Before we discuss 5G, let us review one of the most fundamental equations of the telecommunication engineering: the Friis’ transmission equation1.
Telecommunication Engineering 101
The Friis transmission equation describes the RF signal power transmitted from an emitting antenna to a receiving antenna, such as between a 5G base station to a mobile phone or vice versa. The distance between the receiver and transmitter is R, and f is the carrier frequency of the transmitted signal. Friis’ equation, shown in the figure above, says that the power of the signal received by our cell phone (Pr) is proportional to the power emitted by the transmitter (Pt); in our example, the base station is the transmitter. The power also depends on the frequency of the signal and the distance between the receiver and transmitter antennas. However, this dependency is quadratic, meaning, if the frequency is doubled, the power decreases four times; if the distance increases ten times, the power decays one hundred times. We care about the signal power received because weak signal power results in dropped calls and increased susceptibility to interference.
The complete version of the Friis’ transmission equation includes additional terms but they are not shown here for the sake of simplicity. Also, here we consider the simplest communication scenario: there are two antennas communicating with each other without any obstruction in the path or a reflector nearby. Despite these simplifications, Friis’ equation is very useful to understand the influence of distance and frequency over power received (Pr).
Role of Frequency in 5G
Different countries are considering different electromagnetic frequency bands for 5G. These frequencies range from 600 MHz to 71 GHz. Dividing them into three main groups, i.e. low-band (0.6 GHz - 3.7 GHz), mid-band (3.7 – 24 GHz), and high-band (24 GHz and higher), helps with the frequency related discussions.
Courtesy of Qualcomm Technologies, Inc. .
In general, bandwidth of the signals increase with frequency. As a result, higher frequency signals can transmit larger amounts of data than lower frequency signals. The bad news is that high frequency signals do not travel as far as low frequency signals, for three reasons. First, the power of the signal received at an antenna decays quadratically with increasing frequency, as previously discussed. Second, the amount of power diminishes as the signal travels through air which increases with frequency, e.g., from low-band to high-band. Third, the longer the path the waves travel, the higher the loss they experience. How can we overcome this challenge? The Friis’ transmission equation provides the answer. The first thing is to decrease the distance between receiver and transmitter antennas. This is why 5G networks will be ultra-dense cellular networks meaning that they will have small cells installed every few blocks and closer to the ground than today's tower-top radios scattered every few miles. The second thing we can do is to use better antennas. The complete version of the Friis’ transmission equation includes additional terms representing the gain of antennas. Using multiple antennas instead of a single antenna is one simple way of increasing the gain. This is why current mobile communication systems use multiple antennas, also known as antenna arrays. With 5G, the number of these antennas will be increased to hundreds. These massive multiple input, multiple output (MIMO) antennas  will boost and multiply signals anywhere 5G is offered. Through various types of modulations, these massive MIMO antennas will enable wireless networks to transmit and receive more than one data signal simultaneously over the same radio channel. The MIMOs’ ability to create ultra-narrow beams will be useful to reduce the interference, which limits the capacity of cellular networks, but at the same, it will make it more difficult to handle high-speed mobile users .
Best Antenna is the Nearest Antenna
Despite low-level device-to-device (D2D) communication techniques such as Push-to-Talk, Bluetooth, and WiFi-Direct, most current cellular technologies do not support direct over-the-air communication between end users. A revolutionary aspect of 5G will be enabling D2D communication without or with partial involvement of the network infrastructure, such as mobile access points or mobile base stations. This feature will be the game changer. Hundreds of cars travelling on the same highway or hundreds of devices operating in a manufacturing facility will exchange information directly, without transmitting to a distant base station or through a core network. This will also decrease the overall power consumption of wireless communications: when you use your cell-phone, instead of making a connection with a base station, your device will communicate with a much closer 5G device. That device will pass your data to next nearest device and this will continue until your data reaches to the base station. Let’s say; the data exchange is achieved across a chain of ten 5G phones each separated by ten feet. Application of Friis’ transmission equation estimates this communication requires just 10 % of the power that would have been used to transmit the 100 feet distance between the first and last antennas. As a result, 5G will enable high-speed data transfer at a lower energy cost. However, this great feature comes with three problems: security/privacy, interference, and pricing . The parties sending and receiving the data must be assured their data is not accessible to the relay, and the relay must be assured the data it is handling is benign. Interference between the cellular network and device tiers and interference among users in the device tier will require novel interference management strategies and resource allocation schemes. The pricing of D2D services is another subject, which requires further study. What incentives should be considered to encourage people to allow their devices to be used as a relay in 5G network? What kind of in-network and out-network pricing schemes should mobile network provides consider to maximize their profit?
Health Concerns: RF Signals from Cell Phones
Researchers have conducted studies of many types to find out whether RF fields at levels produced from common devices, principally cellphones, might adversely affect health, including whether they may cause cancer. Numerous national and international scientific and governmental organizations [6-20] have reviewed epidemiologic, in vivo, and in vitro studies of health and biological endpoints in association with the RF exposures that would be encountered in typical environments accessible to the public (i.e., exposures below the level that raises body temperature). These agencies concluded that the outcomes of these studies do not provide sufficient evidence to conclude that RF exposure causes any adverse health effect. Despite this consensus, research continues to test hypotheses, fill in data gaps, and make sure that even a small risk is not overlooked. For example, recent studies on RF exposure in rats and mice conducted by the National Toxicology Program (NTP)  at the request of the U.S. Food and Drug Administration (FDA) reported the presence of tumors in male rats. However, the level and duration of exposure were much greater than what humans experience with even the highest level of cell phone use and the NTP has noted that “the results should not be directly extrapolated to humans.” The tumors were also observed in only a few animals. Thus, further research will be needed to confirm and explain the results of the NTP studies. As human and rats biologically are not the same, it also is not clear whether these findings have any relevance to our exposures to RF signals at far lower levels. However, we do know what will be different when 5G arrives and what we can do to minimize its health impacts on humans, if it has any.
Health Concerns: 5G
The low frequency band of forthcoming 5G networks, where the carrier frequency is less than 3 GHz, is not very different from the frequency bands used in current wireless systems. The small cells planned to be deployed by network providers will be placed in close proximity to urban areas and will be transmitting much less energy compared to today’s base stations. Hence, from the spectral point of view, we do not expect a significant difference in exposure to wireless networks’ operations at low frequencies. However, exposures will be quite different at higher frequencies, especially in the high-band.
The high-band of the 5G frequencies fall into the “millimeter wave” (mmW) category, i.e. the wavelength of the electromagnetic wave is in the range of millimeters. These waves are mostly absorbed within 1 to 2 millimeters of human skin and in the surface layers of the cornea . In the Unites States, the Federal Communications Commission (FCC) determines the rules and regulations related to communication systems as well as the limits of the electromagnetic energy that a device can transmit. In order to protect humans from acute exposure to thermal levels of radiofrequency radiation, FCC allows a maximum 1.0 mW/cm2 of exposure (averaged over 30 minutes for frequencies that range from 1.5 GHz to 100 GHz) for the general public .
While a great deal is known about the interaction of RF signals with materials and the human body, the fact is that because there are few sources of RF fields in the high-frequency band, there is currently little research to point to for confirmation of the current consensus that the adverse effects of RF exposure are caused by tissue heating. Given the concern that some members of the public and some scientists have expressed about exposure to low-band frequencies , it is inevitable that concern also will be raised about exposures to 5G signals that have not been as thoroughly studied. In fact, the World Health Organization has already emphasized the need for high quality scientific studies in this area due to the current widespread use of technology, the degree of scientific uncertainty, and the levels of public apprehension .
Implications of 5G Technologies
The introduction of 5G technologies will require considerable research and development on technical issues [4, 5] and also public reaction to deployment. One can expect that the installation of such a large number of antennas everywhere will be challenged by some members of the public, especially if the antennas are large and unsightly. Opposition may also be fueled by fears about exposure to RF even if 5G does lower general exposure as it replaces current low-frequency RF communication systems. Manufacturers of consumer electronics and test & measurement equipment will need solutions to challenges rising from higher ohmic losses at higher frequencies. Silicon-germanium or other types of alloys might be the next choice of industry by replacing gallium arsenide. Plasmonics, controlling electromagnetic wave propagation via micro- and nano-particles, is likely to be used more frequently at higher frequencies. From wave-generation to amplification and filtering, tiny structures made with noble metals will continue to have a significant role in near-future technologies. New composite materials enabling or preventing the transfer of millimeter waves might create new business opportunities in various industries such as automobile, fashion, cosmetics, and paint and coatings.
What Can Exponent Do For You?
Whether you are a consumer electronics manufacturer looking for a help with your 5G-compatible device design or to learn more about RF health and safety research, Exponent is ready to work for you with its inter-disciplinary teams of engineers, scientists, and regulatory experts.
A few of the many capabilities of Exponent related to 5G technology are:
- Electromagnetic compatibility testing,
- Electromagnetic exposure assessment,
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- FCC and international regulations,
- Material characterization, and
- Product development.
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- Qualcomm Technologies, Inc., "Spectrum for 4G and 5G," December 2017.
- E. G. Larsson, O. Edfors, F. Tufvesson, and T. L. Marzetta, “Massive MIMO for next generation wireless systems,” IEEE Communications Magazine, vol. 52, no. 2, pp. 186-195, Feb. 2014.
- X. Ge, H. Cheng, M. Ghuizani, "5G Wireless Backhaul Networks: Challenges and Research Advances," IEEE Network, vol. 28, no. 6, pp. 6-11, Nov. 2014.
- M. N. Tehrani, M. Uysal, and H. Yanikomeroglu, "Device-to-Device Communication in 5G Cellular Networks: Challenges, Solutions, and Future Directions," IEEE Communications Magazine, vol. 52, no. 5, pp. 86-92, May 2014.
- International Agency for Research on Cancer (IARC). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Preamble. Lyon, France: International Agency for Research on Cancer, 2006.
- Health Canada. Limits of Human Exposure to Radiofrequency Electromagnetic Energy in the Frequency Range from 3 kHz to 300 GHz. Safety Code 6 (2009). Ottawa, Ontario: Health Canada, 2009.
- European Health Risk Assessment Network on Electromagnetic Fields Exposure (EFHRAN). Risk Analysis of Human Exposure to Electromagnetic Fields. Milan, Italy: European Health Risk Assessment Network on Electromagnetic Fields Exposure, 2010.
- European Health Risk Assessment Network on Electromagnetic Fields Exposure (EFHRAN). Report on the Analysis of Risks Associated to Exposure to EMF: In Vitro and In Vivo (Animals) Studies. Milan, Italy: European Health Risk Assessment Network on Electromagnetic Fields Exposure 2010.
- Advisory Group on Non-ionising Radiation (AGNIR). Health Effects from Radiofrequency Electromagnetic Fields. Report of the Independent Advisory Group on Non-ionising Radiation. London, England: Health Protection Agency, 2012.
- Norwegian Institute of Public Health (NIPH). Low-Level Radiofrequency Electromagnetic Fields - An Assessment of Health Risks and Evaluation of Regulatory Practice. English Summary. Oslo, Norway: Norwegian Institute of Public Health, 2012.
- French Agency for Food Environmental and Occupational Health & Safety (ANSES). OPINION of the French Agency for Food, Environmental and Occupational Health & Safety Concerning the Update of the "Radiofrequency Electromagnetic Fields and Health" Expert Appraisal. Maisons-Alfort, France: Agence nationale de sécurité sanitaire de l'alimentation, de l'environnement et du travail, 2013.
- Health Council of the Netherlands (HCN). Mobile Phones and Cancer. Part 1: Epidemiology of Tumors in the Head. The Hague: Health Council of the Netherlands, 2013.
- International Agency for Research on Cancer (IARC). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Volume 102. Non-Ionizing Radiation, Part 2: Radiofrequency Electromagnetic Fields. Lyon, France: International Agency for Research on Cancer, 2013.
- Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR). Preliminary Opinion on Potential Health Effects of Exposure to Electromagnetic Fields (EMF). Brussels, Belgium: Scientific Committee on Emerging and Newly Identified Health Risks, 2013.
- Swedish Radiation Safety Authority (SSM). Research 2013:19. Eighth Report from SSM:s Scientific Council on Electromagnetic Fields. Stockholm, Sweden: Swedish Radiation Safety Authority (SSM), 2013.
- National Toxicology Program, Technical Reports 595 and 596.
- ICNIRP (International Commission for Non-Ionizing Radiation Protection) Standing Committee on Epidemiology: A. Ahlbom, A. Green, L. Kheifets, D. Savitz, and A. Swerdlow, “Epidemiology of Health Effects of Radiofrequency Exposure,” Environmental Health Perspectives, vol. 112, no. 17, pp. 1741-1754, 2004.
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- O. P. Gandhi and A. Riazi, "Absorption of Millimeter Waves by Human Beings and its Biological Implications," IEEE Transactions on Microwave Theory and Techniques, vol. 34, no. 2, pp. 228-235, Feb 1986.
- FCC's Rules and Regulations [47 C.F.R. 1.1307(b), 1.1310, 2.1091, 2.1093]