There has been a continuous debate about harmful effects of Electromagnetic Radiations ever since they came into existence. Most of the research results suggest that there are no harmful effects, if the rules and regulations are followed. But there is a small body of research that suggests that there might be some harmful effects and more research needs to be carried out. This is particularly important now as 5G Wireless Technology is being rolled out around the world and it uses millimeter waves for which we have limited data. Also, 5G would be using much smaller cells meaning that base stations would be closer to human beings.
Those who believe that EM radiation in the millimeter wave is harmless argue that the waves in this band are of the non-ionizing type and the only effect they have on the human body is some localized heating. By non-ionizing it is meant that the photon energy in this band is so low that it cannot knock out an electron from an atom or molecule. Just to emphasize this further it must be mentioned that a photon in this band has an energy of 1.2meV whereas 12eV is required to remove an electron from its parent body i.e. the energy of the photon is 10,000 times less than the minimum required.
Now let us look at the factors that need to be considered when assessing 5G technology and its harmful effects, if any.
1. Transmit Power
The transmit power of a 5G base station can vary from 250mW to 120W depending upon the size of the cell. Compared to 2G/3G/4G towers which could have a maximum transmit power of 20W, this is about six times higher. On a logarithmic scale its 7.8dB higher. But radiation from base stations is not such a big concern since power falls of as the squared of the distance in free space and as fourth power of distance in an urban environment. The more important metric to look at is transmit power of the mobile station. We know that a GSM (2G) mobile station could have a transmit power of up to 1000mW. Compared to this a 5G cell phone has a maximum transmit power of only 200mW, 7dB lower.
2. Cell Size and Base Station Antenna Height
As mentioned earlier the 5G cell sizes are expected to be much smaller, particularly in dense urban environments. A picocell can have a radius of 100m-200m whereas an indoor femtocell can have a radius of 10m or even lesser. Furthermore the height of the base station antenna is going to be much lower as antennas are to be deployed on lamp posts, bus shelters etc. 3G networks reached densities of 4-5 base stations per squared km, 4G networks reached densities of 8-10 for the same area, while 5G networks could achieve densities of 40-50. So exposure to radiation would definitely be higher.
3. Massive MIMO and Beamforming
It is well known that 3G and 4G systems use MIMO technology to get better spectral efficiency, reliability and capacity. 5G systems take this to whole new level by employing 64×64 antenna configurations. Using these antennas a base station can form a beam on a user i.e. it will transmit more power in one direction and transmit lesser or no power at all in the other directions. This means that a base station can reuse the same time slot and frequency in another direction (remember the concept of frequency reuse). But what about exposure to higher powers in the vicinity of the base station? The narrower a beam gets the more Power Density it would have (PD is given in Watts per squared meter). Since the base station would be lower, an unintended signal recipient in the direction of the beam might get exposed to higher Power Density.
4. Power Control
The strength of a wireless communication signal varies greatly as it proceeds from the transmitter to the receiver. There are two main components to this; a distance dependent path loss and fading which occurs due to constructive and destructive interference of the wireless signal. To overcome these effects the transmitter adjusts its transmit power so that a good quality wireless link can be maintained. When the cell size is small, as in 5G (usually), this is easier to implement. But if the cell size is large and user is on the cell edge the transmit power has to be substantially increased which can cause co-channel interference and could be harmful to a cell phone user as well. Last point to note is that most wireless communication systems, including 5G, use Adaptive Modulation and Coding Schemes (MCS) and to achieve a high throughput higher power is transmitted than is necessary just to maintain the link.
5. Near Field and Far Field
Electromagnetic radiation from an antenna can be divided into three regions based upon the distance of the observer from the antenna: a reactive near field, radiative near field and radiative far field. Most of the analysis in the literature assumes that we are in the far field of the antenna where the Electric field, Magnetic field and direction of propagation, are all perpendicular to each other and ratio of Electric to Magnetic field is a constant. But the near field which stretches to about half the wavelength from the antenna (dipole) is not that well understood. One thing that is known is that the Electric field and Magnetic field fall off much more rapidly in the near field than in the far field. At 1GHz the near field is within 15cm of the antenna whereas at 30GHz this is reduced to half a cm. So for millimeter waves we can say that a mobile phone user is in the far field of the mobile antenna most of the time, which is studied in detail and well understood.
6. Penetration in the Human Body
According to studies conducted in the millimeter wave band about 30%-40% of the EM energy is reflected back by the human skin. But the energy reflected back decreases with increasing frequency e.g. at 40GHz, 43% of the energy falling normally on the human body is reflected back while at 100GHz this is reduced to 30% only. On the other hand penetration loss within the human body increases with increasing frequency. It is reported that 90% of the millimeter wave energy is absorbed within the first two layers of the skin, namely epidermis and dermis, which are only a few millimeters in thickness.
7. Superposition of Signals
Most of the studies on effects of exposure to radiation consider one type of radiation, at a particular power level and for a limited time. But real life scenarios are quite different. A typical human being is subjected to a variety of signals at any instant. This may include Bluetooth, WiFi, UWB, 3G/4G/5G, radiation from microwave ovens etc. With the advent of Internet of Things (IoT) its not uncommon for a typical home to have tens of devices, in closed proximity, transmitting simultaneously. So one signal might not be harmful for a human being but when so many signals are combined what effect do they have? Are there any studies that have observed the impact of these devices when operated in close proximity for a number of years? Are results on animals directly applicable to humans? These are some of the questions that need to be answered before adopting this new technology.
Measurement of Radiation
Specific Absorption Rate (SAR)
SAR is the most common parameter used to measure amount of EM radiation that a body is exposed to but it is only useful if you have a volume under consideration. In the USA, FCC requires that phones sold have a SAR level at or below 1.6 watts per kilogram averaged over 1 gram of tissue. The ICNIRP SAR limit for mobile devices is 2 watts/kg averaged over 10 grams of tissue. For long term exposure the limits are much more stringent, particularly for the general public.
Power Density (PD)
If you do not have a defined volume but have a surface area to work with then Power Density is the most useful measure. The PD limit used most often is 1mW per squared centimeter.
The most direct impact of non-ionizing radiation is an increase in temperature, therefore, it may also be used to measure the exposure to EM radiation. The limit for this is 1 degree centigrade increase in temperature for the body part exposed to radiation. This is a useful measure in the near-field of a radiating antenna where the EM energy may be difficult to measure.
Federal Communication Commission (FCC) Advice for General Public
“For users who are concerned with the adequacy of this standard (FCC defined SAR limits) or who otherwise wish to further reduce their exposure, the most effective means to reduce exposure are to hold the cell phone away from the head or body and to use a speakerphone or hands-free accessory. These measures will generally have much more impact on RF energy absorption than the small difference in SAR between individual cell phones“.