Standardization of Computational Techniques for Compliance Evaluation of the Human Exposure in Automotive Environments Bystander and passenger RF exposure from mobile radios with vehicle-mount antennas
Giorgi Bit-Babik and Antonio Faraone
Tilmann Wittig and Alexander Prokop
Motorola Florida Research Laboratory Fort Lauderdale, USA
[email protected]
Computer Simulation Technology, CST GmbH Darmstadt, Germany
[email protected]
Christopher Penney
Andreas Christ
Remcom, Inc. State College, PA, USA
[email protected]
IT’IS Foundation Zurich, Switzerland
[email protected]
Jianxiang Shen and Ji Chen University of Houston Houston, USA
[email protected]
Abstract — This paper presents the recent efforts to develop standards allowing numerical methods and techniques for accurate and representative simulations of human exposure to RF energy in automotive environments. The focus of this work is to standardize numerical and modeling procedures that produce repeatable results based on simulations of the human body exposure to RF energy emitted by mobile radios with vehiclemount antennas. This activity is carried out in the framework of IEEE 1528.1 and IEEE 1528.2 standards development with the goal to provide a robust methodology for evaluating the compliance of vehicle-mount transmitters with respect to international exposure safety standards by means of numerical simulations. Keywords-SAR, bystander, vehicle, antenna, mobile radio
I.
INTRODUCTION
Recent advances in computational electromagnetics and ever increasing processing power of modern computers enable quite sophisticated modeling of complex electromagnetic problems such as the human exposure to EM energy. The ability to model the anatomical details of the human body and evaluate the exposure from various RF energy transmitting devices with sufficient accuracy has eventually led to the present efforts to standardize various aspects of this process. The main goal of this standardization activity, which is carried out within IEEE Technical Committee 34 is to define the methodology to produce repeatable and accurate simulation
results suitable for compliance evaluation of mobile radio transmitters with respect to applicable exposure guidelines and standards such as International Commission on non-Ionizing Radiation Protection (ICNIRP) guidelines [1] and the IEEE C95.1-2005 standard [2]. Both documents define a Specific Absorption Rate (SAR) as the basic dosimetric quantity. Two specific metrics, namely the whole body average SAR and the locally averaged peak SAR (averaged of 10 g of tissue mass [12] or over 1 g tissue mass [3]) are used to limit human exposure to RF energy. Therefore the accurate and consistent SAR evaluation in passenger or bystander body models is a primary goal of the standards. II.
DEVELOPMENT OF THE IEEE 1528.2 STANDARD
While IEEE 1528.1 draft document describes in details the general requirements for Finite-Difference Time-Domain (FDTD) codes in order to be suitable for compliance evaluations, another draft standard, namely IEEE 1528.2 is been developed specifically to address the scenarios with passengers and bystanders exposed to RF energy emitted from vehicle-mount antennas. The IEEE 1528.2 draft defines the specific exposure scenarios, benchmark test conditions as well as details of simulation procedures with the goal to provide a simple, yet robust and conservative evaluation of exposure. Most of the mobile radios that may require SAR compliance simulations are high power transmitters used for
Figure 1. The typical vehicle with trunk mount antenna and related CAD model for simulations
public safety and related applications which operate primarily in the VHF and UHF bands. Therefore the model of a typical police car (Ford Crown Victoria 2005) has been identified as the most suitable candidate for the standard vehicle structure in the current IEEE 1528.2 draft (Fig. 1). A CAD model representing the metal parts of the vehicle body has been developed and all exposure conditions have been defined relative to this structure.
Figure 2. Example of bystander exposure at VHF from trunk mount antenna and related 1-g and 10-g SAR distributions visibly influenced by anatomical compositions of the tissues in the body.
As mentioned above, the most frequent exposure conditions from vehicle-mount antennas are at UHF and VHF bands. Since RF field penetration inside the body at VHF frequencies is significant, the anatomical composition of the human body model in those exposure conditions is important for an accurate SAR evaluation. In many instances, SAR peaks occur deep inside the body following the anatomical structures with relatively higher electric conductivities or specific shape of tissues. An example of this is clearly visible in the Fig. 2. Considering that tissue composition may play a significant role in determining the SAR distributions in the above mentioned exposure conditions, a reference anatomicallycorrect human model has been defined and developed based on the Visible Human body [4, 5]. Using the Varipose® software [6], this human body model was further articulated to create the bystander and passenger models (see Fig. 3). The model features 39 different tissues with frequency dependent electrical properties defined according to the Cole-Cole dispersion model [7-8]. The list of tissue properties can also be found in [9]. III.
EVALUATING UNCERTAINTIES
One of the critical components of a successful standard is a clear definition of all relevant procedures and numerical algorithms that have significant impact on the final assessment, in order to enable repeatable SAR evaluation using different
Figure 3. Passenger and bystander models defined for compliance simulations of exposure form vehicle mount antennas
numerical tools. However, a significant number of uncertainty sources are typically present in the overall evaluation process. Some of them are related to the approximate representation of compliance conditions by means of predefined simulations models while others are due to limited accuracy of simulation tools and variations between the numerical codes themselves. Practice has shown that even with well defined procedures and algorithms there might be significant variations between the results obtained using different software tools especially when an anatomically-correct human body is part of the simulated configuration [10]. To assess and possibly minimize the degree of these variations in numerical simulations specifically related
to human body exposure, it is reasonable to identify and separate the various sources of variations. As the first step, the uncertainty of human body exposure to well defined field needs to be evaluated. Plane-wave exposure (see Fig. 4) is an ideal case since it eliminates the need of considering other possible sources of variation in more complicated exposure conditions, like antennas, vehicle body parts, etc. Three different numerical tools have been employed to simulate a plane wave exposure of the IEEE 1528.2 standard bystander model in the VHF band (150 MHz). Four different exposure conditions with vertically-polarized plane-waves impinging on the body from four different directions (front, right, left, and back) have been analyzed using the XFDTD®, CST MICROWAVE STUDIO® [11], and SEMCAD X® [12] software packages. In addition, the same simulation tool was used independently in two different labs increasing the total number of independent simulations to six. The computed 1-g and 10-g SAR distributions and their peak values, along with whole-body average SAR values have been compared. Evaluation of the 1-g and 10-g local peak SAR was done according to the IEEE standard procedure [2].
IV.
SUMMARY
Standardization of human exposure simulations involving anatomically-correct human body models requires evaluation of possible sources of variations to define consistent and repeatable test procedures applicable for RF safety compliance evaluation. Apart from defining the specific compliance exposure configurations, the standardization of the numerical algorithms such as point SAR computations and SAR averaging is very important. Some initial results suggest that different numerical codes produce very similar whole-body average SAR results for the same exposure scenario that vary only within few percent in all simulated conditions. The 1-g and 10-g peak SAR values, in general, show larger variations, but still within 20-25% from the average values in most cases. In few instances, the variation of 1-g SAR has been much larger which may indicate that there are some differences in SAR computing and averaging algorithms used in different codes. Those differences, as well as possible differences in handling the heterogeneous human body model, need to be further analyzed to minimize the overall uncertainties of SAR evaluations. REFERENCES [1]
Back exposure
Front exposure
Side exposure
Figure 4. Bystander model employed in plane wave exposure evolution using different numerical tools and typical 1-g SAR distribution in sagittal plane for back, front and side incidence.
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