8.7.1 EN-DC X2 Setup

36.4233GPPEvolved Universal Terrestrial Radio Access Network (E-UTRAN)Release 17TSX2 Application Protocol (X2AP)

8.7.1.1 General

The purpose of the EN-DC X2 Setup procedure is to exchange application level configuration data needed for eNB and en-gNB to interoperate correctly over the X2 interface. This procedure erases any existing application level configuration data in the two nodes and replaces it by the one received. This procedure also resets the X2 interface like a Reset procedure would do.

NOTE 1: If X2-C signalling transport is shared among multiple X2-C interface instances, one EN-DC X2 Setup procedure is issued per X2-C interface instance to be setup, i.e. several X2 Setup procedures may be issued via the same TNL association after that TNL association has become operational.

NOTE 2: Exchange of application level configuration data also applies between eNB and en-gNB in case the SN (i.e. the en-gNB) does not broadcast system information other than for radio frame timing and SFN, as specified in the TS 37.340 [32]. How to use this information when this option is used is not explicitly specified.

The procedure uses non UE-associated signalling.

8.7.1.2 Successful Operation

Figure 8.7.1.2-1: eNB Initiated EN-DC X2 Setup, successful operation

Figure 8.7.1.2-2: en-gNB Initiated EN-DC X2 Setup, successful operation

If case of network sharing with multiple cell ID broadcast with shared X2-C signalling transport, as specified in TS 36.300 [15], the EN-DC X2 SETUP REQUEST message and the EN-DC X2 SETUP RESPONSE message shall include the Interface Instance Indication IE to identify the corresponding interface instance. In the current version of this specification an eNB shall not include the Interface Instance Indication IE in the Initiating NodeType IE in the EN-DC X2 SETUP REQUEST message.

If the SFN Offset IE is included in the EN-DC X2 SETUP REQUEST or EN-DC X2 SETUP RESPONSE message, the receiving node shall, if supported, use this information to deduce the SFN0 time offset of the reported cell. The receiving node shall consider the received SFN Offset IE content valid until reception of an update of the IE for the same cell(s).

eNB initiated EN-DC X2 Setup:

An eNB initiates the procedure by sending the EN-DC X2 SETUP REQUEST message to a candidate en-gNB. The candidate en-gNB replies with the EN-DC X2 SETUP RESPONSE message. The initiating eNB shall transfer the complete list of its served cells to the candidate en-gNB. The candidate en-gNB shall reply with the complete list of its served cells or if supported, a partial list of its served cells together with the Partial List Indicator IE, according to the received information in Cell and Capacity Assistance Information IE in EN-DC X2 SETUP REQUEST message. If Supplementary Uplink is configured at the candidate en-gNB, the candidate en-gNB shall include in the EN-DC X2 SETUP RESPONSE message the SUL Information IE and the Supported SUL band List IE for each served cell where supplementary uplink is configured.

If the EN-DC X2 SETUP REQUEST message contains the Protected E-UTRA Resource Indication IE, the receiving en-gNB should take this into account for cell-level resource coordination with the eNB. The en-gNB shall consider the received Protected E-UTRA Resource Indication IE content valid until reception of a new update of the IE for the same eNB.

The protected resource pattern indicated in the Protected E-UTRA Resource Indication IE is not valid in subframes indicated by the Reserved Subframes IE, as well as in the non-control region of the MBSFN subframes i.e. it is valid only in the control region therein. The size of the control region of MBSFN subframes is indicated in the Protected E-UTRA Resource Indication IE.

If the Partial List Indicator IE is set to "partial" in the EN-DC X2 SETUP RESPONSE message from the en-gNB, the eNB shall, if supported, assume that the en-gNB has included in the List of Served Cells NR IE a partial list of cells.

If the EN-DC X2 SETUP REQUEST message contains the TNL Transport Layer Address info IE, the receiving en-gNB shall, if supported, take this into account for IPSEC tunnel establishment.

If the EN-DC X2 SETUP RESPONSE message contains the TNL Transport Layer Address info IE, the receiving eNB shall, if supported, take this into account for IPSEC tunnel establishment.

If the NR Cell PRACH Configuration IE is included in the Served NR Cell Information IE contained in the EN-DC X2 SETUP RESPONSE message, the eNB may store the information.

If the CSI-RS Transmission Indication IE is contained in the EN-DC X2 SETUP REQUEST message, the en-gNB may use this information for neighbour NR cell’s CSI-RS measurement.

If the Additional Measurement Timing Configuration List IE is contained in the EN-DC X2 SETUP REQUEST message, the en-gNB may use this information for neighbour NR cell’s CSI-RS measurement.

If the Served Cell Specific Info Request IE is included in the EN-DC X2 SETUP REQUEST message, the en-gNB shall, if supported, include the Additional Measurement Timing Configuration List IE for the requested NR cells in the EN-DC SETUP RESPONSE message.

If the Intended TDD DL-UL Configuration NR IE is contained in the NR Neighbour Information IE in the EN-DC X2 SETUP REQUEST message, en-gNB should take this information into account for cross-link interference management. The en-gNB shall consider the received Intended TDD DL-UL Configuration NR IE content valid until reception of an update of the IE for the same cell(s).

Interaction with the eNB Configuration Update procedure:

The receiving eNB may forward the Intended TDD DL-UL Configuration NR IE received in the Served NR Cell Information IE in the EN-DC X2 SETUP RESPONSE message to neighbouring eNBs by triggering the eNB Configuration Update procedure.

Interaction with the EN-DC Configuration Update procedure:

The receiving eNB may forward the Intended TDD DL-UL Configuration NR IE received in the Served NR Cell Information IE in the EN-DC X2 SETUP RESPONSE message to neighbouring en-gNBs by triggering the EN-DC Configuration Update procedure.

en-gNB initiated EN-DC X2 Setup:

An en-gNB initiates the procedure by sending the EN-DC X2 SETUP REQUEST message to a candidate eNB. The candidate eNB replies with the EN-DC X2 SETUP RESPONSE message. The initiating en-gNB shall transfer the complete list of its served cells or if supported, a partial list of its served cells together with the Partial List Indicator IE in the EN-DC X2 SETUP REQUEST message to the candidate eNB. The candidate eNB shall reply with the complete list of its served cells.

If Supplementary Uplink is configured at the en-gNB, the en-gNB shall include in the EN-DC X2 SETUP REQUEST message the SUL Information IE and the Supported SUL band List IE for each served cell where supplementary uplink is configured.

If the EN-DC X2 SETUP RESPONSE message contains the Protected E-UTRA Resource Indication IE, the receiving en-gNB should take this into account for cell-level resource coordination with the eNB. The en-gNB shall consider the received Protected E-UTRA Resource Indication IE content valid until reception of a new update of the IE for the same eNB.

If the Partial List Indicator IE is set to "partial" in the EN-DC X2 SETUP REQUEST message from the en-gNB, the eNB shall, if supported, assume that the en-gNB has included in the List of Served Cells NR IE a partial list of cells.

If the Cell and Capacity Assistance Information IE is present in the EN-DC X2 SETUP RESPONSE message from the eNB, the en-gNB shall, if supported, store the collected information to be used for future interface management.

If the EN-DC X2 SETUP REQUEST message contains the TNL Transport Layer Address info IE, the receiving eNB shall, if supported, take this into account for IPSEC tunnel establishment.

If the EN-DC X2 SETUP RESPONSE message contains the TNL Transport Layer Address info IE, the receiving en-gNB shall, if supported, take this into account for IPSEC tunnel establishment.

If the NR Cell PRACH Configuration IE is included in the Served NR Cell Information IE contained in the EN-DC X2 SETUP REQUEST message, the eNB may store the information.

If the CSI-RS Transmission Indication IE is contained in the EN-DC X2 SETUP REQUEST message, the eNB should take it into account when forwarding neighbour NR cell’s CSI-RS configuration.

If the Additional Measurement Timing Configuration List IE is contained in the EN-DC X2 SETUP REQUEST message, the eNB should take it into account when forwarding neighbour NR cell’s CSI-RS configuration.

If the Served Cell Specific Info Request IE is included in the EN-DC X2 SETUP REQUEST message, the eNB shall, if supported, include the Additional Measurement Timing Configuration List IE for the requested NR neighbour cells, capable of performing EN-DC with the served E-UTRA cell in the EN-DC X2 SETUP RESPONSE message.

If the Intended TDD DL-UL Configuration NR IE is contained in the NR Neighbour Information IE in the EN-DC X2 SETUP RESPONSE message, en-gNB should take this information into account for cross-link interference management. The en-gNB shall consider the received Intended TDD DL-UL Configuration NR IE content valid until reception of an update of the IE for the same cell(s).

Interaction with the eNB Configuration Update procedure:

The receiving eNB may forward the Intended TDD DL-UL Configuration NR IE received in the Served NR Cell Information IE in the EN-DC X2 SETUP REQUEST message to neighbouring eNBs by triggering the eNB Configuration Update procedure.

Interaction with the EN-DC Configuration Update procedure:

The receiving eNB may forward the Intended TDD DL-UL Configuration NR IE received in the Served NR Cell Information IE in the EN-DC X2 SETUP REQUEST message to neighbouring en-gNBs by triggering the EN-DC Configuration Update procedure.

8.7.1.3 Unsuccessful Operation

Figure 8.7.1.3-1: eNB Initiated EN-DC X2 Setup, unsuccessful operation

Figure 8.7.1.3-2: en-gNB Initiated EN-DC X2 Setup, unsuccessful operation

If the candidate receving node cannot accept the setup it shall respond with an EN-DC X2 SETUP FAILURE message with appropriate cause value.

If the Message Oversize Notification IE is included in the EN-DC X2 SETUP FAILURE, the initiating node shall, if supported, deduce that the failure is due to a too large EN-DC X2 SETUP REQUEST message and ensure that the total number of served cells in following EN-DC X2 SETUP REQUEST message is equal to or lower than the value of the Message Oversize Notification IE.

If case of network sharing with multiple cell ID broadcast with shared X2-C signalling transport, as specified in TS 36.300 [15], the EN-DC X2 SETUP REQUEST message and the EN-DC X2 SETUP FAILURE message shall include the Interface Instance Indication IE to identify the corresponding interface instance.

8.7.1.4 Abnormal Conditions

If the first message received for a specific TNL association is not an EN-DC X2 SETUP REQUEST, EN-DC X2 SETUP RESPONSE, or EN-DC X2 SETUP FAILURE message then this shall be treated as a logical error.

If the initiating node does not receive either EN-DC X2 SETUP RESPONSE message or EN-DC X2 SETUP FAILURE message, the initiating node may reinitiate the EN-DC X2 Setup procedure towards the same candidate node, provided that the content of the EN-DC X2 SETUP REQUEST message is identical to the content of the previously unacknowledged EN-DC X2 SETUP REQUEST message.

If the EN-DC X2 SETUP FAILURE message includes the Time To Wait IE the initiating node shall wait at least for the indicated time before reinitiating the EN-DC X2 Setup procedure towards the same peer node.

If the initiating node receives an EN-DC X2 SETUP REQUEST message from the peer entity on the same X2 interface:

– In case the initiating node answers with an EN-DC X2 SETUP RESPONSE message and receives a subsequent EN-DC X2 SETUP FAILURE message, the initiating node shall consider the X2 interface as non operational and the procedure as unsuccessfully terminated according to sub clause 8.7.1.3.

– In case the initiating node answers with an EN-DC X2 SETUP FAILURE message and receives a subsequent EN-DC X2 SETUP RESPONSE message, the initiating node shall ignore the EN-DC X2 SETUP RESPONSE message and consider the X2 interface as non operational.