## 2 Background  
### Evolution of V2X Communication Systems  
The initial ITS applications were known collectively as Day 1. It includes all of those services whose Quality of Service (QoS) requirements can be met by today's state-of-the-art technologies and are mature enough to be deployed in practice [4; 5]. The primary objective of Day 1 services is to share status and attribute information between V2X-enabled actors and then send the driver different sound or light signals (i.e., warning and informational messages) based on the data received. They mainly operate with CAM (Cooperative Awareness Message) [6] and DENM (Decentralized Event Notification Message) [7] message types. Day 1 services do not yet have a role in the automatic control of the vehicle but only assist and complement the driver's decision-making in the case of an unexpected event [4; 8]. Besides the two main protocols, further message types are distinguished. SPATEM (Signal Phase and Timing Extended Message) [9] provides traffic-lights status, MAPEM (MAP (topology)) Extended Message) [9] contains road topology information, and IVIM (Infrastructure to Vehicle Information Message) [9] presents up-to-date area information. Day 2 applications include solutions that, in addition to the basic information flow, use their own sensors to continuously scan the world around them to provide more accurate information to drivers and other vehicles [5]. One of the most common communication protocols used by Day 2 applications is the CPM (Cooperative Perception Message) [10]. CPMs are transmitted to share information about the presence of road users and other objects detected and recognized by the transmitting ITS station sensors. Based on calculations presented in Reference [11], it was found that if approximately one-third of all vehicles on the road are equipped with V2X systems, then nearly 100 percent of road users could be covered by CP and similar services, thanks to collective perception features that disseminate information also about road users without V2X. Service advertisement messages such as Service Announcement Message (SAM) [12] and WAVE Service Advertisement (WSA) [13] are also used by Day 2 services in the EU and US, respectively. Both of them allow C-ITS devices to have knowledge of a particular service of interest that other ITS stations provide. With the help of these protocols in a multi-channel environment, we can make sure on which channel a specific service is available without switching to that channel. Vulnerable Road Users Awareness Messages (VAMs) are used to convey relevant information about the presence and behavior of vulnerable road users to nearby vehicles. These messages help vehicles detect and respond to the presence of pedestrians, cyclists, or motorcyclists in their vicinity. The purpose of Maneuver Coordination Message (MCM) is to increase safety and coordination among vehicles, where cooperative maneuvers can improve the overall traffic flow (Table 1). V2X communication technology has been rapidly evolving, and several developments are expected to shape its future beyond Day 2. For example, Precise Awareness Message (PAM) [14] was proposed, aiming at increasing CAM capabilities with high precision awareness with extreme transmission rates. This messaging service was designed to have the lowest possible channel footprint, enabling a transmission rate as high as 100 Hz. Another example is cooperative driving, which involves vehicles communicating with each other to share information about their location, speed, and driving intentions, such as lane changes or turns. This information is shared through V2X communication channels, allowing vehicles to anticipate and react to each other's movements, which can help reduce the risk of collisions and improve traffic flow [5].