# V2X Communication over Cellular Networks: Capabilities and Challenges
## 4 Delay Analysis for V2X Communications
### 5G-NR Delay
#### 4.2.2 User Plane (Uu Interface)  
The overall user plane latency for 5G is different for the UL and the DL. It is worth mentioning that for DL, only the unicast mode is supported as of now by 3 GPP standards. The description for MBMS referenced in Section 3 is only a proposal adapted from [24]. 3 GPP has not yet standardized the MBMS architecture for 5G-NR. For the UL procedure, grant-free transmissions can be used in order to reduce the delay [28]. Since the UL grant is omitted, the UE only needs to process the data and wait for frame alignment. The packet transmission delay depends on the length of the OS needed. The gNB processes the data through PUSCH. However, grant-free transmissions face the inherent problem of potential collisions on the receiver's side. For the DL, the gNB processes the data and waits for frame alignment. The packet transmission delay depends on the OS length. In case there is an error in the transmission/reception, the Hybrid Automatic Repeat request (HARQ) protocol is used for proper reception, which is based on retransmissions. In UL, the gNB schedules the retransmission through the Physical Downlink Control \begin{table}
\begin{tabular}{c c} \hline
**Description** & **Time (ms)** \\ \hline RACH scheduling period & 0 \\ \hline RACH preamble transmission & 1 \\ \hline Preamble detection and transmission of RA response & 3 \\ \hline UE processing & 4 \\ \hline Transmission of RRC Connection Resume Request & 1 \\ \hline eNB processing (L2 and RRC) & 3 \\ \hline Transmission of RRC Connection Resume & 1 \\ \hline UE processing (RRC, including grant reception) & 7 \\ \hline Transmission RRC Connection Resume Complete and UP data & 0 \\
**Total** & **20** \\ \hline \end{tabular}
\end{table} 
Table 8: Control plane delay for the INACTIVE-CONNECTED state with the LTE Radio Access Network (RAN) and 5G Core [28]. Channel (PDCCH). Upon the decoding of the control information from PDCCH from the UE, the latter needs to repeat the UL procedure. In DL, the UE transmits an ACK/NACK to indicate proper reception or failure. The user plane procedure can be calculated as follows [30]:  
\[\tau=\ \tau_{1}+\mathrm{p}(\tau_{2}+\tau_{3}), \tag{4}\]  
where \(\mathrm{p}\) is the probability of retransmission. The first transmission can be calculated as follows:  
\[\tau_{1}=\ t_{\mathrm{gNB}}+\ t_{\mathrm{FA1}}+\mathrm{t}_{\mathrm{TTI}}+\ t_{ \mathrm{UE}}, \tag{5}\]  
where \(\mathrm{t}_{\mathrm{gNB}}\) is the processing time in the gNB (same as the control plane), \(\mathrm{t}_{\mathrm{FA1}}\) is the time needed for frame alignment, \(\mathrm{t}_{\mathrm{TTI}}\) is the actual transmission of data, and \(\mathrm{t}_{\mathrm{UE}}\) is the processing time in the UE:  
\[\mathrm{t}_{\mathrm{UE}}=\ \frac{\max\Bigl{[}\mathrm{N}_{1}(2048+144)\cdot \kappa\cdot 2^{-\mu}\cdot\frac{\mathrm{T}_{c}}{2048^{-\mu}\cdot\Delta_{t}^{ \prime}}\ 0\ \Bigr{]}}{2}, \tag{6}\]  
where \(\mathrm{N}_{1}\) depends on SCS and can be found in Table 5.3-1 and Table 5.3-2 in [37] for UE processing capability 1 and UE processing capability 2, respectively. If an error occurs, the UE needs to ask for a retransmission (HARQ) and the time can be calculated as follows:  
\[\tau_{2}=\ t_{\mathrm{UE}}+\ t_{\mathrm{FA2}}+\mathrm{t}_{\mathrm{HARQ}}+ \mathrm{t}_{\mathrm{gNB}}, \tag{7}\]  
where \(\mathrm{t}_{\mathrm{HARQ}}\) requires 1 OS. After receiving and processing the HARQ request, the gNB retransmits the content (in the case of DL):  
\[\tau_{3}=\ t_{\mathrm{gNB}}+\ t_{\mathrm{FA3}}+\mathrm{t}_{\mathrm{TTI}}+\ t_{ \mathrm{UE}}. \tag{8}\]  
In [40], 3 GPP employs a specific numerology (i.e., UE capability 2, non-slot-based PDSCH/PUSCH allocation of 2 OS) to meet the advanced V2X UC requirements. The values are presented in Table 9.  
The previous analysis for LTE considered the E2E values. However, this is not available in 5G systems, since in the technical specifications and the literature, there are no agreed values for the core network delay. In [22], 5 GAA assumes a 3 ms delay each time a packet traverses through an interface in the EPC.