Rapid parametric SAR reconstruction from a small number of measured E-field data : validation of an ellipsoidal model

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1 1 EUROPEAN COOPERATION IN THE FIELD OF SCIENTIFIC AND TECHNICAL RESEARCH EURO-COST SOURCE: SAGEM SA, France COST 273 TD(03)098 Paris, France 2003/May/21-23 Rapid parametric SAR reconstruction from a small number of measured E-field data : validation of an ellipsoidal model Benoît Derat SAGEM SA 2 rue du petit Albi Osny Cergy-Pontoise Cedex, FRANCE Phone : Fax : benoit.derat@sagem.com Olivier Merckel*, Jean-Charles Bolomey* and Gilles Fleuryº Supélec/Dre*, Supélec/Meº 3 rue Joliot-Curie Gif-sur-Yvette, France olivier.merckel@supelec.fr

2 2 Abstract: The Specific Absorption Rate (SAR) designates the electromagnetic power density deposited per unit mass of biological tissue. SAR measurements are required to assess the compliance of mobile phones with existing standards and recommendations. In this paper, a parametric reconstruction of the E-field distribution from a small number of measurements is shown to allow a significant reduction of the duration of SAR measurements. The measurement time is indeed reduced to less than a minute, while the standard way takes about 10 minutes for a complete phone-phantom configuration involving several hundreds E-field data points. This technique, which is fully compatible with most of the existing dosimetric setups has been initially validated on a truncated spherical phantom. It is extended here to the SAM phantom, which is imposed by the CENELEC pren50361 standard. Keywords: SAR, rapid, propagation, parametric reconstruction, ellipsoidal model. 1. Introduction The experimental electromagnetic dosimetry of mobile phones has been much developed these past years. Most of the existing dosimetric facilities utilize automatic positioning systems to move an E- Field detected probe, with the help of robotized arms [1], or three axes displacement systems in order to achieve SAR (Specific Absorption Rate) measurements (Fig. 1). The European Standard pren50361 [2] details the way to measure the SAR in a head-like phantom, and stipulates that the maximum value of the SAR averaged in 10 g must not be larger than 2W/Kg. The dielectric properties of the liquid filling the phantom are expected to produce SAR values similar to those calculated in human head models. According to the European standard, 12 various configurations (frequency, phone phantom arrangement) must be considered for a dual-band phone. Consequently, a complete phone test lasts now about half a day, pointing out a new concern: the rapidity of SAR measurement. The rapid SAR measurement method proposed here is fully compatible with popular instrumentation and, hence, can be directly implemented on most existing SAR measurement facilities using mechanical scanning of an E-field probe [4]. The number of electric field data is reduced to approximately 30 samples, what allows to reduce the measurement time (acquisition and postprocessing) to less than a minute. This approach has been already validated in the case of the truncated spherical phantom of the CENELEC recommendation, which was prevailing before the norm [3]. The relative simplicity of the E-field distribution in the phantom, the most often shaped as a potatoe, and its reasonable reproducibility from one mobile phone to another one, have suggested to consider a generic ellipsoidal model to describe this field distribution. The reduced number of parameters of this model, namely 11, allows to reconstruct the electric field in the whole data volume, from a small number of measurements. The acquisition time is then drastically reduced, while the parametric inversion can be performed within very reasonable computation time (on the order of a few seconds, without any optimization). The results presented in the last section show SAR reconstructions of many commercial phones from three different sets of data : i) the truncated spherical phantom [5] and ii) the SAM phantom in the Supélec facility, and iii) the SAM phantom in the SAGEM dosimetric facility. These results aim to show that the parametric reconstruction method can be applied on various phantom shapes, and whatever the dosimetric setup. The knowledge of a maximum of 36 E-field measurements is sufficient for obtaining accurate SAR values. The reconstruction algorithm does not require the knowledge of the frequency, the position of the phone, nor the shape of the phantom.

3 3 (a) Figure 1: Truncated hemispherical phantom in Supélec (a) and SAM phantom in SAGEM (b) used for the SAR measurement of phones. 2. Ellipsoidal model The ellipsoidal model used for describing the E-field distribution in a phantom is the following one: ( ) = α( κ) ( ) + ( ) + ( ) 2 (b) where r(x, y, z) is the vector of positions of the E-field measurement points in the phantom. The vector ( ) = ( χ) ( β) ( α) ( ) is obtained with the rotation matrices built from the Euler rotation angles,,. p e (E 0, k, x s, y s, z s, a, b, c,,, ) is the vector of parameters, where E 0 represents the field amplitude, k is similar to the propagation constant, x s, y s and z s are the coordinates of the centre of the ellipsoid, and a, b and c its semi-axes. The configuration represented below (Figure 2) shows an ellipsoidal envelope, cut by a measurement plane inside the hemi-spherical phantom a little upper from the bottom. The part of the ellipsoid reconstructed outside the phantom shell, evidently, cannot be used to describe the real propagation. The physical sense of such a description matches with reality only inside the liquid. Figure 2: Representation of the ellipsoidal model of the electric field, inside the phantom, with the data acquisition plane. The real cut shown at the right is issued from a real measurement The values of the parameters are determined by fitting the measured values with those predicted by the model. The determination of the parameters requires a non-linear iterative procedure (Levenberg- Marquardt) to be used, with appropriate initialization and constraints for ensuring convergence.

4 4 For the sake of comparison, Figure 3 shows, in the particular case of the truncated spherical phantom, the standard 726 E-field points distribution used in the Supélec facility and the reduced set of points (36 and 21) used for the parametric reconstruction. The parametric approach can be used for determining 1g and 10g averaged SAR values. However, the sampling points configurations shown on Figure 3 have been more particularly optimized for the 10g case. That is the reason why the results presented hereafter correspond only the 10g case. Figure 3: E-field points distribution in the truncated spherical phantom (standard method with 726 and reduced with 36 and 21 points) 3. Results on real phones data Supélec truncated hemispherical phantom Data The reconstruction processes have been tested on 128 measurements corresponding to 60 different dual band (GSM 900 and DCS 1800) mobile phones. Figure 4 presents the reconstructed maximum averaged SAR in 10 g (European standard) from 21 and 36 points versus the averaged standard SAR value obtained from 726 E-field data points, in the case of the truncated spherical phantom.. It must be noticed that the quality of the reconstruction is bound to the geometrical form of the real field, but also to its intensity. The method is then less efficient when the measured field strength is near the probe's sensitivity. The mean error in the reconstruction is a few % in the case of 36 sampling points, and may be compared with the global error made with standard dosimetric facilities, which is commonly recognized to be of the order of 30 % to 40 % for 90% confidence. A significant decrease of the accuracy can be observed in the case of 21 sampling points. a b Figure 4: Truncated spherical phantom at Supélec. Reconstructed [21 points (a) and 36 points (b)] versus standard [726 points] SAR integrated in 10g.

5 5 Supélec SAM phantom data Preliminary results shown in Figure 5 present the SAR reconstructed from 32 sampling points in the SAM phantom, versus the SAR calculated from the whole E-field data, i.e. 605 points in the Supélec facility. A total number of 42 phone measurements have been made, for three dual-band phones, in the GSM 900 and DCS 1800 frequency bands, and for different configurations, such as left and right head, upper, middle and lower band, and cheek or tilt position. The correlation between the results of the standard approach and of the parametric reconstruction technique looks quite good, showing that this technique can be applied whatever the shape of the phantom used. Figure 5: SAM Phantom at Supélec. Reconstructed [32 points] versus standard [605 points] SAR integrated in 10g. SAGEM SAM phantom data The aim of these measurements was, firstly, to increase the number of tested phones, but also to validate the portability of the parametric reconstruction method to a new dosimetric facility. Indeed the SAGEM dosimetric facility (ANTENNESSA) utilizes a different mesh of measurement points with respect to Supélec. Furthermore, algorithms of zoom scanning and post-processing are also quite different from those used in Supélec. A statistical analysis of the position of the maximum local SAR on the SAM phantom surface provided by ANTENNESSA has allowed to reduce the amount of points used for a standard SAR calculation in SAGEM, e.g. 325 points. All these differences between the Supélec and SAGEM setups may impact the observed parametric reconstruction efficiency. Indeed, the E-field points used for implementing the parametric reconstruction are extracted from existing full measured data set. Figure 6 presents a comparison of the SAR calculated from the 325 E-field sampling points with the one reconstructed from a 21 and 36 sampling points. A total of 66 phone tests have been made for 12 dual-band phones of any kind (clamshells or barphones with integrated or external antenna), for three channels (low-mid-up) in the GSM 900 or DCS 1800 bands, and for different configurations (each standard configuration has been tested at least a few times). The good correlation between the full measurements and the parametric reconstruction procedure for the 10g averaged SAR confirm the reliability of this method for any kind of SAR assessment system using the SAM phantom.

6 6 a b Figure 6: SAM Phantom at SAGEM. Reconstructed [21 points (a) and 36 points (b)] versus standard [325 points] SAR integrated in 10g. 4. Conclusion The parametric approach proposed in this paper is compatible with standard dosimetric measurement systems using E-field detected probes. It allows to calculate the electric field and the SAR from a reduced number of measured data points. The local SAR and the maximum averaged SAR can be directly obtained from the knowledge of the 11 parameters of the ellipsoidal model. This parameters are estimated from a maximum number of 36 amplitude-only E-field points. The accuracy of the parametric reconstruction approach has been assessed on more than 100 phone tests, either with truncated spherical or SAM phantoms, on two different SAR measurement setups. By comparison with the results deduced from the standard number of points, the error made on the 10g averaged SAR is about 5%. In the same time, the parametric approach allows to reduce the measurement time for a given configuration to less than 1 minute, whereas the data processing time is negligible. It is suggested that the parametric reconstruction algorithms and codes developed by Supélec could be further validated, thanks to a multi-centre assessment procedure. A successful confirmation of the efficiency of the parametric approach could make it a good candidate for future standard SAR measurement procedures, without any additional cost with respect to existing instrumentations. 5. References [1] Schmid, T., Egger, O., Kuster, N., Automated E-Field scanning system for dosimetric assessments, IEEE Transactions on Microwave Theory and Techniques, vol. 44, n 1, January [2] pren50361, Basic standard for the measurement of Specific Absorption Rate related to human exposure to electromagnetic fields from mobile phones (300 MHz 3 GHz), CENELEC TC211 European standard, Bruxelles, June [3] Manning, M., Massey, P., Rapid SAR Testing of Mobile Phone Prototype Using a Spherical Test Geometry, IEE Technical Seminar on Antenna Measurements and SAR (AMS 2002) May, , Loughborough University, UK. [4] Merckel, O., Fleury, G., Bolomey, J.-Ch., Rapid SAR Measurement via Parametric Modeling, 5th International Congress of the European BioElectromagnetics Association, p75-77, Helsinki, Finland, September [5]prES 59005, Considerations for evaluation of human exposure to Electromagnetic Fields (EMFs) from Mobile Telecommunication Equipment (MTE) in the frequency range 30 MHz 6 GHz, European standard, CENELEC, Bruxelles, December 1997.