Increased use of so-called massive MIMO antennas is expected with the further development of mobile communications networks leading to the latest mobile communications generation 5G to achieve cell-wide high data transmission rates. These beamforming and beam-sweeping antennas provide the principle of directing one or more antenna beams at potential users or terminals. Therefore, user-specific signals from the base station are no longer radiated cell-wide but locally as beams, meaning both that the same frequency and time resources can be reused in the same cell but in a different beam, and that the locally limited radiation results in a more favourable signal-to-noise ratio within the beam and less interference outside the beam. However, it also results in the fact that the local electromagnetic exposure in a cell can significantly depend on where active users are located.

In terms of prospective electromagnetic radiation protection, the possibility of beamforming and beam-sweeping of the antenna radiation pattern leads to challenges in the metrological determination of exposure which were not met by the base station antennas used so far. Therefore, a static radiation characteristic of the antennas can no longer be assumed; in fact, the time-dependent antenna pattern depends on the cell utilization and its spatial angle-dependent distribution. Furthermore, due to the higher antenna gain associated with beamforming, an increased maximum electromagnetic exposure for persons in the vicinity of the base station must be expected compared with conventional mobile communications technologies.

Considering this background, the German Federal Office for Radiation Protection (BfS) commissioned a study to RWTH Aachen University, EM-Institut GmbH and TU Ilmenau, RF and Microwave Research Group, dealing with measurement methods for electromagnetic exposure detection at 5G Massive MIMO base stations and with exposure detection at ten base stations with ten measurement points each. This study has now been completed and the most important results are presented below.

In terms of the exposure to be measured, it is important to distinguish between the instantaneous exposure present at the time of measurement and the maximum possible exposure; the latter results from maximum operational utilization of the transmitter and is decisive for approval of the facility in Germany according to radiation protection regulations.

Determining the maximum exposure is a particular challenge, since it applies only when the base station directs a beam with maximum antenna gain and maximum transmission power to the measurement point. As a rule, it cannot be assumed that this will occur during the measurement period, and therefore it may be more expedient to measure permanently emitted signalling signals, which are used, for example, to register cell phones in the 5G cell, on a frequency-selective or code-selective basis and to extrapolate the maximum possible exposure on this basis. However, it must be considered that the signalling signals (broadcast beam) at 5G massive MIMO base stations are emitted with a different antenna pattern than the data signals (traffic beam), which account for the majority of the exposure at 5G. Therefore, the extrapolation must also include a correction factor to consider the antenna gain difference between the antenna patterns of traffic beam and broadcast beam, which must be determined based on the antenna diagram data of the antenna manufacturers.

A different method was used in this project to determine the maximum exposure: A 5G terminal was used to "pull" a traffic beam from the 5G base station to be measured to the measurement point. This was achieved by running a transmission speed measurement application (Speed App Netflix FAST) on the cell phone used, which demanded maximum resources from the base station. The 5G base stations tested were subject to very low utilization at the time of the measurements, thus ensuring that the resource utilization of the 5G base station was actually at a maximum within the measurement bandwidth of the hand-held spectrum analyzer [1] used for the measurements.

The measurements were carried out at ten 5G Massive MIMO base stations operating in the 3.4...3.7 GHz frequency range, and took place in the Länder Bavaria, North Rhine-Westphalia, and Hesse. Systems of the network operators Deutsche Telekom, Vodafone, and Telefónica were analysed using system technology from the manufacturers Huawei, Ericsson, and Nokia. At each of the ten base stations, the exposure was recorded at ten measuring points located at a horizontal distance of 5 m to 1,100 m (median 170 m) from the system tested. The measurement points were selected in such a way that the azimuth angles to the base station were as different as possible compared with a balanced ratio between measurement points according to LOS and NLOS of the base station antenna. The measuring points were not selected at random but deliberately determined after on-site inspection in order to ensure the best possible variety of different measuring points.

The maximum possible exposure level reached 0.2 % to 28.9 % of the field strength limits (61 V/m) specified for this frequency band in the 26th Ordinance on the Implementation of the Federal Immission Control Act; corresponding to electric field strengths between 0.15 V/m and 17.6 V/m. The median was 4.7% or 2.9 V/m. Figure 2 shows the probability distribution of the measured values as they relate to the exhaustion of the limit values. In addition to the 5G exposure measured in the current study, Figure 2 also includes a distribution diagram of a similar study in 2013, when maximum exposure at LTE-800 and LTE-1800 base stations was measured at measurement points selected using a similar methodology [3]. The exposure determined for LTE at that time ranged from 0.02 to 7.3% with a median of 1.1% of the field strength limits. Figure 2 shows that the exposure from 5G Massive-MIMO are on average (median) higher by a field strength factor of 4.3 compared to the LTE study in 2013. However, this could be attributed to the significantly increased signal bandwidth compared to LTE (up to 88.2 MHz for 5G compared to typically 9 or 18 MHz for LTE) as well as the higher maximum antenna gain. Nevertheless, even after the introduction of 5G, only a small percentage of the limits will be utilized.