LTE CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

Confidential Information of Huawei. No Spreading Without Permission









CS Fallback has a simpler network architecture compared with VoIP over IMS.

SRVCC is short for Single Radio Voice Call Continuity



In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of

voice and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide

these services LTE reuses CS infrastructure when the UE is served by E-UTRAN.

A CS Fallback enabled terminal is redirected to 2G/3G network after it initiates a CS service

such as voice call .

CS Fallback applies to scenarios in which the coverage area of the

UTRAN/GERAN/CDMA2000 1xRTT network is the same as or includes that of the E-UTRAN.





To support CS Fallback, the SGs interface is required, so as to let the MME perform a UE

location update over the SGs interface so that the core network of the UTRAN or GERAN

learns about the UE location.



After a UE is powered on in the E-UTRAN, it initiates a combined EPS/IMSI attach

procedure.

If a UE is camping on an E-UTRAN cell, it periodically initiates a combined TAU/LAU

procedure, which allows for simultaneous UE location updates both in the MME and in the

core network of the UTRAN or GERAN.



The Combined EPS/IMSI Attach Procedure is shown in the previous slide:

After the RRC connection setup, the UE sends an Attach Request message to the

MME, requesting a combined EPS/IMSI attach procedure. This message also

indicates whether the CS Fallback or SMS over SGs function is required.

The MME allocates an LAI to the UE, and then it finds the MSC/VLR for the UE

based on the TAI-LAI mapping. If multiple PLMNs are available for the CS domain,

the MME selects a CS PLMN based on the selected PLMN information reported by

the eNodeB. Then, the MME sends the MSC/VLR a Location Update Request

message over the SGs interface so that the core network of the UTRAN or GERAN

learns about the UE location, which contains the new LAI, IMSI, MME name, and

location update type.

The MSC/VLR performs the location update procedure in the CS domain.

The MSC/VLR responds with a “Location Update Accept” message that contains

information about the VLR and temporary mobile subscriber identity (TMSI). The

location update procedure is successful.

At last, the UE is informed that the combined EPS/IMSI attach procedure is

successful by RRC Connection Reconfiguration message. (If the network supports

SMS over SGs but not CS Fallback, the message transmitted to the UE contains the

information element (IE) SMS-only. The message indicates that the combined

EPS/IMSI attach procedure is successful but only SMS services are supported.)



This table lists the advantages and disadvantages of the preceding mechanisms in terms of

impacts on the UEs, impacts on the networks, CS service access delay, and PS service

interruption time.



Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback

procedure, which is shown in the slide:

The UE sends the MME an NAS message Extended Service Request to initiate a CS service.

The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS Fallback

procedure (If the MME supports the LAI-related feature, the MME also delivers the LAI to

the eNodeB).

The eNodeB determines whether to perform a blind redirection based on the setting of

BlindHoSwitch under the HoModeSwitch parameter, as follows:

If BlindHoSwitch is turned off, the eNodeB instructs the UE to perform inter-RAT

measurements. Then, the UE sends measurement reports to the eNodeB.

If BlindHoSwitch is turned on, the eNodeB does not instruct the UE to perform

inter-RAT measurements.

The eNodeB sends an RRC Connection Release message to instruct the UE to perform a

redirection. The message contains information about a target UTRAN frequency. If flash CS

Fallback is available, the RRC Connection release message includes information about a

target UTRAN frequency,PSC and their associated system information, In this way, the UE

can quickly access the target UTRAN without the need to perform the procedure for

acquiring system information of the target UTRAN cell. Then, the UE can directly initiate a

CS service in the UTRAN cell.

Then, the eNodeB initiates an S1 UE context release procedure.

The UE may initiate an LAU, a combined RAU/LAU, or both an RAU and an LAU in the

target cell and initiates a CS call establishment procedure in the target UTRAN cell.



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UE Uplink Transfer Message:

The message is sent by UE to MME to request a CS service.

The exact request information is in the NAS PDU.



UE Context Modification Request:

The message is sent by MME to eNodeB to request a CS Fallback.



UE Context Release Request:

The message is sent by eNodeB to MME to release the UE from EUTRAN.



RRC Connection Release:

The message is sent by eNodeB to UE to request a “Redirection”, that is to execute a CS

Fallback.

In the release message, eNodeB instructs the UE to a Inter-RAT system(UMTS in this case).



Flash CS Fallback is defined in 3Gpp R9 .With this function, SIB can be included into

the ”RRC connection Release” during the redirection procedure. This is achived by the RIM

procedure. with RIM, eNodeB can get information from GERAN/UMTS.

The RIM procedure supports two information exchange modes: Single Report and Multiple

Report. In Single Report mode, the source sends a request, and then the target responds

with a single report. In Multiple Report mode, the target responds with a report after

receiving a request from the source, and it also sends a report to the source each time the

system information changes.

The Multiple Report mode is enabled or disabled by setting the RimSwitch parameter,

while the Single Report mode is not configurable



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The RIM procedure and parameter settings are described as follows:

If a neighboring UTRAN cell supports the Multiple Report mode and

UTRAN_RIM_SWITCH under the RimSwitch parameter is turned on, the RIM

procedure in Multiple Report mode is performed as follows: After an E-UTRAN cell

is set up, the eNodeB sends a request for system information to neighboring

UTRAN cells. After a neighboring UTRAN cell receives a request or the system

information changes, this cell sends the system information to the eNodeB.

If an eNodeB supports flash CS Fallback, it requires the system information of

neighboring UTRAN cells to perform a redirection. If the serving cell does not have

that information, the eNodeB must initiate an RIM procedure in Single Report

mode to acquire the system information.



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The UE sends the MME an NAS message Extended Service Request to initiate a CS

service.

The MME sends an S1-AP Request message to instruct the eNodeB to initiate a CS

Fallback procedure (If the MME supports the LAI-related feature, the MME also

delivers the LAI to the eNodeB).

The eNodeB determines whether to perform a blind redirection based on the

setting of BlindHoSwitch under the HoModeSwitch parameter, as follows:

If BlindHoSwitch is turned off, the eNodeB instructs the UE to perform

inter-RAT measurements. Then, the UE sends measurement reports to the

eNodeB.

If BlindHoSwitch is turned on, the eNodeB does not instruct the UE to

perform inter-RAT measurements.

The eNodeB initiates the preparation phase for a PS handover. If the preparation is

successful, the eNodeB instructs the UE to perform a handover.

After the handover, the UE may initiate a CS call establishment procedure with an

LAU or combined RAU/LAU procedure in the UTRAN.

The UE’s context in EPS is released.






The MSC sends a Paging Request message from the CS domain to the MME over

the SGs interface. Then, either of the following occurs:

If the UE is in idle mode, the MME sends a Paging message to the eNodeB.

Then the eNodeB sends a Paging message over the Uu interface to inform

the UE of an incoming call from the CS domain.

If the UE is in active mode, the MME sends the UE an NAS message to

inform the UE of an incoming call from the CS domain.

The UE sends an Extended Service Request message containing a CS Fallback

Indicator after receiving the paging message from the CS domain.

The MME instructs the eNodeB over the S1 interface to perform CS Fallback.

The subsequent steps are similar to the originated CS Fallback to UTRAN. The

difference is that the UE sends a Paging Response message from the UTRAN cell.



This table lists the advantages and disadvantages of the preceding mechanisms in terms of

impacts on the UEs, impacts on the networks, CS service access delay, and PS service

interruption time.

The procedures of CS Fallback to GERAN are similar with those of to UTRAN, just the

CCO/NACC is particularly for GSM.

During CS Fallback based on CCO/NACC, the eNodeB receives a CS Fallback Indicator from

the MME, and then it sends a Mobility From EUTRA Command message to the UE over the

Uu interface. The message contains information about the operating frequency, ID, and

system information of a target GERAN cell. The UE searches for the target cell based on

the information it received, and then it performs initial access to the cell to initiate a CS

service.



The Cell Change order (CCO) procedure with Network Assisted Cell Change (NACC) is an

alternative to the RRC Connection Release with Redirection procedure used for CS Fallback.

The main difference is that the UE is moved to the target RAT whilst in RRC Connected

Mode, also MME can get some response(UE Context Required) from GSM so as to trigger

the UE context release procedure.

In this CS Fallback procedure, the eNodeB sends a “Mobility From EUTRA Command”

message over the Uu interface to indicate the operating frequency and ID of the target

GERAN cell. If the source cell has the system information of the target cell, the system

information is also carried in the message.







The eNodeB’s configuration may affect following steps：

1, Triggering phase

After receiving a CS Fallback Indicator from the MME, the eNodeB decides whether

to perform CS Fallback in the form of a blind handover based on the UE capabilities

and the setting of the blind handover switch. Compared with measurement-based

handovers, blind handovers have shorter access delays but a lower handover

success rate.

2, Measurement phase

The eNodeB delivers the inter-RAT measurement configuration to the UE, and the

UE performs inter-RAT measurements as instructed.

If CS Fallback fails to be performed for the UE in the form of a measurement-based

handover due to measurement timeout or exceptions, the eNodeB initiates a blind

handover.

3, Decision phase

The eNodeB evaluates the measurement results and generates a list of candidate

cells.



After a UE initiates a CS service in an E-UTRAN cell, the MME sends the eNodeB an S1-AP

Request message that contains a CS Fallback Indicator, notifying the eNodeB that the UE

should be transferred to the target network. The target networks to which CS Fallback can

be performed are specified by the HoAlgoSwitch parameter.

After receiving the CS Fallback Indicator, the eNodeB checks the blind-handover switch

setting and UE capabilities for each RAT. It behaves as follows:

If the BlindHoSwitch check box under the HoModeSwitch parameter is cleared,

blind handovers are prohibited. Under this condition, if the UE is capable of

measuring an RAT, the eNodeB delivers the inter-RAT measurement configuration

to the UE, instructing the UE to perform measurements on this RAT.

If the BlindHoSwitch check box under the HoModeSwitch parameter is selected

or the UE is incapable of measuring an RAT, the eNodeB does not instruct the UE

to perform measurements on this RAT.

If the eNodeB has not received any measurement reports within 3 seconds after

measurements are started for an RAT, or if a measurement-based handover to the

RAT fails to be performed for the UE due to exceptions, the eNodeB will instruct

the UE to perform a blind handover to another RAT that has not been measured.



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BlindHoSwitch check box is under the Handover Mode switch parameter. If it is checked,

the eNodeB does not instruct the UE to perform measurements on this RAT, and eNodeB

selects the target for the CS Fallback.



The eNodeB checks all inter-RAT neighboring cells of the cell serving the UE and considers

the following three sets (in descending order of priority) based on the LAI:

− Set 1 consists of neighboring cells with the same LAI as the received one.

− Set 2 consists of neighboring cells with the same PLMN ID as that indicated in the

received LAI.

− Set 3 consists of neighboring cells located in the same PLMN as the serving PLMN of

the UE.

The eNodeB selects the CS Fallback target cell from the set that has the highest priority

among all non-empty sets. If there are multiple neighboring cells in the highest-priority

non-empty set or the LAI is not indicated, the eNodeB selects the neighboring cell with the

highest blind handover priority as the CS Fallback target cell.

The blind handover priority is based on the blind handover priority of the target RAT and

blind handover priority of the specific cell. The former takes precedence over the latter.

If the three sets are empty, the eNodeB delivers the inter-RAT measurement configuration

to the UE. This CS Fallback will be performed in the form of a measurement-based

handover.



If the LAI is indicated by MME, The eNodeB delivers information about an inter-RAT

frequency in the measurement configuration to the UE if both the following conditions are

met:

− The PLMN ID of any inter-RAT neighboring cell on this frequency is the same as the

PLMN indicated in the LAI or the serving PLMN of the UE.

− The UE is capable of measuring this frequency.

Note that only the frequencies that the UE is capable of measuring are delivered in the

measurement configuration.



MOD CSFALLBACKBLINDHOCFG

By default, UTRAN is configured with the highest priority, GERAN is configured with the

second priority.



MOD UTRANNCELL

MOD GERANNCELL



After having received the B1 measurement reports for CS Fallback, the way in which the

eNodeB determines the target cell for the CS Fallback varies depending on whether an LAI

has been received, as follows:

If no LAI has been received, the eNodeB generates a list of candidate cells for CS

Fallback based on the received reports. It selects the cell with the highest signal

quality from this list.

If an LAI has been received, the eNodeB considers the candidate cells in sets 1, 2,

and 3. It sorts cells in each list by signal quality, and it selects a target cell from the

three lists in descending order of priority.



MOD CSFALLBACKHO

The received signal code power (RSCP) threshold for CS Fallback to UTRAN is specified by

CS FallbackHoUtranB1ThdRscp.

− The Ec/No threshold for CS Fallback to UTRAN is specified by CS

FallbackHoUtranB1ThdEcn0.

− The received signal strength indicator (RSSI) threshold for CS Fallback to GERAN

is specified by CS FallbackHoGeranB1Thd.

− The pilot strength threshold for CS Fallback to CDMA2000 1xRTT is specified by

CS FallbackHoCdmaB1ThdPS.

The time-to-trigger for event B1 related to CS Fallback to UTRAN/GERAN/CDMA2000

1xRTT is specified by the CS FallbackHoUtranTimeToTrig, CS

FallbackHoGeranTimeToTrig, and CS FallbackHoCdmaTimeToTrig parameters,

respectively



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The triggering of event B1 indicates that the signal quality is higher than a specified threshold in at least one

neighboring cell. 3GPP TS 36.331 defines the entering and leaving conditions of event B1 as follows:

Entering condition: Mn + Ofn - Hys > Thresh

Leaving condition: Mn + Ofn + Hys < Thresh

Where:

Mn is the measurement result of the neighboring cell.

Ofn is the frequency-specific offset for the frequency of the neighboring cell. It is

contained in the associated measurement object IE in the measurement configuration.

Hys is the hysteresis for event B1. The hysteresis values for CS Fallback to UTRAN,

GERAN, and CDMA2000 1xRTT are contained in the measurement configuration.

Thresh is the threshold for event B1.Thresh are set based on the measurement

quantities for each RAT.

If the entering condition of event B1 is continuously met during a period known as time-to-trigger, the UE

reports information about the cells that meet the condition to the eNodeB.

The following parameters related to event B1 for CS Fallback are the same as those related to event B1 for

inter-RAT handovers:

Frequency-specific offset for the frequency of the neighboring cell

Hysteresis

Reporting interval

Maximum number of cells contained in one report

Number of periodic reports





Parameter description



MO Parameter ID Description

GeranNcell BlindHoPriority

Indicates the priority of the neighboring cell during blind handovers. Blind handover is a process in which the eNodeB instructs a UE to hand over to a specified neighboring cell. There are 32 priorities altogether. The priority has a positive correlation with the value of this parameter. Note that the value 0 indicates that blind handovers to the neighboring cell are not allowed.

UtranNCell BlindHoPriority

Indicates the priority of the neighboring cell during blind handovers. Blind handover is a process in which the eNodeB instructs a UE to hand over to a specified neighboring cell. There are 32 priorities altogether. The priority has a positive correlation with the value of this parameter. Note that the value 0 indicates that blind handovers to the neighboring cell are not allowed.

CSFallBackHo

CsfbHoGeranB1Thd

Indicates the RSSI threshold for event B1 that is used in CS fallback to GERAN. A UE sends a measurement report related to event B1 to the eNodeB when the RSSI in at least one GERAN cell exceeds this threshold and other triggering conditions are met. For details, see 3GPP TS 36.331.

CSFallBackHo

CsfbHoGeranTimeToTrig

Indicates the time-to-trigger for event B1 that is used in CS fallback to GERAN. When CS fallback to GERAN is applicable, this parameter is set for UEs and used in the evaluation of whether to trigger event B1. When detecting that the signal quality in at least one GERAN cell meets the entering condition, the UE does not send a measurement report to the eNodeB immediately. Instead, the UE sends a report only when the signal quality continuously meets the entering condition during the time-to-trigger. This parameter helps decrease the number of occasionally triggered event reports, the average number of handovers, and the number of wrong handovers, and thus helps prevent unnecessary handovers. For details, see 3GPP TS 36.331.

CSFallBackHo

CsfbHoUtranB1ThdEcn0

Indicates the Ec/No threshold for event B1, which is used in CS fallback to UTRAN. When CS fallback to UTRAN is required, this parameter is set for UEs and used in the evaluation about whether to trigger event B1. This parameter indicates the Ec/No requirement for the UTRAN cells to be included in the measurement report. A UE sends a measurement report related to event B1 to the eNodeB when the Ec/No in at least one UTRAN cell exceeds this threshold and other triggering conditions are met. For a cell with large signal fading variance, set this parameter to a large value to prevent unnecessary handovers. For a cell with small signal fading variance, set this parameter to a small value to ensure timely handovers. For details, see 3GPP TS 36.331.





CSFallBackHo

CsfbHoUtranB1ThdRscp

Indicates the RSCP threshold for event B1, which is used in CS fallback to UTRAN. When CS fallback to UTRAN is applicable, this parameter is set for UEs and used in the evaluation about whether to trigger event B1. This parameter indicates the RSCP requirement for the UTRAN cells to be included in the measurement report. A UE sends a measurement report related to event B1 to the eNodeB when the RSCP in at least one UTRAN cell exceeds this threshold and other triggering conditions are met. For details, see 3GPP TS 36.331.

CSFallBackHo

CsfbHoUtranTimeToTrig

Indicates the time-to-trigger for event B1 that is used in CS fallback to UTRAN. When CS fallback to UTRAN is applicable, this parameter is set for UEs and used in the evaluation of whether to trigger event B1. When detecting that the signal quality in at least one UTRAN cell meets the entering condition, the UE does not send a measurement report to the eNodeB immediately. Instead, the UE sends a report only when the signal quality continuously meets the entering condition during the time-to-trigger. This parameter helps decrease the number of occasionally triggered event reports, the average number of handovers, and the number of wrong handovers, and thus helps prevent unnecessary handovers. For details, see 3GPP TS 36.331.

ENodeBAlgoSwitch

HoAlgoSwitch

Indicates the collective switch used to enable or disable handover and CS fallback algorithms.

Flash CS fallback to UTRAN: If the switch for CS fallback to UTRAN is turned off, this switch does not take effect;

Flash CS fallback to GERAN: If the switch for CS fallback to GERAN is turned off, this switch does not take effect.

ENodeBAlgoSwitch

HoModeSwitch

Indicates the switches corresponding to the inputs based on which the eNodeB determines handover policies. Note that EutranVoipCapSwitch will be removed in the later versions. In this version, the setting of this switch is still synchronized between the M2000 and the eNodeB, but it is no longer used internally. Therefore, avoid using this switch.

CSFallBackBlindHoCfg

InterRatHighestPri

Indicates the high-priority system to be considered in blind handovers. It is UTRAN by default.






InterRatLowestPri

Indicates the low-priority system to be considered in

blind handovers. It is CDMA2000 by default. If this

parameter is set to NULL, the system with the low blind

handover priority is not specified. In this case, only blind

handovers to the high- and medium-priority systems are

applicable.


InterRatSecondPri

Indicates the medium-priority system to be considered in blind handovers. It is GERAN by default. If this parameter is set to NULL, the system with the medium blind handover priority is not specified. In this case, only blind handovers to the high-priority system are applicable.

CSFallBackHo

LocalCellId Indicates the local ID of the cell. It uniquely identifies a cell within a BS.

ENodeBAlgoSwitch

RimSwitch

Indicates the collective switch for the RAN information

management (RIM) function.

UTRAN_RIM_SWITCH: Indicates the switch used to

enable or disable the RIM procedure that requests

event-driven multiple reports from UTRAN cells.

If this switch is turned on, the eNodeB can send RAN-

INFORMATION-REQUEST protocol data units (PDUs) to

UTRAN cells to request multiple event-driven reports.

If this switch is turned off, the eNodeB cannot send RAN-

INFORMATION-REQUEST PDUs to UTRAN cells to

request multiple event-driven reports.

GERAN_RIM_SWITCH: Indicates the switch used to

enable or disable the RIM procedure that requests

event-driven multiple reports from GERAN cells.

If this switch is turned on, the eNodeB can send RAN-

INFORMATION-REQUEST PDUs to CERAN cells to


If this switch is turned off, the eNodeB cannot send RAN-

INFORMATION-REQUEST PDUs to GERAN cells to




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LTE CS Fallback Feature