## 2 Background
### V2X Evolution of Enhanced Perception for ADAS and Autonomous Driving  
As it was introduced above, ADAS and autonomous driving technologies rely on sensor infrastructures to discover the ego vehicle's environment. The output of these sensors is processed and appropriate in-vehicle systems respond accordingly and instantaneously. This scheme enables the vehicle to detect, e.g., a forthcoming collision event and automatically initiate a response action (such as breaking) significantly faster than a human driver would ever be capable of. Even though modern sensor solutions provide a 360-degree field of view, they still pose severe limitations. For example, it is impossible for them to see around corners or across multiple crossroads nor to warn drivers of potential hazards before they appear. Moreover, sensor systems often rely on artificial intelligence to process the data they collect, which may end up in misinterpretation of sensor inputs in certain circumstances. Vehicle-to-Everything (V2X) communication tackles the above issues of automotive sensor architectures by implementing a novel way of perception that relies on cooperative information sharing among road users and road infrastructure. Unlike legacy ADAS applications that depend on sensors modeling human senses, the V2X-based successor techniques will take the paradigm to the next level by moving beyond the current limitations of line-of-sight capability and operating distance. In fact, V2X is considered a new generation of sensors: it is just like radar or lidar but with an active, communicating party on the other side, making V2X the only sensor with non-line-of-sight abilities. Furthermore, V2X can operate and perform perception tasks independently of any weather and lighting conditions, paving an innovative path of reliably and precisely mapping the physical environment into its digital representation. The V2X toolset will effectively help the vehicles comprehend any situations possible on the road, playing an essential role in all four main stages of general autonomous driving procedures. First, V2X as a sensor will contribute to discovering objects, other road users, etc., by extensively sharing status, attribute, and sensor information. Second, V2X, as a pillar of cognition, will help to recognize any specific scene in the environment. For that, intention data exchange will be applied. Third, V2X will also improve autonomous driving decisions by taking part in the selection of the best maneuver the vehicle needs to take utilizing transmitted trajectory/maneuver data. Fourth, V2X will aid actuation procedures by defining how road users must behave while on the move: V2X will be used to confirm that the decided maneuver is the correct one and facilitate actuation effects using coordination data exchange. In the above innovative application areas of V2X, an evolution regarding the use of dynamic data by ADAS and autonomous driving systems can be seen. We are moving from solutions that ensure the efficient supply of information to the human user (driver) towards the possibilities that can be realized with cooperation between autonomous vehicles. The significant development steps of this evolution are manifested in the supported use cases that we group into \"Days\" [13]. V2X applications implementing Day 1 use cases support awareness driving. Awareness means that the V2X protocol stack, through its specialized Facilities-layer message services [14], makes the driver and the vehicle aware of the primary status/attribute and event information relevant to the environment. This is done by receiving and processing periodic or event-driven messages from its partners' messages. The common feature of the Day 1 applications is that they are relatively simple, and based on human drivers, so they do not interfere with driving; they only provide drivers with additional information.