# Evolution of V2X Communication and Integration of Blockchain for Security Enhancements
## 3 Evolution of V2X Technologies
### Evolution of Non-Cellular V2X from 802.11p to 802.11 bd  
The IEEE 802.11p standard was launched in 2010 based on the IEEE 802.11a wireless local area network (WLAN) standard. DSRC performance is adequate for most vehicle safety applications requiring end-to-end latency around 100 ms as long as the vehicle density is low [23]. However, DSRC scalability and performance degrades if the vehicle density increases, which is due to two main factors, i.e., packet collisions from simultaneous transmissions and the hidden node problem. To overcome these issues, IEEE 802.11 and the 3 GPP are focusing on next-generation V2X systems; and, have created a task force called IEEE 802.11 next-generation V2X (NGV). A new and updated IEEE 802.11 bd was introduced for V2X communications. IEEE 802.11 bd will be an advanced form \begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Use Case** & **Communication Mode** & **Pyaload (Bytes)** & **Max. Delay (msec)** & **Datar (Mbps)** & **Minimum Range (m)** & **Reliability** \\ \hline Advanced driving & V2V, V2I & 300–12,000 & 3\(\sim\)100 ms & 10–50 & 360–500 & 90–99.999 \\ \hline Remote driving & V2N & - & 5 ms & UL25/DL:1 & - & 99.999 \\ \hline Vehicle platooning & V2V, V2I & 50–6000 & 10\(\sim\)500 ms & 50–65 & 80–350 & 90–99.99 \\ \hline Extended sensors & V2V, V2I, V2P & 1600 & 3\(\sim\)100 ms & 10–1000 & 50–1000 & 90–99.999 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advanced use cases and Quality of Service (QoS) requirements of Vehicle to Everything (V2X) applications [22]. of existing 802.11p by leveraging the MAC and PHY options of 802.11h/ac/ax [24; 25]. It overcomes the issues related to 802.11p in terms of MAC throughput, interoperability, Doppler shift, etc. Table 3 summarizes the advancements in 802.11 bd over 802.11p based on features and mechanisms. Moreover, the 802.11 bd should be able to meet the following requirements:  
* Coexistence: the 802.11 bd standard must coexist with 802.11p. The 802.11 bd should detect the 802.11p data transmissions and comply with channel access and vice-versa. * Interoperability: the 802.11 bd and 802.11p standards must be interoperable i.e., devices, information systems or applications should be interoperable in such a way that 802.11p devices can detect and decode at least one of the transmission modes from 802.11 bd devices and vice-versa. * Fairness: the 802.11 bd and 802.11p standards must have fair communication and access capabilities in co-channel configurations. * Backward compatibility: the 802.11 bd must be backward compatible in such a way that at least one mode of 802.11 bd must be interoperable with 802.11p. Some of the key mechanisms of 802.11 bd are as follows:  
1. Midambles: the 802.11p PHY layer was extracted directly from 802.11a, decreasing the spacing of the sub-carrier from 312.5 KHz by a factor of two. In 802.11p, the 156.25 KHz subcarrier spacing generated a trade-off between multipath fading and relative Doppler spread for average vehicle speeds [26]. To overcome this issue, the 802.11 bd suggests the use of midambles, similar in structure and function to the preamble except for their position within the frame. The preamble, which is at the start of the frame, is used for the initial estimation of channels. In 802.11 bd, the midambles can be inserted with appropriate frequency between OFDM data symbols. 2. Re-transmissions: one of the approaches to enhance reliability is to allow one or more packet re-transmissions in both 802.11p and 802.11 bd, as shown in Figure 4. For 802.11p applications, the initial transmission and its re-transmissions act as separate packets, and the packets are transmitted successfully as long as one packet is received successfully. Original transmissions and re-transmissions can be either transmitted within the same channel access or by using different contention procedures [27].