## 2 Background
### V2X Evolution of Enhanced Perception for ADAS and Autonomous Driving
The most crucial Day 1 use cases include those ADAS applications that increase the cooperative awareness of road users by relying on the status and attribute information delivered by the Cooperative Awareness Messages (CAM) sent periodically (with a frequency of 1-10 Hz) based on the \"see and be seen\" principle [15]. These include applications that support safe passage through intersections, turning, changing lanes, and overtaking, as well as blind spot monitoring and collision warning. Cooperative Adaptive Cruise Control (C-ACC) can also be classified here because the joint implementation of acceleration/deceleration,in the direction of travel, is also based on the CAM Day 1 message service. Preparation for unexpected and potentially dangerous events is helped by the information carried in event-driven Decentralized Event Notification (DENM) messages, with which road users can warn each other about road works, weather conditions, stationary or slow vehicles, crossing prohibited signs, and different similar dangerous situations [16]. The use cases referred to as Day 1.5 refer to situations where status/attribute information is shared and used in multimodal applications. In the next step, Day 2 will lead to the era of \"sensing driving\" that relies on shared sensor data, expanding the possibilities of Day 1 applications in several aspects. The V2X messages communicated here no longer only provide information about sender road users, but also ensure the sharing of digital representations (objects) detected by them, thus facilitating the operation of new ADAS applications with additional capabilities. An excellent example is the Collective Perception Message (CPM) protocol and message service [17]. With the help of CPM, we can share the objects detected by the sensors and monitoring systems of the vehicle or the infrastructure and/or the free (\"empty\") area between the objects (\"free space\"). The sensor information and detections conveyed in CPM messages can be used as input for sensor fusion procedures, which, using this additional data and running various refinement algorithms and techniques, can form a much more accurate mapping of the world surrounding the vehicle, the current characteristics of the environment, and also receive real-time sensor data from relevant areas, which may be blocked in front of their own sensors or due to limited range. With the approaches available in the Day 2 phase, there is an opportunity for data from various objects that are not fundamentally capable of V2X communication to be included in the cooperative domain and thus increase the detection area. After the Day 2 phase, the applications of the so-called cooperative and synchronized cooperative driving will presumably become the next evolutionary steps, which will gain an increasing role with the spread of vehicles reaching more advanced levels of autonomous driving. The essential innovation foreseen in Day 3 and Day 3+ V2X applications is the introduction of the sharing of vehicle intention and coordination data. Cooperative Automated Vehicles (CAVs) will be able to exchange such information, so they can coordinate their maneuvers with other road users or the infrastructure and prevent conflicts. This includes, for example, the AGLOSA (Automated Green Light Optimized Speed Advisory) application, which can adjust the speed of controlled CAVs to the rate of phase transitions of traffic lights by actively and adaptively controlling the pace [13]. In the information exchange of the relevant messaging services, traffic participants will share their planned trajectory, with which traffic can be much more continuous and smooth than at present, and accidents caused by misinterpretation of driving intentions, or even just unnecessary slowing down, can be avoided. The above-introduced V2X evolution and example application areas all point to the fact that ADAS and autonomous driving solutions will increasingly rely on cooperative communication, and for some, V2X will already be an absolutely essential requirement.