# Evolution of V2X Communication and Integration of Blockchain for Security Enhancements
## 2 V2X Technology
### State-Of-The-Art V2X Communications
#### 2.2.2 Cellular V2X  
The Cellular Vehicle to Everything (C-V2X) was developed by the 3 GPP based on V2X RAT in Rel.14 that gives highest priority for modifications of radio access suitable for V2X. LTE for vehicles (LTE-V) was introduced in vehicle networks as an alternative technology for intelligent transport, in addition to DSRC. Telecom companies and the automotive industry approved LTE-V for vehicle connectivity based on LTE. The LTE-V promised low costs, rapid development, and installation by leveraging existing cell towers to make the public transport network more efficient and accessible. Soon, the telecom companies and the automotive industry approved LTE-V for vehicle connectivity based on LTE-A. There is some research on using unlicensed bands in 5G networks via LTE and WLAN aggregation in order to alleviate spectrum scarcity [11; 12]. Several vehicles use an on-board system to connect to the cellular network for telematics, GPS systems, infotainment, fleet management, etc. For commercial applications, such as voice or data access, C-V2X can use V2N mode based on existing licensed cellular networks. The cellular network provides access to the cloud through commercial licensed spectrum using a network slicing architecture for the vertical industry. C-V2X describes transmission modes that allow for direct V2X connections over the PC5 interface through the sidelink channel. The PC5 refers to the point of contact where the vehicular nodes connect with other nodes directly over the same channel, where it does not require connection with the base station. Based on Rel.14 [13] and Rel.15 specification, the 3 GPP announced C-V2X technology. The Rel.14 included two additional transmission sidelink modes to enable V2X communications with low latency. The two transmission modes are mode 3 and mode 4, as shown in Figure 3[14]. The C-V2X can perform in both within-coverage and outside-of-coverage environments, i.e., C-V2X can work with both the conventional LTE air interface and the sidelink air interface [15]. 1. V2X based on LTE-Uu Air Interface: LTE-Uu is the standard air interface for connecting User Equipment (UE) and an eNodeB. Each UE that supports the LTE-Uu protocol relays its signal on uplink to the eNB, and the eNB transmits the signal to the destination UE on downlink. The eNB will use semi-persistent scheduling to lower the scheduling overhead that is involved with V2X uplink transmission. In semi-persistent scheduling, the eNB allocates resources to a userover several subsequent transmissions, as most of the traffic is periodic and has similar packet sizes [16]. 2. V2X based on PC5 Air Interface: according to the the 5G Automotive Association (5 GAA), the device-to-device communication mode (i.e., PC-5) must be enabled in C-V2X for direct safety message communication assuring privacy. This mode also works well in the ITS 5.9 GHz band without a paid subscription, and provides privacy. In addition, for commercial applications, such as voice or data access, C-V2X can use V2N mode based on existing licensed cellular networks. The PC5 air interface allows for direct UE communications without requiring each packet to proceed through the eNodeB. The user nodes can exploit the PC5 interface when eNodeB is either present or absent. * C-V2X sidelink mode 3: in C-V2X side-link mode 3, the scheduled mode functions only in the presence of a base station or an eNB. The allocation of resources is carried out in a centralized manner by the cellular network. Some of the mode 3 mechanisms are semi-persistent scheduling, UE reports based scheduling, and cross carrier scheduling. However, this mode has an issue in high mobility highway scenarios, where the vehicles should be connected with the eNB. * C-V2X sidelink mode 4: C-V2X side-link mode 4 functions independently in the absence of eNB support for directly interacting using the PC5 side-link radio interface using a 5.9 GHz frequency band similar to the DSRC. It is also known as autonomous mode.