5G NR : Dual Connectivity (DC) Definition & 5G-NR (EN-DC) Bearer Concept

 As per 3GPP 36.300, Dual Connectivity is a mode of operation where a a multiple Rx/Tx  capable UE in RRC CONNECTED mode can be configured to utilize the radio resource of two distinct schedulers, located in two eNBs namely Master eNB and Secondary eNB connected via a non-ideal back-haul over the X2 interface.

Dual Connectivity Properties:

• Dual Connectivity (DC) allows a UE to simultaneously transmit and receive data on multiple component carriers from two cell groups via master eNodeB (MN) and secondary eNodeB (SN)
• DC is for non-ideal backhaul, e.g., relatively large delay between nodes
• DC can increase user throughput, provide mobility robustness, and support load-balancing among eNodeBs
• In DC, user traffic is split between carriers in PDCP layer and data bearer is known split bearer
• DC and CA are not mutually exclusive; they can be implemented togther
• In DC, a UE has two C-RNTI: one in MCG and C-RNTI in SCG, it has an always-active cell in MCG and another always-active cell in SCG, and it has separate PUCCH resource in MCG and SCG
• The configuration between master eNB (MeNB) and secondary eNB (SeNB) is independent, e.g., bandwidth, number of component carriers, frame structure of each carrier (FDD or TDD), etc.
• In control plane, there is only one S1-MME connection and it is between MME and MeNB.
• RRC connection only terminates at MeNB; there is no RRC entity in SeNB. All SeNB-related RRC configuration is transmitted to MeNB which then transmits RRC message to UE.

Here we will discuss about EN-DC bearer concept.
Split Bearer ( MCG Split, SCG Split )

• MCG ( Master Cell Group ) Bearer
• SCG ( Secondary Cell Group ) Bearer

Assumption is reader is aware about LTE bearer concept.

MCG ( Master Cell Group ) Bearer: A bearer terminating at MN node (in case of EN-DC, more specifically at eNB)
SCG ( Secondary Cell Group ) Bearer: A bearer terminating at SN node (in case of EN-DC, more specifically at gNB)

Here terminating means, which node (MN/SN) PDCP is being used for the bearer. If PDCP (E-PDCP/NR-PDCP: MN node support both E-PDCP and NR-PDCP) is used from MN node then we call it MN terminating. If NR-PDCP is used from SN node then we call it SN terminating.

A MCG bearer can be configured with either E-PDCP or NR-PDCP. Thus MCG bearer can be configured in two variation (E-UTRA PDCP -> E-UTRA-RLC/MAC, E-UTRA-PHY) and (NR-PDCP -> E-UTRA-RLC/MAC, E-UTRA-PHY)
Whereas a SCG bearer is always configured with NR-PDCP only (NR-PDCP -> NR-RLC/MAC, NR-PHY)

Split Bearer: A bearer for which traffic is routed via LTE and/or NR bearer. Split bearer is further either SCG split or MCG split.
MCG Split: Traffic is splitted at MN node, refer Fig 0.
SCG Split: Traffic is splitted at SN node, refer Fig 0

Definition for Dual Connectivity.

• Master eNB (MeNB) is the eNB which terminates at least S1-MME or both S1-MME and S1-U
• Secondary eNB (SeNB) is the eNB that is providing additional radio resources.
• Master Cell Group is a group of serving cells associated with the MeNB, comprising of the PCell and optionally one or more SCells.
• Secondary Cell Group is a group of serving cells associated with the SeNB, comprising of PCell and optionally one or more SCells.
• MCG Bearer is a bearer whose radio protocols are only located in the MeNB to use MeNB resources only
• SCG Bearer is a bearer whose radio protocols are only located in the SeNB to use SeNB resources.
• Split Bearer is a bearer whose radio protocols are located in both the MeNB and the SeNB to use both MeNB and SeNB resources

How n/w configure these bearer in case of EN-DC

1. During LTE attach, n/w configure one DRB. This DRB can be configured either via EUTRA-PDCP or NR-PDCP. Please refer Fig 2 and Fig 3, both snippet are taken from RRC reconfiguration message during LTE attach procedure. Here we are trying to show n/w configure the DRB with NR-PDCP and this NR-PDCP is MN terminating (Fig 3, keyToUse is master)

Fig 2: Snippet of RRC Reconfiguration during LTE attach where EUTRA-RLC is associated with DRB ID (1)

Fig 3: Snippet of RRC Reconfiguration during LTE attach where DRB ID (1) is configured with NR-PDCP

So by referring Fig 2 & 3, n/w configure a standalone MCG bearer with NR-PDCP.

2. Now during NR cell addition, n/w may configure either standalone SCG bearer or split bearer ( a split bearer may be further either MCG split or SCG split ), here we are taking the example of SCG split bearer, refer Fig 4.

Fig 4: Snippet of RRC Reconfiguration during NR cell addition where DRB is same as in case of LTE attach but keyToUse IE value changed

By referring Fig 2 (EUTRA-RLC associated with DRB ID 1 during LTE attach time ) and Fig 5 (NR-RLC is associated with same DRB ID 1 during NR cell addition time).

As both EUTRA and NR is associated with same DRB ID(1), hence it is case of split bearer and NR-PDCP is terminating at SN node (Fig 4, keyToUse), hence it is case of SCG split bearer