## 3 Materials and Methods  
### Ray-Launching Simulation Technique  
With the aim of analyzing the impact of the different channel impairments present in an urban V2X communication link at 28 GHz frequency band, an in-house developed 3D-RL has been used. The algorithm has been widely used and validated for urban complex scenarios and V2X links for frequency bands below 6 GHz. For instance, in [53], an urban environment is simulated by means of the in-house 3D-RL algorithm, coupled with a microscopic vehicular movement simulator, showing good agreement with experimental measurements for the frequency band of 5.9 GHz. On the other hand, in [54], with the aid of the 3D-RL tool, the radio wave characterization for ISM 2.4 GHz and 5 GHz Wireless Sensor Networks (WSNs) deployed within a smart city scenario has been assessed and validated. It has also been validated for wireless propagation in closed environments [55], interference analysis [56] or electromagnetic dosimetry evaluation in wireless systems [57]. A detailed description of the algorithm and its convergence analysis can be found in [58]. In this work, a further step is proposed, to analyze the impact in the physical layer of the frequency band of 28 GHz, which arises as a potential band for the next generation of vehicular communications. The in-house developed 3D-RL is based on Geometrical Optics (GO) and the Uniform Theory of Diffraction (UTD). The 3D-RL approach basis consist to launch a significant set of rays that travel from the transmitter to the receiver position considering the direct ray (LOS), reflected and diffracted rays according to the geometry and morphology of the considered scenario. The whole channel information is achieved by means of the resulting complex impulse response. However, to obtain accurate results, a thorough description of the site-specific propagation scenario is required. The application of GO can cause some channel prediction impairments in edges and discontinuities areas, which are resolved in the algorithm with the consideration of the diffraction phenomenon based on UTD. Thus, the diffraction coefficients on the obstacles edges or diffractive elements have been considered to predict the shadowing areas' field. The electric field E created by GO and the diffracted electric field created by UTD are calculated by [59]. \[E_{\rm GO}^{\perp\parallel}=\sqrt{\frac{P_{rad}D_{t}(\theta_{t},\phi_{t})\eta _{0}}{2\pi}}\frac{e^{-j\beta_{0}\tau}}{r}\mathbf{X}^{\perp\parallel}\mathbf{ L}^{\perp\parallel} \tag{1}\]  
\[E_{UTD}^{\perp\parallel}=\epsilon_{0}\frac{e^{-j\beta_{1}}}{s_{1}}\mathbf{D} ^{\perp\parallel}\sqrt{\frac{s_{1}}{s_{2}(s_{1}+s_{2})}}e^{-j\beta_{2}} \tag{2}\]  
where \(\beta_{0}=2\pi f_{c}\sqrt{\epsilon_{0}\mu_{0}}\), \(\epsilon_{0}=8.854\times 10^{-12}\) F/m, \(\mu_{0}=4\pi 4\times 10^{-7}\) H/m and \(\eta_{0}=120\pi\) ohms. The radiated power of the transmitter antenna is denoted as \(P_{rad}\). \(D_{t}(\theta_{t},\phi_{t})\) is the directivity where rays are launched as defined in the spherical coordinate system at an elevation angle \(\theta_{t}\) and an azimuth angle \(\phi_{t}\). \(\mathbf{X}^{\perp\parallel}\) and \(\mathbf{L}^{\perp\parallel}\) are the polarization ratio and path loss coefficients for each polarization, \(r\) the distance in free space and \(fc\) the transmission frequency. In Equation (2), \(\mathbf{D}^{\perp\parallel}\) are the diffraction coefficients for each polarization, \(e_{0}\) is the free-space field strength, \(k\) is the propagation constant and \(s_{1}\), \(s_{2}\) are the distances from the source to the edge and from the edge to the receiver point. The launching rays are predefined in a solid angle that considers the radiation diagram of the transceivers. Parameters such as frequency of operation, radiation patterns of the antennas, number of multipath reflections, separation angle between rays, and cuboid dimension can be considered to be inputs in the 3D-RL software.