## 4 Discussion
For such solutions, an even larger spread of the BS transmit power over different propagation paths is to be expected. The experimental data obtained for 8 CSI-RS ports codebook-based beamforming are therefore expected to provide a conservative estimate of the actual transmit power of relevance for EMF assessments compared with other (existing or upcoming) implementations. Although the spreading of energy is deemed to increase when traffic increases, the results in Section 3 show that \(E\!IRP_{\text{act}}\) is also well below the theoretical maximum for a limited number of active users. Out of about 13 million samples, including instances for which only one user was active within a six-minute interval, not a single case was found when any of the BSs would constantly schedule the same beam with the maximum power. Mobile networks are in fact deployed to provide services to a myriad of UEs, with enough margins to address occasional traffic peaks without reaching traffic congestion. When only a few users are served, the amount of traffic, and therefore the BS time-averaged transmit power, will account for a small fraction of the available maximum. Moreover, even the same UE might be served by different beams over time as a consequence of the changes in the propagation conditions or due to simultaneous transmission of parallel data streams (when two or more layers are transmitted to the same UE, the allocated power will be simultaneously spread among different beams). Network measurements provide an efficient tool to gather comprehensive information on the actual EMF levels of live network for several BSs and long periods. This is even more relevant for BS implementing beamforming, as the gathering of data directly from the network manager allows one to study the distribution of the transmitted signal in the spatial domain rather than at a fixed location. At the same time, measurements conducted in-situ are complementary and equally important, as they provide a direct measure of the typical EMF exposure levels in areas accessible by the general public. Although results from in-situ measurement campaigns addressing EMF exposure for massive MIMO are already available (e.g., [10; 11]), including 5G (e.g., [3; 4; 6]), additional data are desirable while 5G services become widely spread. The maximum \(E\!IRP_{\text{act}}\) reported in Section 3, obtained from an overestimate of \(P_{\text{avg}}\) was found to be 8.8 dB below the theoretical maximum. This value would drop to \(-15\) dB if \(P_{\text{avg}}\) was estimated from the mean of the six-minute time-averaged transmit power distribution reported for every fifteen-minute interval. \(E\!IRP_{\text{act}}\) was determined with respect to the peak instantaneous EIRP including the effect of the TDD downlink duty cycle (TDC). This allows for a direct comparison with other studies available in the literature for which the TDC factor was also included when evaluating the actual power of massive MIMO BSs. The experimental values of \(E\!IRP_{\text{act}}\) observed in this work are below the maximum actual power levels previously estimated by statistical and numerical models (e.g., [8; 9]). According to a technical report from the International Electrotechnical Commission [15], the actual maximum transmit power predicted for a massive MIMO site with horizontal beam scanning would only correspond to about 25% (\(-6\) dB) of the theoretical maximum (including the effect of 75% TDD downlink duty cycle). Such values are consistent with measurements previously conducted on massive MIMO BSs of commercial 4G networks [10; 11]. In [10], the actual maximum exposure was found to be about 8.5 dB lower than the theoretical maximum by using network counters like those utilized in this paper. In [11], field measurements conducted in front of a massive MIMO LTE antenna, led to exposure values (when scaled for the maximum BS utilization) ranging from 10 dB to 7 dB below the theoretical maximum.