# Clustering-Based Data Dissemination Protocol Using the Path Similarity for Autonomous Vehicles
## 2 Related Works and Problems
### Data Dissemination Scheme Based on Clustering and Probabilistic Broadcasting  
In May 2018, [PERSON] announced the data dissemination scheme using broadcasting and clustering based on connection probability [34]. In this study, the authors proposed clustering and probabilistic broadcasting (CPB), which is the main concept when constructing clusters based on cumulative connection probabilities through continuous connectivity verification. This paper is significant in that it is a clustering algorithm that maximizes connectivity in unstable V2X environments. However, the data rates they define are not consistent with the actual road environment. \[P_{ij}^{f}=1 -F_{d}(r_{\tau},m,\psi)=e^{-\frac{m_{T}}{4}\sum_{i=1}^{m}\frac{( \frac{m_{T}}{4})^{i-1}}{(i-1)^{i}}} \tag{1}\] \[p(x|d_{0},\ t) =\Pr(x\leq D(t)\leq x+dx|d_{0})\] (2) \[=\frac{1}{\sigma\sqrt{2\pi}t}\sum\limits_{n=-\infty}^{\infty}[ \exp\{\frac{\mu x_{n}^{n}}{\sigma}-\frac{[x-d_{0}-x_{n}^{n}-\mu t]^{2}}{2\sigma ^{2}t}\}\] \[-\exp\{\frac{\mu x_{n}^{n}}{\sigma}-\frac{[x-d_{0}-x_{n}^{n}-\mu t ]^{2}}{2\sigma^{2}t}\}]\]  
The authors define the probability \(P_{ij}^{f}\) that packet transmission between nodes \(i\) and \(j\) succeeds as Equation (1). In Equation (1), \(r_{\tau}\) is the threshold of the signal strength that can be received, \(\psi\) is the average signal strength, and \(m\) is the fading parameter. \(m\) is 1 if the distance between vehicles is  
Figure 1: 802.11p WAVE Protocol Stack. more than 150 m, 1.5 when it is more than 50 m and 3 if it is less than 50 m. \(F_{d}\) presents the probability density function of possibility that the nakagami-m fading will occur. That is, the probability that fading due to the nakagami-m distribution will not occur [35]. Additionally, the authors assumed that each vehicle follows the Wiener process. Equation (2) is used to derive the probability \(p(x|d_{0},\ t)\) that the vehicle will be at a certain radius. \[LC_{ij}=F_{i,j}(t)=1-\int_{0}^{R}p(x|d_{0},\ t)dx \tag{3}\]  
Equation (3) defines the link connectivity \(LC_{ij}\) using \(P_{ij}^{f}\) derived from Equation (1) and \(p(x|d_{0},\ t)\) derived from Equation (2). The link quality \(lq_{ij}\) defined in the paper is calculated as Equation (4). \[lq_{ij}=P_{ij}^{f}LC_{ij} \tag{4}\]  
After the clusters are configured, they determine the forwarding probability as follows. First, a timer is started for a \(t_{int}\) interval which is a preset data transmission interval. Then, when the cluster member receives a disseminated message while the timer is running, it increments the counter \(t_{\text{c}}\). The transmission probability \(P\) is determined according to the numerical value of the counter \(t_{\text{c}}\) as shown in Equation (5). \[P=\left\{ \begin{array}{ll}1,\ \ C=1\\ \frac{2}{C^{2}},\ \ 2\leq C\leq Threshold\\ 0,\ \ C>Threshold\end{array} \right. \tag{5}\]  
Note that \(P\) is a probabilistic variable that determines whether a cluster member will deliver it probabilistically when it receives a message, and does not mean a transmission success rate. That is, if \(P_{ij}^{f}\) in Equation (1) represents the success rate of the physical transmission of the communication, then \(P\) in Equation (5) is a variable of the probabilistic transfer algorithm which is one of the core solutions of CPB. Here, \(Threshold=\lfloor 6V_{r}\rfloor\), and \(V_{r}\) is the speed ratio, which is the current vehicle speed \(V_{c}\) divided by the vehicle maximum speed \(V_{f}\). The authors estimate this value using the number of vehicles on the road, but in this paper, we used the actual vehicle speed and the maximum speed when reproducing. However, in reality, V2X communication not only includes various types of fading such as coherent fading, k-type fading as well as nakagami-m fading, but also an environment in which multiple constraints are applied in addition to fading. Therefore, it is difficult to verify the efficiency with simple arithmetic calculations.