7.6 Dual Connectivity

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

In DC, the configured set of serving cells for a UE consists of two subsets: the Master Cell Group (MCG) containing the serving cells of the MeNB, and the Secondary Cell Group (SCG) containing the serving cells of the SeNB.

When a UE is configured with CA in the MCG, the same principles as described in clause 7.5 apply to MCG.

For SCG, the following principles are applied:

– At least one cell in SCG has a configured UL CC and one of them, named PSCell, is configured with PUCCH resources;

– When SCG is configured, there is always at least one SCG bearer or one Split bearer;

– Upon detection of a physical layer problem or a random access problem on PSCell, or the maximum number of RLC retransmissions has been reached associated with the SCG, or upon detection of an access problem on PSCell (T307 expiry) during SCG change, or when exceeding the maximum transmission timing difference between CGs:

– RRC connection Re-establishment procedure is not triggered;

– All UL transmissions towards all cells of the SCG are stopped;

– MeNB is informed by the UE of SCG failure type;

– For split bearer, the DL data transfer over the MeNB is maintained.

– Only the RLC AM bearer can be configured for the split bearer;

– Like PCell, PSCell cannot be de-activated and cannot be in dormant SCell state (see clause 11.2);

– PSCell can only be changed with SCG change (i.e. with security key change and, unless RACH-less HO is configured, with RACH procedure);

– Neither direct bearer type change between Split bearer and SCG bearer nor simultaneous configuration of SCG and Split bearer are supported.

With respect to the interaction between MeNB and SeNB, the following principles are applied:

– Logical channel identities are independently allocated by the MeNB and the SeNB.

– The MeNB maintains the RRM measurement configuration of the UE and may, e.g. based on received measurement reports or traffic conditions or bearer types, decide to ask a SeNB to provide additional resources (serving cells) for a UE.

– Upon receiving the request from the MeNB, a SeNB may create the container that will result in the configuration of additional serving cells for the UE (or decide that it has no resource available to do so).

– For UE capability coordination, the MeNB provides (part of) the AS configuration and the UE capabilities to the SeNB.

– The MeNB and the SeNB exchange information about UE configuration by means of RRC containers (inter-node messages) carried in X2 messages.

– The SeNB may initiate a reconfiguration of its existing serving cells (e.g., PUCCH towards the SeNB).

– The SeNB decides which cell is the PSCell within the SCG.

– The MeNB does not change the content of the RRC configuration provided by the SeNB.

– In the case of the SCG addition and SCG SCell addition, the MeNB may provide the latest measurement results for the SCG cell(s).

– Both MeNB and SeNB know the SFN and subframe offset of each other by OAM or UE measurement, e.g., for the purpose of DRX alignment and identification of measurement gap.

When adding a new SCG SCell, dedicated RRC signalling is used for sending all required system information of the cell as for CA described in clause 7.5, except for the SFN acquired from MIB of the PSCell of SCG.