, pp. 270-279. A Publication of the Measurement of Output Power Density from Mobile Phone as a Function of Input Sound Frequency Emanuele Calabrò and Salvatore Magazù Department of Physics, University of Messina Viale D Alcontres, 31 I-98166 Messina, Italy Received: May 15, 2013 Accepted: August 15, 2013 ABSTRACT Measurements of power density emitted by a mobile phone were carried out as a function of the sound frequency transmitted by a sound generator, ranging from 250 to 14000 Hz. Output power density was monitored by means of the selective radiation meter Narda SRM 3000 in spectrum analysis mode, and the octave frequency analysis of each tone used for the experimental design was acquired by the sound level meter Larson Davis LxT Wind. Vodafone providers were used for mobile phone calls with respect to various local base station in Southern-Italy. A relationship between the mobile phone microwaves power density and the sound frequencies transmitted by the sound generator was observed. In particular, microwaves power density level decreases significantly at sound frequency values larger than 4500 Hz. This result can be explained assuming that discontinuous transmission mode of global system for mobile communications is powered not only in silence-mode, but also at frequencies larger than 4500 Hz. KEYWORDS: Mobile phone; microwaves; power density; sound frequency; spectrum analysis. INTRODUCTION In the past few years, the use of mobile phones has increased considerably and it is considered an essential means of communication. Nevertheless, this explosive growth of mobile communication has brought up several problems about possible health effects related to exposure in the range of radio-frequencymicrowaves (RF-MWs) from mobile phones and base stations. Indeed, RF-MW electromagnetic radiation is used in mobile phones to transmit information between handsets and base stations. In spite of the great number of studies carried out up to now, the possibility of induction of biological and health effects by low energy levels MWs emitted by mobile phones remains a controversial issue. Indeed, knowledge about the adverse effects of RF and MW radiation on human health, or the biological responses to their exposure, is still limited. As cellular telephone technology has advanced, the modulation patterns have become increasingly complex and difficult to investigate possible health effects of mobile phone radiations on organic systems. A response in many types of neurons in the avian central nervous system was observed due to RF-MW radiations exposure [Beasond and Semm, 2002]. It was shown that MWs emitted 270
by mobile phones can change blood brain barrier permeability and produce oxidative damage in brain tissues [Fritze et al., 1997; Salford et al., 2003]. Heat-shock proteins expression of human neuronal-like cells were observed to change after exposure to MW radiations [Calabrò et al., 2012a]. Similar results led to the recommendation to minimize exposure to MWs according to the guidelines for exposure limits to electromagnetic fields (EMFs) of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [ICNIRP, 1998]. Otherwise, it was shown that the exposure limits to electromagnetic fields (EMFs) recommended by the ICNIRP can be exceeded during a call on a mobile phone [Calabrò and Magazù, 2010]. Moreover, it was observed that exposure to MWs can produce alterations also within the limits suggested by the Guidelines of ICNIRP; indeed mobile phone MWs can also affect simple organic systems, such as a protein s structure, inducing alterations into protein s linkages and protein s aggregation [Calabrò and Magazù, 2012b, 2013]. These results have suggested the study of some mechanisms to minimize exposure to MWs during mobile phone calls. For instance, a relationship between the emitted EMF intensity and the Leq intensity of the sound transmitted from a mobile phone was observed by Ozgur and Güler [2010], that found a critical value of sound intensity to which an increase in EMF intensity corresponds. The aim of this work was to verify the existence of a threshold frequency above which the intensity of the MWs power density emitted by a mobile phone decreases regardless of the sound which is used. Mobile phone systems generally use different frequency bands and coding methods, but global system for mobile communications (GSM) is the most widely used system for mobile phone communications in the world, which operates in the 900-MHz and 1800-MHz bands in Europe and Asia and in the 850-MHz and 1900-MHz bands in the United States. GSM system is based on different techniques, involving communication techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA) and discontinuous transmission (DTX). GSM phones transmit pulsed signals, which consist of short carrier wave bursts of 580 μs of duration, at repetition frequency of 217 Hz. The duty factor is 1/8 for the mobile phone and varies from 1/8 to 8/8 for the base station. Indeed, the GSM signal is pulsemodulated at a frequency of 217 Hz with a frame length of 4.6 ms and each frame is divided into eight slots with a pulse width of 0.580 ms, allowing eight simultaneous calls on the same channel. TDMA allows eight users to share the same channel, that is the same frequency band. A large number of base stations is generally used in a city and each carrier also runs one central office called the Mobile Telephone Switching Office (MTSO). Mobile phones have special codes associated with them which are used to identify the phone, the phone s owner and the service provider. The control channel is a special frequency that the phone and the base station use to talk to one another about things like call set-up and channel changing. When the system identification code (SID) is received, the phone compares it to the SID programmed into the phone and if the SIDs match; then the two mobile phones communicate. Furthermore, FDMA puts each call on a separate frequency, allowing 124 MW channels of 200 khz wide, which can be used simultaneously. Each base station use a different set of channels to serve mobile phones to avoid interference with neighboring base stations. There are lower frequency pulsed components associated with Time Division 271
Multiple Access (TMDA) at 217 Hz and 8.34 Hz and a 2 Hz. TDMA assigns each call a certain portion of time on a designated frequency. It allows several users to share the same RF channel by dividing the data stream into time slots allocated to each user. Using TDMA, a narrow band that is 30 khz wide and 6.7 milliseconds long is split time-wise into three time slots, so that each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. CDMA gives a unique code to each call and spreads it over the entire available frequencies bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned a unique sequence code. Finally, DTX is produced when the user is connected but not speaking. This mode allows to generate lower intensity electromagnetic field (EMF) reducing exposure to MWs radiation. This operation mode of the GSM system suggested to investigate if sounds can be transmitted in a particular frequency range during DTX function, and to verify the possibility of using this operative mode to transmit a signal from a mobile phone minimizing EMFs exposure during mobile phone working. However, a relevant number of phones calls were carried out to provide reliable results which do not depend on the code assigned to the call. MATERIALS AND METHODS A software synthesizer was used to generate sound tones at frequencies in the range from 250 Hz to 14000 Hz. Each sound produced by the synthesizer was received by a mobile phone working and transmitted to another mobile phone. A selective radiation meter Narda SRM 3000 was used to perform spectral analysis of MWs emitted by the mobile phone at a distance of 3.5 cm from the phone during the transmission of the sound. The device was linked through a cable to a Narda three axis antenna, covering the frequency range from 75 MHz to 3 GHz. In spectrum analysis mode, all the frequency components in the environment can be detected and measured. An integrating sound level meter type Larson Davis LxT was also used to perform the octave frequency analysis for monitoring the Leq intensity of sounds during exposure. The Narda SRM-3000 device can automatically define a suitable RBW, depending on the selected frequency span. This function gave the value RBW = 6 MHz for the spectral analysis performed in this study. The average mode was chosen as result type, and the average of the measured values were taken over a number of 16 results. In addition, time analysis mode was also used to monitor the power density and the related electric and magnetic field components, at a fixed frequency, during exposure. The mobile phone models Nokia 1200, Nokia 1208, Samsung SGH-C140, Samsung Star-II, and Sony Ericcson W350 were used in the experimental design of this study. RESULTS AND DISCUSSION Measurements of MWs during mobile phones working were carried out at several local base station in Southern-Italy, because each base station uses a different set of channels to serve mobile phones as mentioned in the first section. Measurements were performed during exposures around 1765 MHz and 905 MHz of Wind and Vodafone providers, respectively. In the first section it was explained that GSM signals are pulse-modulated at 217 Hz and every 26 th pulse is idle by definition, producing a modulation component at 8 Hz, which represent the talk mode. This system produces a DTX mode in which the power is switched off when a user stops speaking either because he is listening or because 272
neither user is speaking. The use of this system allows to minimize exposure to EMFs during a call. Indeed, the speaker is only exposed to MWs fields emitted by the phone for the time of the conversation. In silence mode DTX for saving battery power produces additional pulsing at 2 Hz and in standby mode when the phone is switched on without an active call; the carrier frequency pulses less periodically at below 2 Hz. Thus, the GSM signal have different modulation components of the frequencies at 2, 8, 217, 1733 Hz. Furthermore, the different spectral composition of talk, silence, and standby produces the output power to which the amount of radiation energy absorbed by adjacent tissue is related. Indeed, it was evidenced by [Hyland, 2000; Hung et al., 2010] that the relation Talk- SAR > Silence-SAR > Standby-SAR 0 W/kg may be considered verified for mobile phones working. Time analysis of H component during exposure to mobile phone MWs in talkmode and silence-mode was carried out at the frequency of 1765 MHz and represented in Figure 1. During the last minute of the mobile phone working the conversation was interrupted, giving rise to a silence-mode, as indicated in Figure 1. Time analysis evidenced that a significant decrease of the H-field intensity occurred in silencemode, confirming that DTX mode reduces significantly EMFs emitted by a mobile phone during a call. Sound frequency generated by a software synthesizer in the range from 250 Hz to 14000 Hz were transmitted to a mobile phone during a call of 3 min for each tone used. The tones used were obtained varying the frequency of the synthesizer from 250 Hz to 14000 Hz with a step of 200 Hz. The overall sound intensity of each tone was fixed at 72 db and was monitored by a Larson Davis Lxt, so that the frequency was the unique variable. The power density generated by each tone was measured at a distance of 3.5 cm from mobile phone by a Narda SRM 3000 device in spectrum analysis mode. Power density output was integrated from 1.65 GHz to 1.85 GHz. A relevant number of mobile phone calls and measurements were carried out by means of the mobile phone models indicated Figure 1. Time analysis of H component at the frequency of 1765 MHz during a mobile phone exposure. The last minute of time analysis (on the right of figure) represents an interruption in the conversation. 273
in the section Materials and Methods and using Wind and Vodafone providers, so that significant results can be obtained. All the power density integrated values were measured as a function of the sound frequencies generated by the synthesizer and were represented in Table I and plotted in Figure 2. Each value reported represents the mean ± SEM (standard deviation of the mean) of ten exposures using the mobile phone models and providers reported in Section Materials and Methods. A sigmoidal fit was used for power density measurements, that was represented in Figure 2. A significant decrease in intensity of power density emitted by a mobile phone was observed to occur at frequencies larger than 4500 Hz, from an average value of 400 mw/m 2 to values lower than 100 mw/m 2. A statistical analysis applied to measurements provided that power density relative to sound frequencies upper than 4500 Hz resulted significantly decreased (p < 0.01) in comparison to power density values related to sound frequencies lower than 4500 Hz. Obviously, in this study decrease in MWs power density occurred regardless of the sound tone used. Power density integrated value was measured also in silence mode. Values ranging from 105 mw/m 2 to 35 mw/m 2 were obtained. Looking at Table I, it appears that the integrated power density values generated at sound frequencies larger than 4500 Hz are comparable with power density measured in silence mode; this is reported in the last row of the table. Representative spectra analyses of power density measured at some frequencies were reported in Figure 3. Looking at the integrated areas corresponding to frequencies larger than 4500 Hz (Figure 3c- 3d) it appears that they are comparable with a typical integrated area obtained in silence mode (Figure 3e). This result can be explained assuming that DTX mode is powered not only in silence- Figure 2. The average intensity of output power density from a mobile phone as a function of input sound frequency. The values reported are the average of a relevant number of measurements performed during mobile phone calls. The curve in red color represents a sigmoidal fit of measurements. The decrease in intensity of output power density occurred at frequencies larger than 4500 Hz. 274
Table I. Output power density (integrated from 1650 to 1850 MHz) as a function of the sound frequencies generated a synthesizer at the intensity Leq= 72 db. Each value reported represents the mean ± SEM of ten exposures using the mobile phone models and providers reported in the section Materials and Methods. Input sound frequency (Hz) Output power density (mw/m 2 ) Input sound frequency (Hz) Output power density (mw/m 2 ) 250 414 ± 15 4200 400 ± 25 400 421 ± 15 4400 386 ± 35 600 420 ± 15 4600 139 ± 20 800 421 ± 15 4800 121 ± 20 1000 425 ± 20 5000 93 ± 15 1200 402 ± 15 5200 85 ± 20 1400 423 ± 20 5400 84 ± 15 1600 428 ± 20 5600 81 ± 15 1800 427 ± 20 5800 73 ± 15 2000 415 ± 15 6000 76 ± 15 2200 370 ± 35 6400 71 ± 15 2400 410 ± 15 6700 75 ± 15 2600 414 ± 15 7000 91 ± 25 2800 423 ± 20 7500 79 ± 15 3000 417 ± 15 8000 78 ± 15 3200 408 ± 15 9000 73 ± 15 3400 360 ± 30 10000 75 ± 15 3600 427 ± 15 12000 78 ± 15 3800 406 ± 15 14000 64 ± 15 4000 390 ± 25 Silence mode 70 ± 35 Stand-by mode 13 ± 5 Figure 3a. Representative spectra analyses of output power density integrated in the range 1650-1850 MHz, measured at the sound frequencies of 2000 Hz. 275
Figure 3b. Representative spectra analyses of output power density integrated in the range 1650-1850 MHz, measured at the sound frequencies of 4600 Hz. Figure 3c. Representative spectra analyses of output power density integrated in the range 1650-1850 MHz, measured at the sound frequencies of 10000 Hz. 276
Figure 3d. Representative spectra analyses of output power density integrated in the range 1650-1850 MHz, measured at the sound frequencies of 14000 Hz. Figure 3e. Representative spectra analyses of output power density integrated in the range 1650-1850 MHz, measured in silence mode. 277
Figure 3f. Representative spectra analyses of output power density integrated in the range 1650-1850 MHz, measured in stand-by mode. mode but also at sound frequencies larger than about 4500 Hz. CONCLUSION In this study, a relationship between the power density generated by a mobile phone and the sound frequencies transmitted by a sound synthesizer was investigated. In particular, it was observed that microwave power density level decreased significantly at sound frequency values larger than 4500 Hz. Measurements were repeated using different mobile phone models and using Wind and Vodafone providers of a relevant number of local base stations in Southern- Italy, so that power density decreasing can be considered significant with p < 0.01. This result can be explained assuming that discontinuous transmission mode of global system for mobile communications is powered not only in silence-mode, but also at frequencies larger than 4500 Hz. This important result should lead to the design of acoustic filters that can minimize the MWs intensities emitted by a mobile phone during a call. REFERENCES Beasond R. C. and Semm P., (2002) Responses of neurons to an amplitude modulated microwave stimulus. Neuroscience Letters, vol. 33, pp. 175-178. Calabrò E., and Magazù S. (2010) Monitoring Electromagnetic Field Emitted by High Frequencies Home Utilities. Journal of Electromagnetic Analysis & Applications, vol. 2(9), pp. 571-579. Calabrò E., Condello S., Currò M., Ferlazzo N., Caccamo D., Magazù S., and Ientile R. (2012a) Modulation of HSP response in SH-SY5Ycells following exposure to microwaves of a mobile phone. World Journal of Biological Chemistry, vol. 3(2), pp. 34-40. Calabrò E., and Magazù S. (2012b) 278
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