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DECT and GSM RF exposure measurements

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    DECT and GSM RF Exposure Measurements Dina SimuniC and Milan &vkoviC* University of Zagreb, Faculty of Electrical Engineering and Computing, Dept. for Radiocommunications, Unska 3,10000 Zagreb, Croatia *Ericsson-NikolaTesla, Krapinska 45,10000 Zagreb, Croatia 1. Introduction Measurements of DECT (Digital Enhanced Cordless Telecommunications system) and GSM (Global System for Mobile Communications) signals have been performed in the working environment in order to find out whether RF exposure limits are exceeded in frequency bands occupied by two mentioned systems. In the building, DECT system replaces classic wired telephony by installed DECT base stations, directly connected to PSTN (Public Switched Telephone Network). Most of base stations are in corridors, but some are very close to working tables. Coverage of a base station in the building is between 50 and 100 m; outside is up to 300 m. As reference levels for RF exposure limits IEEE values are used. 2. DECT and GSM Systems DECT has a total spectrum allocation of 20 MHz, from 1880.928-1898.208 MHz. That allocation is divided into ten carriers, each separated by 1.728 MHz. Speech and data can be transfened with a speed of 32 kbps. Since the used access is TDMA (Time Division Multiple Access), all DECT equipment is capable of working on any DECT frequency, which ensures that the cordless equipment can always choose the best possible channel. Multicarrier TDMA scheme is based on a 10 ms frame having 24 timeslots, with a common scheme for transmitting and receiving data packets of various length at 1152 kpbs within one or more of those timeslots. Since the radio signals are transmitted in bursts, there is a difference between the peak power and the average power value. When a DECT system transmits a burst, it has a peak power up to 250 mW. If it transmits during one timeslot out of 24, the average power is approximately 10 mW. The usual average output power of a base station can change from 10 mW to 125 mW, depending on a number of calls, which base station controlls. If there are two or three calls, then the average power is approximately 20-30 mW. GSM is a paneuropean cellular system, which is more and more used in Croatia. The system uses two frequency bands: one for uplink: 890-915 MHz and for downlink: 935-960 MHz, with channel spacing of 200 H z , which gives 124 channel pairs for usage. GSM is also a TDMA system, with a frame of 4.615 ms, divided into 8 timeslots of 577 ps duration. This gives 217 timeslots pe seconds, while transmitting. 3. Measurements Measurements are performed according to Fig. 1. 0-7803-6369-8/00/$10.0020000 IEEE 1096 Fig. 1 Measurement scheme In the scheme spectrum analyzer Anritsu MS710C (with a frequency range of 10 lcHz - 23 GHz) is installed on an electromagnetic transparent tripod. Calibrated half-wave dipoles are used as antennas in two different frequency ranges (DECT and GSM). For DECT measurements dipole is 0.07 m long and for GSM 0.15 m. As it can be seen from Fig. 1, DECT base station is positioned on 2.5 m height. First measurements have been performed at a distance d = 1.5 m from the base station in the corridor, and then also in the working area, behind the wall (distance d = 3.6 m). In order to perform measurements, it was necessary to find the nearlfar field distance, according to the wellknown formula: r = 2D2A (1) where r is the far field distance, D is dipole length, and h is a wavelength. At GSM frequencies r = '. 14 m, at DECT frequencies r = 0.06 m. I'herefore, we can conclude that we can measure electric field at distances d > 1.5 m in the far field and then calculate the power density from the relation: S = E21377(W/m2) (2) 3.1 DECT signal measurements As mentioned, DECT measurements have been performed at several locations: d = 1.5 m from the base station, close to a working table d = 4.5 m from the base station at heights h = 1.I-1.7 m and on the working table (d = 5.5 m, h = 1 m). The care has been taken that the calibrated dipole antenna is as far as possible from metallic surfaces, for the sake of measurement accuracy. All three orthogonal polarizations have been measured, i.e. polarization parallel with x-axis, marked as a-polarization (horizontal); one parallel with y-axis, marked as b-polarization (vertical) and the one parallel with z-axis, marked as cpolarization. 3.1.I Measurements of DECT signal in corridor Frequency spectrum of a-polarization measured in the corridor (d = 1.5m) is shown in Fig. 2. For the two other polarizations (b- and c-) measured output spectrum had the same shape, but other maximum values. As recommended in IEEE SCC 28. C9j.3-1991 Revision: Draji Recommended Practice for Measurements and Computations with Respect to Human Exposure to Radio Frequency Electromagnetic Fields [4] the height and width were changed, in order to deduce about the spatial averaged value. The values dependent on height for b-polarization are shown on Fig. 3. On the spectrum analyzer resolution bandwidth (RBW) was chosen as 3 MHz, because of the DECT channel bandwidth (1.728 MHz). REF -1 0 -20 .. - 3 0 E -40 m 0 -50 d -60 -70 -80 I I I 1 I 1880 928 1898 208 f(M Hz) Fig. 2 DECT fiequency spectrum on spectrum analyzer 1097 height h(m) 1 12 13 14 15 16 17 - ' - 2 0 " -22 5 -25 '' " I " 'I , ' Polarization a b C Measured -50 dBm -55 dBm -61dBm Power -37 5 - -40' ' ' I Fig3. Measuredpower at d=1.5 mfrom DECT base station, depending on heights h=l.l-l.7mfor b-polarization Results for all three polarizations show that the measured power values take the values between -26 dBm and -42 dBm. In order to compare these values with the RF exposure standards, it is necessary to calculate power density. Power density (W) and power (P) are interrelated with effective area, so it may be written: W=PIA, (3) where effective area of a half-wave dipole is: A, = 0.13 k2 (4) Relations (3) and (4) give for the maximum value of -26 dBm power density of 0.76 mW/mz and for the minimum of -42 dBm power density of 19.2 pW/m2. Since we are looking for the worst case exposure, the maximum will be compared to RF exposure limits. 3.1.2 Measurements of DECT signal in the working area The next measurements are performed in the working environment, which is separated from the base station by a wall (Fig. 1). The first measurement is performed very close to the wall (d = 3.6 m; h = 1.9 m). The measured values are given in Table 1: ' Polarization a b C Measured -65 dBm -40 dBm -64dBm uower Table 2. Measured power at d=4.5 m, h=1.6 m from DECT base station The third measurement refers to a position on a working table (d=5.5 m, h =1 m), shown in Table 3. Polarization a b C Measured -65 dBm -66 dBm -62dBm power In this area, the maximum of -40 dBm is measured at d=4.5 m, h=1.6 m. This value corresponds to 0.03 mWlm2.Minimum is -66 dBm, which gives 76 nW/m2. Here it is important to comment on the fact that the values, as well as spatial distribution of electromagnetic waves, show that standing waves occur. Standing waves are formed due to summation of direct and reflected electromagnetic waves. 3.2 GSMsignal measurements GSM signal measurements are performed on the same positions as DECT measurements. The same spectrum analyzer was used with another, GSM dipole antenna with 1 = 0.15 m. Frequency range of measured signal was 890 - 960 MHz. 1098 Resolution bandwidth (RBW) was for this measurement 300 kHz, because the channel spacing is 200 kHz. 3.2.1 GSM measurements in corridor Results for a-, b- and c-polarization give peak value of -47 dBm at 890 MHz (when the mobile unit was switched on). The maximum level was -52 dBm (without anybody talking on the mobile phone). This gives the maximum power density of 1.44 pW/m2,measured in GSM frequency band. 3.2.2 GSM measurements in working area Results for all three polarizations in working area at distance d = 3.6 m and height of 1.9 m show that the maximum is -52.5 dBm, whereas at d = 4.5 m, h = 1.6 m the maximum is -58 dBm. At d=5.5 m, h=l m for a-, b- and c-polarizations maximum is -57.3 dBm (0.13 pW/m2). The maximum measured value in the working area is found when the mobile unit is switched on. Therefore, this value will be compared to RF exposure limits. 4. Comparison of results with RF exposure limits Possible adverse effects of electromagnetic fields to human health have caused concem in working population. In order to estimate eventual overexposure, results of measurements will be compared to values of IEEE Standard [ 5 ] . IEEE Standard has two levels: controlled and uncontrolled environment. Since in this case persons who are workers are not aware of the potential for exposure as a concomitant of employment, the levels applicable for uncontrolled environment are taken for comparison purposes. In this frequency range (300-3000 MHz), permissible power density is f/1500 (mW/cm2), with averaging time of 30 minutes. This would Y [lean that at the central GSM frequenc 225 MHz permissible level is 6.17 W/m . AOft entral DECT frequency (1890 MHz) the maximum permissible level of power density is 12.6 W/m2. According to [4], the measured values for borh systems were spatially averaged over an area of 0.35 m width and 1.25 m height, perpendicular to earth. Comparison for GSM gives the following result: the maximum occurs with the switched on mobile unit very close to measuring equipment and it is 1.44 pW/m2. The ratio between the measured maximum and permissible level (6.17 W/m2) is 0.23* Comparison for DECT gives ratio between the measured maximum (0.76 mW/m2) and permissible level (12.6 W/m2)of 0.06*103. 5. Conclusion Far-field measurements with spectrum analyzer and two different dipole antennas have been performed. The dipoles were tuned for GSM and DECT frequency range. The results show that measured values in working area and corridor are significantly lower (five orders of magnitude) than maximum permissible exposure levels in corresponding frequency ranges, given in IEEE Standard. References [ I ] J.A. Phillips, G. Mac Namee: Personal Wireless Communication with DECT and PWT, Artech House Mobile Communications Library, 1998 Artech House, Inc. [2] The Mobile Communications Handbook, edited by J.D. Gibson, CRC Press & IEEE Press, 1996 [3] C.A. Balanis: Antenna Theory, Analysis and Design, John Wiley & Sons, Inc., 1997, New York, Chichester, Brisbane, Toronto, Singapore [4] IEEE SCC 28. C95.3-1991 Revision. Draft Recommended Practice for Measurements and Computations with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz, 1999 [5] IEEE C95.1-1991 Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz , Standards Coordinating Committees. 1992 1099

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