# V2X Communication over Cellular Networks: Capabilities and Challenges
## 4 Delay Analysis for V2X Communications
### LTE-A Delay
#### 4.1.1 Control Plane  
Control plane latency is related to the establishment of a signaling connection of a UE with the network (Radio Resource Control (RRC) connection establishment), paging, and handover (HO). The delay during these control functions may seriously affect the performance of an autonomous driving application, especially if the vehicles involved do not already have a connection to the network (i.e., IDLE RRC state). RRC connection establishment switches the RRC protocol from the IDLE to the CONNECTED state [27]. To do this, the UE initially performs the Random Access Channel (RACH) procedure. It transmits a preamble to the Evolved Node B (eNB) to request resources to transmit in the UL channels. Then, once the resources are granted from the eNB,the UE performs the RRC connection establishment procedure [31]. This procedure may include security tasks. The overall procedure is decomposed in its steps and is presented in Table 3, together with the time required for each one of them. The time required for steps 11-14 is left null. This is due to the fact that steps 11-14 are performed concurrently with the steps 15-17 and are expected to finish before them, thus making the calculation of the delay for these steps irrelevant. The overall delay of 50 ms can be considered as a significant delay, especially for V2X UCs that require a fast response time, even from vehicles that are in an RRC IDLE state. Paging is the procedure performed by the network to locate a UE with the accuracy of a cell. It is an RRC procedure that occurs when incoming information (data, system information, etc.) is intended for a UE. As defined in [27], paging latency is considered from the point that a packet has already arrived at an eNB. The paging cycle varies from 320 ms to 2.54 s; an additional 2.5 ms of transmission and processing time is considered [27]. Upon reception of a paging message, a UE has to follow the RRC connection establishment procedure as described above. As the combined delay of paging and RRC connection establishment appears to be high, in relation to V2X UCs, V2X applications should take this into serious consideration. For vehicles already involved in active communication (i.e., RRC CONNECTED), the disruption time during the execution of HOs needs to be considered. This procedure relates to the transfer of the UE context from one cell to another, the establishment of radio resources in the new cell, and possibly the switching of the core network's established data paths. In LTE, the HO procedure is of the break-before-make type, meaning that there is a data interruption period where data transmission is paused; the interruption varies from 17 to 50 ms and it may even go up to hundreds of ms [33]. The HO procedure consists of the following phases, described in detail in Table 4:  
* The HO preparation, starting with the transmission of the RRC measurement report from the UE to serving eNB. The eNB processes the request, coordinates with the neighboring eNB, and sends the HO command to the UE. * The HO execution, which starts upon the reception of the HO command message from the UE. During this phase, the data transmission stops and the UE proceeds to RRC reconfiguration. Consequently, the UE attempts to reach the target cell via the RACH. The procedure is finalized when the UE transmits the RRC Connection