# V2X Communication over Cellular Networks: Capabilities and Challenges
## 4 Delay Analysis for V2X Communications
### 5G-NR Delay
#### 4.2.3 User Plane (PC5 Interface)  
3 GPP in 5G-NR Release 16 devoted significant effort to the development of solutions related to V2X direct communications, in order to overcome some of the key deficiencies of LTE PC5, related to unicast communication among UEs and QoS management. In parallel,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mode**} & **Resource** & \multirow{2}{*}{**OS**} & \multicolumn{2}{c}{**Initial Error**} & \multicolumn{2}{c}{**UE Capability 2**} \\  & **MappingType** & & **Probability** & **15 KHzSCS** & **30 KHzSCS** & **60 KHzSCS** \\ \hline Downlink & & & \(p=0\) & 0.49 & 0.29 & 0.23 \\ (ms) & \multirow{2}{*}{B} & \multirow{2}{*}{2} & \(p=0.1\) & 0.60 & 0.35 & 0.28 \\ \cline{3-5}  & & & \(p=0\) & 0.52 & 0.30 & 0.24 \\ \cline{3-5}  & & & \(p=0.1\) & 0.62 & 0.36 & 0.28 \\ \hline \hline \end{tabular}
\end{table}
Table 9: User plane delay for 5G-NR [28].  
enhancements have been provided to reduce the latency and increase the reliability by reducing collisions [41].  
As mentioned in Section 1, in 5G-NR, sidelink communication comprises Mode 1 and Mode 2, which are direct enhancements of LTE Mode 3 and Mode 4, respectively. Therefore, 5G-NR Mode 1 refers to communication where the UE transmissions are scheduled by the gNB, whereas in Mode 2, the UE autonomously selects resources [42].  
In 5G-NR, the UE can transmit on a per slot basis (the frame length is 10 ms and the subframe 1 ms), which has a length of 1, 0.5, 0.25, and 0.125 ms for subcarrier spacing of 15 (FR1), 30 (FR1), 60 (FR1 and FR2), and 120 kHz (FR2), respectively, thus facilitating a reduction in the E2E delay [42].  
Regarding the resource allocation, in Mode 1, there are two options. In the first one, the UE performs the scheduling grant request once the data are available, whereas in the latter, the UE requests the schedule grant request in advance (so-called grant-free transmission) [42]. In the grant-free case, the UE reserves the resources in a periodic manner. As in LTE's Mode 3 transmission, if the UE is in the IDLE state, it should turn to CONNECTED. This procedure, however, is only needed in the first scheduling grant request, and for the rest of the transmissions, it will not introduce an additional delay. The delay cost of this case is the same as in the Uu transmissions.  
In Mode 2, the UE can start transmitting from the IDLE state. Once the data are available, the UE performs the sensing procedure and the resource selection to identify the less occupied resources and select the most appropriate to use [41]. Then, using the Sidelink Control Information (SCI) message, it reserves the particular resource(s) to be used. In order to reduce the latency, in the case of collisions, the UE may reserve resources for the retransmissions from the initial selection. In case other UEs with more urgent services need to transmit a message (or a set of messages), they may reserve an already reserved resource; in this case, the UE initially accessing the spectrum resources will need to reselect resources, thus increasing the latency. Interestingly, to perform QoS control (especially in Mode 2), the UEs may report (according to their configuration) the channel busy ratio and the channel occupation ratio to the gNB; the latter according to these will define the set of spectrum and sensing parameters of the UE, thus impacting the delay [41].