# Compp-DDPG: Computation Offloading with Privacy Preservation in a Vehicular Edge Network
## 3 Model of VEN with Privacy Preservation
### Privacy Model  
In the secure communication of VEN, the privacy protection of ICV's identity and location are very important. There are two privacy preservation schemes in the Internet of Vehicles, pseudonymous-based scheme, and signature-based scheme. According to the European Telecommunications Standards Institute (ETSI) standard, pseudonyms are considered a main method for the security and privacy preservation in secure communication of intelligent transportation system [35]. The pseudonym is a digital certificate issued to an ICV by the Certificate Authority (CA) based on its real identity after the ICV is registered with the CA. Using pseudonyms rather than real identities in the process of vehicular communication can effectively protect the identity and the location privacy of the ICV to some extent.  
Pseudonymous privacy preservation scheme is widely used in the IoV. Pseudonym exchanging and pseudonym changing are two methods in this scheme. Pseudonymexchanging is when an ICV's pseudonym expires, the ICV negotiates with another ICV to exchange their pseudonyms. Although pseudonym exchanging saves on pseudonym resources, the situation where the ICV cannot reach an agreement with its surrounding ICVs may occur, which may not be able to meet the ICV's timely pseudonym update requirement. By contrast, it is much simpler to implement ICV's pseudonym updating by pseudonym changing. The CA has to suffer more computational and communicational stress and requires additional storage in the method of pseudonym changing. However, in our considered model, the CA is integrated into the MEC server, which provides the CA's required computation, communication, and storage resources. In addition, the signature privacy preservation scheme in the IoV includes the group signature and ring signature. The group signature scheme can effectively implement anonymous communication, but it is difficult to update system parameters effectively when new members join the group and remove malicious members. Ring signatures can provide spontaneous anonymity, but the difficulty is how to form a ring between ICVs.  
Therefore, pseudonym changing is the most appropriate method pf the above-mentioned methods for the proposed VECN scenario. Here we assume that ICVs should change their pseudonyms under certain conditions to reduce the risk of a single pseudonym being associated with the real identity of the ICV [36].  
Let \(N_{i(t)}\) indicate whether the ICV-\(i\) changes its pseudonym at time slot \(t\).  
\[N_{i(t)}=\left\{\begin{matrix}0,\text{ICV}-i\text{ does not change the pseudonym}\\ 1,\text{ ICV}-i\text{ changes the pseudonym}\end{matrix}\right.. \tag{14}\]  
\(N_{(t)}\) depicts the number of ICV units changing the pseudonym at time slot \(t\).  
\[N_{(t)}=\sum_{i=1}^{V}N_{i(t)}. \tag{15}\]  
According to the information entropy principle, the maximum privacy \(P_{max}\) that an ICV can obtain at time slot \(t\) is [37]  
\[P_{max}=\sum_{a=1}^{N}P_{a|b}\log_{2}P_{a|b}, \tag{16}\]  
where \(P_{a|b}\) is the case that the pseudonym is changed from \(b\) to \(a\), and the pseudonym is available when \(P_{a|b}=\frac{1}{N_{(t)}}\).  
There is a certain cost when changing the ICV's pseudonym. Here, we comprehensively consider the cost in two parts. One part is the cost of changing the radio's routing and addressing table when acquiring a new pseudonym, and another is the time cost of keeping silent during the process of changing the pseudonym [38]. It is assumed that there is no limit to the number of pseudonym changing in this work, and the costs are collectively denoted as \(P_{loss}\). The real-time loss of ICV-\(i\)'s privacy at time slot \(t\) is  
\[P_{loss}=\vartheta*(t-t_{c,i}), \tag{17}\]  
where \(\vartheta\) represents the location privacy loss constant, and \(t_{c,i}\) indicates the last time slot that the pseudonym has been changed. The privacy of ICV-\(i\) at time slot \(t\) is  
\[P_{i(t)}=P_{max}-P_{loss}=\sum_{d=1}^{V}\frac{1}{N_{(t)}}\log_{2}\frac{1}{N_{ (t)}}-\vartheta*(t-t_{c,i}). \tag{18}\]