Accurately assessing exposure to radio frequency electromagnetic fields from 5G networks

Throughout the world, the rollout of 5G networks is either currently taking place or is about to take place, and extensive growth of 5G services and subscriptions is expected in the coming years. To address the increasing demand for mobile network capacity and coverage, 5G uses advanced antenna technologies and new allocated frequency bands. This white paper provides information related to human exposure to radio frequency electromagnetic fields (RF EMF) from the base stations in the new 5G networks and describes how to accurately assess compliance with established limits.

With the introduction of a new generation of mobile networks, questions are naturally being raised about health and safety aspects related to the RF EMF exposure from the radio equipment, which includes 5G. Members of the public and local authorities have expressed an interest in knowing whether the RF EMF exposure from 5G will be different from the previous generations of mobile communications. Service providers and regulators need to understand how the RF EMF exposure levels from 5G networks can be accurately determined to assess compliance with applicable safety regulations. Regarding the RF EMF compliance assessments of 5G new radio (NR) base stations with advanced antennas, the challenge is how to consider the dynamic change of beam patterns that serve users in different places. In this white paper, these questions and challenges are addressed.

Like all radio communications, including radio and television broadcasting, satellite communications, and previous generations of mobile networks, 5G uses radio waves to transfer information between base stations and connected devices. Radio waves are a form of electromagnetic fields that are transmitted and received by antennas. They belong to the radio frequency part of the electromagnetic spectrum, as shown in Figure 1. 5G uses frequency bands assigned by regulators ranging between 600MHz and 40GHz, which are within or adjacent to the ranges that are already used by previous generations of mobile networks, satellite communications, and other radio applications. Radio waves, including the new higher bands used by 5G, called millimeter waves, are very different from the electromagnetic fields in the upper part of the spectrum, called ionizing radiation, with frequencies more than 100,000 times higher than those of the radio waves used for communication. Ionizing radiation is known to have frequencies (and photon energy) high enough to break chemical bonds, which can cause tissue damage. Radio waves do not have such properties.

5G is designed to increase the spectrum efficiency compared to earlier generations. This means that less bandwidth and less energy are needed to transfer a certain amount of information. 5G also uses only a very small fraction of the transmit power for non-data signaling, and it uses advanced antenna technology to direct the energy where it is needed (beamforming). All of this together means that the 5G radio technology is very efficient and that levels of RF EMF are, for comparable services, lower than from earlier technologies.

Radio equipment needs to meet regulatory requirements related to RF EMF exposure. In most countries, the applied limits have been adopted from guidelines provided by the International Commission on Non-Ionizing Radiation Protection (ICNIRP), which is an independent international expert group formally recognized by the World Health Organization. The limits recommended by the ICNIRP are based on reviews of all relevant scientific literature and have been set with significant margins to protect from substantiated short-term and long-term health effects of exposure to RF EMF. Most regulations are still based on the ICNIRP limits from 1998 [1], which have been confirmed to be protective. In 2020, the ICNIRP published updated guidelines [2] considering the latest available scientific research and introducing some additions and changes. Some countries have already adopted the new ICNIRP limits and others will follow soon. The ICNIRP guidelines are technology-independent, meaning that the same limits apply for all radio technologies, that is, also for 5G.

As concluded by the ICNIRP, the substantiated health effects of RF EMF are related to local or whole-body temperature elevations that high exposure levels for an extended period of time can cause, known as thermal effects. Compliance with the limits ensures that the real exposure that people may experience is always far below these levels and that radio communication equipment, including 5G, is safe. The ICNIRP has also concluded that no non-thermal health effects (that is, effects below the limits and not associated with temperature elevations) have been established as being caused by RF EMF, including cancer.

The customer product information for radio products includes information about exclusion zones to consider when installing base stations to ensure compliance with international RF EMF exposure limits for the general public and workers. For 5G radios, the necessary distance to keep to public areas varies from less than a few centimeters for low-power indoor products to a few meters for outdoor micro products mounted on walls and poles, and up to about 20m for macro products installed on rooftops, masts, and towers. The exclusion zones are in most cases determined based on calculations, but measurements may also be used, especially for radio products with very low output power levels. These zones are not applicable for 5G user devices such as mobile phones, which are compliant with the RF EMF exposure limits when being used near the head or body.

Applicable limits and classic calculation methods

The RF EMF limits applicable for base stations are typically expressed in terms of power density (unit W/m²) or electric field strength (unit V/m) levels. Figure 2 below shows the power density limits for the general public that are prescribed in many countries globally (from the ICNIRP 1998 guidelines). These so-called reference levels are frequency- dependent, but from 2GHz to 300GHz defined at a constant level of 10W/m² (or 61V/m expressed as electric field strength). Importantly, they are also associated with an averaging time, for example, 6 or 30 minutes, meaning that it is the averaged power density over the specified time that is to be compared with the limit.

International standards such as IEC 62232 [3] from the International Electrotechnical Commission (IEC) describe the methodology to be used to evaluate the RF EMF exposure from individual base stations and base station sites, based either on calculations or measurements. With knowledge of the input power to the base station antenna P (W), the antenna gain G (direction-dependent), and the power density limit S_lim (W/m²), the RF EMF compliance distance CD (in meters) in different directions can be calculated using the free- space formula:

Figure 3 illustrates the RF EMF compliance distances for different values of PG, called Equivalent Isotropic Radiated Power (EIRP), and for the ICNIRP general public power density limit S_lim of 10W/m².

Calculations using the above formula are a common way to assess the RF EMF exposure from base station antennas, specifically classic passive panel antennas. When adding 5G radios and antennas to an existing base station site, the total RF EMF exposure from all antennas and technologies (2G, 3G, 4G, and 5G) has to be considered for assessment of compliance with limits and regulations.

Solutions to address RF EMF compliance assessment challenges for 5G massive MIMO base stations

Regulations typically require that the highest possible power and gain values are used when calculating the RF EMF exposure to ensure conservative results. For classic base station antennas with static antenna radiation patterns, the RF EMF evaluations are relatively straightforward using the calculation method described above. However, for massive multiple-input multiple-output (MIMO) 5G radios some challenges need to be addressed. These are:

• how to consider the dynamic change of antenna radiation patterns, that is, that the base station can use a large number of different beam patterns to serve users located in different places in the most efficient way
• how to consider that only a fraction of the total transmit power is contributing to the RF EMF exposure in a certain direction, that is, that different beam patterns will be used for a limited time, which is much lower than the EMF averaging time of 6 or 30 minutes

Solutions to these challenges are presented below.

Antenna beam pattern envelopes

Antenna pattern files are available for 5G massive MIMO radio products, for both individual broadcast and traffic beams, and a combination of the maximum gain values of the traffic beams, so-called envelope pattern files. Figure 4 shows examples of traffic beam envelope patterns in the azimuthal and elevation planes for a mid band (3.5GHz) product with the individual boresight traffic beam indicated. Envelope files are available for different beamforming schemes, that is, reciprocity-based as well as codebook-based beamforming.

By applying such envelope beam patterns together with the configured transmit power, RF EMF compliance boundaries (exclusion zones) can be created from calculations using the free-space formula. In the case of time division duplex (TDD), the transmit power should be scaled by the TDD duty cycle. For example, with a frame structure leading to 75 percent downlink transmission, the configured power used should be multiplied by 0.75 (-1.2dB). For 5G NR, the fraction of the power used for broadcast beam transmission is tiny (less than one percent) meaning that only the traffic beam envelopes need to be considered in the calculations.

Figure 5 shows an example of RF EMF compliance boundaries for the general public and occupational exposure determined using traffic beam envelopes and the configured power. This corresponds to the theoretical maximum, assuming that all the power can be transmitted continuously in any direction within the steering range of the radio product. However, this will not happen in a real 5G network.

Power reduction factors for accurate RF EMF exposure assessments

In a real 5G network with massive MIMO base stations, the antenna patterns are changing rapidly, and beams are formed to optimize the transmission to the served devices. Since the RF EMF limits are associated with an averaging time of 6 or 30 minutes, calculations using time-averaged antenna patterns gives the most accurate RF EMF exclusion zones.Figure 6 shows an example of an instantaneous traffic beam pattern (blue curve) and the time-averaged pattern for six-minute periods (red curve) based on measurements in a live commercial 5G 3.5GHz network using codebook-based beamforming [4]. The average antenna gain in any direction is several dB lower than the instantaneous maximum. This means that the actual maximum RF EMF exposure is significantly lower than the theoretical maximum obtained by applying the traffic beam envelope and the configured maximum power. A statistical model has been developed to determine the expected difference between the theoretical and actual maximum RF EMF exposure levels from massive MIMO antennas [5].

Based on these research studies it is recommended that a power reduction factor (PRF) of 0.25 is used when assessing the RF EMF exposure from mid band 16T, 32T, and 64T 5G NR massive MIMO base stations. This means that the power or EIRP should be multiplied by 0.25 (reduced by 6dB) in calculations of RF EMF compliance boundaries using traffic beam envelopes. In this PRF, the power reduction due to a TDD duty cycle of 0.75 has been included. Without this factor, the recommended PRF is 0.32.

This power reduction factor is valid for a traffic load of 100 percent. The real time-averaged traffic load is typically well below 100 percent, which means that the actual RF EMF exposure is even lower than what is obtained with the recommended PRF.

Figure 7 shows the reduction of the size of the RF EMF exclusion zones when applying the recommended power reduction factor of 0.25 for the same radio product as illustrated in Figure 5.

The concept of actual maximum RF EMF exposure and the use of power reduction factors has been introduced in IEC and ITU reports [6, 7] and will be covered in the next edition of the international standard IEC 62232 that is expected to be published in 2022. Although the established PRFs were derived based on very conservative assumptions, it is recommended in these documents that software features are used to monitor and/or control the time- averaged power when applying this concept. This may also be a regulatory requirement in some countries.

Software features to control the power transmission

A set of software features is available that can be used to control the time-averaged power transmission of massive MIMO radio products to ensure that it is not exceeding values used to determine the size of actual maximum RF EMF exclusion zones or to limit the beam steering range.

The EMF power lock mid band feature monitors and controls the sector carrier-wide, time-averaged transmitted power of a 5G new radio advanced antenna systems (NR AAS) radio. The purpose of this feature is to keep the time-averaged transmitted power below a configurable level. This level as well as the averaging time are determined by the service provider to meet EMF regulatory requirements when the radio is deployed at a site. Counters are available for monitoring of the time-averaged power. A similar feature is also available for LTE products (the intelligent power emission control feature). The functionalities of these features are described in [8].

The codebook subset restriction function (as part of the massive MIMO mid band feature for NR), provides the possibility to limit the directions used for beamforming. In that way, the vertical or horizontal extension of the RF EMF exclusion zones can be limited if needed.

Ensuring compliance with RF EMF limits and regulations is a necessity when rolling out 5G, but understanding what the typical levels of exposure are and will be is also important, especially from the perspective of communicating with stakeholders. Measurements were recently taken in a large number of street-level locations around base stations in a commercial 5G network with massive MIMO base stations operating in the 3.5GHz band [9]. It was found that the contribution from the 5G network to the total environmental RF EMF exposure was less than 10 percent even in the case of 100 percent induced traffic and that the maximum exposure levels from the 5G base stations were 150 to 200 times below the international limits set by the ICNIRP.

The high spectrum efficiency and the advanced antenna technologies used by 5G NR lead to lower levels of RF EMF exposure than from earlier generations of mobile networks for comparable services. The base stations in 5G NR networks need to comply with the same RF EMF safety regulations as other radio equipment, and the limits cover all frequency bands used by 5G, including those in the millimeter-wave range. International RF EMF exposure guidelines have recently been published based on the latest available scientific research, with conservative limits that have been or will be adopted in national regulations. Assessing compliance with RF EMF limits may be a challenge for massive MIMO 5G base stations due to dynamic beam steering, but solutions such as envelope beam pattern files together with recommended power reduction factors are available to enable accurate evaluations. The typical overall environmental RF EMF exposure will remain at a small fraction of international limits even with 5G being deployed since the contribution from 5G is relatively small.