# V2X Communication over Cellular Networks: Capabilities and Challenges
## 6 Open Issues and Future Trends  
Based on the previous analysis, it is evident that current cellular networks are able to support a significant number of V2X UCs, but are still unable to fully support the vision of fully autonomous driving under any environmental conditions. Indeed, LTE networks can provide for services that have moderate requirements in terms of throughput and delay, but need to address major challenges, such as the physical layer structure, synchronization issues, MBMS challenges, and resource allocation and security issues, which are thoroughly analyzed in [44], before the massive exploitation of vehicular communications. However, not even 5G networks can fully support UCs with stringent requirements with today's technological solutions. Figure 6: Sidelink (SL) spectrum needs per gNB sector in the urban grid environment. As far as the delay is concerned, the analysis we provided in this paper has clearly indicated that in LTE, the control plane delay required to switch to the CONNECTED state is circa 50 ms (or 20 ms if the 5G core network is used), whereas the user plane delay varies from 7 to 10 ms for unicast communication over the Uu interfaces up to 26 or 55 ms if the sidelink is to be used. Additionally, a considerable 26 ms user plane delay is experienced if MBMS is used. LTE network's limitations, evaluated through network simulations, are presented in [45], providing the limits of different parameter sets with regard to the requirements of V2X-based applications. As expected, LTE is not a suitable solution for tackling the advanced 3 GPP requirements for fully autonomous driving. Design principles for latency-aware V2X services, such as \"one-OFDM-symbol\" TTI (1/14 TTI) and sustained RRC connections during V2X services, are proposed in [46] to satisfy the feasibility of V2X services in terms of latency. 5G-NR has considerably improved the control plane latency (reduced it to 11.3 ms) and is able to provide just a 1 ms delay for user-plane communication inside the radio access network (i.e., measured as the time required to transmit data from a vehicle to a BS and back to another vehicle). However, the control plane delay is significant and usually not taken into consideration when evaluating the delay for V2X communications. The assumption that all vehicles will always be connected to the network at all times does not seem so realistic due to the scarcity of the spectrum. Therefore, in advanced cases, the extra delay of 11.3 ms required to switch from the RRC INACTIVE to CONNECTED state may result in a significant deviation from the expected performance of the network. A potential solution would be that for some UCs, a connectionless transmission of brief messages for a short period of time could be used to eliminate this delay. This will significantly impact the way V2X services will be designed to operate and will have a big impact on standardization activities. As existing requirements suggest that the desired V2X communication delay for the user plane should be in the range of 3 to 10 ms for advanced UCs, it is rather significant to support local breakout schemes and avoid traffic passing through the core network or reaching centralized application servers, where statistical multiplexing from a huge number of vehicles may fail to guarantee the required QoS. It is important to remember that for most V2X UCs, the data have a local significance and they do not need to propagate across large distances. Therefore, future cellular networks should further investigate solutions to support local break-out schemes and find solutions for fast data transmission in specific cases (e.g., collision avoidance in urban crossroads [47]). Mobile edge computing (MEC) is a key technology for addressing this challenge. The authors in [48] evaluate C-V2X communications assisted by MEC for the vulnerable road users use case in a two-lane freeway scenario. The evaluation considers LTE coverage (i.e., Uu interface) with the MEC host collocated with a serving eNB.