# PC5-Based Cellular-V2X Evolution and Deployment
## 3 C-V2X Standard and Application Scenarios
### NR-V2x  
5G NR (New Radio)-V2X as phase 3 of 3 GPP V2X is backward compatible at upper layers of LTE-V2X. To meet the low latency and reliability requirements for the advanced V2X services, NR-V2X is designed to support these applications. As a V2N type application, 5G URLLC (Ultra-Reliable Low-Latency Communication) network slicing can provide  
Figure 3: C-V2X 3 GPP standard and deployment milestone [10].  
autonomous driving advanced functions for L3 (Conditional Automation) and L4 (Highly Automation) of Figure 4[13] with higher QoS (Quality of Service) profile.  
Some of these advanced application scenarios require the transmission of periodic traffic. So besides broadcast, two new communication types, unicast and groupcast, are introduced in 5G NR-V2X. Similar to LTE-V2X, 5G NR-V2X [14] also defines two types of sidelink communication modes: Mode1 and Mode2. The NR-V2X Mode 1 defines the mechanisms that allow direct vehicular communications while the cellular network base station allocates radio resources to the vehicle by the Uu interface. Mode 2 supports direct vehicular communications via PC5 interface under the out-of-coverage of the cellular network. 3 GPP Rel-16 has been officially frozen in July 2020 [15]. In the developing of 3 GPP NR Release 17 [16], new sidelink communication relaying architecture is proposed to support some of the advanced V2X services. The difference between LTE-V2X and NR-V2X is highlighted in Table 1.  
Since PC5-based C-V2X Mode4 [11] does not require a cellular network, normally two kinds of wireless devices **RSU (Road Side Unit)** and **OBU (On Board Unit)** are sufficient to deploy C-V2X V2I/V2V/V2P application scenarios:  
1. **RSU**: Wireless transmitting device can provide direct sidelink communication via PC5 interface without cellular networks. Road signs, traffic lights, and IP camera information can be broadcasted in real-time to the vehicle via RSU in the predefined area. Another useful scenario is that RSU can be equipped with a SIM card to transfer road information via cellular networks to create additional applications for public safety.
2. **OBU**: Installed in a vehicle as a wireless communication device to enhance autonomous driving vehicles' sensor function by direct communication with RSU and other OBUs. OBU is responsible for broadcasting vehicle's location, direction, and speed information to other defined devices while receiving other vehicle's data as input for its internal algorithms to avoid possible accidents.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Items** & **LTE-V2X** & **NR-V2X** \\ \hline Specification & 3 GPP Rel-14/Rel-15 & 3 GPP Rel-16/Rel-17 \\ \hline Latency & Low latency: 10\(\sim\)100 ms & Ultra-low latency: 1 ms [17] \\ \hline PC5 Message Type & Broadcast & Broadcast, Unicast and Groupcast \\ \hline Application Scenario & Safety related/Enhanced & Advanced Application \\ \hline Cellular Network Coverage & Uu and PC5 Mode3 & Uu and PC5 Model \\ \hline Out of Cellular Network & PC5 Mode4 & PC5 Mode2 \\ Coverage & \hline \end{tabular}
\end{table}
Table 1: Difference between LTE-V2X and NR-V2X.  
Figure 4: The 5 levels of driving automation [13].