## 2 Related Work  
Wireless communications in the mmWave band promise an increase in transfer rates while decreasing communications latency. However, high transmission losses, and poor diffraction and signal penetration pose obstacles to the complete arrangement of mobile communication systems in these frequency bands. The complete understanding of all the characteristics behind the transmission in the mmWave frequency bands is fundamental for the application of these high-throughput wireless communication systems. This multi-gigabit communication will allow sharing raw sensor data between ITS elements. Despite this, its application in transport systems is currently limited to radar arrangements [13]. Although the use of mmWave as a communication system in the vehicular environment is a concept already established almost two decades ago [34], it is only recently that its practical application for these purposes seems more real. However, the empirical-based propagation models are not able to provide accurate space-time or angle-delay results, which are key characteristics for simulation of these systems. Using geometric methods allows the analysis of these systems natively, in which techniques such as RT/RL will be useful to address complicated propagation conditions in high-frequency regimes [35]. The use of these techniques for the analysis of wireless links at these frequency bands can be verified in works such as those shown in Table 1, where references like [36; 37; 38; 39] make use of these approaches in a wide range of scenarios spanning indoor and outdoor environments. Figure 1: Schematic overview of the main contributions of the 3D-RL mmWave framework. Specifically, in vehicular environments, RT/RL is a valuable tool to analyze the wireless communication channel. In mmWave frequencies, works such as [40] use the RT technique to analyze aspects of vehicular communication under NLOS conditions. In such work, a professional RT tool called Wireless Insite is selected and the results gathered in simulations are compared with statistics obtained during measurement campaigns. The environment is staged under V2I communication conditions, and parameters such as Path Loss exponent, shadowing factor, and RMS DS are some of those obtained in the work. Another communication environment for transport systems is the high-speed trains (HST) scenarios that are rarely explored. The work presented by [41], uses a calibrated and validated 3D-RT algorithm to explore the characteristics of the 25.25 GHz wireless channel in HST scenarios. Multiple layouts are simulated, and parameters such as Path Loss, PDP, RMS DS, and Doppler effects are some of those obtained during RT simulations. In addition, another RT algorithm is used to characterize the influence of typical objects in the context of trains to infrastructure communication in the work conducted by [42]. The research carried out at 28 GHz, uses a calibrated algorithm and experimental measurement campaigns. Both simulation and the measurement campaign uses an omnidirectional antenna and a Close-in (CI) free-space reference distance model to fit the data. Path Loss parameters and RMS DS are obtained in different scenarios, which are expanded with the assistance of RT simulations. In [43], the wireless communication channel is characterized in a V2I environment in the mmWave frequency bands. The work is also assisted by a RT algorithm, validated by experimental measurement campaigns at 60 GHz, and then used in 22.6 GHz simulations. Through the comparison of parameters such as received power and RMS DS, the algorithm precision is validated. As part of the conclusions drawn in such research, a relationship is found between the RMS DS and traffic flow density, highlighting that this factor increases when the traffic density rise, related to the addition in the number of multipath generated by the vehicles. Another work presented in [44], based on the characterization of the wireless channel in a V2I scenario, also employs the assistance of a deterministic technique.