## 3 Content Acquisition Process and Caching Algorithm Optimization  
### Content Acquisition Process  
In vehicular networks, vehicles can obtain desired content from neighboring vehicles as well as RSUs, and the content acquisition process is illustrated in Figure 2. In our strategy, as shown in Figure 3, there are five cases of content acquisition. In case 3 and case 3, the request vehicles obtain desired content from other vehicles on opposite lanes. Request vehicles and content source vehicles drive in opposite directions, which can easily lead to unsuccessful content transmission due to communication duration times being too short. For example, assuming that the speed of vehicles is 50 km/h and the coverage range of \begin{table}
\begin{tabular}{c l} \hline \hline
**Symbol** & **Description** \\ \hline \(I\) & The number of street \\ \(D\) & The width of lane \\ \(E\) & The length of lane \\ \(\lambda_{i}\) & The parameter of Poisson Point Process for the number of vehicles on lane \(i\) \\ \(M\) & The number of the roadside units \\ \(R_{m}\) & The roadside unit with sequence number of \(m\) \\ \(R^{rstu}\) & The coverage range of roadside unit \\ \(R^{max}\) & The maximum transmission rate of roadside unit \\ \(R^{a}\) & The average travel distance of vehicle under the coverage range of roadside unit \\ \(\lambda_{m}^{rstu}\) & The parameter of Poisson Process for the request number arriving at \(R_{m}\) \\ \(L\) & The number of content in content library \\ \(\gamma\) & The parameter of Zipf distribution \\ \(W\) & The size of content \\ \(V_{x}\) & The vehicle with sequence number of \(x\) \\ \(H_{x}\) & The number of vehicles in the same lane as \(V_{x}\) under the coverage range of \(V_{x}\) \\ \(N\) & The maximum number of content that vehicle’s caching capacity can accommodate \\ \(r^{v}\) & The coverage range of vehicle \\ \(C_{m,i}\) & The cluster with vehicles have next prediction lane \(i\) under the coverage range of \(R_{m}\) \\ \(C_{m,i}^{input}\) & The number of vehicle in \(C_{m,i}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The definitions of common symbols. vehicle is 50 m, the communication duration time will be around 7.2 s. If the transmission rate is 1 Mbit/s, then the transmission volume of content is no more than 7.2 Mbit. However, the volume of desired content of vehicle users always exceeds 7.2 Mbit, which means that the situation of unsuccessful transmission occurs easily. So, our strategy caches content chunks using multiple vehicles, which shortens the volume of content transferred from the content source vehicle to the request vehicle and increases the probability of successful content transmission. The process of content acquisition for vehicle users is as follows. Figure 2: The flowchart of content acquisition. The request vehicle will first searches for desired content among vehicles that are a one-hop distance from itself in the same lane. If the desired content exists, then the content source vehicle will directly transfer content to the request vehicle. As shown in Figure 3, \(V_{2}\) caches the content that \(V_{1}\) requests, and \(V_{2}\) directly transfers the content as it receives the request from \(V_{1}\). If the request vehicle fails to find desired content among vehicles that are a one-hop distance from itself in the same lane, then the request will be delivered to vehicles that are a two-hop distance from the request vehicle in the same lane. If the desired content exists, then the content source vehicle will transfer the content to a relay vehicle, and then the relay vehicle will transfer the content to the request vehicle. As shown in Figure 3, \(V_{5}\), which is a two-hop distance from \(V_{3}\), caches the content \(V_{3}\) desires. \(V_{5}\) first transfers the content to the relay vehicle \(V_{4}\), and then \(V_{4}\) transfers the content to \(V_{3}\).