4.7.6 Signalling procedures

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

4.7.6.1 RN attach procedure

Figure 4.7.6.1-1 shows a simplified version of the attach procedure for the RN. The procedure is the same as the normal UE attach procedure TS 23.401 [17] with the exception that:

– The DeNB has been made aware of which MMEs support RN functionality via the S1 Setup Response message earlier received from the MMEs;

– The RN sends an RN indication to the DeNB during RRC connection establishment;

– After receiving the RN indication from the RN, the DeNB sends the RN indicator and the IP address of the S‑GW/P-GW function embedded in the DeNB, within the Initial UE Message, to an MME supporting RN functionality;

– MME selects S-GW/P-GW for the RN based on the IP address included in the Initial UE Message;

– During the attach procedure, the EPC checks if the RN is authorised for relay operation; only if the RN is authorised, the EPC accepts the attach and sets up a context with the DeNB; otherwise the EPC rejects the attach.

The RN is preconfigured with information about which cells (DeNBs) it is allowed to access.

Figure 4.7.6.1-1: RN attach procedure

4.7.6.2 E-RAB activation/modification

Figure 4.7.6.2-1 shows a simplified version of the DeNB-initiated bearer activation/modification procedure. This procedure can be used by the DeNB to change the EPS bearer allocation for the RN. The procedure is the same as the normal network-initiated bearer activation/modification procedure TS 23.401 [17] with the exception that the S-GW/P‑GW functionality (steps 1 and 6) is performed by the DeNB.

Figure 4.7.6.2-1: DeNB-initiated bearer activation/modification procedure

4.7.6.3 RN startup procedure

Figure 4.7.6.3-1 shows a simplified version of the startup procedure for the RN. The procedure is based on the normal UE attach procedure TS 23.401 [17] and it consists of the following two phases:

I. Phase I: Attach for RN preconfiguration.
The RN attaches to the E-UTRAN/EPC as a UE at power-up and retrieves initial configuration parameters, including the list of DeNB cells, from RN OAM. After this operation is complete, the RN detaches from the network as a UE and triggers Phase II. The MME performs the S-GW and P-GW selection for the RN as a normal UE.

II. Phase II: Attach for RN operation.
The RN connects to a DeNB selected from the list acquired during Phase I to start relay operations. For this purpose, the normal RN attach procedure described in clause 4.7.6.1 is applied. After the DeNB initiates setup of bearer for S1/X2, the RN initiates the setup of S1 and X2 associations with the DeNB (see clause 4.7.4). In addition, the DeNB may initiate an RN reconfiguration procedure via RRC signalling for RN-specific parameters.
After the S1 setup, the DeNB performs the S1 eNB Configuration Update procedure(s), if the configuration data for the DeNB is updated due to the RN attach. After the X2 setup, the DeNB performs the X2 eNB Configuration Update procedure(s) to update the cell information.
In this phase the RN cells’ ECGIs are configured by RN OAM.

Figure 4.7.6.3-1: RN startup procedure

4.7.6.4 RN detach procedure

Figure 4.7.6.4-1 shows a simplified version of the detach procedure for the RN operation in case no UE is connected to the RN cells.

1. The detach procedure is the same as the normal UE detach procedure TS 23.401 [17].

2. The DeNB performs the X2 eNB Configuration Update procedure(s) to update the cell information.

3 The DeNB performs the S1 eNB Configuration Update procedure(s), if the configuration data for the DeNB is updated due to the RN detach.

Figure 4.7.6.4-1: RN detach procedure

4.7.6.5 Neighbouring Information Transfer

The X2 eNB Configuration Update procedure (see clause 20.2.2.8) is used by the DeNB to also transfer application level configuration data of a single neighbouring eNB to the RN. Upon reception of an ENB CONFIGURATION UPDATE message, if the served cells contained in the message belong to the neighbour eNB rather than the DeNB, the RN shall regard the X2 interface between DeNB and the neighbour eNB as available. The RN will update the X2 availability, the corresponding GU Group ID and other information of the neighbour eNB according to the message.

4.7.6.6 Mobility to or from RN

In case of Handover between RN and neighbour eNB, in addition to the procedures specified in clause 10.1.2.1.1, the following also applies.

– The DeNB may inform the RN of any GUMMEI of the UE’s serving MME in the INITIAL CONTEXT SETUP REQUEST and S1 HANDOVER REQUEST messages. Considering this information as well as the GU Group ID of the neighbour eNB and the X2 interface availability between DeNB and neighbour eNB, the RN initiates either S1 or X2 handover for the UE. In case the GUMMEI information is not available to the RN, the RN attempts X2 handover for the UE (see clause 19.2.2.5); upon X2 handover failure, S1 handover may be initiated.

– The S1/X2 HANDOVER REQUEST is received by the DeNB, which reads the target cell ID from the message, finds the target node corresponding to the target cell ID, and forwards the message toward the target node if appropriate.