4.9 Support for Dual Connectivity

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.9.1 General

E-UTRAN supports Dual Connectivity (DC) operation whereby a multiple Rx/Tx UE in RRC_CONNECTED is configured to utilise radio resources provided by two distinct schedulers, located in two eNBs connected via a non-ideal backhaul over the X2 interface (see TR 36.842 [59] and TR 36.932 [60]). The overall E-UTRAN architecture as specified in clause 4 and depicted in Figure 4-1 is applicable for DC as well. eNBs involved in DC for a certain UE may assume two different roles: an eNB may either act as an MeNB or as an SeNB. In DC a UE is connected to one MeNB and one SeNB.

NOTE: This clause only concerns intra-E-UTRAN DC. Dual connectivity between E-UTRAN and NR is specified in TS 37.340 [76].

4.9.2 Radio Protocol Architecture

In DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three bearer types exist: MCG bearer, SCG bearer and split bearer. Those three bearer types are depicted on Figure 4.9.2-1 below. RRC is located in MeNB and SRBs are always configured as MCG bearer type and therefore only use the radio resources of the MeNB.

NOTE: DC can also be described as having at least one bearer configured to use radio resources provided by the SeNB.

Figure 4.9.2-1: Radio Protocol Architecture for Dual Connectivity

4.9.3 Network Interfaces

4.9.3.1 E-UTRAN Control Plane for Dual Connectivity

Inter-eNB control plane signalling for DC is performed by means of X2 interface signalling. Control plane signalling towards the MME is performed by means of S1 interface signalling.

There is only one S1-MME connection per DC UE between the MeNB and the MME. Each eNB should be able to handle UEs independently, i.e. provide the PCell to some UEs while providing SCell(s) for SCG to others. Each eNB involved in DC for a certain UE controls its radio resources and is primarily responsible for allocating radio resources of its cells. Respective coordination between MeNB and SeNB is performed by means of X2 interface signalling.

Figure 4.9.3.1-1 shows C-plane connectivity of eNBs involved in DC for a certain UE: the S1-MME is terminated in MeNB and the MeNB and the SeNB are interconnected via X2-C.

Figure 4.9.3.1-1: C-Plane connectivity of eNBs involved in Dual Connectivity

4.9.3.2 E-UTRAN User Plane for Dual Connectivity

For dual connectivity two different user plane architectures are allowed: one in which the S1-U only terminates in the MeNB and the user plane data is transferred from MeNB to SeNB using the X2-U, and a second architecture where the S1-U can terminate in the SeNB. Figure 4.9.3.2-1 shows different U-plane connectivity options of eNBs involved in DC for a certain UE.

Different bearer options can be configured with different user plane architectures. U-plane connectivity depends on the bearer option configured:

– For MCG bearers, the S1-U connection for the corresponding bearer(s) to the S-GW is terminated in the MeNB. The SeNB is not involved in the transport of user plane data for this type of bearer(s) over the Uu.

– For split bearers, the S1-U connection to the S-GW is terminated in the MeNB. PDCP data is transferred between the MeNB and the SeNB via X2-U. The SeNB and MeNB are involved in transmitting data of this bearer type over the Uu.

– For SCG bearers, the SeNB is directly connected with the S-GW via S1-U. The MeNB is not involved in the transport of user plane data for this type of bearer(s) over the Uu.

Figure 4.9.3.2-1: U-Plane connectivity of eNBs involved in Dual Connectivity

NOTE: if only MCG and split bearers are configured, there is no S1-U termination in the SeNB.

4.9.3.3 Support of HeNBs for Dual Connectivity

The following scenarios for Dual Connectivity involving HeNBs are supported as listed in Table 4.9.3.3-1.

Table 4.9.3.3-1: Support of HeNBs for Dual Connectivity

MeNB	SeNB
eNB	open access HeNB
eNB	hybrid access HeNB

Membership Verification for the hybrid access HeNB is performed between the MeNB and the MME and is based on membership status information reported by the UE and the CSG ID.

If the cell served by the SeNB is a shared hybrid cell, the UE reports the subset of the broadcasted PLMN identities passing PLMN ID check and the Permitted CSG list of the UE includes an entry comprising of the concerned PLMN identity and the CSG ID broadcast by the cell served by the SeNB. The MeNB performs PLMN ID check for the PLMNs reported by the UE and selects one if multiple pass the PLMN ID check. If the cell served by the SeNB belongs to a different PLMN than the PLMN serving for the UE in the MeNB, the information provided to the MME for membership verification needs to contain the PLMN-ID of the hybrid cell served by the SeNB as well. Finally the MME verifies the CSG membership according to the received CSG ID, the selected PLMN ID and stored subscription CSG information of the UE.

In case the UE has been admitted with SCG resources configured with the split bearer option from a hybrid HeNB and a SeNB Change is performed within the coverage area of the MeNB towards another hybrid HeNB which has the same CSG ID as the first one, the MeNB may re-use the result of the membership verification performed for the first HeNB.

4.9.3.4 Support of SIPTO@LN and LIPA for Dual Connectivity

This version of the specification supports SIPTO@LN and LIPA for Dual Connectivity according to the following logical architecture:

– SIPTO@LN with co-located L-GW in the MeNB. The MeNB and the MME support the functions described in clause. 4.8.2 with the following change:

– For SCG bearer option, the MeNB sets GTP TEID and Transport Layer Address in S1 UL GTP Tunnel Endpoint IE in the SENB ADDITION REQUEST message and SENB MODIFICATION REQUEST messages as the correlation ID received from the MME and the IP address of the collocated L-GW respectively.

Figure 4.9.3.4-1: SIPTO@LN with co-located L-GW in MeNB – split and SCG bearer options.

– SIPTO@LN with co-located L-GW in the SeNB. For this scenario, only the SCG bearer option is supported for the SIPTO bearer. The SeNB signals its L-GW IP address using the SeNB Addition Preparation procedure, or the MeNB obtains such address via OAM. The MeNB signals the "SIPTO correlation id" to the SeNB using the SeNB Addition Preparation and SeNB Modification Preparation procedures. The functions described in clause 4.8.2 are supported with the following changes:

– The MeNB supports the transfer of the L-GW IP address of SeNB over S1-MME to the EPC within every Uplink NAS Transport procedure;

– The SeNB supports basic P-GW functions in the collocated L-GW such as support of the SGi interface corresponding to SIPTO@LN;

– Additional support by the SeNB of first packet sending, buffering of subsequent packets, internal direct L-GW-eNB user path management and in sequence packet delivery to the UE;

– The SeNB supports the necessary restricted set of S5 procedures corresponding to the support of SIPTO@LN function as specified in TS 23.401 [17];

– The MeNB supports the notification to the EPC of the L-GW uplink TEID(s) or GRE key(s) for the SIPTO@LN bearer(s) over S5 interface within the restricted set of procedures to be forwarded over S1-MME and further used as "SIPTO correlation id" for correlation purposes between the L-GW and the SeNB;

– The SeNB supports triggering SIPTO@LN PDN connection release by the collocated L-GW after an SeNB change or MeNB to eNB handover is performed.

Figure 4.9.3.4-2: SIPTO@LN with co-located L-GW in SeNB

– SIPTO@LN with stand-alone gateway: the MeNB and the SeNB belong to the same LHN (i.e. they have the same LHN ID). The MeNB and the SeNB exchange their LHN ID using the X2 Setup procedure or via OAM. The MeNB initiates the SeNB Modification Preparation procedure in order to support the MME-triggered S-GW relocation without UE mobility. The MeNB and the MME support the functions described in Sec. 4.8.3.

– LIPA: the logical architectures for LIPA correspond to the logical architectures for SIPTO@LN with co-located L-GW in the SeNB.

– Before handover, the MeNB shall initiate the SeNB Modification Preparation procedure or the UE Context Release procedure to release radio and control plane related resources associated to the LIPA bearer.