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In order for MR-DC to work, the two RATs serving a single UE must be connected to a single core network in the connected state. The configuration method of the serving cell is as follows: the primary cell group contains the cells served by the primary node, and the secondary cell group contains the cells served by the secondary node. There are three bearer types: MCG bearer, SCG bearer and split bearer. From a network perspective, each bearer (MCG, SCG or Split bearer) can terminate on the MN or SN. After
configures SCG, there will always be at least one SCG hosting and one Split hosting. In MR-DC, MCG bearers are usually defined as radio bearers with RLC bearers only in MCG. For the user plane:
- For MCG bearers, SN does not participate in transmitting user plane data of such bearers on Uu.
- For Split bearer, PDCP data is transmitted between MN and SN through X2-U/Xn-U. SN and MN participate in transmitting data of this bearer type on Uu.
- For SCG bearers, the MN does not participate in transmitting the user plane data of such bearers on Uu.
The core network is not aware of the mapping of DC bearers between UE and RAN. If the addition of a bearer requires adding or modifying an SN user plane tunnel to the CN, the CN process needs to be called to add a new user plane tunnel. However, all control signaling between the UE and the CN (eg, NAS signaling) is handled by the MN itself, and the MN only has control plane signaling to the CN (eg, S1-AP or NG-AP signaling). For the Split bearer, if it involves an SCG bearer terminated at the SN and an MCG bearer terminated at the MN, the PDCP data is transmitted between the MN and SN through the MN-SN user plane interface, and then transmitted to the CN through the MN user plane interface.
Figure 12.8 explains how the user plane protocols defined in 3GPP TS 37.340 work in the UE.
Figure 12.8 User plane protocol architecture of UE under MR-DC
For the control plane, the UE has only a single RRC state, which is based on the MN's RRC and the corresponding single control plane connection between the MN and the core network. Each RAN node has its own RRC entity: E-UTRA if the node is an eNB; NR if the node is a gNB. These entities generate the protocol data unit (PDU) of the RRC layer and then send it to the UE. Figure 12.9 (see 3GPP TS 37.340) illustrates the UE control plane connection protocol structure.
Figure 12.9 UE control plane protocol architecture, EPC or 5GC MR-DC
Since the MN has a control plane connection to the core network, all functions related to the UE and the core network (such as mobility and management, session management, location reporting, access restrictions, user plane management) are handled by the MN.
UEs that support MR-DC use different capabilities (e.g., specific radio-related information), which are conveyed in the system through so-called UE capability containers. These capability information sets are required to establish appropriate DC support between UE and MN and between UE and SN. These capability containers contain a common set of capabilities related to MR-DC, which can be accessed by both MN and SN, but there are also some capability containers that are unique to a certain radio access technology (RAT) and are therefore only relevant to nodes that support this radio access technology. These different containers are called MR-DC capability container, E-UTRAN capability container and NR capability container.
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