## 2 Test Setup and Simulation Environment
### Simulation Scenarios  
The topology of the simulated communication scenarios can be seen in Figure 1. It is worth reiterating here that two different scenarios were investigated in this study, i.e., one involving a fixed infrastructure density and the other one focusing on a varying infrastructure density in order to study the impact of infrastructure density on the V2I C-V2X communication performance, which are clearly highlighted in Figure 1. We simulated vehicular communications in the vicinity of a large, signalized intersection in the city of Zilina, Slovakia. This specific intersection has been selected due to its strategic location, where intercity and long-distance traffic crosses at the same spot, causing frequent congestion and traffic safety issues. Being a part of the frequent international passenger and freight transport route from the Czech Republic and Poland to eastern parts of Slovakia, the road infrastructure we have selected for our simulation experiment is also heavily loaded in terms of traffic. This fact gives us a chance to benchmark both C-V2X communication technologies in a wide variety of traffic conditions, while maintaining realistic traffic volumes. Selecting this intersection and its surrounding infrastructure comes with yet another practical benefit - the communication and traffic models developed within this study can be further used to evaluate the impact of 4G- and 5G-based V2X services on the traffic performance of this challenging road infrastructure. Vehicles entered the simulation 1.8 km from the intersection (Figure 1 point \"A\" in the map). We have simulated a V2I communication from a vehicle to a Roadside Unit (RSU) located at the intersection. High-level simulation scenario parameters are detailed in Table 1.  
Communication from a vehicle to the RSU was initiated once the Euclidean distance between the vehicle and the RSU dropped below a predefined perimeter. Vehicles continued to periodically send communication messages to the RSU with a specific message generation frequency until they left the network (Figure 1 point \"B\" in the map). The message generation frequency has been set to follow the standard used for the CAM [30]. It is worth noting here that individual communication messages were sent at uniformly spaced time intervals and were generated 2, 4, 6, 8 and 10 times per second. So, for instance, in the case of 2 messages per second, the time interval was 500 ms. We have not considered background traffic in this study because it is foreseen in the literature [31; 32] that the \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Simulated technologies & LTE infrastructural, LTE device-to-device in mode 3, 5G infrastructural and 5G device-to-device in mode 3 \\ Application protocol & Periodic vehicle-to-infrastructure fixed-length message \\  & transmission service \\ Transport protocol & User Datagram Protocol \\ Message length & 300 bytes (including a security header) \\ Number of cells & 4/8 1  \\ Base station spacing & 800/400 metres 2  \\ Simulation length & 600 s \\ Number of repetitions & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation scenario parameters. Figure 1: Visualization of the simulation scenario. Base stations depicted in black represent the cellular infrastructure used to simulate the fixed infrastructure density scenario. To study the impact of infrastructure density on the performance of Cellular-V2X (C-V2X) technologies, i.e., the varying infrastructure density scenario, four additional base stations (in red) were added. channels purely dedicated for C-ITS services in 4G-based and 5G-based C-V2X will be deployed in the real implementations. All the simulation runs simulated 600 s of traffic. A 100 s long initialization period was introduced to populate the network with vehicles and achieve the target traffic intensity. To study their impact on the communication network performance, we varied the values of the message generation frequency, communication perimeter and traffic intensity.