Every new mobile communication generation comes with the deployment of additional radio frequency bands to support the massive growth of data volumes. With 4G LTE, there are more than 50 frequency bands standardized (if we combine LTE FDD and TD-LTE). But should we be worried at all those cell towers around us and the potential effects on our health?
There are often stories about villages or neighbors complaining after the installation of a new telecommunication towers near their plot of land. Where to draw the line between the actual health hazard that an active mobile antenna can cause and the fear of the neighborhood who sees that tall and unaesthetic installation (the “NIMBY” effect)?
The illustration above is extracted from a French regional channel doing a news report in a village where a cheese producers complains that his cattle does not produce milk since a new telecom tower has been erected near his farm.
In this video, we can see a local electrician showing the farmers how he can receive digital television (sic) using his Televes H45 digital spectrum analyzer (sic). According to the datasheet of this device, it is calibrated to decode digital terrestrial TV (DVB-H, DVB-C, DVB-T) and satellite TV (DVB-S2), and has nothing to do with mobile communications on GSM, 3G or LTE.
How far can the signal of a tower go?
When looking at the propagation model of electromagnetic waves, we learn that the power of the signal is divided by 4 (-6 dB) every time the distance is multiplied by 2, or divided by 100 (-10 dB) every time the distance is multiplied by 10. The chart below shows how fast the power decreases with the distance.
When we look at the complete formula for free space path loss (FSPL), we see that it also depends on the frequency used to transmit the signal. The higher the frequency, the quicker the attenuation of the signal will be:
In the news report cited above, the telecom installation company mentions that the base station is transmitting at a power of 1W (note: usually a transceiver of one sector of a telecom tower has a maximum output power of 20W).
Assuming the tower is at a distance d = 500m from the farm, and that it is transmitting a 3G signal on a frequency f = 2100 MHz, and considering an antenna gain of +3dB and cable losses of -1 dB for the tower, the received signal, in outdoor conditions, will be at around 0.0008 uWatt, which is more than 1,200 times lower than the initial 1W (around -90 dB).
Which one is more dangerous, my phone or the tower?
This is where things become interesting. Often people are concerned about the giant telecommunication towers being erected in their neighbourhood, but they care less about their mobile phone in their pocket or on the table near them.
The thing is, mobile communications are bi-directionnal, so if the tower has to transmit something to the mobile phone, the mobile phone has to send back a message with a signal powerful enough to be received by the tower.
To do this, the mobile phone is constantly measuring the link quality with the tower it is hooked on, and adapts the transmitting power accordingly. In GSM, the maximum transmitting power by the mobile is 2W (33 dBm) and can reduce if the signal reception is good (down to 3 mW or 5 dBm). In 3G similar ranges apply also.
Variation of the transmit power of a GSM and of an UMTS terminal (Source)
Based on the simplified measurement model proposed by this study, we can compare the range of received signal level values obtained the two following scenarios:
Scenario 1 – Good Outdoor Coverage. 2G phone located at 0.5m from the user having a good coverage of -70 dBm vs 2G Cellsite transmitting at 20W on 1800 MHz band located at 500m distance. Given the good signal level, the mobile phone will transmit at around +5 dBm. So from the user this is the theorical signal strength:
- RxLev from cellsite = -46.5 dBm (~0.022 uWatt)
- RxLev from phone = -27.5 dBm (~1.759 uWatt)
Scenario 2 – Bad Outdoor Coverage. 2G phone located at 0.5m from the user having a poor coverage of -95 dBm vs 2G Cellsite transmitting at 20W on 1800 MHz band located at 1,000m distance. Given the poor signal level, the mobile phone will transmit at around +25 dBm.
- RxLev from cellsite = -52.5 dBm (~0.006 uWatt)
- RxLev from phone = -7.55 dBm (~0.176 mWatt)
As we can see in the theorical example above, the variation of signal level received from the mobile phone by the user is much more important(x100), while the variation of signal level received from the base station being further is relatively small. And the signal strength of the mobile phone from the user position is much stronger in the bad coverage situation.
Since the model has been extremely simplified, we should not focus too much on the absolute values, but rather on the variations observed between scenarios. Also, we can expect the impact to be even worse in Indoor situation as the signal received from the cellsite will be further attenuated by walls and building structure (like Scenario 2).
This article only covers the signal propagation principles and formulas to evaluate the received signal levels from either a cellphone or a cellsite. It is proposing a model which is simplified and not as accurate as field measurements. Moreover, it does not cover scientific studies done to evaluate the potential impact of electromagnetic exposure of the human body.
Finally, it is important to remember that mobile signal is only one of the many electromagnetic signals that we have around us nowadays. Radio, TV, WiFi routers at home are examples of sources of exposition among others.
Regarding the mobile signal, the main observation, from a link-budget perspective, is that we should care more about the mobile phone first, and then about the cellsite. In particular, if the coverage conditions at your location are not good, your mobile phone will transmit at maximum power while being just next to you (and you will see as an evidence the battery life of your phone reduced dramatically).
Disclaimer: the content of this article reflects my personal views and may in no way represent the views of my current, former or future employers. Statements expressed here are reflects of my personal experience and do not intend to be of proofed scientific nature. Comments and criticism are welcome to discuss and challenge this point of view.