# V2X Communication over Cellular Networks: Capabilities and Challenges
## 4 Delay Analysis for V2X Communications
### 5G-NR Delay  
As mentioned in the introductory sections, 3 GPP has considered that only the initial UCs of V2X will be tackled by the 4G system. The 5G system should handle the more demanding UCs. Therefore, 5G-NR introduces flexible resource allocation in the physical layer [36], including different Subcarrier Spacing (SCS), Orthogonal Frequency Division Multiplexing Symbol (OS) durations, and Cyclic Prefixes (CPs), usually referred to as numerologies. In the time domain, physical transmissions correspond to frames with a 10 ms duration, which in turn are divided into ten subframes with a 1 ms duration, as in LTE. One slot consists of 14 OS and subframes might have several slots, depending on the numerology (i.e., SCS). For example, a subframe is equal to one slot in the case of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mode** & **Scheduling** & **Ster** & **Component** & **Description** & **Min.** & **Average** & **Max.** \\ \hline \multirow{8}{*}{3} & \multirow{4}{*}{1} & \multirow{4}{*}{Sidelink Scheduling} & \multirow{4}{*}{Sidelink Scheduling} & Same as the first nine steps from (Table 5) & & \\  & & & & Min: SR/2 (SR = 1 ms) & & \\  & & & & Average: SR/2 (SR = 10 ms) & & \\  & & & & Max: SR (SR = 10 ms) & & \\ \cline{2-6}  & \multirow{2}{*}{2} & Transmission of data & Data and SCI are transmitted at the same & & & \\  & & + SCI & subframe (1 TII) & & 1 & 1 \\ \cline{2-6}  & & 3 & & Receiver UE L1/L2 processing time & 1.5 & 1.5 & 1.5 \\ \cline{2-6}  & & 4 & & Receiver UE upper layer processing & 3 & 3 & 3 \\ \cline{2-6}  & & 5 & **Total** & & 22 & 26.5 & **31.5** \\ \hline \multirow{8}{*}{3} & \multirow{4}{*}{1} & \multirow{4}{*}{Sidelink SPS} & Min: The transmission of SCI and data can & & & \\  & & & be aligned with packet arrival time & & & \\  & & & & Average: SPS period/2 & & \\ \cline{2-6}  & \multirow{2}{*}{1} & \multirow{2}{*}{scheduling period} & Max: The UE misses the last transmission & & & \\  & & & & opportunity and need to wait for one SPS & & \\ \cline{2-6}  & \multirow{2}{*}{Seeduling} & \multirow{2}{*}{Processing} & \multicolumn{2}{c}{Transmitter UE L1/L2 processing} & 1.5 & 1.5 & 1.5 \\ \cline{2-6}  & & 3 & & Receiver UE L1/L2 processing & 1.5 & 1.5 & 1.5 \\ \cline{2-6}  & & 4 & & Receiver UE upper layer processing & 3 & 3 & 3 \\ \cline{2-6}  & & 5 & **Total** & & **-** & **7** & **56** & **106** \\ \hline \multirow{8}{*}{4} & \multirow{4}{*}{-} & \multirow{4}{*}{1} & Timing of sensing & Min: T1 = 1 & & & \\  & & and resource & Average: (T2 \(-\) T1)/2 = (100 \(-\) 1)/2 = 49.5 & 1 & 49.5 & 100 \\ \cline{1-1}  & & & & Max: T2 = 100 & & \\ \cline{1-1} \cline{2-6}  & 2 & & Transmitter UE L1/L2 processing & 1.5 & 1.5 & 1.5 & 1.5 \\ \cline{1-1} \cline{2-6}  & 3 & & & Receiver UE L1/L2 processing & 1.5 & 1.5 & 1.5 \\ \cline{1-1} \cline{2-6}  & 4 & & & Receiver UE upper layer processing & 3 & 3 & 3 \\ \cline{1-1} \cline{2-6}  & 5 & **Total** & & **-** & **7** & **55.5** & **106** \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Sidelink delay in LTE-A [22]. 14 OS using 15 KHz SCS (i.e., fixed transmission parameters in the LTE frame structure). A physical resource block (PRB) is composed of 12 consecutive subcarriers for one OS, contrary to LTE, which corresponds to a fixed slot of 0.5 ms (i.e., PRB = 12 subcarriers \(\times\) 1 slot). Resource mapping types [37] indicate how the Physical Uplink Shared Channel (PUSCH) and PDSCH are allocated throughout the slot. For PUSCH, mapping type A indicates that the allocation always occurs at OS 0 in the slot, while in mapping type B, it might start midway through the slot. PDSCH follows the same frame structure for both mapping types. Therefore, Demodulation Reference Signals (DM-RS) are used in PDSCH allocation for channel estimation and the demodulation of physical channels. Type A implies that DM-RS are located in the second or third symbol of a slot and type B implies that DM-RS are located in the first symbol of the PDSCH allocation. Having the DM-RS at the beginning of the allocation makes the demodulation faster.