## 1 Introduction  
The first 5G NR (new radio) commercial networks were launched in 2019, and the number of 5G subscriptions globally reached 13 million by the end of 2019 [1]. Like any other radio access technology, 5G base stations (BSs) have to comply with radiofrequency (RF) electromagnetic field (EMF) exposure limits, such as those specified by the International Commission on Non-Ionizing Radiation Protection [2]. EMF measurement surveys of 5G BSs have been conducted in some countries [3, 4], and typical exposure levels in areas accessible by the general public were found to be thousands of times below the limit values and similar to those of other existing mobile technologies (2G, 3G, and 4G).  
5G makes use of beamforming and beam steering to adapt the antenna radiation pattern to the time-varying traffic and the radio propagation conditions. This allows it to efficiently direct the energy where needed, providing enhanced performance to the users. To be able to steer the beams over a range of angles, BS products make use of array antennas characterized by many elements, generally known as massive MIMO antennas. Compared with conventional non-beamformed systems, massive MIMO BSs are characterized by a larger variability of transmitted signals in time and space. In order to take this into account, specific measurement techniques to assess the exposure levels in proximity of 5G massive MIMO BSs have been developed [5, 6, 7].  
The effects of beamforming and traffic variation on the EMF exposure levels have been studied by means of numerical simulations and statistical models [8; 9]. Based on conservative assumptions on the data traffic and on the user distributions, these studies estimate the BS maximum power level contributing to EMF exposure in any beam direction. Since the beams are steered in order to provide service to the connected users, the time-averaged power in each beam direction was found to be well below the instantaneous peak (or theoretical maximum). According to [8], the 95 th percentile of the transmitted power per beam would range between 7% and 22% of the theoretical maximum. These results have been verified by measurements conducted on massive MIMO BSs [10; 11] of 4G LTE-Advanced networks. While a substantial amount of research available to date already shows that massive MIMO BSs do not constantly operate at the maximum power, experimental data specific for 5G live networks are yet not available.  
In this study, a large amount of experimental data of relevance for EMF exposure was collected from a 5G NR commercial network. The dynamic of the spatial distribution of the transmit power serving 5G users was continuously monitored for 25 massive MIMO BSs over 24 h. Data were extracted from network counters and events, which directly provide information on the BS baseband operations. In addition to the time-averaged transmit power, information on the beams scheduled by the BSs when serving the connected users is obtained and processed. By this knowledge, the spatial distribution of the transmit power was determined. The method is developed in Section 2 and results on the actual power are given in Section 3. A discussion, including implications of the results on EMF exposure from massive MIMO BSs is provided in Section 4 and some conclusions are drawn in Section 5.  
This work is of interest to support the ongoing revision of IEC 62232 [5], which provides methods to conduct EMF compliance assessment of massive MIMO BSs by taking the effects of beamforming into consideration. The availability of 5G experimental data can be relevant to regulators when defining guiding principles on the usage of the actual maximum power to avoid unrealistic estimates of the EMF exposure levels.