IRAT Strategies

This section describes the principles in terms of where the UEs should camp in Idle mode and the recommended strategy for CS and PS during a call. It is assumed in this document that an LTE or WCDMA system have back-up from another radio access technology (RAT) if coverage is lost. For LTE the back-up would be WCDMA or GSM, and for WCDMA, it is GSM.

The proposed strategies for RAT interworking aim at minimizing the number of system transitions and obtaining the highest possible throughput for packet-switched (PS) services. System transitions increase the probability of call drops, longer outage times for PS services, as well as lower throughput.

For circuit-switched (CS) voice calls, the general strategy is to keep the call in WCDMA as long as there is WCDMA coverage. Load issues in any of the systems could suggest certain deviation from this strategy. CS voice call in GSM should not do handover to WCDMA, except when GSM coverage is lost, for example when user goes into a building where there is only WCDMAcoverage.

For PS services, the general strategy is to use the RAT capable of providingthe highest bit rates to the largest extent possible. This results in prioritizing LTE for PS if LTE is available and terminal capability allows it, and WCDMA otherwise. Therefore, it is recommended to use priority-based cell reselection in Idle mode to move Idle mode UE in GSM to LTE and WCDMA and from WCDMA to LTE. Priority-based cell reselection only makes sure that the RAT with higher priority has adequate quality, and not comparing that to the quality of the current low-priority RAT.

Once being in LTE, a LTE capable terminals that want to perform CS servicescan be temporarily redirected to WCDMA (or GSM) using CS Fallback (CSFB). Non-LTE capable terminals should use WCDMA as the preferred RAT to camp on and initiate PS and CS services, conditioned on the adequate quality of WCDMA signal for Idle mode camping. The recommended parameter settings for LTE, WCDMA, and GSM support those strategies. Parameters and recommended values to implement the strategies are given further.

Overview of IRAT strategy

The strategy is implemented through parameters in GSM, WCDMA and LTE. Idle mode parameters cover users in the following states of the different RATs:

• WCDMA: IDLE, URA_PCH and CELL_FACH

• LTE: RRC_IDLE

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• GSM: Idle and Packet transfer mode Dedicated mode parameters cover users in the following states:

• WCDMA: CELL_DCH (CS and PS)

• LTE: RRC_CONNECTED

• GSM: CS voice call users

The handover (HO) or cell change (CC) is controlled by the network, based on measurement reports from UE, while cell reselection is controlled by UE, based on configuration parameters broadcast by the network. Figure 1 shows the possible IRAT transitions between the different states.

Mobility for a PS connection is controlled by idle mode parameters in GSM and dedicated mode parameters in WCDMA. Thus, the triggering thresholds for IRATHO/CC must be matched between GSM idle mode and WCDMA dedicated mode to avoid back and forth mobility between GSM and WCDMA (W2G and G2W ping-pong). Similarly, triggering thresholds controlling mobility

between WCDMA and LTE must be matched, in both Idle and Dedicated mode,

to avoid W2L and L2W ping-pong.

The following features are recommended when implementing the proposed strategy.

• The WCDMA feature GSM Handover and Cell Reselection (FAJ 121 154) provides the support needed for the UE to perform cell reselection from WCDMA to GSM. See Idle Mode and Common Channel Behavior.
• The WCDMA feature LTE Cell Reselection (FAJ 121 1474) provides the support needed for the UE to perform cell reselection from WCDMA to LTE. See Idle Mode and Common Channel Behavior.
• The WCDMA feature LTE Cell Reselection Dedicated Priority (FAJ 1213588) prevents LTE cell reselection for UEs that are LTE-capable but has no LTE subscription with the operator. See LTE Cell Reselection Dedicated Priorities.
• The WCDMA feature Packet Switched Handover from LTE (FAJ 121 1611) enables the transfer of one or more already established PS connections from LTE to UTRAN. PSHO from LTE is only performed in connected mode. See Packet Switched Handover from LTE.
• The LTE features Coverage-triggered WCDMA Session Continuity (FAJ121 0493) and Coverage-triggered GERAN Session Continuity (FAJ 1210495) provide the support needed for the UE to perform Idle mode cell reselection from LTE to WCDMA and GSM. These features also allow redirection to WCDMA or GSM for ongoing PS connections in LTE when bad coverage is detected in LTE. In this latter case, the mechanism is Release-with-Redirect where the PS call is released in LTE and re-established in WCDMA. See Coverage-triggered WCDMA Session
• The LTE feature Coverage-Triggered WCDMA IRAT Handover (FAJ 1210897) that provides the support needed for the UE to perform a seamless handover from LTE to WCDMA in connected mode when bad coverage is detected in LTE. No interruption for the PS call is caused by handover as in Release-with-Redirect, which makes it a recommended feature to use. This feature, however, has the feature 'Coverage-triggered WCDMA Session Continuity' as a pre-requisite.
• The LTE feature Mobility Control at Poor Coverage (FAJ 121 3013) that provides improvement in the procedure to determine when an UE in LTE connected mode should start IRAT and Inter-frequency measurement and when it should perform IRAT or Inter-frequency handover or Release-with-Redirect. See Mobility Control at Poor Coverage, 
• The LTE feature CS Fallback to GERAN and UTRAN (FAJ 121 0856) releases the UE and redirects it to WCDMA or GSM when a CS voice call needs to be set up. The feature is recommended together with the WCDMA feature Release with redirect to LTE (FAJ 121 2174) that will return the device to LTE after a CS Fallback. See CS Fallback to GSM and WCDMA.
• The WCDMA feature CS Fallback from LTE (FAJ 121 1610) allows UTRAN to establish a CS speech call that is transferred from LTE based on Fallback mechanism.
• In GSM, from G10A onwards, the feature GSM-LTE Cell Reselection (FAJ123 155) is introduced and the feature GSM-UMTS Cell Reselection and Handover (FAJ 121 57) is updated to support priority-based cell reselection.
• The GSM feature Fast Return to WCDMA after Call Release (FAJ 123 149) is recommended to use to reduce the time in GSM where there is WCDMA coverage. Further, the feature Combined cell re-selection triggering GSM to WCDMA (FAJ 121 933) introduces network support for an improved GSM to WCDMA cell reselection algorithm, resulting in increased likelihood to be served in a WCDMA network.

Idle Mode IRAT strategy
The overall idle mode strategy is that the UEs should camp on the RAT providing the best possible packet data performance in accordance with the capability of the UE until coverage is lost. This means LTE capable terminals camp on LTE and terminals not capable of LTE camp on WCDMA. UEs only capable of GSM camp on GSM. It is recommended to use priority based cell reselection, with the priority order LTE, WCDMA and GSM where LTE has the highest priority. IRAT related features are used for moving the UE in case the requested service is not supported by the network the UE is camping on. Figure below shows the different transitions between the RATs involved in achieving the recommended idle mode priority based strategy.

• A: Cell reselection from WCDMA to LTE should be done as soon as the RSRP level is above the minimum criteria for cell reselection. A margin should be used to prevent ping-pong IRAT cell reselection back to WCDMA. In WCDMA border cells no neighbor relations to LTE should be defined.
• B: Cell reselection from LTE to WCDMA shall only be done when the signal strength or quality level are below the minimum criteria in LTE and above sufficient levels in WCDMA including a margin.
• C: Cell reselection from GSM to WCDMA should be done as soon as possible as the WCDMA cell is above the minimum criteria plus a margin to avoid ping-pong IRAT cell reselection. The UE will continuously search for higher priority WCDMA (and LTE) cells.
• D: Cell reselection from WCDMA to GSM shall only be done when the WCDMA cell becomes unsuitable for camping. Measurements on GSM should only start just before the WCDMA cells' minimum criteria. The Idle mode UEs in WCDMA should be kept there until camping is not allowed on WCDMA before cell reselection to GSM occurs.

URA_PCH IRAT mobility Strategy
Cell reselection in URA_PCH will follow the Idle mode strategy.

CELL_FACH IRAT mobility strategy
It is recommended the that cell reselection in CELL_FACH should follow the strategy for CELL_DCH state. This means that GSM measurements on CELL_FACH should be turned off in WCDMA core cells. For more information of scenarios where there are border cells between different WCDMA frequency layers, see Additional WCDMA Carrier Deployment.

CELL_DCH IRAT mobility Strategy
The recommended IRAT strategy for UEs in WCDMA CELL_DCH state is to keep the UEs in WCDMA and to minimize unnecessary IRAT measurements. CS and multi-RABs containing CS could start IRAT measurements but only in border cells where the WCDMA coverage is limited. IRAT measurements for PS only RABs should be avoided because it is better to stay on WCDMA until the users have finished the PS data transfer before any IRAT measurements and transitions are initiated. Further there is an elevated risk of dropped calls during the IRAT process and the user packet throughput is usually much higher in WCDMA compared to GSM. The recommended IRAT HO strategy is below figure.

• A: PS HO, Single Radio Voice Call Continuity (SRVCC) and WCDMA session continuity from LTE to WCDMA should be done from LTE border cells only. 
• B: Handover should only be allowed for border cells. For WCDMA core cells all IRAT HO should be limited by setting the hoType to "NONE". For border cells IRATHO shall be allowed for CS Speech and Multi-RAB CS + PS, but not allowed for PS only. This is achieved by setting hoType to "GSM_PREFERRED". The IRATCC for PS can then be limited by using very low values for the parameters serviceOffset2dEcNo and serviceOffset2dRscp for all UeRc's that are serving PS only. When more than one WCDMA carrier is involved, see Additional WCDMA Carrier Deployment on how to set hoType.
• C: GSM packet transfer mode IRAT cell reselection should be allowed to WCDMA but not CS HO as long as there is GSM coverage. It is better to finish the CS call first and then do the IRAT cell reselection. The CS HO should be switched off using the parameters QSC, MSRL and ISHOLEV, 

Call Setup in LTE
For LTE terminals, the proposed strategy is to keep PS-only services in LTE. If CS service is not supported in LTE it can be set up in WCDMA (or GSM) using the feature “CS Fallback to GSM and WCDMA” (CSFB).


Call Setup in WCDMA
For non-LTE terminal, or in case LTE coverage is not available, the call (PS or CS) is set up in WCDMA.
CS Speech Call Setup in GSM
CS speech calls established in GSM should remain in GSM except when GSM coverage is lost, for example when user goes into a building where there is only WCDMA coverage. The reason is to minimize the number of IRATHOs.
High demand for CS Speech in combination with PS data could motivate diverging from this general recommendation where CS Speech+PS call in GSM can be moved to WCDMA Multi-RAB. In this case, the IRATHO for the CS Speech component from GSM to WCDMA should be allowed. However, the risks related to IRAT transitions should be balanced against the benefits of multi-RAB access. Furthermore, with the use of priority-based cell reselection together with the recommended GSM feature 'Fast return to WCDMA after CS call release', there will not be many UE in GSM that uses CS+PS.
Packet Call Setup in GSM
To maximize throughput for PS connections, calls in GSM Packet transfer mode should do IRAT cell change to WCDMA (or LTE, depending on UE capability) as soon as coverage and quality is acceptable. Connections in GSM idle mode and packet transfer mode are controlled by the same parameters, and the parameter setting must therefore consider the desired behavior in both idle and
packet transfer modes. However, with the use of priority-based cell reselection together with the recommended GSM feature 'Fast return to WCDMA after CS call release', there will not be many UEs in GSM that initiate PS services.

IRAT mobility strategy during state transitions
If the UE has been redirected to a lower priority RAT due to the setup of CS Speech, such as in the case of Circuit-switch Fall-back (CSFB) from LTE to WCDMA, the recommendation is to reselect back to higher priority RAT as soon as possible but only during a state transition, for example transition from CELL_DCH to CELL_FACH state.
It is therefore recommended to activate the WCDMA feature Release with redirect to LTE (FAJ 121 2174) to redirect the UE back to LTE after CSFB. Use this feature only from WCDMA cells where there is overlapping LTE coverage. It is recommended to use the feature Fast Return to WCDMA after Call Release (FAJ 123 149) if the UE has been redirected from WCDMA to GSM during a call set-up in an area where there is WCDMA coverage.