## 5 MR-DC Architectures for Multi-RAT V2X
When building on the multi-connectivity operation options using E-UTRA and NR radio access technologies proposed by 3 GPP Technical Specification (TS) 37.340 [7], several MR-DC multi-RAT scenarios may be envisioned, which encompass both LTE V2X sidelink communications and NR sidelink communications. However, note that in Rel-16 only the Master Node (MN) is allowed to control and configure the UEs performing NR sidelink communications and or LTE V2X sidelink communications [7]. This implies that the Secondary Node (SN) is not allowed to use (initiate) an E-UTRA Radio Resource Control (RRC) Connection Reconfiguration procedure or an NR RRC Reconfiguration with sidelink fields for LTE sidelink or NR sidelink. That the MN would be the entity in charge of receiving this control signaling yields three option (Figure 13):  
1. NR-E-UTRA Dual Connectivity (NE-DC): standalone gNB, a 5G NR node, connected to the 5G Core (5 GC), and sidelink communications controlled/configured by the MN (a 5G gNB);
2. NG-RAN E-UTRA-NR Dual Connectivity (NGEN-DC): standalone next-generation eNB (ng-eNB), which is an E-UTRAN node (LTE radio) with the capability to connect to the 5 GC, and sidelink communications controlled/configured by ng-eNB (MN);
3. E-UTRAN NR Dual Connectivity (EN-DC): standalone LTE eNB connected to the Evolved Packet Core (EPC), and sidelink communications controlled/configured by eNB (MN). An interesting conclusion to the considerations made in this paper is that a smart approach to capacity provisioning fosters a more sustainable growth in the adoption of V2X for ITS. Therefore, it seems promising to regionally manage the multi-RAT nature of a heterogeneous network offering V2X so as to diversify RAT selection decisions associated to pre-defined geographical regions matching the statistical triggering of MR-DC procedures. This approach may be justified given that, typically, the reliability experienced by vehicular UEs is spatially correlated. Furthermore, this strategy may be the best one when the data available from specific UEs is scarce, but their recent geographical position is known, and it may also be deduced from prior knowledge about the successive geographical triggering of MR-DC procedures. Figure 13: Multi-RAT V2X together with MR-DC architectures: UE configured for either NE-DC (**left**), NGEN-DC (**center**), or EN-DC (**right**). For example, Figure 14 shows a UE (green vehicle) along a road covered by a 5G NSA deployment with EN-DC architecture. As shown, the UE is expected to experience several MR-DC procedures as it enters the region with 5G coverage, crosses the boundaries between MNs, between SNs and leaves the region with 5G non-standalone coverage. A possible approach to clarify multi-RAT selection decisions is to segment the area into geographical zones according to the overall reliability and range related KPIs experienced for each of the RATs considered, and associate each zone with a specific multi-RAT decision potentially satisfying the QoS requirements. In the case that a RAT has a good reliability, it could be of use for long-range sensitive use cases (e.g., the cooperative maneuvering) where remote alerts and reaching vehicles far away in a reliable way is a must. Furthermore, the QoS conditions of multi-RAT may be the basis for recommending, e.g., different automation levels according to the expected performance of the 5G communication system for the support of autonomous driving in the different geographical zones. Our future work is aimed at simulating the performance of MR-DC architectures with different connection interfaces in non-standalone 5G networks. Thus, coverage distances are expected to improve and network reliability will increase due to redundant information being sent over several interfaces. This will enable new use cases to be offered efficiently.