Dual Connectivity - Terminology

There is a lot of new terms used in a connection to Dual Connectivity

The signaling connection for the UE is anchored at the Master Node. The cell UE is using in the MN node is called PCell Primary Cell. The UE also use the cell from the Secondary Node and this cell is called PSCell Primary Secondary Cell.

All radio resources used within MN are refered as MCG Master Cell Group, the resources from the SN are referred as SCG Secondary Cell Group. Figure 1 illustrates all those terms.

Figure 1 PCell, PSCell, MCG and SCG

While using PCell and PSCell, or rather Dual Connectivity, it is also possible to use Carrier Aggregation within both Master Node and Secondary Node. One or more SCells Secondary Cells might be configured. This is shown in Figure 2.

Figure 2 PCell, PSCell and SCells

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Dual Connectivity - Types

There are couple options how the Dual Connectivity could be configured in 4G and 5G. First of all Dual Connectivity in 4G shall be mentioned. Two eNodeB connected with X2 interface. Both of those connected to EPC. Figure 1 LTE DC As for the migration towards 5G several more options are available: EN-DC The first option is the eNodeB connected to gNodeB with X2 and both of nodes are connected to EPC via S1 interfaces. However the gNodeB in this option is only using S1-U towards SGW. The eNodeB has the MN role and the gNodeB has the SN role. This option is called EN-DC (E-UTRA-NR Dual Connectivity). This and three other options described below are grouped in MR-DC (Multi-Radio Dual Connectivity). Figure 2 EN-DC Overview NE-DC In this option called NE-DC (NR-E-UTRA Dual Connectivity) the gNodeB is MN and the eNodeB is SN. Nodes are connected with Xn. Figure 3 NE-DC Overview NGEN-DC Yet another option one is called NGEN-DC (NG-RAN E-UTRA-NR Dual Connectivity). It is the

Dual Connectivity Introduction

Dual Connectivity allows the phone to be connected simulatenously to two radio base stations. What can be achievied with DC? The phone may send and received data from both nodes at the same time. User plane tranmission is aggregated (Figure 1). Figure 1 User Plane aggregation in Dual Connectivity It shall be mentioned that two nodes in the RAN shall be connected directly. What is more one node in DC relation will be called Master Node (MN) and the other one will be called Secondary Node (SN). The signalling connection is anchored in the MN. In a one of the cases this will allow to have User Plane and Control Plane separation as it is shown in Figure2. Figure 2 User Plane and Control Plane separation One other case is when the direction of transmission could be separated. The transmission in the Uplink direction (UL) can be done through SN and tranmission in Downlink direction (DL) can be done through MN. This is depicted in Figure 3 Figure 3 UL/DL separation

Xn Handover

Initialy transmission in both UL and DL directions is done via source gNB. The UE sends measurement report to inform the gNB that signal from other cell is better than the one in the current cell. After report reception the source gNB takes a decision to trigger handover procedure towards target gNB. The target gNB starts to prepare resources to serve this UE and when ready acknowledges this to source gNB. The source gNB sends handover command message with information about target cell. At the same time the source gNB suspends the data transmission and starts forwarding data to target gNB. The UE needs to perform random access procedure towards the target cell. After successful random access procedure the UE confirms the handover towards target gNB. Target gNB starts the DL data transmission and informs the CN about completed handover. Data tunnels from Core Network are switched to target gNB. The target gNB informs source one that it may release UE resources.

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