Laboratory measurements: maximum SAR
When checking mobile phone compliance, the ANFR seeks to determine the maximum SAR that a terminal can generate. Therefore, in a controlled laboratory environment isolated from all mobile networks, the phone is blocked at its maximum power. This configuration is applied to each frequency band and for each technology (2G, 3G, 4G or 5G) in each of the positions to be measured.
The SAR values given in the test results illustrate very precise measurement conditions corresponding to reasonably foreseeable use conditions..
These laboratory-measured SAR values thus indicate the maximum mean power the phone can produce for each technical configuration (frequency band, technology).
As for the SAR displayed in stores, it is reduced to a single value per SAR type (head, trunk and limbs): the highest SAR reached during the tests, i.e. for only one of the frequency bands and one of the technologies the phone can use.
Classic use conditions: actual SAR
Laboratory measurements are not an indication of what happens during the usual use of the device. Indeed:
- for voice communications, statistically, the phone only emits for about 50% of the time as it does not emit when the person is listening; furthermore, the average call time is less than 3 minutes;
- for data use (internet or video), the use times are certainly longer, but the phone, which generally receives more data than it sends (videos, emails, etc.), rarely emits for more than 10% of the time during a session;
- The SAR value displayed at points of sale is for a single frequency (800 MHz for example) and specific technology (4G for example) for which the maximum was detected. In reality, however, the phone does not run in those conditions all the time, it often switches frequency or technology;
- finally, for all uses, the phone rarely emits at its full power as it is permanently interacting with the network to best adapt to its conditions.
The actual SAR therefore does not match the maximum SAR very often.
As the SAR is proportional to the phone’s emitted power, the actual SAR can be assessed by measuring the power used by the terminal. It can be estimated using professional software installed on the phones available to the general public.
Interaction between the phone and the network
When the phone is connected to a mobile network, it permanently dialogues with the operator cell towers, which actually control its emitted power. This system was especially developed starting with 3G. It is known as adaptive power control: the mobile phone continuously reports the radioelectric conditions it encounters to the network and the network recommends an emission level to the phone, i.e. its actual SAR.
When radio conditions are good (shown by the number of bars on the phone screen), the actual SAR emitted by the terminal is much lower than the maximum SAR. If radio conditions deteriorate, the power emitted by the phone increases and may, if necessary, reach the maximum SAR.
The adaptive power control system is described in published standards covering network and mobile operations (1). The effect of the system is to modulate the phone’s power (and therefore the actual SAR) under actual use conditions. It is extensively described in the literature ([1] to [7]).
An illustration of the results found using the ANFR measurements [8] is shown below for 4G. The test conditions cover use of the mobile for voice conversations.
The graph shows many points that, one the one hand, show the power received by the terminal (abscissa) correlated to the distance between the cell tower and the terminal, and on the other hand, the power emitted by the phone (ordinates), which is proportional to the actual SAR produced by the device. The unit of measure, which is the same for both cases, is the dBm. 0 dBm is the equivalent of 1 mW of power. It is a logarithmic unit; an increase of 3 dBm corresponds to a power variation of 10^(0,3) = 2, i.e. the doubling of the power. As for an increase of 30 dBm, it is the equivalent of a variation of 10^3 = 1,000, or power one thousand times higher.
On observe ainsi en ordonnées que la puissance émise varie entre 23 dBm (niveau équivalant au DAS maximum mesuré en laboratoire) et -30 dBm pour les extrêmes. Cela correspond à des variations d’une amplitude de 23 – (-30) = 53 dB, soit un ratio de 10^(5,3) ≈ 200 000.
En moyenne, dans la zone située à droite dans la zone inférieure du graphique, l’ordonnée des points est de l’ordre de -16 dBm (centre des deux barres verticales les plus à droite). Dans ces situations, qui correspondent à d'excellentes conditions de réception (toutes les barres du téléphone seraient alors affichées), le DAS réel apparaît beaucoup plus faible que le DAS maximum. La proportion entre ces deux valeurs est de 23 – (-16) = 39 dB, soit 10^(3,9) ≈ 8 000.
L’écart réel est en réalité plus important, car il ne serait constaté que si le téléphone émettait en permanence pendant 6 minutes (conditions de mesure du DAS maximal), ce qui n’est jamais réalisé en conversation vocale (cf. supra). En outre, il convient de souligner que les points figurant sur la courbe traduisent le maximum de puissance atteint pour chaque intervalle de temps par le téléphone, et non pas la moyenne de la puissance émise pendant cette période.
Pour mieux approcher le ratio que reflète ce graphique, il faut ainsi prendre en compte la durée moyenne de communication vocale (3 minutes, sur une durée de mesure de DAS de 6 minutes, soit un facteur 2), le temps de parole et donc d’émission du téléphone (50 %, soit un nouveau facteur 2). Il convient donc d’introduire un facteur 2 x 2 = 4 pour évaluer l’écart. En conclusion du fait du dispositif d’adaptation de puissance, dans le cas illustré ci-dessus, le DAS réel varie dans une proportion de 1 à 800 000 par rapport au DAS maximum, et en moyenne dans d'excellentes conditions de réception, d’un facteur de 1 à 32 000.
Cette analyse n’a été conduite que dans le cas d’une communication vocale en 4G.
(1) GSM : ETSI TS 145 005 UMTS : ETSI 125.214 LTE : ETSI TS 136.213
Références
[1] "Output Power Levels of 4G User Equipment and Implications on Realistic RF EMF Exposure assessments" P. Joshi, D. Colombi, B. Thors, L.-E. Larsson and C. Törnevik, IEEE Access, vol.5, 2017
[2] "Power level distributions of radio base station equipment and user devices in a 3G mobile communication network in India and the impact on assessments of realistic RF EMF exposure" P. Joshi, M. Agrawal, B. Thors, D. Colombi, A. Kumar, and C. Törnevik, IEEE Access, vol. 3,2015.
[3] "Output power distributions of terminals in a 3G mobile communication network" T. Persson, C. Törnevik, L.-E. Larsson, and J. Lovén Bio-electromagnetics, vol. 33, no. 4, May 2012.
[4] "Duality between uplink local and downlink whole-body exposures in operating networks" A. Gati, E. Conil, M.-F.Wong, and J.Wiart, IEEE Trans. Electromagn. Compat., vol. 52, no. 4, Nov. 2010.
[5] "Exposure induced by WCDMA mobiles phones in operating networks" A. Gati, A. Hadjem, M. F. Wong, and J. Wiart, IEEE Trans. Wireless Commun., vol. 8, no. 12, Dec. 2009.
[6] "Analysis of the influence of the power control and discontinuous transmission on RF exposure with GSM mobile phones" J. Wiart, C. Dale, A. V. Bosisio, and A. L. Cornec, IEEE Trans. Electromagn. Compat.,vol. 42, no. 4, Nov. 2000.
[7] "Output Power Levels of 4G User Equipment and Implications on Realistic RF EMF Exposure Assessments" P. Joshi, D. Colombi, B. Thors, L.-E. Larsson, and C. Törvenik, IEEE Access, February 19, 2017.
[8] Rapport technique sur les déploiements pilotes de petites antennes en France