## 5 MR-DC Architectures for Multi-RAT V2X  
The progressive adoption of 5G by a LTE operator is key for fostering sustained network growth and tackle crucial, safety-related aspects such as network communications reliability. Already considered by 3 GPP Release 16 (Rel-16) in their coexistence scenarios,  
Figure 11: PDR as a function of propagation distance under different packet transmission rates and vehicle density in LTE mode 3. TS 22.186 [27] paves the way for introducing solutions with multiple RATs (multi-RATs). Note that the coexistence aspects stated by Rel-16 make possible the simultaneous use of multiple 3 GPP RATs for direct sidelink transmissions--both NR, and Evolved Universal Terrestrial Radio Access (E-UTRA) based--but there is no mutual inter-operation of NR and LTE sidelink with each other. As an example, Figure 12 shows a cross-border scenario (such as the studied by the European Commission funded 5G-CARMEN project [28]) with MR-DC, and multi-RAT in a generic 5G Non-StandAlone (NSA) deployment. Here, cooperative maneuvering procedures are expected to begin among road vehicles 2 km ahead (some of them driving on the opposite side of the transnational border) within 1 min of the reception of a back-situation awareness message concerning the arrival of an emergency vehicle. Assuming that multiple technology interfaces may be potentially involved (NR Uu, NR PCS, LTE Uu, and LTE PCS) the cooperative maneuvering action requires that several vehicles attached to diverse networks would be able to determine, in advance, in a fast way, and at a certain point in time, whether they are in risk of re-selecting or switching to another network technology. Besides, unforeseen circumstances, like transient radio coverage fluctuations, etc., reasonably worsen the complexity of predicting in a timeline the likely future onset of such network events. Three possible, potentially coexisting, solutions to achieve robustness through redundancy are shown in Figure 12:  
1. Selection of multiple interfaces: use of NR PC5 in addition to the NR Uu interface communications (e.g., simultaneous transmissions via PC5 and Uu interface);
2. Dual RAT Uu transmissions: (NR and LTE Uu communications);
3. Multi-RAT: mutual inter-operation of NR and LTE sidelink communications;
4. Cross-RAT: coexistence of LTE and 5G sidelink with no inter-operation. In these scenarios, it is essential to assess the reliability of the redundant interfaces. Managing this also seems natural because there are mechanisms, such as marking the packets as redundant via enhancements in the protocol stack, that allow discarding redundant packets that no longer serve their purpose. In addition, when managing such multi-RAT V2X networks, trustworthiness on the available network interfaces impacts on the communication network deploying problem and defines the sustainable growth of capacity provisioning by an MNO. It seems reasonable to anticipate the existence of more reliable road areas that will ensure automated driving, for example, where the Uu interface serves vehicles uninterruptedly, assisted by a dependable PC5 interface, assuring  
Figure 12: Example of joint Cooperative Maneuvering and Back-Situation Awareness involving multi-RAT V2X. an up-to-date map of neighboring vehicles; but also degraded areas will exist, where the quality of the redundant communication channels will drop and may trigger lowering a degree of maneuvering assistance, losing any assistance at all, resorting to on-board sensors, or to manual driving. Thus, a multi-RAT V2X use case-aware approach for improved reliability that has in-built consideration of the performance characteristics motivated by the use of 3 GPP MR-DC seems interesting. It may also be reasoned that being aware of the underlying 3 GPP radio access support for DC UEs could be useful to assure reliability.