# UAV-Assisted Caching Strategy Based on Content Cache Pricing in Vehicular Networks
## 3 System Model
### Caching Model and Content Delivery Strategy  
In the scenario of this paper, the NO leases some contents from the CP and caches them in the RSUs and pays a fee to the CP, while the UAV caches the leased content from the RSUs without paying an additional fee. The set of \(R_{m}\)-caching decision indicator variables is denoted by \(Y_{m}^{R}=\left\{y_{m,1}^{R},\cdots y_{m,i}^{R},\cdots,y_{m,l_{m}}^{R}\right\}\), where \(y_{m,i}^{R}\) is the caching indicator variable of \(R_{m}\) for content \(f_{j}\), \(m\in\left\{l,r\right\}\) and \(y_{m,i}^{R}=1\) if \(R_{m}\) caches content \(f_{j}\), otherwise \(y_{m,i}^{R}=0\). The UAV will select the content with high popularity from the content rented by the NO to cache. If the node caches the content requested by the vehicle user it is a cache hit, otherwise it is a cache miss. This paper involves both caching nodes, RSU and UAV, and the UAV only caches content leased by the RSU. Thus, for each content there are three caching cases: not cached by any caching node, cached only by RSU, and cached by both RSU and UAV. If the content is not cached by any node, the user of the vehicle requesting it will be on hold until it  
Figure 2: The relationship between the long-term optimization period of the caching policies and the short-term optimization period of the UAV’s trajectory. leaves the road. Then, the user will receive the content from the CP via the RSU relay. If the content is only cached by the RSU, the vehicle user obtains the content via the RSU. If the content is cached by both the RSU and the UAV, the vehicle user requesting it receives the content from the UAV first, as the transmission speed of the UAV is greater than that of the RSU. This situation is more complex and the delivery process can be divided into three cases as shown in Figure 3. Taking the delivery decision for content \(f_{i}\) as an example, the analysis is as follows:  
Figure 3: Content delivery process when both UAV and RSU cache content \(f_{i}\). (**a**) Both RSUs and UAVs cached \(f_{i}\); (**b**) the UAV and one RSU cache \(f_{i}\), the other does not, and the UAV stays in the section where \(f_{i}\) is cached; (**c**) the UAV and one RSU cache \(f_{i}\), the other does not, and the UAV stays in the section where \(f_{i}\) is not cached. For the case where both RSUs and the UAV have cached \(f_{i}\), take the example of a UAV staying on the road section where \(R_{l}\) is located, as shown in Figure 3a. When a vehicle drives in from the \(R_{l}\) side of the road section, if the vehicle user requests \(f_{i}\) when it is not in the UAV coverage range, such as \(V_{1}\), the user will first obtain \(f_{i}\) from \(R_{l}\) and, if it is still not obtained when driving into the UAV coverage range (rectangle block 2), it will continue to obtain \(f_{i}\) from the UAV. If the vehicle user requests \(f_{i}\) while within the coverage of the UAV, e.g., \(V_{2}\), the user will first obtain \(f_{i}\) from the UAV and, if it is still not fully obtained when driving out of the UAV coverage, the remainder will continue to be obtained from \(R_{l}\), and, if it is still not fully obtained when driving out of the \(R_{l}\) section, it will continue to obtain \(f_{i}\) from \(R_{r}\). If the vehicle user requests \(f_{i}\) after driving through the coverage area of the UAV, e.g., \(V_{3}\), it first obtains \(f_{i}\) from \(R_{l}\) and continues to obtain \(f_{i}\) from \(R_{r}\) if it is still not fully acquired when driving out of the section where \(R_{l}\) is located. When a vehicle drives in from the \(R_{r}\)-side section, as in \(V_{4}\), it first acquires f from \(R_{r}\), continues to acquire \(f_{i}\) from \(R_{l}\) if it has not yet fully acquired the content and is not in the coverage area of the UAV when it drives out of the \(R_{r}\) section, and continues to acquire \(f_{i}\) from the UAV if it has not yet fully acquired it when it drives into the coverage area of the UAV. The delivery process is the same when the UAV stays in the section where \(R_{r}\) is located.