8.14 CS Handover between GAN A/Gb mode and GERAN/UTRAN mode

3GPP43.318Generic Access Network (GAN)Release 17Stage 2TS

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

8.14.1 CS Handover to GAN A/Gb mode

8.14.1.1 GERAN to GAN CS Handover

Figure 26: GERAN to GAN CS Handover

The description of the GERAN to GAN CS 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 MS implementation specific threshold; and

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

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

1. The MS begins to include GAN cell information in the Measurement Report message to the GERAN. The MS reports the highest signal level for the GAN cell {ARFCN, BSIC}. 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, GERAN decides to handover to the GAN cell, using an internal mapping of {ARFCN, BSIC} to CGI. The GERAN starts the handover preparation by sending a Handover Required message to the CN, identifying the target (GAN) cell.

3. The CN requests the target GANC to allocate resources for the handover, using Handover Request message.

4. The target GANC acknowledges the Handover Request message, using Handover Request Acknowledge message, indicating it can support the requested handover, and provides a Handover Command message that indicates the radio channel to which the mobile station should be directed.

5. The CN forwards the Handover Command message to the GERAN, completing the handover preparation.

6. GERAN sends Handover Command message to the MS to initiate handover to GAN. The Handover Command message includes among other parameters information about the target GAN such as BCCH ARFCN, PLMN colour code and BSIC. The MS does not switch its audio path from GERAN to GAN until handover completion, i.e. until it sends the GA-CSR HANDOVER COMPLETE message, to keep the audio interruption short.

7. The MS accesses the GANC using the GA-CSR HANDOVER ACCESS message, and provides the entire Handover Command message received from GERAN. Information in the Handover Command message (e.g. Handover Reference) allows the GANC to correlate the handover to the Handover Request Acknowledge message earlier sent to the CN and identify the successful completion of the handover.

8. The GANC sets up the bearer path with the MS, using the same steps as in steps 9 to 14 of Mobile Originated Call Flow as defined in sub-clause 8.10.

9. The MS transmits the GA-CSR HANDOVER COMPLETE message to indicate the completion of the handover procedure at its end. It switches the user from the GERAN user plane to the GAN user plane.

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

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

12. The target GANC indicates the handover is complete, using the Handover Complete message. If it had not done so before, the CN now switches the user plane from source GERAN to target GAN. The CN may use the target CGI used in the Handover procedure for charging purposes.

13. Finally, the CN tears down the connection to the source GERAN, using Clear Command message.

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

8.14.1.2 UTRAN to GAN CS Handover

Figure 26b: UTRAN to GAN CS Handover

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 included the Support of Handover to GAN indication in the UE RAC; and

– the UE has been ordered by the RNC to make inter-RAT measurements, and

– if the MS is in GAN preferred mode with an Event 3A configured, the MS handles parameters associated with the Event 3A in a GAN specific manner (as described in 3GPP TS 44.318) for the reporting of the GAN cell {ARFCN, BSIC}; and

– when the UE is in GERAN/UTRAN/E-UTRAN-preferred mode and an event 3A has been configured for the GAN cell (and possibly for GERAN cells), the UE shall only send a measurement about the GAN cell, when this event is triggered and no GERAN cells from the neighbour cell list of the UE satisfy the triggering condition of this Event (as described in 3GPP TS 25.331);

– the UTRAN provides information on neighbouring cells such that one of the {ARFCN, BSIC} in the neighbour list matches the {ARFCN, BSIC} associated with the GANC, as provided in the AS-related component of the system information obtained from GANC. The selection of the RF channel numbers (ARFCNs) used for the UTRAN to GAN handover procedure should not correspond to a channel from a frequency band supported by any UE, to avoid UEs that do not require compressed mode from unnecessarily powering up their GSM receivers.

1. The MS begins to include information about a GAN cell in the Measurement Report message sent to the RNC. The MS reports the highest signal level for the GAN cell {ARFCN, BSIC}. This is not the actual measured signal level on GAN, rather an artificial value (i.e., GSM carrier RSSI = 63) allowing the UE to indicate preference for the GAN.

2. Based on MS measurement reports and other internal algorithms, the RNC decides to initiate handover to the GAN cell, using an internal mapping of {ARFCN, BSIC} to CGI. The RNC starts the preparation phase of the Relocation procedure by sending a Relocation Required message to the CN, identifying the target (GAN) cell.

3. The CN requests the target GANC to allocate resources for the handover, using Handover Request message.

4. The target GANC acknowledges the handover request message, using Handover Request Acknowledge message, indicating it can support the requested handover, and including a Handover Command message that indicates the radio channel to which the mobile station should be directed.

5. The CN sends the Relocation Command message to the RNC, completing the relocation preparation.

6. The RNC sends HANDOVER FROM UTRAN COMMAND message to the MS to initiate handover to GAN. The HANDOVER FROM UTRAN COMMAND message includes the RR HANDOVER COMMAND message, specified in 44.018, which contains other parameters about the target GAN such as BCCH ARFCN, PLMN colour code and BSIC. The MS does not switch its audio path from UTRAN to GAN until handover completion, i.e. until it sends the GA-CSR HANDOVER COMPLETE message, to keep the audio interruption short.

7. The MS accesses the GANC using the GA-CSR HANDOVER ACCESS message, and provides the entire HANDOVER FROM UTRAN COMMAND received from RNC. The handover reference in the HANDOVER FROM UTRAN COMMAND message allows the GANC to correlate the handover to the Handover Request Acknowledge message earlier sent to the CN and identify the successful completion of the handover.

8. The GANC sets up the bearer path with the MS, using the same steps as in steps 9 to 14 of Mobile Originated Call Flow as defined in sub-clause 8.10.

9. The MS transmits the GA-CSR HANDOVER COMPLETE to indicate the completion of the handover procedure from its perspective. It switches the user from the UTRAN user plane to the GAN user plane.

10. The GANC indicates to the CN that it has detected the MS, using Handover Detect message. The CN can optionally now switch the user plane from the source RNC to the target GANC.

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

12. The target GANC indicates the handover is complete, using the Handover Complete message. If it has not done so before, the CN now switches the user plane from source RNC to target GAN.

13. Finally, the CN tears down the connection to the source RNC, using Iu Release Command.

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

8.14.2 CS Handover from GAN A/Gb mode to GERAN

Figure 27: CS Handover from GAN A/Gb mode to GERAN

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

– the MS is on an active call on the GAN A/Gb 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.

The handover from GAN to GERAN procedure is always triggered by the MS.

1. The GANC may send a GA-CSR 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-CSR HANDOVER 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-CSR HANDOVER 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 CN the need for handover, using Handover Required, and including the GERAN cell list provided by the MS. The GANC may include only a subset of the cell list provided by the MS.

4. The CN selects a target GERAN cell and requests it 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 CN through the Handover Request Acknowledge message.

6. The CN signals the GANC to handover the MS to the GERAN, using Handover Command message, ending the handover preparation phase.

7. GANC transmits the GA-CSR HANDOVER COMMAND to the MS including the Handover from GAN Command IE. The Handover from GAN Command IE 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 CN in response to the Handover Required.

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

10. The CN 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 CN the completion of the handover, via Handover Complete message. The CN 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 CN, via the GERAN.

15. On receiving the confirmation of the completion of the handover, the CN indicates to the GANC to release any resources allocated to the MS, via the Clear 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-CSR RELEASE message.

17. GANC confirms resource release to CN using the Clear Complete message.

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

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

8.14.3 CS Handover from GAN A/Gb mode to UTRAN

Figure 28: CS Handover from GAN A/Gb mode to UTRAN

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

– the MS is on an active call on the GAN A/Gb mode; and

– the MS is capable of operating in all of the GAN A/Gb, 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.

The Inter-RAT handover from GAN procedure is always triggered by the MS.

2. The MS sends the GA-CSR HANDOVER INFORMATION message to the Serving GANC including a list of candidate target UTRAN and GERAN cells, in order of preference for handover, and includes 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 a target RAT, the inter-RAT handover procedure is performed. The Serving GANC starts the handover preparation by signaling to the CN the need for handover, using Handover Required and including the target RNC-ID that the GANC derives from the target UTRAN cell it selects from the list of candidate target UTRAN cells provided by the MS. The GANC may include only a single RNC-ID in the Handover Required message. The GANC also includes the Source RNC to Target RNC Transparent Container that contains the selected target UTRAN cell ID.

4. The CN starts the inter-RAT handover procedure towards the target RNC identified by the Serving GANC. The CN requests from the target RNS to allocate the necessary resources using Relocation Request.

5. The target UTRAN builds a UTRAN RRC Handover to UTRAN Command message providing information on the allocated UTRAN resources and sends it to the CN through the Relocation Request Acknowledge message.

6. The CN signals the Serving GANC to handover the MS to the UTRAN, using Handover Command message (which includes the UTRAN RRC Handover to UTRAN Command message), ending the handover preparation phase.

7. The Serving GANC transmits the GA-CSR HANDOVER COMMAND to the MS including the Handover from GAN Command IE. The Handover from GAN Command IE includes the GERAN RRC Inter System to UTRAN Handover Command message, which encapsulates the UTRAN RRC Handover to UTRAN Command message.

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

9. The target UTRAN confirms the detection of the handover to the CN, using the Relocation Detect message.

10. The CN 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 Handover to UTRAN Complete.

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

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

14. On receiving the confirmation of the completion of the handover, the CN indicates to the Serving GANC to release any resources allocated to the MS, via the Clear 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-CSR RELEASE message.

16. The Serving GANC confirms resource release to CN using the Clear Complete message.

17. The MS confirms resource release to the Serving GANC using the GA-CSR 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 A/Gb mode to UTRAN mode (i.e., after step 11).