9.14 CS Handover between GAN Iu mode and GERAN/UTRAN mode

3GPP43.318Generic Access Network (GAN)Release 16Stage 2TS

9.14.1 CS Handover from GERAN to GAN

The description of the GERAN to GAN Handover procedure assumes the following:

– the MS is on an active call on the GERAN; and

– its mode selection is GAN-preferred, or if GERAN/UTRAN/E-UTRAN-preferred, the RxLev from the current serving cell drops below a defined threshold; and

– the MS has successfully registered with a GANC, allowing the MS to obtain GAN system information; and

– the GANC has directed the MS to operate in GAN Iu mode; and

– the GERAN provides information on neighbouring 3G cells such that one of the cells in the 3G neighbour list matches the 3G cell information associated with the GANC, as provided in the AS-related component of the system information obtained from the GANC.

9.14.1.1 Normal case: IMSI is present in Relocation Request message

Figure 55: CS Handover from GERAN to GAN (IMSI in Relocation Request)

1. The MS has successfully registered with the GANC and begins to include GAN Iu mode cell information in the Measurement Report message to the GERAN. The MS reports the highest signal level for the GAN Iu mode cell. This is not the actual measured signal level on GAN, rather an artificial value (i.e., RxLev = 63), allowing the MS to indicate preference for the GAN.

2. Based on MS measurement reports and other internal algorithms, the GERAN BSC decides to handover to the GAN Iu mode cell. The BSC starts the handover preparation by sending a Handover Required message to the MSC, identifying the target 3G RNC (GANC).

3. The MSC requests the target GANC to allocate resources for the handover using the Relocation Request message. The MS is identified by the included IMSI parameter.

4. The Iu bearer is established per standard Iu procedures. In the case of the ATM-based Iu-cs interface, this may include the exchange of ALCAP signalling between the GANC and the MSC to setup the ATM virtual circuit. For both ATM and IP-based Iu-cs interface types, Iu bearer establishment may also include the Iu UP initialization exchange, if Iu UP support mode is required as indicated by the MSC in the RANAP Relocation Request message.

5. The GANC sends the GA-RRC RELOCATION REQUEST message to the MS including bearer path setup information such as:

– RAB ID (received from MSC in step 3)

– Multi-rate codec configuration.

– UDP port & the IP address for the uplink RTP stream.

– Voice sample size.

6. The MS sends the GA-RRC RELOCATION REQUEST ACK to the GANC indicating that the MS has successfully established resources for the call and including the UDP port for the downlink RTP stream.

7. The GANC builds a Handover to UTRAN Command message and sends it to the MSC through the Relocation Request Acknowledge message.

8. The MSC forwards the Handover to UTRAN Command message to the GERAN BSC in the BSSMAP Handover Command message, completing the handover preparation.

9. The GERAN BSC sends the Intersystem to UTRAN Handover Command message, containing the Handover to UTRAN Command message, to the MS to initiate handover to GAN Iu mode. The MS does not switch its audio path from GERAN to GAN until handover completion (i.e., until it sends the GA-RRC RELOCATION COMPLETE message) to keep the audio interruption short.

10. The MS transmits the GA-RRC RELOCATION COMPLETE message to indicate the completion of the handover procedure. It switches the user from the GERAN user plane to the GAN Iu mode user plane.

11. The GANC indicates to the MSC that it has detected the MS, using Relocation Detect message. The MSC can optionally now switch the user plane from the source GERAN to the target GAN.

12. Bi-directional voice traffic is now flowing between the MS and CN, via GANC.

13. The GANC indicates the handover is complete, using the Relocation Complete message. If it had not done so before, the MSC now switches the user plane from source GERAN to target GAN.

14. Finally, the MSC tears down the connection to the source GERAN, using Clear Command message.

15. The source GERAN confirms the release of GERAN resources allocated for this call, using Clear Complete message.

9.14.1.2 Exception Case: No IMSI in Relocation Request

Figure 55a: CS Handover from GERAN to GAN (No IMSI in Relocation Request)

1-2. Same as steps 1-2 in clause 9.14.1.1.

3. The MSC requests the target GANC to allocate resources for the handover using the Relocation Request message. The IMSI parameter is not included in the message since it is not available at the MSC.

4. Same as step 4 in clause 9.14.1.1.

5-7. Same as steps 8-10 in clause 9.14.1.1.

8. The MS accesses the GANC using the GA-RRC RELOCATION ACCESS message, and provides the entire Intersystem to UTRAN Handover Command message received from GERAN. Information in the Intersystem to UTRAN Handover Command message (e.g. Handover Reference) allows the GANC to correlate the handover to the Relocation Request Acknowledge message sent to the MSC in step 5.

9-11. The GANC sets up the bearer path with the MS, using the procedure as in steps 10 to 12 in clause 9.10.

12. The MS transmits the GA-RRC RELOCATION COMPLETE message to indicate the completion of the handover procedure. It switches the user from the GERAN user plane to the GAN Iu mode user plane.

13. The GANC indicates to the MSC that it has detected the MS, using the Relocation Detect message. The MSC can optionally now switch the user plane from the source GERAN to the target GAN.

14. Bi-directional voice traffic is now flowing between the MS and MSC, via the GANC.

15. The target GANC indicates the handover is complete, using the Relocation Complete message. If it had not done so before, the MSC now switches the user plane from source GERAN to target GAN.

16. Finally, the MSC tears down the connection to the source GERAN, using Clear Command message.

17. The source GERAN confirms the release of GERAN resources allocated for this call, using Clear Complete message.

9.14.2 CS Handover from UTRAN to GAN

The description of the UTRAN to GAN CS handover procedure assumes the following:

– the MS is on an active call on the UTRAN; and

– the MS has successfully registered with a GANC, allowing the MS to obtain GAN system information; and

– the GANC has directed the MS to operate in GAN Iu mode; and

– the UTRAN provides information on neighbouring cells such that one of the cells in the neighbour list matches the cell associated with the GANC, as provided in the AS-related component of the system information obtained from GANC.

9.14.2.1 Normal Case: IMSI is present in Relocation Request message

Figure 56: CS Handover from UTRAN to GAN (IMSI in Relocation Request)

1. The MS begins to include information about a GAN Iu mode cell in the Measurement Report message sent to the RNC. See Annex B.2.1 for further information.

2. Based on MS measurement reports and other internal algorithms, the RNC decides to initiate handover to the GAN Iu mode cell. The RNC starts the preparation phase of the Relocation procedure by sending a Relocation Required message to the MSC, identifying the target (GAN Iu mode) cell.

3-6. Same as steps 3-6 for CS handover from GERAN to GAN in clause 9.14.1.1.

7. The GANC acknowledges the relocation request message, using Relocation Request Acknowledge message, indicating it can support the requested handover, and including a Physical Channel Reconfiguration message that indicates the radio channel to which the MS should be directed.

8. The MSC sends the Relocation Command message to the RNC, completing the relocation preparation.

9. The RNC sends the PHYSICAL CHANNEL RECONFIGURATION message to the MS to initiate handover to GAN Iu mode. The MS does not switch its audio path from UTRAN to GAN until handover completion (i.e., until it sends the GA-RRC HANDOVER COMPLETE message) to keep the audio interruption short.

10-13. Same as steps 10-13 for CS handover from GERAN to GAN in clause 9.14.1.1.

14. Finally, the MSC tears down the connection to the source RNC, using Iu Release Command.

15. The source RNC confirms the release of UTRAN resources allocated for this call, using Iu Release Complete.

9.14.2.2 Exception Case: No IMSI in Relocation Request

Figure 56a: CS Handover from UTRAN to GAN (No IMSI in Relocation Request)

1-2. Same as steps 1-2 in clause 9.14.2.1.

3. The MSC requests the target GANC to allocate resources for the handover using the Relocation Request message. The IMSI parameter is not included in the message since it is not available at the MSC.

4. Same as step 4 in clause 9.14.2.1.

5-7. Same as steps 7-9 in clause 9.14.2.1.

8. The MS accesses the GANC using the GA-RRC RELOCATION ACCESS message, and provides the entire Physical Channel Reconfiguration message received from UTRAN. Information in the Physical Channel Reconfiguration message allows the GANC to correlate the handover to the Relocation Request Acknowledge message sent to the MSC in step 5.

9-11. The GANC sets up the bearer path with the MS, using the procedure as in steps 10 to 12 in sub-clause 9.10.

12. The MS transmits the GA-RRC RELOCATION COMPLETE message to indicate the completion of the handover procedure at its end. It switches the user from the UTRAN user plane to the GAN user plane.

13. The GANC indicates to the MSC that it has detected the MS, using the Relocation Detect message. The MSC can optionally now switch the user plane from the source UTRAN to the target GAN.

14. Bi-directional voice traffic is now flowing between the MS and MSC, via the GANC.

15. The target GANC indicates the handover is complete, using the Relocation Complete message. If it had not done so before, the MSC now switches the user plane from source UTRAN to target GAN.

16. Finally, the MSC tears down the connection to the source RNC, using Iu Release Command.

17. The source RNC confirms the release of UTRAN resources allocated for this call, using Iu Release Complete.

9.14.3 CS Handover from GAN Iu mode to GERAN

The procedure description in this sub-clause assumes the following:

– the MS is on an active call in GAN Iu mode; and

– the GERAN becomes available; and

– the MS mode selection is GERAN/UTRAN/E-UTRAN-preferred; or

– the MS mode selection is GAN-preferred and the MS begins to leave GAN coverage, based on its local measurements, received RTCP reports, as well as any uplink quality indications received from the GANC.

Figure 57: CS Handover from GAN Iu mode to GERAN

1. The GANC may send a GA-RRC UPLINK QUALITY INDICATION if there is a problem with the uplink quality for the ongoing call. Uplink Quality Indication is information sent by the GANC to the MS indicating the crossing of a uplink quality threshold in the uplink direction. Whenever the MS receives an indication of bad quality, it should start the handover procedure, as described in the next step. Alternatively, MS can use its local measurements or received RTCP reports, to decide to initiate the handover procedure.

2. The MS sends the GA-RRC RELOCATION INFORMATION message to the GANC including a list of target GERAN cells, identified by CGI, in order of preference (e.g. ranked by C1 path loss parameter) for handover, and the received signal strength for each identified GERAN cell. This list is the most recent information available from the GSM RR subsystem. In addition, the GA-RRC RELOCATION INFORMATION message may include a list of target UTRAN cells ranked in order of preference for handover, and the received signal strength for each identified UTRAN cell.

3. If the Serving GANC selects a target GERAN cell, the handover to GERAN procedure is performed. The Serving GANC starts the handover preparation by signalling to the MSC the need for handover, using Relocation Required, and including the selected GERAN cell from the list provided by the MS.

4. The MSC requests the target GERAN to allocate the necessary resources, using Handover Request.

5. The target GERAN builds a GERAN RRC Handover Command message providing information on the channel allocated and sends it to the MSC through the Handover Request Acknowledge message.

6. The MSC signals the GANC to handover the MS to the GERAN, using the Relocation Command message which includes the encapsulated GERAN RRC Handover Command message, ending the handover preparation phase.

7. GANC transmits the GA-RRC RELOCATION COMMAND to the MS including the UTRAN RRC Handover from UTRAN Command message, which encapsulates the GERAN RRC Handover Command message.

8. The MS transmits Handover Access containing the handover reference element to allow the target GERAN to correlate this handover access with the Handover Command message transmitted earlier to the MSC in response to the Handover Required.

9. The target GERAN confirms the detection of the handover to the MSC, using the Handover Detect message.

10. The MSC may at this point switch the user plane to the target BSS.

11. The GERAN provides Physical Information to the MS (i.e., Timing Advance) to allow the MS to synchronize with the GERAN.

12. The MS signals to the GERAN that the handover is completed, using Handover Complete.

13. The GERAN confirms to the MSC the completion of the handover, via Handover Complete message. The MSC may use the target CGI used in the Handover procedure for charging purposes.

14. Bi-directional voice traffic is now flowing between the MS and MSC, via the GERAN.

15. On receiving the confirmation of the completion of the handover, the MSC indicates to the GANC to release any resources allocated to the MS, via the Iu Release Command.

16. If the GANC has not already received the GA-RC DEREGISTER message from the MS, the GANC commands the MS to release resources, using the GA-RRC RELEASE message.

17. GANC confirms resource release to MSC using the Iu Release Complete message.

18. The MS confirms resource release to the GANC using the GA-RRC RELEASE COMPLETE message.

19. The MS may finally deregister from the GANC, using GA-RC DEREGISTER message. Note that the MS may send the GA-RC DEREGISTER message at any time after transitioning from GAN Iu mode to GERAN mode (i.e., after step 12).

9.14.4 CS Handover from GAN Iu mode to UTRAN

The procedure description in this sub-clause assumes the following:

– the MS is on an active call in GAN Iu mode; and

– the MS is capable of operating in all of the GAN, GERAN and UTRAN modes; and

– the UTRAN becomes available; and

– the MS is in GERAN/UTRAN/E-UTRAN-preferred mode; or

– the MS mode selection is GAN preferred and begins to leave GAN coverage, based on its local measurements, received RTCP reports, as well as any uplink quality indications received from the GANC.

Figure 58: CS Handover from GAN Iu mode to UTRAN

1. The GANC may send a GA-RRC UPLINK QUALITY INDICATION if there is a problem with the uplink quality for the ongoing call. Uplink Quality Indication is information sent by the GANC to the MS indicating the crossing of a uplink quality threshold in the uplink direction. Whenever the MS receives an indication of bad quality, it should start the handover procedure, as described in the next step. Alternatively, MS can use its local measurements or received RTCP reports, to decide to initiate the handover procedure.

2. The MS sends the GA-RRC RELOCATION INFORMATION message to the Serving GANC including a list of candidate target UTRAN and GERAN cells, in order of preference for handover, and the received signal strength for each identified cell. The UTRAN cells are identified by the PLMN ID, the LAC and the 3G Cell identity (defined in 3GPP TS 25.331 [40]).

NOTE: The choice of the candidate target UTRAN cells is out of the scope of this technical specification.

3. If the Serving GANC selects UTRAN as the target RAT, the CS handover to UTRAN procedure is performed. The Serving GANC starts the handover preparation by signalling to the MSC the need for handover, using Relocation Required and including the target RNC-ID that the GANC derives from the target UTRAN cell it selects from the list of target UTRAN cells provided by the MS. The GANC may include only a single target RNC-ID in the Relocation Required message. The GANC also includes the Source RNC to Target RNC Transparent Container that contains the selected target UTRAN cell ID.

4. The MSC starts the relocation procedure towards the target RNC identified by the Serving GANC. The MSC requests from the target RNC to allocate the necessary resources using Relocation Request.

5. The target RNC builds a Physical Channel Reconfiguration message providing information on the allocated UTRAN resources and sends it to the MSC through the Relocation Request Acknowledge message.

6. The MSC signals the Serving GANC to handover the MS to the UTRAN, using the Relocation Command message (which includes the Physical Channel Reconfiguration message), ending the handover preparation phase.

7. The Serving GANC transmits the GA-RRC RELOCATION COMMAND to the MS including the Physical Channel Reconfiguration message.

8. Target RNS achieves uplink synchronization on the Uu interface.

9. The target RNC confirms the detection of the handover to the MSC, using the Relocation Detect message.

10. The MSC may at this point switch the user plane to the target RNS.

11. The MS signals to the UTRAN that the handover is completed, using Physical Channel Reconfig Complete.

12. The UTRAN confirms to the MSC the completion of the handover, via Relocation Complete message. If the user plane has not been switched in step 10, the MSC switches the user plane to the target RNS.

13. Bi-directional voice traffic is now flowing between the MS and MSC, via the UTRAN.

14. On receiving the confirmation of the completion of the handover, the MSC indicates to the Serving GANC to release any resources allocated to the MS, via the Iu Release Command.

15. If the Serving GANC has not already received the GA-RC DEREGISTER message from the MS, the Serving GANC commands the MS to release resources, using the GA-RRC RELEASE message.

16. The Serving GANC confirms resource release to MSC using the Iu Release Complete message.

17. The MS confirms resource release to the Serving GANC using the GA-RRC RELEASE COMPLETE message.

18. The MS may finally deregister from the Serving GANC, using GA-RC DEREGISTER message. Note that the MS may send the GA-RC DEREGISTER message at any time after transitioning from GAN Iu mode to UTRAN mode (i.e., after step 11).