- Research Article
- Open Access
Real-Time Propagation Measurement System and Scattering Object Identification by 3D Visualization by Using VRML for ETC System
© 2009 Minseok Kim et al. 2009
Received: 25 April 2008
Accepted: 27 September 2008
Published: 29 October 2008
In the early deployment of electric toll collecting (ETC) system, multipath interference has caused the malfunction of the system. Therefore, radio absorbers are installed in the toll gate to suppress the scattering effects. This paper presents a novel radio propagation measurement system using the beamforming with 8-elmenet antenna array to examine the power intensity distribution of the ETC gate in real time without closing the toll gates that are already open for traffic. In addition, an identification method of the individual scattering objects with 3D visualization by using virtual reality modeling language will be proposed and the validity is also demonstrated by applying to the measurement data.
In the early deployment of ETC system, multipath interference has caused the malfunction of the system. Therefore, electromagnetic absorbers are installed on some objects, such as canopy, sidewall, poles, and so forth, in the toll gate to suppress the scattering. Thanks to the electromagnetic absorbers , the malfunction significantly decreased but the periodical investigation into the corrosion is still required for the maintenance and replacement after the installation. Since the measurement has been taken only for the spatial distribution of the power of the combined signal in conventional method using a spectrum analyzer, the sufficient examination has not been carried out from the diagnostic point of view.
This paper proposes a novel radio propagation measurement system for the ETC gates. The most significant feature of this system is that the measurement can be made on the vehicle passing through the gates using the real ETC signal transmitted from the ETC roadside station, thus there is no need to close the toll gates. In practice, it is greatly useful to maintain and manage the toll gates already open for traffic. This system can identify the individual scattering object by using the power intensity around the ETC gate with 3D (three-dimensional) visualization method using VRML (virtual reality modeling language) as well as the spatial power distribution in real time. In this paper, the measurement principle and the developed system configuration will be described. The evaluation by the field experiment will be presented as well.
2. Measurement System
2.1. Hardware Setup
8-element uniform linear array
25.77 mm (0.5 wavelength)
Circularly polarized MSA
Maximum input power
Baseband or low IF
2.2. Measurement Concept
3. Signal Processing
ETC signal specification (ARIB STD-T75 ).
3.3. System Calibration
So far each antenna in an antenna array has been assumed to have an ideal omnidirectional property. However, in general, the actual response of the antenna array significantly deviates from the assumed ideal model. In addition, the amplitude and phase response for each channel in the multichannel receiver has been assumed to be identical. But in reality, it is not feasible. Therefore it is generally needed to measure the actual responses of the antennas and receivers in advance. It is called system calibration.
Considering the responses of array antenna and RF cable do not vary so much over time and space, total responses including those of receivers are measured in a radio anechoic chamber. Strictly speaking, the radiation pattern of each antenna element, mutual coupling and deviation among RF cables should be properly calibrated. As far as this system employs a linear antenna array, the deviation of the radiation pattern of each antenna element cannot be eliminated by calibration processing since there exists cone ambiguity in the beamforming. Therefore we measured the calibration data only on the plane determined by the directional vector toward the maximum radiation pattern and the array axis. Moreover, the receiver is greatly affected by temperature, thus on-site calibration is necessary. For receiver calibration, we used the one-to-eight power divider calibrated by a vector network analyzer in advance.
where , , and denote a reference signal for calibration, a noise vector with zero mean and variance of and the number of calibration data, respectively. In this study, the calibration data at every were collected.
For precise verification of the ETC signal specification, we calculated the absolute received power level applying equivalent isotropic received power (e.i.r.p.).
4. Virtual Reality Modeling Language; VRML
This study proposes the scattering object identification method using 3D visualization with the virtual reality modeling language (VRML) which is the language for displaying 3D objects with a VRML viewer. In some literatures, VRML has been used for the modeling tool for visualization  and simulation . However, the significant feature of this study is that VRML is made use of for the purpose of scattering object identification based on the experiment.
Since the popularization of world wide web (WWW), there has been an effort to enhance the content of web documents with advanced 3D graphics and interaction with them. The term virtual reality markup language (VRML) was first used at a European web conference in 1994 where a need for a 3D web standard was addressed. Afterward, the name of the standard was changed to virtual reality modeling language to emphasize the role of graphics.
The brief history review of VRML is well described in  as following. The VRML1.0 specification was originated from the prototype 3D web interface "Labyrinth," developed in 1994. The open inventor ASCII file format from Silicon Graphics Inc. was the basis of VRML1.0. This first release of VRML supports the complete description and manipulation of 3D static scenes. The VRML architecture group (VAG) formed in 1995 issues the RFP (request for proposal) for VRML2.0, which was officially released at the Siggraph 96 conference. In 1997 VRML2.0 was stated as an International Standard: VRML97. Now animation, viewpoint binding, texture animation, timers and scripting are Now animation, viewpoint binding, texture animation, timers and scripting are supported, making advanced 3D web content possible. In 1999 the Web 3D consortium joins the W3C (WWW consortium) to form X3D (extensible 3D) as the next generation VRML standard, being fully compatible with VRML97 content and based on XML (extensible markup language).
VRML is a scene graph-based 3D system. All information that defines a 3D scene is stored in so-called "nodes" of the graph. Most of the nodes have special purposes, like holding geometry data, appearance information or transformation rules. VRML includes an event oriented system which enables the nodes to interact with each other and with the user. Because it is a fully descriptive ASCII code language, VRML files can easily be produced and edited by any text editor.
The VRML97 standard continues to be improved by the Web 3D consortium. The newly released X3D standard is the successor to VRML97. X3D is an extensible standard that provides compatibility with existing VRML content and browsers .
5. Scattering Object Identification by 3D Visualization
5.1. Basic Idea
In ETC gate environment, propagation mechanism including specular reflection, edge diffraction, corner diffraction and non-specular scattering as well as direct wave can be considered. However, in general, the other paths than direct and specular reflection paths are comparatively weak. For the specular reflection, the image method can be applied to the identification of the equivalent source location that can be found by the measurement data at different positions in the same manner as the direct waves.
An infinite number of equivalent sources at exist on the conical surface and the observation point becomes its vertex as shown in Figure 10. Hence the equivalent source cannot be uniquely determined, which is called cone ambiguity. If it can be uniquely determined, we can identify the scattering object by tracing the peaks on the beamforming spectrum along moving path and approximating to the curve from the relationship of . However, this method is applicable only if the length of the lit zone is sufficiently long that we can observe a change of AoA in the beamforming spectrum.
5.2. VRML File Generation and Manipulation Procedure
In this study, MATLAB, Mathworks, was used for producing the spherical beampatterns and VRML file generation. The virtual reality toolbox product uses VRML97 standard (VRML2.0) to deliver a unique, open 3D visualization solution for MATLAB.
Import model in MATLAB. The toll gate model is converted into 3ds format to import to graphics in MATLAB. In importing procedure, some extra steps are required. Several user-contributed open sources available at the MATLAB Central web site . One of them, model3d was used to import the model. It can support Autocad (.dxf) and 3D Studio Max (.3ds) formats (model3d was contributed by Steven Michael of the MIT Lincoln Laboratory).
Overdraw spherical beampattern. Draw the spherical beampattern at the exact measurement position over the gate model in MATLAB.
Save as VRML. Save as the VRML format using the standard "vrml" function of MATLAB.
Edit viewpoint and orientation. In generated VRML model, the viewpoint and orientation is manually edited by applying the measurement position (viewpoint) and view angle (orientation) by any text editor.
Load on VRML viewer. There are many free downloadable VRML/X3D viewers. Herein, BS Contact VRML/X3D 7.02, Bitmanagement Software GmbH , was used.
Identify the scattering object. Some movement operations such as examine, pan, walk, slide, and so forth are available in the VRML viewer. We can see the surroundings through the spherical beampattern at the center of the sphere in virtual reality world.
6. Measurement Examples in ETC Gate
Over-reach problems caused by scattering in ETC lane have been studied in [2, 4]. For example, if the car is passing through the ETC gate and a big object like water sprinkler is also passing through the adjacent ETC lane at the same time, the signal power can be over-reached out of the specified communication zone due to the scattering and hence the malfunction can occur by this leaked power.
This system employed a linear array antenna with finite number of elements and computed the 1D beamforming for the array axis. To improve the identification resolution, multiple measurements changing the direction of the array axis can be considered. For example, if an additional beamforming is done horizontally to the moving direction ( -axis) as shown in Figure 10, the 2D (azimuth and elevation) identification becomes possible using the vertical beamforming ( -axis) as well that considered so far.
Although two crossing lines formed by two conical surfaces from each measurement are obtained, the line above ground can be uniquely determined because the reception by ground reflection is negligible in this antenna geometry. However, if there are sources, candidates can exist, thus the identification complexity will greatly increase.
On the other hand, in this study, the identification method using 3D models was proposed. However, as a matter of fact, the 3D models are not always available. To answer this practical question, it can be also thought to use the pictures of the panorama view of the gate taken synchronously to the data capture timing by a fish-eye camera as in  instead of 3D model of the toll gate. It typically needs the hardware reconfiguration, and hence, in this time, the further evaluation remains as a future study.
This paper proposed a novel real-time radio propagation measurement system for the ETC toll gates without closing the gate. The measurement can be made on the vehicle passing through the gates using real ETC signal transmitted by ETC roadside station. This system can obtain the angular power intensity distribution around the ETC gate in real time and identify the individual scattering object with 3D visualization method based on VRML. We also proposed a 3D visualization based identification method for the individual scattering object. Moreover, the validity of the system and identification method was demonstrated by the field experiment in a highway ETC gate.
Azimuth and Elevation Conversion Relationship
The authors would like to thank East Nippon Expressway Company Limited for the support during the field measurements in Kasama Nishi IC. The authors would also like to express the appreciation to Mr. K. Ri, Mr. Y. Choi, and T. Kan for their help in the experiment.
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