# V2X Communication over Cellular Networks: Capabilities and Challenges
## 4 Delay Analysis for V2X Communications
### LTE-A Delay
#### 4.1.2 User Plane (Uu Interface)  
User plane delay is the time needed by a UE from the transmission of a packet to its reception by another end device. During this procedure, a packet usually traverses through the core network (including the Evolved Packet Core (EPC) and V2X AS), which contributes to the total E2E delay. This delay consists of UL, DL, and network processing delays. The UL delay in the radio access network is the time required for a message to be transmitted from a UE, until its reception by the eNB [27]. Three distinct cases can be identified in relation to UL transmissions, namely dynamic scheduling with/without a buffer status report (BSR) and semi-persistent scheduling (SPS) [27; 34]. For dynamic scheduling, the UE sends a Scheduling Request (SR) through the Physical Uplink Control Channel (PUCCH) to the BS. The SR period is typically 10 or 1 ms; the average waiting time (SR/2) is used for evaluation [27]. Upon the reception of the SR by the eNB, the latter will process it and provide the UL grant to the UE, which will use it for the UL transmission. In the case where the UE provides the BSR, an additional time of 8 ms is required. For the SPS case, the eNB transmits the UL grant to the UEs, without the reception of SR, so the total time required for the SR procedure can be omitted. Therefore, the total delay for the SPS depends on the SPS period. The minimum SPS period is 10 ms, which should be augmented from the time required to transmit the data to identify the overall delay [27]. The afore-described analysis is presented thoroughly in Table 5. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **HO Phase** & **Description** & **Time (ms)** \\ \hline \multirow{8}{*}{**Uplink Control**} & \multirow{8}{*}{**UU**} & RRC: Measurement report & 11 \\  & & HO decision & 15 \\  & & X2: HO request & 10 \\  & & Admission control & 22 \\  & & X2: HO request ACK & 10 \\  & & RRC: HO command & 6 \\  & & Process HO command & 15 \\  & & Total HO preparation & 89 \\ \hline \multirow{8}{*}{**Uplink Control**} & \multirow{8}{*}{**UU**} & UE reconfiguration & 20 \\  & & Synchronization & 3.5 \\  & & UE allocation and TA transmission & 8 \\  & & RRC: Reconfiguration Complete & 11 \\  & & Total HO execution & 42.5 \\ \cline{1-1}  & & **TIT alignment** & 0.5 \\ \cline{1-1}  & & Data transmission from target & 1 \\ \cline{1-1}  & & Decode & 3 \\ \cline{1-1}  & & **Total data interruption time** & 47 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Handover delay in LTE-A [33]. In the DL, LTE can support both unicast and multicast/broadcast transmissions. The multicast/broadcast transmission mode is provided through MBMS, which is categorized as two service types: MBMS Single Frequency Networks (MBSFN) and Single Cell Point-To-Multipoint (SC-PTM). The former one is based on multi-cell transmission, meaning that UEs can receive the same message through different cells. In this case, the UE can exploit the multi-cell transmission, by combining the received messages, in the same way as multipath works, thus increasing the total reliability. In SC-PTM transmissions, the transmission area is restricted in one cell. The scheduling delay for SC-PTM is lower, since synchronization from multiple cells is avoided. Table 6 presents the DL delay, reported in [27], and includes both unicast and multicast/broadcast transmission. In general, the same values are considered for both transmission modes, except the scheduling periods. For MBSFN, the scheduling period refers to the Multicast channel Scheduling Period (MSP) and the SC-PTM scheduling period (SSP) is employed for single cell transmission. The former uses the Multicast Traffic Channel (MTCH) to convey the data. In contrast, whilst SC-PTM delivers the data through the Physical Downlink Shared Channel (PDSCH). Regarding the delay in the core network, this is also referred to as network processing delay. In LTE, the packet needs to traverse through network components (i.e., Serving Gateway S-GW and Packet Data Network Gateway P-GW) to reach the destined eNB.