform Theory of Diffraction) techniques are already widely applied to site-specific radio propagation modeling for wireless applications. This paper briefly presents the software tool Radio-Tracer that uses efficient algorithms to calculate dynamically the exact visibility graph of an environment taking into account the position of both transmitter and receivers and the multi-path propagation model. A fast ray-tracing engine finally guarantees that all ray-paths with up to a desired number of interactions are found. Uniform theory of diffraction is applied once all ray-paths have been calculated to compute electromagnetical parameters such as power delay profiles or overall variations in order to characterize the wireless channel. The kernel of Radio-Tracer has been extended to deal with the special needs of a land mobile satellite environment where diffractions of horizontal edges become important and where the number of possible satellite positions and sample points on a mobile route become quite large. INTRODUCTION With the increasing need to improve the accuracy in propagation predictions and the requirement to adequately assess the wide-band characteristics of wireless channels (e.g. power delay profiles (PDP), correlation bandwidths, directions of arrival of multi-path echoes), site-specific analysis tools have proliferated. The use of these tools has also been boosted by the availability of detailed city or building maps in electronic format. These tools, in order to be of practical use, need to be sufficiently fast in delivering the required results. Very efficient algorithms and data structures from the field of computational geometry and ray-tracing methods from the field of computer graphics have been combined and implemented in the simulation program RadioTracer. The fast and practical tool delivers the necessary geometric information for the multi-paths propagation of waves in a suburban, urban or indoor environment based on Fermats principle. Applying deterministic models (e.g. [1, 2, 3]) for the propagation mechanisms and
Arno Formella Universit¨at des Saarlandes FB 14 Informatik, D–66041 Saarbr¨ucken, Germany phone: +49 (0) 681 302 5537, fax: +49 (0) 681 302 4290 e-mail:
[email protected] URL: http://www-wjp.cs.uni-sb.de/formella
performing a large set of simulation experiments good statistical descriptions of the mobile radio channel have been obtained that show close correlations with measured data sets. The program is presented during the Workshop in an instructive demonstration. Radio-Tracer is used within the ESA-ESTEC global navigation satellite project (GNSS2) as a deterministic simulation tool for a snapshot based statistical model of a multi-satellite scenario [4]. RADIO-TRACER KERNEL The kernel of Radio-Tracer can be divided into two main parts: the geometric part and the electromagnetic part. Both are described briefly in the sequel. For a more detailed description see [4, 5, 6]. The geometric part of Radio-Tracer is based on the polar sweep paradigm and on a fast ray-tracing engine. The basic idea behind the sweeping technique is to rotate a plane perpendicular to the ground around a point of interest, e.g. the transmitter, and to calculate all intersections of the plane with the environment maintaining in an efficient data structure those surfaces being visible to the point. The polar sweep algorithm is used to calculate dynamically the visibility graph of the environment according to the specified transmitting and receiving points. The graph stores in each of its nodes all visible surfaces being visible from the point (i.e. transmitter, receiver, and topmost points of vertical edges) corresponding to the node. Two nodes of the graph are connected by an edge when their corresponding points see each other. (See for instance [7] for an introduction to computational geometry and graph algorithms.) Dynamic calculation of the graph means that only those parts of the visibility graph are calculated which are actually needed in a specific simulation run. This method greatly reduces the simulation time compared to the brute force approach. The calculation of the exact visibility of all points including image points guarantees that all reflective surfaces and all diffractive edges are found. The visibility graph can be seen as a compact coding of all possible ray-paths being of interest for the given environment. The ray-tracing engine is used to further con-
Figure 1: Sample Environment including some RayPaths fine the ray-paths, because the visibility graph considers in the case of diffraction on vertical edges only the top most point (i.e. the one with larger visibility). In this raytracing routine additional obstruction and attenuation can be included. For instance, the reflection on the ground can be handled and the accumulated distance a ray-path crosses trees can be calculated. In other words, Radio-Tracer is guaranteed to find all multi-paths up to a given number and type of interactions. Once all ray-paths have been identified in the geometric part of the program, high frequency electromagnetic techniques (such as UTD, Uniform Theory of Diffraction) are applied in the electromagnetic part of the program to the ray-paths to compute the amplitude, phase, delay, and polarization of each ray. A final step in this type of propagation analysis is to perform the coherent combination of all rays reaching the receiver taking into account antenna patterns and system bandwidths. PROGRAM OVERVIEW The simplified environmental objects which can be handled by the current version of Radio-Tracer include: flat top houses with an arbitrary shaped base polygon; roof top houses with rectangular shaped base polygon; trees with spherically shaped crown approximation. Fig. 1 shows a typical suburban environment including some ray-paths between the transmitter and receiving points along a mobile route. Radio-Tracer considers the following environment related propagation mechanisms: direct ray as line-of-sight, ray-paths with an arbitrary number of diffractions on vertical edges, ray-paths with an arbitrary number of reflections on vertical walls, all combinations of diffractions on convex vertical edges and reflections on vertical walls, first order diffractions on convex horizontal edges,
ray-paths with diffractions on adjacent vertical and horizontal edges in ray-paths without reflections, all ray-paths may include an additional reflection on the ground, all ray-paths may include attenuation according to the accumulated distance the ray-path crosses trees, first order scattering from different points in trees. The transmitter and receiver antennas can be chosen out of a variety of antennas (e.g. dipole, monopole, isotropic). Several polarization patterns (such as vertical or horizontal) are available. The locations of possible receiving positions can be specified as single points, as sampling points along a mobile route (the sampling distance is chosen automatically according to the given frequency), or as a two dimensional grid. Besides system limits due to available memory size and simulation time, Radio-Tracer has no restrictions on the number of objects in the environment, the number of receiving points, or the number of interactions along a raypath. It is a valuable tool for comparative studies of the magnitude of the different contributions of the ray-paths in a scenario where stochastical or empirical models do not provide sufficient information. TYPICAL SESSION Radio-Tracer can be run in two different modes: interactive mode and non-interactive mode. The non-interactive mode is provided in order to run batch jobs carrying out a large number of simulations without further interaction by the user. However, the following description considers only a typical interactive session. Once Radio-Tracer has been started, the main window appears where all two-dimensional input and output takes place. Fig. 2 shows the main menu bar. Usually a user performs four major steps: 1 The environmental data file and a possibly preset configuration file are loaded. All parameters— such as antenna positions, receiver types, interaction types—for this specific simulation run are entered or modified. 2 All ray-paths are calculated and stored into a file. (In case that the number of ray-paths is not prohibitive large, a VRML-based viewer can be used to visualize the ray-paths in the environment. This provides an interactive feedback to the user (Fig. 1).) 3 The electromagnetic calculations are performed. For the GNSS-2 version, the generated output data is adapted to the needs of the post-processor and the files are copied to the appropriate locations for later usage. 4 The calculated field parameters can be visualized in a variety of output formats. For instance, Fig. 3 shows the overall variations in dBm along a mobile route.
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Figure 2: Main Menu-Bar of Radio-Tracer
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Figure 4: Igloo Model for Multi-Satellite Environment
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Figure 3: Overall Variations along a Mobile Route IGLOO MODEL Radio-Tracer is used in the GNSS-2 project and within the COST255 action as a prediction tool for the mobile channel in a multi-satellite environment. Possible positions of a satellite are modeled in an igloolike manner (see Fig. 4) where points on the igloo specify directions towards the satellites. The environment under study is located in the center of the igloo. For each satellite direction and each sampling point on the mobile route all ray-paths with a certain number of interactions are computed. Eventually, the igloo data base containing for each transmitter/receiver-pair the multi-path information is passed to a post-processing module which is not described in this paper. Possible outputs of the postprocessing module are, for instance, statistics about availability of satellite connections for certain possible constellations. VALIDATION The output of Radio-Tracer has been validated through various comparisons with measured data. For instance, an indoor measurement was made using a Hewlett Packard network analyser 8753D, which serves as transmitter and receiver at the same time. The equipment performs the S21 measurements in the frequency domain. To go from frequency to time domain, the HP network analyser uses the Chirp FFT. To achieve a practical comparison between the measurements of the network analyser and the simulation results, the narrow-band simulations were converted into a wideband power delay profile. To simulate the wide-band aspect, for each location and for each sample frequency in the frequency band (e.g. 201 frequency samples between
a start frequency and an end frequency) the simulation was performed separately and the results were composed to the full frequency response from the channel in a similar way the analyser achieves it in the measurements. By performing an inverse FFT on this frequency response, the power delay from the channel is obtained in the same manner as if a pulse associated with the specified bandwidth would have been applied to the channel. Each frequency response was obtained by adding the complex contribution of each ray-path found in the simulation. Because of the short-term fading effects, it is nearly impossible to reproduce exactly the values from a measurement with the help of simulations. The slightest difference in the location of the antenna may result in a very different power delay profile. Therefore, both in the measurements and in the simulations, an average in the frequency domain over 20 random experiments was made. The locations were chosen in both cases in an area of diameter equal to the wavelength around the principal antenna position. Afterwards, the chirp inverse FFT has been computed to obtain the power delay profiles. As an example, averaged simulated and measured power delay profiles obtained in the environment given in Fig. 5 is shown in Fig. 6. Monopole antennas for 900 MHz were used for the measurements. They were put on a metal base plane with a diameter of 30 cm mounted on a pole 1.25 m above ground. The bandwidth used was 600 MHz around 900 MHz. The environment was modeled with concrete and steel (conductor) materials. Additionally, a filter with the antenna frequency characteristics was introduced correcting the differences between simulated (ideal) and measuring (real) antenna. The wide-band, short-term statistics of the measurements and the simulations are presented in Tab. 1. The values shown are given in nanoseconds. As it can be observed that a close agreement between the statistics of the measurements and the simulations has been achieved. EXTENSIONS The current version of Radio-Tracer has been tailored to meet the specific needs within the GNSS-2 project. However, the kernel of the program is written in such a way
Average Delay Delay Spread Delay Window (50%) Delay Window (75%) Delay Window (90%) Delay Interval (9 dB) Delay Interval (12 dB) Delay Interval (15 dB)
Measurement 21.83 9.42 6.18 10.00 18.18 185.53 185.94 186.24
No Freq. Charac. 24.00 13.43 6.16 14.70 40.62 152.34 153.35 153.68
Freq. Charac. 23.96 13.44 6.11 14.95 39.20 152.34 153.18 153.77
Table 1: Power delay profiles in nanoseconds generated by the simulation and the measurement at location A width bandwidth of 600 MHz around 900 MHz. CONCLUSION This paper introduced Radio-Tracer, a software tool to study multi-path effects in micro-cell, macro-cell and land mobile satellite environments. A very brief insight in the underlying algorithms has been given. Some samples of typical input and output parameters have been presented. Further information on Radio-Tracer can be obtained over the web or directly from the authors. References Figure 5: Map of the environment that many interesting features can be incorporated easily. Subsequent versions of Radio-Tracer will contain a module with additional features for indoor environments. The computation of coverage maps will be included as well (a preliminary version is already available). Further extensions will include: interactions with non-plane grounds, larger variety of tree shapes, additional scattering objects, multiple transmitting antennas and their interference patterns, introduction of measured antenna patterns, and a more sophisticated set of output values.
[1] J. Walfish and H. Bertoni, “A Theoretical Model of UHF Propagation in Urban Environments”, in IEEE Trans. Ant. and Prop., vol. 36, no. 12, pp. 17881796, 1988. [2] T.S. Rappaport, S.Y. Seidel, and K.R. Schaubach, “Site Specific Propagation Prediction for PCS System Design Wireless Personal Communications”, Kluwer Academic Publishers, 1993. [3] G.A.J. van Dooren, “A Deterministic Approach to the Modeling of Electromagnetic Wave Propagation in Urban Environments”, PhD Thesis, University of Eindhoven, The Netherlands, 1994. [4] Space Engineering and IDS, “Propagation Model for the Land Mobile Satellite Radio Channel in Urban Environment”, Final Report, ESA Contract No. 9788/92NL/LC(SC), in press. [5] F. Aguado Agelet, F. P´erez Font´an, and A. Formella, “Fast Ray-Tracing for Microcellular and Indoor Environments”, in IEEE Trans. on Magnetics, vol. 33, no. 2, pp. 1484-1487, March 1997.
Figure 6: Power delay profiles for location A at 900 MHz: measured (black), simulated without filter (red), simulated with filter (green), relative power (dB) vs. time (ns) [for gray scale: from dark to light]
[6] F. Aguado Agelet, F. P´erez Font´an, and A. Formella, “Indoor and Outdoor Channel Simulator based on Ray Tracing”, in IEEE Vehicular Technology Conference, Phoenix, Arizona, May 1997. [7] T.H. Corman, Ch.E. Leiserson, and R.L. Rivest, “Introduction to Algorithms”, MIT Press, 1994.
