## 3 Our Definition of Awareness in V2X Communications  
Although a plethora of beaconing approaches in [6; 7; 8; 9; 20] have been presented in the last two decades, it is surprising that there is almost no consensus in both academia and industry on the precise definition of awareness in V2X communications. Instead, it is conventionally understood as the ability of a vehicle to continuously perceive and share real-time information about its own kinematic state and those of its neighbor vehicles to enhance driving safety and traffic efficiency [1; 2]. However, we argue that awareness cannot be explained by such a monolithic view; it must be understood as a multi-faceted concept that comprehensively considers vehicle detection, tracking, and maintaining a safe distance, all of which are crucial for ensuring driving safety and traffic efficiency. Given neighbor vehicles in the transmission range \(D_{T}\), the goal of this section is to present our definition of awareness in terms of three key components:  
* **Vehicle detection** refers to the ability to recognize the presence of a neighbor vehicle. If the ego vehicle receives at least one beacon from a neighbor vehicle in \(D_{T}\), the neighbor vehicle is considered successfully detected; otherwise, its detection is deemed to have failed. * **Vehicle tracking** involves continuously monitoring and updating the kinematic states of a neighbor vehicle. If the tracking error is within threshold \(E_{T}\), the neighbor vehicle is considered successfully tracked; otherwise, its tracking is considered to have failed. * **Minimum awareness failure distance (mAFD)** is the distance between the ego vehicle and the nearest neighbor vehicle that has failed to be detected or tracked. Increasing the mAFD is crucial for maintaining road safety and preventing accidents. Notice that each neighbor vehicle can be in one of the following three states: _undetected_ (vehicle detection failure), _untracked_ (successful vehicle detection but tracking failure), and _tracked_ (successful vehicle tracking). To further clarify the definition of mAFD, we present an example of the awareness of the green-colored ego vehicle at times \(t_{1}\), \(t_{2}\), and \(t_{3}\) (\(t_{1}<t_{2}<t_{3}\)), as illustrated in Figure 3. The transmission range \(D_{T}\) is represented by a green transparent area. At time \(t_{1}\), the ego vehicle successfully receives the beacon from pink-colored neighbor vehicle 1 (NV1), while it fails to receive the beacon from gray-colored neighbor vehicle 2 (NV2), as shown in Figure 2(a). Then, the initial states of NV1 and NV2 at time \(t_{1}\) are tracked and undetected, respectively. Figure 2(b) shows that the states of NV1 and NV2 at time \(t_{2}\) remain unchanged from time \(t_{1}\). This is due to the ego vehicle's failure to receive a new beacon from NV2 and because the tracking error between NV1 and estimated NV1 (white vehicle) remains within the tracking threshold \(E_{T}\). At time \(t_{3}\), the state of NV1 changes to untracked because a new beacon from NV1, containing the lane-changing behavior, is not successfully received at the ego vehicle. Here, we devise three awareness performance measures, as follows:  
* **Minimum detection failure distance (mDFD)** is the distance to the nearest undetected vehicle. This measure helps assess the critical proximity at which the ego vehicle is unaware of nearby vehicles, posing the most significant safety risk. * **Minimum tracking failure distance (mTFD)** is the minimum distance to the nearest untracked vehicle. This measure evaluates the risk associated with vehicles that, although detected, have their kinematic information inaccurately monitored, potentially leading to unsafe driving maneuvers. * **Maximum tracking error (MTE)** is the largest tracking error over all tracked neighbor vehicles. While it does not pose a risk of accidents, this measure indicates the upper bound of the tracking inaccuracy of tracked neighbor vehicles. Finally, the mAFD is given by the shorter distance between the mDFD and the mTFD.