## 3 System Model
### Task Processing Model
#### 3.3.2 Computing on the RSU  
When the vehicle \(un\) offloads tasks to the associated RSU \(n\) (i.e., \(\eta_{un}=n\)) for processing, the task processing delay includes both computation delay and task transmission delay. Similar to [32; 33], we assume that the radio access system of each RSU is based on OFDM (Orthogonal Frequency Division Multiplexing), which allocates orthogonal bandwidth resources to different vehicles, ensuring that no interference occurs during task offloading. Then, we formulate the transmission rate from vehicle \(un\) to RSU \(n\) is given by  
\[r_{un}^{\
u 2r}=\beta_{un}B_{n}\text{log}_{2}(1+\frac{p_{un}h_{un}}{\sigma^{2}}), \tag{3}\]  
where \(p_{un}\) is the transmission power of vehicle \(un\) and \(h_{un}\) is the channel gain between vehicle \(un\) and RSU \(n\). \(\sigma^{2}\) denotes the noise power. Additionally, \(B_{n}\) denotes the available bandwidth of RSU \(n\), while \(\beta_{un}\) represents the bandwidth ratio allocated to vehicle \(un\) by RSU \(n\).  
According to the CSTO mechanism, when the vehicle performs task offloading, it only needs to transmit the sensing data that are not redundant with the RSU \(n\). Therefore, the transmission delay from the vehicle \(un\) to the RSU \(n\) can be expressed as  
\[t_{un}^{n,\
u 2r}=\frac{(1-O_{un})d_{un}\lambda_{un}}{r_{un}^{\
u 2r}}. \tag{4}\]  
Note that \(O_{un}=1\) indicates complete redundancy in the sensing data between vehicle \(un\) and RSU \(n\). In this case, the vehicle needs to transmit a command containing its position information to RSU \(n\) for coordinate transformation. Given the small size of the command, we neglect its transmission delay [28].  
Considering that the task computed on the RSU undergoes a queuing process, we model the task computation process on the RSU as an M/M/1 queue. We denote \(C_{n}\) as the computing resource of the RSU \(n\). Then, the tasks' computation delay from the vehicle \(un\) offloading to the RSU \(n\) is expressed as  
\[t_{un}^{n,comp}=\frac{f_{un}d_{un}\lambda_{un}+O_{un}X_{tra}d_{un}\lambda_{un}} {C_{n}-\sum_{k\in\mathcal{N}}\sum_{l\in\mathcal{U}_{k}}(f_{lk}+O_{lk}X_{tra}) d_{lk}\lambda_{lk}}, \tag{5}\]  
where \(X_{tra}\) is the computation intensity of coordinate transformation, and \(O_{un}X_{tra}d_{un}\lambda_{un}\) denotes the computation required for coordinate transformation of redundant sensing data between vehicle \(un\) and RSU \(n\)[14]. \(f_{un}d_{un}\lambda_{un}\) denotes the computation required for the task of the vehicle \(un\), and \(\sum_{k\in\mathcal{N}}\sum_{l\in\mathcal{U}_{k}}(f_{lk}+O_{lk}X_{tra})d_{lk} \lambda_{lk}\) represents the total computational load of all tasks offloaded to the RSU \(n\), including tasks from associated vehicles, as well as those transmitted from other RSUs. Therefore, the processing delay for offloading tasks from vehicle \(un\) to RSU \(n\) is given by  
\[t_{un}^{n,rsu}=t_{un}^{\
u 2r}+t_{un}^{n,comp}. \tag{6}\]  
When RSU \(n\) lacks sufficient computational capacity to process the tasks of vehicle \(un\), it can further transmit the tasks to another RSU \(v\) (i.e., \(\eta_{un}=v,v\
eq n\)). Additionally, since the transmission rate of fiber-optic communication can reach 8 Gbps and the task size is typically only tens of Mb, we ignore the transmission delay of vehicle tasks between RSUs [16]. Then, we denote \(t_{un}^{v,comp}\) as the tasks computing the delay for offloading from vehicle \(un\) to RSU \(n\), and further offloading to RSU \(v\), which can be calculated based on (5).  
To summarize, the processing delay for offloading from the vehicle \(un\) to RSU \(n\) and then to RSU \(v\) is given by  
\[t_{un}^{v,su}=t_{un}^{v2r}+t_{un}^{v,comp}. \tag{7}\]