# V2X Communication over Cellular Networks: Capabilities and Challenges
## 6 Open Issues and Future Trends
Additional transport and core network delays are adopted from a realistic LTE environment with commercial terminals. The gain reported compared to conventional LTE (i.e., w/o MEC) is in the range of 66-80%. However, the massive deployment of MEC or local UPFs is expected to significantly increase the cost and management complexity of the network. Additionally, issues related to the security of data located closer to the edge of the network should be addressed. In the case of multiple operators, the core network delay becomes even higher, making some UCs rather unlikely to be deployed in multi-operator environments. RAN sharing or regional roaming could be considered as potential solutions. However, this requires appropriate agreements between the operators that are not always easy to achieve. The design of a 5G MBMS is of paramount importance as its counterpart 4G system has been designed for Mobile Broadband (MBB) communication and its use will incur intolerable delays. This is expected to be addressed by 3 GPP in Release 17 [49]. Potential future solutions may take advantage of additional contextual information. Note that in fully autonomous driving, a network entity (e.g., the V2X AS) is expected to know the path a vehicle will follow in the near future. Sharing this contextual information with the network control functions (e.g., RRC, scheduling, etc.) may minimize the delay (e.g., control plane delay such as paging or handover) and simultaneously optimize the experienced QoS. Further network enhancements applied to minimize the delay should be considered to avoid treating packets at the IP layer as the information is so localized that it could be communicated in a layer-2 switch function to the neighboring vehicles or the neighboring BSs [50]. This requires the 5G gNBs to operate as switches in the user plane, which is a considerable difference from today's systems. Additionally, the case of border crossing should be considered when it comes to delay analysis. Current standardization solutions [51] support inter-operator HO, but these solutions are rather complex and require additional agreements between the operators, since they need to expose their AMF deployment to other operators (which may be competitors). In regions like Europe, where vehicles may cross multiple borders in a short period of time, simpler solutions would be highly desirable. If vehicles are to be supported by multiple operators then some unneeded delay may be experienced that is not easily addressable until some out-of-the box ideas are considered, such as force regional roaming [52], which may have some interesting implications in the operator business models themselves. In future scenarios, especially in urban areas where the density of vehicles will be high, the capacity is going to be the major issue. LTE will struggle to support a fraction of vehicles, even for the most moderate usage of UCs. Apparently, such a problem will not appear from day one, but as more and more vehicles start communicating, this problem will become evident. Fully autonomous driving is highly unlikely to be easily supported, even with the current 5G technology, as indicated from our analysis. Most of the advanced V2X UCs have modest spectrum requirements. However, use cases that demand the exchange of a significant amount of information (e.g., high-definition videos and maps) will create significant challenges. To address this, one possible way forward would be to merge several UCs that attempt to improve the vehicle's perception of the environment. Moreover, smart techniques employed to compress the information needed to be transmitted are expected to play an important role. The extensive use of the multicasting/geo-casting mode of communication will further minimize the need for the spectrum. PC5 communication (both in LTE and 5G) will improve the capacity situation, given that short-range communication will enable higher frequency reuse, but it will not fix the problem.