# V2X Communication over Cellular Networks: Capabilities and Challenges
## 4 Delay Analysis for V2X Communications
### 5G-NR Delay
#### 4.2.1 Control Plane  
In order to reduce the need for repeated UE context retrieval from the network, every time a UE performs an RRC Connection Request, 3 GPP's Release 15 introduces a new state in respect to the RRC protocol called RRC INACTIVE [38]. The basic feature of this state is that a UE maintains the connection with the core network. The Access Stratum context is stored in the UE and the gNB. This new state is beneficial for the overall delay, since the signaling for the transition in the RRC CONNECTED state is shortened and the UE context is already available in the gNB. According to [28], control plane latency is defined as the time required for the transition of the UE from the RRC INACTIVE to RRC CONNECTED state. The calculation of the control plane latency is based on the following assumptions. The waiting time for the transmission of a preamble through RACH is considered to be T\({}_{1}\) = 0 ms. However, the start time for the transition from the \"battery efficient state\" is not defined, so the RACH scheduling period could be included, affecting the overall delay. The time required to transmit the preamble T\({}_{2}\) depends on different Physical Random Access Channel (PRACH) symbol lengths (i.e., 2, 6, and 14). The next step is preamble detection and processing in the gNB, calculated as T\({}_{3}\) = t\({}_{\text{gNB}}\)/2. The processing time in the gNB is given by [37], calculated as  
\[\text{t}_{\text{gNB}}=\ \frac{\max\Bigl{[}\text{N}_{2}(2048+144)\cdot\kappa \cdot 2^{-\mu}\cdot\frac{\text{T}_{\text{c}}}{2048-\Delta_{t}}\,\ 0\ \Bigr{]}}{2}, \tag{2}\]  
where N\({}_{2}\) depends on SCS and can be found in Table 6.4-1 and Table 6.4-2 in [37] for UE processing capability 1 and UE processing capability 2, respectively; \(\mu\) is the numerology; and \(\kappa=\frac{\text{T}_{\text{s}}}{\text{T}_{\text{s}}}=64\) is a constant to relate the NR basic time unit T\({}_{\text{c}}\) = 0.509 ns and LTE basic time unit T\({}_{\text{s}}\). After the time required for the first frame alignment T\({}_{\text{FA1}}\), the gNB sends the RA response and the total time T\({}_{4}\) depends on the configuration (slot or non-slot). The UE processing delay T\({}_{5}\) involves the decoding of the scheduling request, Cell Radio Network Temporary Identifier (C-RNTI) assignment, and the L1 encoding of RRC resume request. The UE processing time is calculated [39] as  
\[\text{T}_{5}=\ \text{N}_{\text{T},1}+\ \text{N}_{\text{T},2}+\text{T}_{\text{wait}}, \tag{3}\]  
where N\({}_{\text{T}}\),1 (Table 5.3-1 [37]) is the time required to transmit N1 symbols for PDSCH reception with processing capability 1 and an additional DM-RS configuration and N\({}_{\text{T},2}\) (Table 6.4-1 [37]) is the time required to transmit N2 symbols for PUSCH reception with processing capability 1. Moreover, T\({}_{\text{wait}}\) is the average waiting time between the reception and transmission of data, which is assumed to be 0.5 ms.  
Once the RACH procedure has been completed and after frame alignment T\({}_{\text{FA2}}\), the UE transmits the RRC Resume Request message and the total time T\({}_{6}\) also depends on the configuration (slot or non-slot). The assumption for the gNB processing the L2 and RRC request is T\({}_{7}\) = 3 ms. After frame alignment T\({}_{\text{FA3}}\), gNB responds with the RRC resume message alongside the UL grant and the time T\({}_{8}\) depends on the configuration. The UE processing (L2 and RRC) is T\({}_{9}\) = 7 ms. The time required for transmission of the RRC Resume Complete message is omitted, since the transmission occurs at the beginning of the subframe with the actual data T\({}_{10}\) = 0 ms. The above procedure results in different values for the control plane delay that range from 11.3 to 17 ms, depending on the TTI length, subcarrier spacing, resource mapping type, and UE capability, analyzed in [28]. Note that a UE can support an RRC INACTIVE state, even if attached to an eNB, but only if the eNB is connected to the 5G Core network [31].