## 3 Motivation and System Model
### System Model  
A vehicle has multiple wireless interfaces such as Long-Term Evolution (LTE), 5G, DSRC, Wi-Fi, etc. We denote the wireless interfaces of a vehicle by \(IF_{set}=\{IF_{1},IF_{2},\ldots,IF_{n}\}\). A vehicle connects to the cloud server via V2C communication through the Internet, as shown in Figure 1. A communication path between a vehicle and a cloud server includes both wireless links, from the vehicle to the base station or access point, and wired links to the Internet. A typical application of data transmission in a vehicle is the periodical delivery of sensing results and vehicle position information to the cloud server.  
Let \(MaxPathNum\) denote the maximum number of active paths in a multipath connection that a vehicle can utilize. The maximum number of active paths is limited by the total number of wireless interfaces \(|IF_{set}|\) in a vehicle.  
Suppose that each vehicle has a target reliability defined by the successful rate of packet delivery. The target reliability value is determined by the demand for reliable communication according to the position and safety status of the vehicle [31]. For example, at a road intersection that has a potential risk of collision, a vehicle requires high reliability in V2C communication.  
The reliability of V2C communication is measured by the successful delivery rate of packets from a vehicle to the cloud server. Hence, the reliability value is in the range of \([0,1]\). With multipath-based packet delivery, reliability is the successful delivery rate of packets in a multipath connection from a vehicle to the cloud server. To simplify the problem we are addressing, we assume that the target reliability is constant for each vehicle.  
A vehicle initiates V2C communication by periodically sending sensing, speed, and position information to the cloud server. V2C communication requires UDP-based and low-delay transmission. We assume that the delay is small in packet transmission without ACK and retransmission. The cloud server identifies packet duplication when receiving multiple duplicated packets transmitted from different paths, assuming the sender of a vehicle can label the duplicated packet with a time stamp and an associated ID.  
In the proposed system, a vehicle exchanges the schedule information of periodically transmitted messages with the cloud server. The basic schedule information includes the start and end times of the message transmission, transmission interval (e.g., 100 ms), and target reliability in terms of successful packet reception rate. Based on the information exchange of the transmission schedule, the cloud server carries out packet loss counting, assessment of receiving reliability, and evaluation of path adjustments.  
In accordance with practical multipath transport protocols, we present a general approach to path utilization in the proposed system as follows. A vehicle creates \(k\) paths, from which the multipath scheduler chooses \(n\) paths for transmitting data packets with duplication. Essentially, \(k\) is set to \(MaxPathNum\). The monitoring of reliability for each created path is carried out at the cloud server during data delivery. Suppose a path is monitored for \(m\) percent of the time during V2C transport. The percentage m can be either 100 percent or the ratio of time the path is in an active state for data delivery. (When m  
Figure 1: System model.  
is 100 percent and a path is not always chosen as an active path to deliver data packets, we assume path reliability is monitored via probing-based packet delivery rates, such as periodical ping operations).