1 COST 2100 TD(09) 814 Valencia, Spain 2009/May/18-19 EUROPEAN COOPERATION IN THE FIELD OF SCIENTIFIC AND TECHNICAL RESEARCH ———————————————— EURO-COST ———————————————— SOURCE: University of Bologna, Fondazione Ugo Bordoni, ARPA Emilia Romagna Italy Evaluation of exposure levels generated by WiMax systems Marina Barbiroli Claudia Carciofi Doriana Guiducci Silvia Violanti DEIS - University of Bologna Villa Griffone I-40037 Pontecchio Marconi Bologna ITALY Phone: + 39 051 846854 Fax: + 39 051 845758 Email: marina.barbiroli@unibo.it Fondazione Ugo Bordoni Villa Griffone I-40037 Pontecchio Marconi Bologna ITALY Phone: + 39 051 846854 Fax: + 39 051 845758 Email: {ccarciofi,dguiducci}@fub.it ARPA Emilia RomagnaSezione di Piacenza Via XXI Aprile, 48 I-29100 Piacenza Tel: 0523/489666 Fax: 0523/482480 Email: sviolanti@arpa.emr.it 2 Evaluation of exposure levels generated by WiMAX systems Marina Barbiroli DEIS – University of Bologna Villa Griffone I-40037 Pontecchio Marconi (BO) ITALY email: marina.barbiroli@unibo.it Silvia Violanti ARPA Emilia-Romagna Sezione di Piacenza Via XXI Aprile, 48 I-29100 Piacenza email: sviolanti@arpa.emr.it Claudia Carciofi Doriana Guiducci Fondazione Ugo Bordoni Villa Griffone I-40037 Pontecchio Marconi (BO) ITALY email: {ccarciofi, dguiducci}@fub.it Abstract In this work the impact of WiMAX systems is investigated in terms of electromagnetic field exposure evaluations. This study is performed in collaboration with the Regional Protection Agency (ARPA), in order to identify some guidelines to simplify the environmental evaluation procedures. According to the information given by the operators, typically WiMAX installations have been analyzed by means of simulation tools. The obtained results show that exposure due to these systems is compliant with the limits provided by the regulation. The ongoing activity foresees also the realization of measurement campaigns to validate simulation results. 1. Introduction The recent assignment of WiMAX frequency rights in Italy has brought to an increasing demand for new base station (BS) installations. New WiMAX operators are now starting to deploy their network for Broadband Wireless Access (BWA) as an alternative to wired connections both in densely populated areas and in rural areas, where broadband access is lacking (the so called Digital Divide areas). It is currently debated whether the necessary deployment of a large number of base stations will encounter the same hostile perception from the population, often stimulated by an excessive perception of risk inherent to electromagnetic field exposure, already seen in the case of cellular systems. In Italy the scenario is even more complex then in other European countries, since the regulation is stricter by far than what recommended by the European Council [1] according to the ICNIRP guidelines [2]. Exposure limits are in fact enforced by law [3] [4] to 20 V/m (3÷3000 MHz) and 40 V/m (3÷300 GHz) in generic areas accessible to the public and to 6 V/m (irrespective of frequency) where the average daily permanence shall exceed four hours and, in general, in any highly attended public place. This paper gives an evaluation of electromagnetic field (EMF) exposure due to WiMAX system, considering typical installations in real scenarios. The exposure levels generated by WiMAX Base Stations is firstly evaluated computing the corresponding “respect volume”, defined as the portion of space in proximity of a source outside which the compliance with current Italian regulation is guaranteed, then a more accurate estimate of the impact is also performed by means of level curves, using more accurate models. Simulations were performed with the ArGIS tool, distributed by Wireless Future s.r.l. [5], where a 3D ray tracing model is 3 embedded [6][7], which is suitable for all kinds of sources, such as macro- and microcells of mobile radio systems (GSM/GPRS, UMTS, etc.), broadcast transmitters, radio relays and so on. Moreover, the activity issued in this paper is performed in collaboration with the Regional Protection Agency (ARPA), in order to identify some guidelines to classify typical WiMAX site installations and simplify the environmental evaluation procedures, mandatory to authorize BSs deployment. WiMAX installations have been classified according to the information given by the operators, regarding the type of antenna (omnidirectional, directive), antenna height and location (e.g. on rooftop or on trellis), transmitted power. To this end we have identified trellis-, rooftop- and street lamp- installations, which can be equipped with omni- or directional-antennas. Firstly some main characteristics of WIMAX systems relevant to this work’s extent are summarized in section 2. Then section 3 reports the results obtained for the chosen WiMAX system installations. 2. The WiMAX system The IEEE 802.16 standard [7], dubbed WiMAX (Worldwide Interoperability for Microwave Access), is a metropolitan area network (MAN) system used to provide broadband access, alternative to fixed access for last mile connection. One of its advantages lies in its capability to implement point-to-multipoint networks that can support different kinds of traffic with different Quality of Service (QoS) requirements, that make it suitable to cater for the needs of heterogeneous users. The standard comes in two versions: IEEE 802.16d that provides broadband services to fixed and nomadic users and IEEE 802.16e that supports full user mobility at vehicular speed. Table 1 recalls the main characteristics of the two versions of the standard. The frequencies of interest in Europe is the licensed 3.4 – 3.6 GHz band. Table 1. RECALLS OF MAIN TECHNICAL CHARACTERISTICS OF WIMAX STANDARD Spectrum management Frequency bands (GHz) Bit rate Modulation/Multiple access IEEE 802.16d Licensed/ Unlicensed 3.5, 5 Up to 40-45 Mbit/s bi-directional (2x7 MHz channel) OFDM IEEE 802.16e Licensed/Unlicensed 2.3, 2.5, 3.5, 3.7, 5 Up to 30-35 Mbit/s bi-directional (10 MHz channel) Scalable OFDMA The OFDM modulation guarantees a high coverage range, because it is suitable to operate even without line of sight and the adaptive modulation ensures high bit rates by choosing the most appropriate scheme for the actual propagation environment. The scalability of the modulation combined with the appropriate access control allows several users (up to some hundreds) to share the bandwidth and to transmit without collisions. Typical WiMAX links include transmission between both fixed and mobile points. The BS antennas have a strong directivity in the vertical plane, while the transmitted power depends on the number of carriers. The OFDM modulation allows good performance with high bit rate and its robustness to interference allows it to operate with small transmitted power. 4 3. Electromagnetic exposure evaluation According to information given by one of the licensed operator deploying its own network in Italy [E-VIA s.p.a. gruppo Retelit s.p.a.], 4 different typical WiMAX sites have been identified: • • • • Piezometric tower Trellis Pole Rooftop All the different classes of installations can be equipped either with 2- or 3- sector BS antennas or with omnidirectional antennas. The basic antennas characteristics are summarized in Table 2. Table 2. ANTENNAS CHARACTERISTICS ARGUS SPX310F Shenglu SL13123A 3.3 – 3.8 3.4 – 3.6 Gain (dBi) 18 11 Polarization ±45° vertical Sectorial 65° omnidirectional 6.5° 9° 760x126x69 (height)≤ 750 Frequecy (GHz) Diagram Type Vertical –3dB beam width Dimension (mm) The electromagnetic impact of each different type of installation has been evaluated considering both sectorial and omnidirectional BS antennas, transmitting respectively a maximum power of 10 Watt for each sector and a total power of 10 Watt for omnidirectional BSs, which are equipped with two antennas for diversity purposes (e.g. MIMO). For sectorial BS installations the operator foresees to use tilt angles varying from 0° to 8°: in this study we focus mainly on the two extreme cases of mechanical tilts equal to 0 and 8 degrees. Simulations have been performed by means of the ARGIS tool. It supports a detailed description of the propagation environment (3D map of buildings, characterisation of their electromagnetic properties) and of the considered field sources (geometrical characteristics, height, power, radiation patterns, tilt, azimuth, etc.). Field predictions can be made either by free space propagation law (for a quick estimate of field levels especially for macrocells or small cells) or by a full three-dimensional ray tracing algorithm [6][7], which is particularly suitable for microcells and indoor picocells. The latter model allows to take into account the interactions (i.e. reflections, diffractions, refractions) of electromagnetic waves with the surrounding environment. The received field is computed as the vector sum of all the contributions (rays) that reach the evaluation point, after a given maximum number of interactions with the environment. For all four different type of WiMAX sites, the exposure levels have been calculated assuming free space propagations conditions, according to the preventative approach usually followed in Italy. 5 Only for the rooftop site a deeper analysis of exposure levels has been performed using the ray-tracing ARGIS tool: field exposure has been investigated for the building on which the BS is located and for the surrounding buildings. The propagation models and formulas, used to identify the areas where Italian exposure limits could be exceeded, are in accordance with the Guidelines CEI 211-10 [9]. For precautionary reasons, the maximum possible transmitted power for the BS is assumed and free space propagation is considered. 4. The respect volume The first analysis on WiMAX installations has been done estimating the respect volume, which is a parallelepiped characterised by the dimensions reported in Figure 1, outside which the electrical field never exceeds a certain threshold (Eth), usually equal to the exposure limit. A more accurate estimate of the EMF impact can also be achieved by means of level surfaces, which define exactly the shape of the respect volume. The parallelepiped clearly inscribes this latter volume, allowing a more conservative estimate of the region where the human presence should not be permitted to avoid excessive exposure. This is the reason why the parallelepiped shape is generally preferred. height width length Figure 1. The respect volume In the current work, the distances used to define the respect volume are computed with the free space propagation law (1): (1) di = P " G(#,$ ) " 30 E[V /m] where di is the i-th distance, E is the electrical field strength, P is the output power, G (θ,ϕ) the antenna gain in the direction of interest. The free space formula allows a quick and simple evaluation of the received field and yields a more conservative estimate than other more refined computation methods. As field limit we imposed the “attention threshold” of 6 V/m defined by the Italian law, even though in many cases the “exposure limit” is relevant, which at 3.5 GHz is 40 V/m. The latter value applies everywhere except in areas where people do not sojourn continually for more than four hours and in public places [3][4]. 6 Table 3 and Table 4 show the dimensions of the respect volume respectively for the three-sector and omdirectional installations. Mechanical tilt is disregarded, since its potential presence only reflects in a different orientation of the parallelepiped in the space. Table 3. RESPECT VOLUME FOR THE THREE-SECTOR ANTENNA ARGUS SPX310F Length (m) Width (m) Height (m) PTX (W) 40 V/m 6 V/m 40 V/m 6 V/m 40 V/m 6 V/m 10 3.5 23.1 2.6 17.4 0.5 1.8 Table 4. RESPECT VOLUME FOR THE OMNIDIRECTIONAL ANTENNA SHENGLU SL13123A TWO ANTENNAS 20λ SPACED , PTX PER ANTENNA 5 W PTX Length (m) Width (m) Height (m) (W) 40 V/m 6 V/m 40 V/m 6 V/m 40 V/m 6 V/m 5+5 2.8 19.4 2.3 18.0 0.6 5.8 The above results are encouraging even in densely populated urban areas, where the BSs can be deployed on building tops or on other infrastructures such as poles, because their close proximity, where exposure levels could be higher, is typically not accessible to the public. It is also to be mentioned that at 3.5 GHz the attenuation due to building walls is much higher than at cellular frequencies, so that the indoor field is strongly attenuated by the penetration loss, thus remaining well below the limits. In case of co-siting among different operators, we must consider the contribution of all field sources. Thus, equation (1) becomes: N N (2) E Tot [V /m] = " E i2 = i=1 " P # G ($,% ) # 30 i i i=1 d where N is the number of sources. In the simple case where all the sources have the same irradiation characteristics, the dimensions of the overall respect volume are obtained by multiplying by N those computed for a single source. 5. Level curves of the electromagnetic field The level curves shown here below, are evaluated on an horizontal plane at the same antenna height and on a vertical plane along one of the maximum radiation directions. The effect of the mechanical tilt has been taken into account, referring to the extreme values of 0 and 8 degrees. Obviously in real installations the more pronounced the tilt, the higher the BS antenna height will be. Level curves are referred to 40 V/m and 6 V/m. 7 5.1 Three-sector antenna ARGUS SPX310F 5.1.1 TILT 0° 50 40 30 20 10 0 40 V/m -10 6 V/m -20 -30 -40 -30 -20 -10 0 Figure 2. 10 20 30 40 Horizontal plane 30 25 20 40 V/m 6 V/m 15 10 5 0 -20 -10 0 10 Figure 3. 5.1.2 20 30 40 50 Vertical plane TILT 8° For mechanical tilt equal to 8 degree, prediction on the horizontal plane are disregarded since of little interest in real cases. 8 25 50 40 V/m 6 V/m 45 40 35 30 -20 -10 0 10 20 Figure 4. 5.2 30 40 50 Vertical plane Omnidirectional antenna Shenglu SL13123A For these evaluations, two omnidirectional antennas have been considered with a 20λ spacing typical for MIMO purposes. Because of the OFDM modulation, the overall electric field has been computed through the power sum of the signals generated by each omnidirectional source. 25 20 15 10 5 40 V/m 0 -5 6 V/m -10 -15 -20 -25 -20 -15 -5 -10 Figure 5. 0 5 10 15 20 25 Horizontal plane 9 25 40 V/m 6 V/m 20 15 10 5 0 -20 -10 Figure 6. 6. 0 10 20 Vertical plane Analysis of rooftop installations with ray-tracing algorithm For the first three types of installations (piezometric tower, trellis, pole) the evaluation of respect volumes ad level curves with free space propagation formula can be considered exhaustive to characterize exposure levels. The analysis of more complex WiMAX sites should require considering the effect of buildings or other objects of the scenario by means of more sophisticated propagation models. Therefore a more detailed analysis is performed for a three-sector rooftop BS, where exposure levels are evaluated using the ARGIS 3D ray-tracing tool. In Figure 7 different exposure levels of a typical rooftop installation have been obtained on an horizontal plane at the same antenna height, assuming a mechanical tilt equal to 0 degree. Simulation results are layered over the building map. For a deeper investigation of possible exposure of the public, EMF field levels have also been calculated in an horizontal plane 2 m above the rooftop ridge, since it represents a potential accessible area. In our evaluation we have assumed the mechanical tilt equal to 0 degree (Figure 8) and 4° (Figure 9), since a more pronounced tilt does not fit rooftop installations. Finally in Figure 10 and Figure 11 the iso-level curves at 40 V/m and 6 V/m are shown in the vertical plane along the maximum radiation direction of the sector 1, respectively for mechanical tilt equal to 0 and 4 degrees. It can be noticed that exposure levels remain below the imposed limits both in the considered horizontal and vertical planes. 10 Figure 7. Horizontal plane at the same antenna height (25.36 m), tilt 0 degree Figure 8. Horizontal plane at 2 m height above the rooftop ridge (22 m) tilt 0° 11 Figure 9. Horizontal plane 2 m above the rooftop ridge (22 m) tilt 4 degree 30 25 6 V/m 40 V/m 20 15 10 5 0 -10 Figure 10. 0 10 20 30 40 50 Vertical plane along the maximum direction, sector 1, tilt 0° 12 30 25 40 V/m 6 V/m 20 15 10 5 0 -10 0 Figure 11. 7. 10 20 30 40 50 Vertical plane along the maximum direction, sector 1, tilt 4° Conclusions and further work This work has presented an overview on the impact of WiMAX systems in terms of exposure to electromagnetic fields on population. Evaluations have been made identifying typical WiMAX sites, which are classified according to the BS antenna type and installation. The obtained results highlight that installations are generally compliant with the exposure limits provided by the regulation, which in Italy is particularly strict. The work is still in progress and future activities will concentrate also on complex urban (dense urban or cositing scenarios) and indoor installations to have a more complete knowledge of realistic EMF exposure levels. The ongoing activity foresees also the realization of measurement campaigns to validate simulation results and further investigate the WiMAX installations impact. Measurements will be realized in collaboration with ARPA, as soon as the number of active BSs becomes significant, as the network deployment in Italy is at its very first stage so far. 8. References [1] 1999/519/EC, Council Recommendation of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz), available on http://eur-lex.europa.eu/en/repert/1530.htm [2] ICNIRP, Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz), Health Physics Vol. 74, No 4, pp 494-522, 1998; revised and extended edition in: R. Matthes, J.H. Bernhardt, A.F. McKinlay (eds.): International Commission on Non-Ionising Radiation Protection 1999, ISBN 3-9804789-6-3 [3] Framework Act, “Legge quadro sulla protezione dalle esposizioni ai campi elettrici, magnetici ed elettromagnetici” n. 36, 22nd February 2001, G.U. n. 55, 7th March 2001 [4] Actuation Decree, DPCM 8th July 2003, G.U. n. 199, 28th August 2003 [5] Wireless Future official site, http://www.wirelessfuture.it/ [6] G. E. Corazza, V. Degli-Esposti, M. Frullone, G. Riva, “A Characterisation of Indoor Space and Frequency Diversity by Ray-Tracing Modelling”, IEEE Journal on Selected Areas in Communication, Vol. 14, No. 3, pp. 411- 419, April 1996 13 [7] V. Degli-Esposti, D. Guiducci, A. de'Marsi, P. Azzi, F. Fuschini “An advanced field prediction model including diffuse scattering”, IEEE Transactions on Antennas and Propagation, Vol. 52, Issue 7, July 2004, pp. 1717 - 1728. [8] IEEE 802.16, IEEE Standard for Local and Metropolitan Area Networks, www.ieee.org [9] Italian Technical Norm CEI 211-10, “Guideline for the realisation of Base Stations in compliance with exposure limits to radiofrequency electromagnetic fields”, available on http://www.ceiuni.it (only in Italian).