## 2 Materials and Methods  
### Ray Launching Technique  
As it has been previously introduced, the EMF exposure simulation tool has already been used in previous works in order to analyze nonionizing radiation exposure in complex indoor environments when multiple cellular technologies coexist. In this work, a step further is proposed in order to analyze an EMF exposure assessment in a crowded indoor complex environment where 5G-FR2 and WLAN 802.11 ay wireless communication systems operating at 28 and 60 GHz, respectively, coexist. For that purpose, the EMF exposure tool has been enhanced in order to consider the distinctive characteristics of these wireless communication systems. The most significant ones are presented in Figure 1 and can be described as follows: (i) possibility to simulate mid-band frequencies and mmWave system operations, (ii) multifrequency operations, (iii) flexible beamforming depending of the number of UEs served, (iv) possibility of considering SU/MU-MIMO in both UL and DL, (v) consideration of different complex scenarios with all the scatterers/obstacles within them, (vi) a geometric-based deterministic channel model (GBDM) based on an in-house three-dimensional ray launching (3D-RL) technique, (vii) feasibility of the densification of the UE and base stations/access points and (viii) UL and DL EMF exposure can be assessed in a high-user density indoor/outdoor environment. GBDM based on full-wave techniques are considered highly accurate, as they take into account all the geometry and obstacles of the environment, but they present the disadvantage of a higher computational cost, which can be unaffordable for typical real-world scenarios [27; 28]. In this sense, GBDM based on ray tracing or ray launching techniques such as the in-house 3D-RL algorithm presented in this work achieve good accuracy with a reasonable computational load for typical real-world scenarios [29; 30]. Traditionally, ray launching and ray tracing are both classified as ray tracing methods, although, more recently, both methods have been distingu  
Figure 1: Schematic view of the distinctive characteristics of the enhanced EMF exposure simulation tool. principally due to different methodologies. The principle of ray launching techniques is to launch sets of test rays from the transmitter to determine the true path by looking for the rays that arrive at the receiver. Conversely, in ray tracing methods, the basic principle is to compute the image of the transmitter or the receiver to find the reflected paths by using walls and furniture. The proposed in-house 3D-RL algorithm is based on Geometrical Optics (GO) and the Uniform Theory of Diffraction (UTD). It has been previously validated in different indoor/outdoor complex environments [31; 32] for different applications, such as vehicular communications [33; 34], Intelligent Transportation Systems (ITS) [35; 36; 37], interference analysis [38; 39], Wireless Local Area Networks (WLAN) [40; 41], Wireless Sensor Networks (WSNs) [42; 43] or Public Land Mobile Networks (PLMN) [23; 24], among others. These applications have been validated for different operating frequencies (from mid-bands to mmWave frequency bands) and different wireless communication system configurations, achieving, in general, a Root Mean Square Error (RMSE) of 3-6 dB, an absolute mean error of 4-5 dB and a Standard Deviation (SD) of 1-7 dB. The algorithm basis is that a certain number of rays is launched from the transmitter to the receiver with a determined angular and spatial resolution. Depending on the UE and access point locations in the considered scenario, the angles for the different beams are defined and then launched from the transmitter (which can be one or more UE or access points), depending on the selected input parameters of the algorithm. The implemented flexible beamforming in the enhanced simulation tool consists of the simulation of a set of spatial orientations (beams) with a defined angular and spatial resolution in the 3D scenario.