8.3.6 Inter-RAT handover to UTRAN

25.3313GPPProtocol specificationRadio Resource Control (RRC)Release 17TS

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

Figure 8.3.6-1: Inter-RAT handover to UTRAN, successful case

8.3.6.1 General

The purpose of the inter-RAT handover procedure is to, under the control of the network, transfer a connection between the UE and another radio access technology (e.g. GSM, E-UTRA or NR) to UTRAN.

This procedure may be used to perform CS handover which applies when there is a signalling connection to the CS domain.

When the UE supports Inter-RAT PS handover, this procedure may be used to perform PS handover which applies when there is only a signalling connection to the PS domain.

This procedure may be used to perform simultaneous CS and PS handover (i.e. DTM Handover) which applies when there are signalling connections to both CS and PS domains.

This procedure may be used to perform PS to CS handover (i.e. SR-VCC Handover) which applies when there is a signalling connection in E-UTRA or NR PS domain to be transferred to UTRA CS domain. For E-UTRA, SR-VCC handover may be performed with a simultaneous PS handover. For NR, SR-VCC handover can only be performed without a simultaneous PS handover.

This procedure may be used to perform CS to PS handover (i.e. rSR-VCC Handover) which applies when there is a signalling connection in GSM CS domain to be transferred to UTRA PS domain. The rSR-VCC Handover may be performed with a simultaneous PS handover.

8.3.6.2 Initiation

The procedure is initiated when a radio access technology other than UTRAN, e.g. GSM, E-UTRA or NR, using radio access technology-specific procedures, orders the UE to make a handover to UTRAN.

A Handover to UTRAN command message is sent to the UE via the radio access technology from which inter- RAT handover is performed.

In case UTRAN decides to use a predefined or default radio configuration that is stored in the UE, it should include the following information in the Handover to UTRAN command message.

– the IE "New U-RNTI" to be assigned;

– the IE "Predefined configuration identity", to indicate which pre-defined configuration of RB, transport channel and physical channel parameters shall be used; or

– the IE "Default configuration mode" and IE "Default configuration identity", to indicate which default configuration of RB, transport channel and physical channel parameters shall be used;

– PhyCH information elements.

NOTE 1: When using a predefined or default configuration during handover to UTRAN, UTRAN can only assign values of IEs "New U-RNTI" and "scrambling code" that are within the special subranges defined exclusively for this procedure. UTRAN may re- assign other values after completion of the handover procedure.

NOTE 2: When using a predefined or default configuration during handover to UTRAN, fewer IEs are signalled; when using this signalling option some parameters e.g. concerning compressed mode, DSCH can not be configured. In this case, the corresponding functionality can not be activated immediately.

NOTE 3: When using a predefined or default configuration, the HANDOVER TO UTRAN COMMAND should not include more than one radio link. If UTRAN includes more than one radio link in the HANDOVER TO UTRAN COMMAND using a predefined or default configuration, the UE behaviour is unspecified.

In case UTRAN does not use a predefined radio configuration that is stored in the UE, it should include the following information in the Handover to UTRAN command message.

– the IE "New U-RNTI" to be assigned;

– the complete set of RB, TrCH and PhyCH information elements to be used.

8.3.6.3 Reception of Handover to UTRAN command message by the UE

The UE shall be able to receive a HANDOVER TO UTRAN COMMAND message and perform an inter-RAT handover, even if no prior UE measurements have been performed on the target UTRAN cell and/or frequency.

The UE shall act upon all received information elements as specified in subclause 8.6, unless specified otherwise in the following.

The UE may:

1> maintain a list of the set of cells to which the UE has Radio Links if the IE "Cell ID" is present.

The UE shall:

1> store a U-RNTI value (32 bits), which is derived by the IEs "SRNC identity" and "S-RNTI 2" included in IE "U-RNTI-short". In order to produce a full size U-RNTI value, a full size "S-RNTI" (20 bits) shall be derived by padding the IE "S-RNTI 2" with 10 zero bits in the most significant positions; and

1> if the HANDOVER TO UTRAN COMMAND message is used to perform anything other than CS handover from GERAN and the message does not contain at least one instance of the IE "RAB Info";

2> the UE behaviour is unspecified.

1> initialise the variable ESTABLISHED_SIGNALLING_CONNECTIONS with the signalling connections that remains after the handover according to the specifications of the source RAT;

NOTE 0: The IE "CN domain identity" in IE "RAB Info" in the HANDOVER TO UTRAN COMMAND message determines towards which CN domain signalling connections are established. The HANDOVER TO UTRAN COMMAND message contains at least one IE "RAB Info" except in the CS domain handover from GERAN case where it may not be present, in which case the UE behaviour is the same as if "CN domain identity" had been received and set to "CS Domain".

1> initialise the variable UE_CAPABILITIES_TRANSFERRED with the UE capabilities that have been transferred to the network up to the point prior to the handover, if any;

1> initialise the variable TIMERS_AND_CONSTANTS to the default values and start to use those timer and constants values;

1> if IE "Specification mode" is set to "Preconfiguration" and IE "Preconfiguration mode" is set to "Predefined configuration":

2> initiate the radio bearer and transport channel configuration in accordance with the predefined parameters identified by the IE "Predefined configuration identity";

2> initiate the physical channels in accordance with the predefined parameters identified by the IE "Predefined radio configuration identity" and the received physical channel information elements;

2> store information about the established radio access bearers and radio bearers according to the IE "Predefined configuration identity"; and

2> set the IE "RAB Info Post" in the variable ESTABLISHED_RABS and the IE "Re-establishment timer" in the IE "RAB Info" in the variable ESTABLISHED_RABS to "useT314".

1> if IE "Specification mode" is set to "Preconfiguration" and IE "Preconfiguration mode" is set to "Default configuration":

2> initiate the radio bearer and transport channel configuration in accordance with the default parameters identified by the IE "Default configuration mode" and IE "Default configuration identity";

2> initiate the physical channels in accordance with the default parameters identified by the IE "Default configuration mode" and IE "Default configuration identity" and the received physical channel information elements;

NOTE 1: IE "Default configuration mode" specifies whether the FDD or TDD version of the default configuration shall be used.

2> set the IE "RAB Info Post" in the variable ESTABLISHED_RABS and the IE "Re-establishment timer" in the IE "RAB Info" in the variable ESTABLISHED_RABS to "useT314".

1> if IE "Specification mode" is set to "Preconfiguration":

2> use the following values for parameters that are neither signalled within the HANDOVER TO UTRAN COMMAND message nor included within pre-defined or default configuration:

3> 0 dB for the power offset P _Pilot-DPDCH bearer in FDD;

3> calculate the Default DPCH Offset Value using the following formula:

3> in FDD:

Default DPCH Offset Value = (SRNTI 2 mod 600) * 512

3> in TDD:

Default DPCH Offset Value = (SRNTI 2 mod 7)

3> handle the above Default DPCH Offset Value as if an IE with that value was included in the message, as specified in subclause 8.6.6.14.

1> if IE "Specification mode" is set to "Complete specification":

2> initiate the radio bearer, transport channel and physical channel configuration in accordance with the received radio bearer, transport channel and physical channel information elements.

1> if IE "Default configuration for CELL_FACH" is set:

2> act in accordance with the default parameters according to section 13.8.

1> perform an open loop estimation to determine the UL transmission power according to subclause 8.5.3;

1> set the IE "START" for each CN domain, in the IE "START list" in the HANDOVER TO UTRAN COMPLETE message equal to the START value for each CN domain as stored in the volatile memory of the ME for each CN domain if the USIM or the SIM is present;

NOTE 2: Keys received while in another RAT (GERAN) or generated when coming from another RAT (EUTRAN) are not regarded as "new" (i.e.do not trigger the actions in subclause 8.1.12.3.1) in a subsequent security mode control procedure in UTRAN, irrespective of whether the keys are already being used in the other RAT or not. If the UE has received new keys in the other RAT before handover, then the START values in the USIM (sent in the HANDOVER TO UTRAN COMPLETE message and in the INTER_RAT_HANDOVER_INFO sent while in the other RAT) will not reflect the receipt of these new keys
If the source RAT is not E-UTRAN and the procedure is not an rSR-VCC handover, and it is not a SR-VCC handover from NR: If ciphering has been active for the CS domain in the source RAT, then during the first security mode control procedure following the Inter-RAT handover to UTRAN procedure, UE activates integrity protection using the integrity key of the key set used. The term "key set used" denotes the key set that was used in the last successfully completed RRC Security Mode Control (UTRAN), RR Cipher Mode Control procedure or GMM Authentication and Ciphering procedure (GERAN) after entering connected mode in UTRAN or GERAN. The term "if ciphering has been active for the CS domain" is defined by a successfully completed RRC Security Mode Control procedure with "Ciphering Mode Info" (UTRAN) or RR Cipher Mode Control procedure (GERAN) for the CS domain. More specifically the state of ciphering refers to the establishment of a security context where the key is loaded from the (U)SIM to the ME even if the security command indicates NULL ciphering ("no ciphering" in GERAN or UEA0 in UTRAN).
If the source RAT is GERAN: Upon performing rSR-VCC(CS to PS), then during the first security mode control procedure following the Inter-RAT handover to UTRAN procedure, UE activates integrity protection using the integrity key of the key set as defined in [40].
If the source RAT is E-UTRAN: Upon performing SR-VCC (PS to CS) or PS handover, then during the first security mode control procedure following the Inter-RAT handover to UTRAN procedure, UE activates integrity protection using the integrity key of the key set used (see 8.1.12.2.2.). The term "key set used" denotes the CS (SR-VCC) or PS (PS handover) key set (KSI, CK, IK) which was mapped using the current SECURITY CONTEXT in EUTRAN as defined in [78] and [79].
If the source RAT is NR: Upon performing SR-VCC (PS to CS), then during the first security mode control procedure following the Inter-RAT handover to UTRAN procedure, UE activates integrity protection using the integrity key of the key set used (see 8.1.12.2.2.). The term "key set used" denotes the CS (SR-VCC) key set (KSI, CK, IK) which is defined in [92].

1> set the value of "THRESHOLD" in the variable "START_THRESHOLD" to the 20 MSBs of the value stored in the USIM [50] for the maximum value of START for each CN Domain, or to the default value in [40] if the SIM is present;

1> if ciphering is indicated in the HANDOVER TO UTRAN COMMAND by the presence of the IE "Ciphering Algorithm":

2> for the handover of CS domain, indicated by the IE "CN domain identity" of the IE "RAB info" of the IE "RAB information to setup" set to "CS domain" or absence of this IE, if ciphering was not active in the radio access technology from which inter-RAT handover is performed:

3> the UE behaviour is unspecified.

NOTE 2a: The handover of the CS domain from GERAN to UTRAN is particular because CS ciphering must be maintained across the inter-RAT handover procedure. For other inter-RAT handover procedures (SR-VCC, rSR-VCC and PS handover) ciphering in UTRAN is not dependent on previous ciphering status in the source radio access technology.

2> if the handover is from EUTRAN and there is no current SECURITY CONTEXT in EUTRAN:

3> the UE behaviour is unspecified.

NOTE 2b: The current EUTRAN security context is defined in [78] and [79]. For EUTRA to UTRA key mapping, this context consists of a minimum of the current KASME, the associated key set identifier (KSI) and the associated downlink NAS COUNT values.

2> set the variable LATEST_CONFIGURED_CN_DOMAIN to the value indicated in the IE "CN domain identity" of the IE "RAB info" of the IE "RAB information to setup" if all instances of the IE indicate the same CN domain, or to the CS domain when this IE is either not present or different instances indicate different CN domains;

2> for the CN domain in variable LATEST_CONFIGURED_CN_DOMAIN set the IE "Status" in the variable CIPHERING_STATUS to "Started";

2> if the variable LATEST_CONFIGURED_CN_DOMAIN is set to "CS domain":

3> set the 20 MSB of the HFN component of the COUNT-C variable for all signalling radio bearers to the "START" value from the IE "UE security information" in the variable INTER_RAT_HANDOVER_INFO_TRANSFERRED.

2> else if the variable LATEST_CONFIGURED_CN_DOMAIN is set to "PS domain":

3> set the 20 MSB of the HFN component of the COUNT-C variable for all signalling radio bearers to the "START" value from the IE "UE security information2" in the variable INTER_RAT_HANDOVER_INFO_TRANSFERRED.

2> if the CN domain indicated in the IE "CN domain identity" of any instance of the IE "RAB info" in the IE "RAB information to setup" is "CS domain":

3> set the 20 MSB of the HFN component of the COUNT-C variable for all CS domain radio bearers to the "START" value included in the IE "UE security information" in the variable INTER_RAT_HANDOVER_INFO_TRANSFERRED;.

3> set the IE "Status" in the variable CIPHERING_STATUS to "Started".

2> if the CN domain indicated in the IE "CN domain identity" of any instance of the IE "RAB info" in the IE "RAB information to setup" is "PS domain":

3> set the 20 MSB of the HFN component of the COUNT-C variable for all PS domain radio bearers to the "START" value included in the IE "UE security information2" in the variable INTER_RAT_HANDOVER_INFO_TRANSFERRED;

3> set the IE "Status" in the variable CIPHERING_STATUS to "Started".

2> set the remaining LSBs of the HFN component of COUNT-C for all radio bearers to zero;

2> not increment the HFN component of COUNT-C for radio bearers using RLC-TM, i.e. keep the HFN value fixed without incrementing every CFN cycle;

2> set the CFN component of the COUNT-C variable for radio bearers using RLC-TM to the value of the CFN as calculated in subclause 8.5.15;

2> for all user radio bearers belonging to the "CS domain" and for signalling radio bearers when the variable LATEST_CONFIGURED_CN_DOMAIN is set to "CS domain":

3> apply the algorithm according to IE "Ciphering Algorithm" with the ciphering key of the key set used and apply ciphering immediately upon reception of the HANDOVER TO UTRAN COMMAND.

2> for all user radio bearers belonging to the "PS domain" and for signalling radio bearers when the variable LATEST_CONFIGURED_CN_DOMAIN is set to "PS domain":

3> If the source RAT is E-UTRA; or

3> if the source RAT is GERAN and the procedure is an rSR-VCC handover:

4> apply the algorithm according to IE "Ciphering Algorithm" with the ciphering key of the PS key set used and apply ciphering immediately upon reception of the HANDOVER TO UTRAN COMMAND.

3> Otherwise:

4> apply the algorithm according to IE "Ciphering Algorithm" with the ciphering key stored in the USIM/SIM and apply ciphering immediately upon reception of the HANDOVER TO UTRAN COMMAND.

NOTE 3: If ciphering is indicated in the HANDOVER TO UTRAN COMMAND, UTRAN should not include the IE "Ciphering mode info" in the SECURITY MODE COMMAND message that starts Integrity protection. Upon performing simultaneous CS and PS handover (i.e. DTM Handover or SR-VCC with a simultaneous PS handover), UTRAN should start Integrity protection for the CS domain first then the PS domain. UTRAN should not include the IE "Ciphering mode info" in the SECURITY MODE COMMAND message that starts Integrity protection for the CS domain and UTRAN should include the IE "Ciphering mode info" in the SECURITY MODE COMMAND message that starts Integrity protection for the PS domain.

NOTE 4: DTM Handover is not possible if the IE "specification mode" is set to "Preconfiguration" due to the existence of a single IE "RAB Info".

1> if ciphering is not indicated in the HANDOVER TO UTRAN COMMAND:

2> for the CN domain(s) included in the IE "CN domain identity" which is included in the IE "RAB info" of the IE "RAB information to setup", or the CS domain when these IEs are not present:

3> set the IE "Status" in the variable CIPHERING_STATUS to "Not Started".

If the UE succeeds in establishing the connection to UTRAN, it shall:

1> if "RAB Info" list contained only one CN domain identity in IE "CN domain identity":

2> if CN domain is set to CS domain only indicate to upper layers that no CN system information is available for any domain other than the CS domain (see NOTE 5);

2> if CN domain is set to PS domain only indicate to upper layers that no CN system information is available for any domain other than the PS domain (see NOTE 6).

1> if "RAB Info" list contained RABs for the PS and CS domain:

2> indicate to upper layers that CN system information is available for the PS and CS domain (see NOTE 7).

NOTE 5: After CS handover or SR-VCC handover without a simultaneous PS handover has been completed, the UTRAN should provide the UE with the CN system information of the PS domain as soon as possible, in order not to delay access to the PS domain.

NOTE 6: After PS handover or rSR-VCC handover has been completed, the UTRAN should provide the UE with the CN system information of the CS and PS domain as soon as possible in order not to delay access to the CS domain and trigger a NAS procedure for the PS domain.

NOTE 7: After DTM handover or SR-VCC handover with a simultaneous PS handover has been completed, the UTRAN should provide the UE with the CN system information of the PS domain as soon as possible in order to trigger a NAS procedure for the PS domain.

1> if the IE "Status" in the variable CIPHERING_STATUS of a CN domain is set to "Started" and transparent mode radio bearers have been established by this procedure for that CN domain:

2> include the IE "COUNT-C activation time" in the response message and specify a CFN value for this IE other than the default, "Now", that is a multiple of 8 frames (CFN mod 8 =0) and lies at least 200 frames ahead of the CFN in which the response message is first transmitted;

2> at the CFN value as indicated in the response message in the IE "COUNT-C activation time" for radio bearers using RLC-TM:

3> set the 20 MSB of the HFN component of the COUNT-C variable common for all transparent mode radio bearers of this CN domain to the START value as indicated in the IE "START list" of the response message for the relevant CN domain; and

3> set the remaining LSBs of the HFN component of COUNT-C to zero;

3> increment the HFN component of the COUNT-C variable by one even if the "COUNT-C activation time" is equal to zero;

3> set the CFN component of the COUNT-C to the value of the IE "COUNT-C activation time" of the response message. The HFN component and the CFN component completely initialise the COUNT-C variable;

3> step the COUNT-C variable, as normal, at each CFN value. The HFN component is no longer fixed in value but incremented at each CFN cycle.

1> if the IE "Status" in the variable CIPHERING_STATUS of a CN domain is set to "Not Started" and transparent mode radio bearers have been established by this procedure for that CN domain:

2> initialise the 20 MSB of the HFN component of COUNT-C common for all transparent mode radio bearers of this CN domain with the START value as indicated in the IE "START list" of the response message for the relevant CN domain;

2> set the remaining LSBs of the HFN component of COUNT-C to zero;

2> do not increment the COUNT-C value common for all transparent mode radio bearers for this CN domain.

1> for 3.84/7.68 Mcps TDD, if the HANDOVER TO UTRAN COMMAND provides for E-DCH/HS-DSCH operation without an uplink DPCH:

2> the UE shall obtain timing advance for the 3.84/7.68 Mcps TDD cell according to [15] before transmitting on the uplink DCCH.

1> if an IE "Logged Measurement Info-FDD" or "Logged Measurement Info-TDD" in variable LOGGED_MEAS_REPORT_VARIABLE is present and registered PLMN is present in the IE "PLMN Identity List" stored in variable LOGGED_MEAS_REPORT_VARIABLE:

2> include IE "Logged Meas Available".

1> if an IE "Logged Connection Establishment Failure Info-FDD" or "Logged Connection Establishment Failure Info-TDD" in variable LOGGED_CONNECTION_ESTABLISHMENT_FAILURE is present and the current Registered PLMN is the same as the PLMN in IE "PLMN Identity" stored in variable LOGGED_CONNECTION_ESTABLISHMENT_FAILURE:

2> include IE "Connection Establishment Failure Info Available".

1> transmit a HANDOVER TO UTRAN COMPLETE message on the uplink DCCH, using, if ciphering has been started, the new ciphering configuration;

1> when the HANDOVER TO UTRAN COMPLETE message has been submitted to lower layers for transmission:

2> if the source RAT was E-UTRA:

3> store the CS "key set used" including the mapped KSI to the USIM CS security key if SR-VCC was performed;

3> store the PS "key set used" including the mapped KSI to the USIM PS security key if PS handover was performed;

2> if the source RAT was GERAN:

3> store the PS "key set used" including the mapped KSI to the USIM PS security key if rSR-VCC was performed;

2> if a SR-VCC handover is performed and the source RAT was NR:

3> store the CS "key set used" including the mapped KSI to the USIM CS security key if SR-VCC was performed;

3> indicate to the upper layer that SR-VCC handover from NR was completed successfully.

2> if the UE supports voice and video SRVCC handover, and a SR-VCC handover is performed:

3> if radio bearer configurations contain Conversational / speech CS RAB:

4> indicate to the upper layer that voice-only SRVCC handover was completed successfully.

3> if radio bearer configurations contain Conversational / unknown / UL:64 DL:64 kbps CS RAB:

4> indicate to the upper layer that voice and video SRVCC handover was completed successfully.

2> if the UE supports rSRVCC handover, and an rSR-VCC handover is performed;

3> indicate to the upper layer that rSR-VCC handover was completed successfully.

2> enter UTRA RRC connected mode in state CELL_DCH;

2> initialise variables upon entering UTRA RRC connected mode as specified in subclause 13.4;

2> update the variable UE_CAPABILITY_TRANSFERRED with the UE capabilities stored in the variable INTER_RAT_HANDOVER_INFO_TRANSFERRED.

1> and the procedure ends.

8.3.6.4 Invalid Handover to UTRAN command message

If the UE receives a Handover to UTRAN command message, which contains a protocol error causing the variable PROTOCOL_ERROR_REJECT to be set to TRUE according to clause 9, the UE shall perform procedure specific error handling according to the source radio access technology. The UE shall:

1> if allowed by the source RAT:

2> transmit an RRC FAILURE INFO message to the source radio access technology; and

2> include the IE "Protocol error information" with contents set to the value of the variable PROTOCOL_ERROR_INFORMATION;

1> Other details may be provided in the specifications related to the source radio access technology.

NOTE: The other RAT may include the above diagnostics information in a subsequent handover request towards the same RNC.

8.3.6.4a Unsupported configuration in HANDOVER TO UTRAN COMMAND message

If the UE does not support the configuration included in the HANDOVER TO UTRAN COMMAND message, e.g., the message includes a pre-defined configuration that the UE has not stored, the UE shall:

1> continue the connection using the other radio access technology; and

1> indicate the failure to the other radio access technology.

8.3.6.5 UE fails to perform handover

If the UE does not succeed in establishing the connection to UTRAN, it shall:

1> terminate the procedure including release of the associated resources;

1> resume the connection used before the handover; and

1> indicate the failure to the other radio access technology.

Upon receiving an indication about the failure from the other radio access technology, UTRAN should release the associated resources and the context information concerning this UE.

8.3.6.6 Reception of message HANDOVER TO UTRAN COMPLETE by the UTRAN

Upon receiving a HANDOVER TO UTRAN COMPLETE message, UTRAN should consider the inter-RAT handover procedure as having been completed successfully and indicate this to the Core Network.