5.8 GAN Handover

3GPP43.129Packet-switched handover for GERAN A/Gb modeRelease 17Stage 2TS

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

5.8.1 Intra-SGSN Handover (GERAN A/Gb mode  GAN mode handover)

5.8.1.1 Intra SGSN PS Handover, Preparation phase

Figure 5.8-1: PS Handover Preparation Phase; Intra-SGSN (GERAN A/Gb mode  GAN mode)

1. While in a GERAN cell with one or more active packet sessions an MS uses neighbour cell list information for measurement and reporting purposes and eventually detects the presence of GAN mode service.

2. The MS performs a successful GAN registration (see 3GPP TS 43.318) and receives the ARFCN/BSIC corresponding to the GANC on which it has registered. The MS may then at some point send a Packet Cell Change Notification message that includes the ARFCN/BSIC received during GAN registration along with an RXLEV of 63 (either immediately if GAN mode is preferred or when the GERAN cell becomes sufficiently degraded if GERAN/UTRAN mode is preferred).

3. The source BSS decides to initiate a PS handover. At this point both uplink and downlink user data is transmitted via the following: TBFs between MS and source BSS, BSSGP PFCs tunnel(s) between the source BSS and SGSN, GTP tunnel(s) between the SGSN and GGSN.

4. The source BSS sends a PS Handover Required (Old TLLI, Cause, Source Cell Identifier, Target Cell Identifier, Source BSS to Target BSS Transparent Container (RN part)) message to the SGSN.

5. The SGSN determines from the Target Cell Identifier whether or not the routing area has changed and that an Intra-SGSN PS handover is required. In case of no change of routing area, the SGSN sends a PS Handover Request (TLLI, Cause, IMSI, Source Cell Identifier, Target Cell Identifier, PFCs To Be Set Up List, Source BSS to Target BSS Transparent Container (RN part)) message to the target GANC. In case the routing area changes, the SGSN shall assign a new P-TMSI for the MS and derive a local TLLI prior to the sending of the PS Handover Request message. All PFCs shall be included in this message regardless of whether or not they are active.

NOTE 1: The BSS PFCs required to be set up are downloaded to the target GANC from the SGSN, i.e. all information required for PFC creation.

6. Based upon the ABQP for each PFC the target GANC makes a decision about whether or not to activate a GA-PSR Transport Channel. If a GA-PSR Transport Channel is allocated it is used to support the packet data flow requirements of all requested PFCs. The details of this decision are implementation specific but could take into account the current GAN cell loading and the ABQP indicated for each requested PFC. The target GANC allocates a GA-PSR Transport Channel as described in 3GPP TS 43.318. After allocating the GA-PSR Transport Channel the target GANC shall prepare the Target BSS to Source BSS Transparent Container for the set up BSS PFCs.

7. The target GANC sends the PS Handover Request Acknowledge (TLLI, List of Set Up PFCs, Target BSS to Source BSS Transparent Container (RN part)) message to the SGSN. Upon sending the PS Handover Request Acknowledge message the target GANC shall be prepared to receive downlink LLC PDUs from the SGSN for all requested PFCs (i.e. the List of Set Up PFCs in this message shall always match the PFCs To Be Set Up List included in the corresponding PS Handover Request message since a GA-PSR Transport Channel is allocated with the intent of supporting all PFCs requested by the SGSN).

When the SGSN receives the PS Handover Request Acknowledge message and it decides to proceed with the handover, the preparation phase is finished and the execution phase will follow.

5.8.1.2 Intra SGSN PS Handover, Execution phase

The execution phase is the same as described for Intra SGSN, GERAN A/Gb mode to GERAN A/Gb mode PS handover except for the following:

– The BSS sends the PS Handover Command message where the RN part does not include any of the optional information.

– After receiving the PS Handover Command message in the source cell the MS determines that the indicated ARFCN and BSIC match those of the GAN cell that it last registered on and therefore does not perform system synchronization in the GAN cell. Instead, the MS enables the transmission of uplink LLC PDUs on the allocated GA-PSR Transport Channel and sends a GA-PSR HANDOVER COMPLETE message to the target GANC. Note that the reception of downlink LLC PDUs is enabled in the MS upon allocation of the GA-PSR Transport Channel during the preparation phase.

– Upon reception of the GA-PSR HANDOVER COMPLETE message from the MS in the GAN cell the target GANC sends a PS Handover Complete (TLLI, IMSI, Target Cell Identifier) message to the SGSN.

5.8.2 Intra-SGSN Handover (GAN mode  GERAN A/Gb mode handover)

5.8.2.1 Intra SGSN PS Handover, Preparation phase

The preparation phase is the same as described for Intra SGSN, GERAN A/Gb mode to GERAN A/Gb mode PS handover except for the following:

– While in a GAN cell an MS sends the GA-PSR HANDOVER INFORMATION message to the GANC indicating a list of target GERAN A/Gb mode cells, identified by CGI, in order of preference for PS handover, and includes the received signal strength for each identified GERAN A/Gb mode cell. The MS decision to send the GA-PSR HANDOVER INFORMATION may be based on local measurements in the MS (see 3GPP TS 44.318 [28]). This list is the most recent information available from the GSM RR subsystem. In addition, the GA-PSR HANDOVER INFORMATION message may include a list of target UTRAN cells ranked in order of preference for PS handover, and the received signal strength for each identified UTRAN cell.

– The source GANC selects a target GERAN A/Gb mode cell and triggers the PS handover preparation phase by signalling to the SGSN the need for PS handover (i.e. it sends the PS Handover Required message) and including the target GERAN A/Gb mode cell.

– When the source GANC decides to trigger a PS handover both uplink and downlink user data is transmitted via the GA-PSR Transport Channel between MS and source GANC, BSSGP PFCs tunnel(s) between the source GANC and SGSN, and GTP tunnel(s) between the SGSN and GGSN.

5.8.2.2 Intra SGSN PS Handover, Execution phase

The execution phase is the same as described for Intra SGSN, GERAN A/Gb mode to GERAN A/Gb mode PS handover except for the following:

– The SGSN sends a PS Handover Required Acknowledge (Old TLLI, List of Set Up PFCs, Target BSS to Source BSS Transparent Container (RN part)) message to the source GANC.

– Before sending the GA-PSR Handover Command message to the MS the source GANC, based on QoS, may try to empty the downlink GANC buffer for one or more PFCs.

– The source GANC interrupts the downlink transmission of LLC PDUs on the GA-PSR Transport Channel and sends the MS a GA-PSR Handover Command message that includes a PS Handover Command (RN part only) created by the target BSS.

5.8.3 Inter-SGSN Handover (GERAN A/Gb mode  GAN mode handover)

5.8.3.1 Inter SGSN PS Handover, Preparation phase

The preparation phase is the same as described for Intra SGSN, GERAN A/Gb mode to GAN mode PS handover except that Inter-SGSN communications are required and are as described for Inter-SGSN A/Gb mode to A/Gb mode PS handover.

5.8.3.2 Inter SGSN PS Handover, Execution phase

The execution phase is the same as described for the Intra SGSN PS Handover, Execution phase in sub-clause 5.8.1.2.

5.8.4 Inter-SGSN Handover (GAN mode  GERAN A/Gb mode handover)

5.8.4.1 Inter SGSN PS Handover, Preparation phase

The preparation phase is the same as described for Inter SGSN, GERAN A/Gb mode to GERAN A/Gb mode PS handover except for the following:

– While in a GAN cell an MS sends the GA-PSR HANDOVER INFORMATION message to the GANC indicating a list of target GERAN A/Gb mode cells, identified by CGI, in order of preference for PS handover, and includes the received signal strength for each identified GERAN A/Gb mode cell. The MS decision to send the GA-PSR HANDOVER INFORMATION message may be based on local measurements in the MS (see 3GPP TS 44.318 [28]). This list is the most recent information available from the GSM RR subsystem. In addition, the GA-PSR HANDOVER INFORMATION message may include a list of target UTRAN cells ranked in order of preference for PS handover, and the received signal strength for each identified UTRAN cell.

5.8.4.2 Inter SGSN, Execution phase

The execution phase is the same as described for the Intra SGSN PS Handover, Execution phase in sub-clause 5.8.2.2.

5.8.5 Inter RAT Handover; Intra SGSN (UTRAN  GAN mode handover)

5.8.5.1 Intra SGSN PS Handover, Preparation phase

Figure 5.8-2: PS Handover Preparation Phase; Intra-SGSN (UTRAN  GAN mode)

1. While in a UTRAN cell with one or more active packet sessions an MS uses neighbour cell list information for measurement and reporting purposes and eventually detects the presence of GAN mode service.

2. The MS performs a successful GAN registration (see 3GPP TS 43.318) and receives the ARFCN/BSIC corresponding to the GANC on which it has registered. The MS may at some point begin sending measurement reports that include the ARFCN/BSIC receieved during GAN registration along with an RXLEV of 63 (either immediately if GAN mode is preferred or when the UTRAN cell becomes sufficiently degraded if GERAN/UTRAN mode is preferred) for the GAN cell and report an RXLEV of 63.

3. The source RNC/BSS decides to initiate a PS handover.

4. The source RNC/BSS sends a Relocation Required (Relocation Type, Cause, Source ID, Target ID, Source BSS To Target BSS Transparent Container (RN part)) message to the 3G/2G SGSN. The source RNC/BSS shall set Relocation Type to "UE Involved in relocation of SRNS". Target ID contains the identity of the target cell.

5. The SGSN determines from the Target ID whether or not the routing area has changed and that an Intra-SGSN PS handover is required. In case of no change of routing area, the SGSN sends a PS Handover Request (TLLI, Cause, IMSI, Source Cell Identifier, Target ID, PFCs To Be Set Up List, Source BSS to Target BSS Transparent Container (RN part)) message to the target GANC. In case the routing area changes, the SGSN shall assign a new P-TMSI for the MS and derive a local TLLI prior to the sending of the PS Handover Request message. All PFCs shall be included in this message regardless of whether or not they are active.

NOTE 1: The BSS PFCs required to be set up are downloaded to the target GANC from the SGSN, i.e. all information required for PFC creation.

When the SGSN receives the PS Handover Request Acknowledge message and it decides to proceed with the handover, the preparation phase is finished and the execution phase will follow.

5.8.5.2 Intra SGSN PS Handover, Execution phase

The execution phase is the same as described for Intra SGSN, UTRAN/GERAN Iu mode to GERAN A/Gb mode PS handover except for the following:

– The RNC sends the Handover from UTRAN Command message to the MS where each message includes a PS Handover Command message where the RN part does not include any of the optional information.

– After receiving the Handover from UTRAN Command message in the source cell the MS determines that the indicated ARFCN and BSIC match those of the GAN cell that it last registered on and therefore does not perform system synchronization in the GAN cell. Instead, the MS enables the transmission of uplink LLC PDUs on the allocated GA-PSR Transport Channel and sends a GA-PSR HANDOVER COMPLETE message to the target GANC. Note that the reception of downlink LLC PDUs is enabled in the MS upon allocation of the GA-PSR Transport Channel during the preparation phase.

– Upon reception of the GA-PSR HANDOVER COMPLETE message from the MS in the GAN cell the target GANC sends a PS Handover Complete (TLLI, IMSI, Target Cell Identifier) message to the SGSN.

5.8.6 Inter RAT Handover; Intra SGSN (GAN mode UTRAN handover)

5.8.6.1 Intra SGSN PS Handover, Preparation phase

The preparation phase is the same as described for Intra SGSN, GERAN A/Gb mode to UTRAN/GERAN Iu mode PS handover except for the following:

– While in a GAN cell an MS sends the GA-PSR HANDOVER INFORMATION message to the GANC indicating a list of target UTRAN cells, identified by a combination of LAI and 3G Cell Identity/CGI, in order of preference for PS handover, and includes the received signal strength for each identified UTRAN cell. The MS decision to send the GA-PSR HANDOVER INFORMATION may be based on local measurements in the MS (see 3GPP TS 44.318 [28]). This list is the most recent information available from the UTRAN RRC/GSM RR subsystem. In addition, the GA-PSR HANDOVER INFORMATION message may include a list of target GERAN A/Gb mode cells ranked in order of preference for PS handover, and the received signal strength for each identified GERAN A/Gb mode cell.

– The source GANC selects a target UTRAN cell and triggers the PS handover preparation phase by signalling to the SGSN the need for PS handover (i.e. it sends the PS Handover Required message) and including the target UTRANcell.

5.8.6.2 Intra SGSN, Execution phase

The execution phase is the same as described for Intra SGSN, GERAN A/Gb mode to UTRAN/GERAN Iu mode PS handover except for the following:

– The SGSN sends a PS Handover Required Acknowledge (Old TLLI, List of Set Up PFCs, Target BSS to Source BSS Transparent Container (RN part)) message to the source GANC.

– Before sending the GA-PSR Handover Command message to the MS the source GANC, based on QoS, may try to empty the downlink GANC buffer for one or more PFCs.

– The source GANC interrupts the downlink transmission of LLC PDUs on the GA-PSR Transport Channel and sends the MS a GA-PSR Handover Command message that includes a Handover to UTRAN Command message (as specified in 3GPP TS 25.331 [17]) created by the target RNC.

5.8.7 Inter RAT Handover; Inter SGSN (UTRAN  GAN mode handover)

5.8.7.1 Inter SGSN PS Handover, Preparation phase

The preparation phase is the same as described for Intra SGSN, UTRAN to GAN mode PS handover except that Inter-SGSN communications are required and are as described for Inter-SGSN UTRAN to A/Gb mode PS handover.

5.8.7.2 Inter SGSN PS Handover, Execution phase

The execution phase is the same as described for the Intra SGSN PS Handover, Execution phase in sub-clause 5.8.5.2.

5.8.8 Inter RAT Handover; Inter SGSN (GAN mode  UTRAN handover)

5.8.8.1 Inter SGSN PS Handover, Preparation phase

The preparation phase is the same as described for Inter SGSN, GERAN A/Gb mode to UTRAN/GERAN Iu mode PS handover except for the following:

– While in a GAN cell an MS sends the GA-PSR HANDOVER INFORMATION message to the GANC indicating a list of target UTRAN cells, identified by a combination of LAI and 3G Cell Identity/CGI, in order of preference for PS handover, and includes the received signal strength for each identified UTRAN cell. The MS decision to send the GA-PSR HANDOVER INFORMATION message may be based on local measurements in the MS (see 3GPP TS 44.318 [28]). This list is the most recent information available from the UTRAN RRC/GSM RR subsystem. In addition, the GA-PSR HANDOVER INFORMATION message may include a list of target GERAN A/Gb mode cells ranked in order of preference for PS handover, and the received signal strength for each identified GERAN A/Gb mode cell.

5.8.8.2 Inter SGSN PS Handover, Execution phase

The execution phase is the same as described for the Intra SGSN, Execution phase in sub-clause 5.8.6.2.