## Materials and Methods
### Time-Average Antenna Array Patterns  
An instantaneous array pattern is calculated as a sum of the patterns of its individual elements, weighted with the components of the transmit vector \(\mathbf{t}\). As all antennas in the BS array are identical dipoles, this gives  
\[A(\theta,\varphi,\mathbf{t})=\sum_{n=1}^{N}A_{dip}(\theta,\varphi)t_{n}\exp(-2\pi i (\mathbf{d}_{n},\mathbf{c}(\theta,\varphi))/\lambda), \tag{10}\]  
where \(A_{dip}\) is a half-wave dipole radiation pattern [19], and \(t_{n}\) denotes the \(n\)th element of \(\mathbf{t}\). Here we do not account for the effect of mutual coupling in the antenna array, i.e., the modification of the free-space antenna element pattern by the currents in the neighboring elements. In the far-field region of a BS, the incident EMF is proportional to the antenna gain in the direction where the measurement is preformed. As mentioned above, ICNIRP specifies [4] an EMF time-averaging interval \(T_{avg}=6\) min for the human exposure assessment. At the same time, it is foreseen that in a typical scenario 5G DL traffic will be transmitted in short bursts (in the order of tens of seconds [6]), switching between sets of UEs that demand it at any given moment. If the served UEs are distributed uniformly enough within the cell, the BS would focus the transmission in many different directions over a sufficiently long time interval. Therefore, a _realistic_ time-average BS antenna array gain is expected to differ significantly from the theoretical maximum one. To quantify how much the time-averaged gain is reduced relative to the theoretical maximum we follow the approach proposed in [6; 7]. We introduce a constant \(T\)--the duration of one connection (\"drop duration\" in [7] or \"scheduling time\" in [6]). We model a network in which independent sets of \(K\) UEs are served for time \(T\) in series with no overlaps. Then, the time-average BS array radiation pattern \(\tilde{A}_{m}^{N,K}(\theta,\varphi)\) is calculated as a weighted mean of the instantaneous patterns \(i\) produced during the averaging interval  
\[\tilde{A}_{m}^{N,K}(\theta,\varphi)=\sum_{i}\omega_{i}A_{m}^{N,K}(\theta, \varphi,\mathbf{t}^{i}), \tag{11}\]where \(m\in\{CB,MRT,ZF\}\) denotes the transmission precoding scheme used at the BS; \(N,K\) are the number of utilized antenna elements and the number of simultaneously served UEs, respectively, and \(\omega_{i}\) is the fraction of the averaging time during which pattern \(i\) was active, varying from 0 (not in the averaging interval) to \(T/T_{avg}\) (fully inside the averaging interval). In the following, for convenience we choose \(T\) such that \(T_{avg}\) is its integer multiple, then \(\omega_{i}=T/T_{avg}\). Next, the normalized gain \(G_{m}^{N,K}\) is given by  
\[G_{m}^{N,K}(T)=\frac{\max_{\theta,\varphi}\left[A_{m}^{N,K}(\theta,\varphi) \right]}{G_{max}^{N}}, \tag{12}\]  
where \(G_{max}^{N}\) is the maximum gain of an array of \(N\) elements. The maximum gain of a planar antenna array is calculated as a product of the maximum antenna element gain and the maximum array factor. The maximum array factor equals to the number of elements in the array [20]. Therefore, for an antenna array composed of identical half-wave dipoles, \(G_{max}^{N}=\max_{\theta,\varphi}[A_{dip}]\cdot N\simeq 1.64\cdot N\). This value is further used in (12) as a normalization factor. The RMS E-field strength at the location of the UE, to which \(G_{m}^{N,K}\) would be directed, can be estimated using a free-space approximation according to the following expression,  
\[E_{RMS}=\frac{\cos\left(\pi-\theta_{max}\right)}{h_{BS}-h_{UE}}\sqrt{\frac{ZPG _{m}^{N,K}G_{max}^{N}}{2\pi}}, \tag{13}\]  
where \(\theta_{max}\) is the polar angle of \(G_{m}^{N,K}\), \(Z\simeq 377\) Ohm is the impedance of free space, \(P\) is the BS total radiated power, and \(h_{BS}=25\) m and \(h_{UE}=1.5\) m are the BS and the UE height above the ground, respectively.