# Two-Level Sharing and Extraction of Sensing Information with Hybrid V2X Communications
## 4 Two-Level Sharing and Extraction of Sensing Information with Local Cooperation among Vehicles  
The overview of the proposed method is illustrated in Figure 2. The proposed procedure of the extraction of object information is a two-level process. The first-level process is called the global collection-extraction process. In this process, the edge server queries and collects sensing information from vehicles to extract the object sensing information, such as the minimum sensing value among vehicles. The second-level process is the local sharing and extraction of sensing information. In this process, vehicles locally share, collect, and extract the object sensing information. After the local extraction of object sensing information, such as the minimum value of the sensing data among vehicles, each vehicle makes a judgement of whether to transmit a report message to the edge server. Only the vehicles that are self-judged to be necessary to report sends its report to the edge server. Moreover, hybrid communication is utilized in the extraction of the sensing information from vehicles. First, long-range communication is utilized for the global collection and extraction of sensing information between the edge server and vehicles. This communication activity involves a broadcast from the edge server and a unicast for reports from vehicles to the edge server. Second, short-range wireless communication is utilized for the local sharing of sensing information among vehicles via a local broadcast. We specify the minimum value of the sensing data among vehicles as the object sensing information of the extraction. Such a minimum value is called MinFactor in this paper. We refer to the reports from vehicles to the edge server as the global reports at vehicles. The edge server starts to extract sensing information at vehicles by sending a query message to the vehicles within a limited region. After that, the edge server waits for the reports from vehicles. On receiving the query message from the edge server, the vehicles begin the local sharing and extraction of sensing information. The flowchart of the proposed scheme is shown in Figure 3. A vehicle at first sets its local record of the MinFactor with a sensing value on its own, denoted by a sensing value of 0. A vehicle starts to exchange the sensing data via the local broadcast. The edge server defines the length of the period for an exchange round. Each vehicle counts the number of rounds during the sensing information exchange. Repetition of the local broadcast of the  
Figure 1: The extraction of sensing information from vehicles. sensing data allows for the spreading of sensing information among vehicles that are not directly connected in the local communication range. After the first round of exchange, each vehicle collects its received sensing results and then broadcasts the new received information if the number of exchange rounds is less than the threshold. Let the set of received message at a vehicle be denoted by \(M_{i}[k]=\{X_{1},X_{2},\ldots,X_{n}\}\), where i is the vehicle ID, \(k\) is the round number, and \(n\) is the number of messages received. The information to exchange at an exchange round \(k\) at a vehicle is hence denoted by \(T[k]=M[k]\setminus M[k-1]\), which is the set difference of \(M[k]\) and \(M[k-1]\). If the number of exchange rounds reaches the threshold, the vehicle stops the exchange procedure of sensing data. Each vehicle then locally extracts the object sensing information. For instance, given a set of collected sensing results of RSSIs obtained from other vehicles, a vehicle searches the minimum RSSI and judges whether its own RSSI value is the minimum RSSI compared with other RSSI values obtained from the exchanges. A vehicle refrains from reporting its sensing result to the edge server if its sensing result is not the MinFactor, such as the minimum RSSI or the minimum free space between vehicles.