Lte cs fallback feature

LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

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

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SRVCC is short for Single Radio Voice Call Continuity

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LTE eRAN3.0 CS Fallback Feature

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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.

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A CS Fallback enabled terminal is redirected to 2G/3G network after it initiates a CS service such as voice call .

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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.

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

ď Ź

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.

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LTE eRAN3.0 CS Fallback Feature

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

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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.

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LTE eRAN3.0 CS Fallback Feature

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

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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.

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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.)

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LTE eRAN3.0 CS Fallback Feature

ď Ź

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.

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LTE eRAN3.0 CS Fallback Feature

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Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback procedure, which is shown in the slide:  

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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: 

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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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LTE eRAN3.0 CS Fallback Feature 

UE Uplink Transfer Message:

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

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The exact request information is in the NAS PDU.

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LTE eRAN3.0 CS Fallback Feature 

UE Context Modification Request:

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

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LTE eRAN3.0 CS Fallback Feature 

UE Context Release Request:

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

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LTE eRAN3.0 CS Fallback Feature 

RRC Connection Release:

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

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

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LTE eRAN3.0 CS Fallback Feature

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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.

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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.

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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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LTE eRAN3.0 CS Fallback Feature

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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.

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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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LTE eRAN3.0 CS Fallback Feature

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Once a LTE UE initiates a voice request, MME informs the eNodeB to perform a CS Fallback procedure, which is shown in the slide: 

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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).

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The eNodeB determines whether to perform a blind redirection based on the setting of BlindHoSwitch under the HoModeSwitch parameter, as follows: 

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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.

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The UE’s context in EPS is released.

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LTE eRAN3.0 CS Fallback Feature

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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 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.

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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.

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LTE eRAN3.0 CS Fallback Feature

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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.

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The procedures of CS Fallback to GERAN are similar with those of to UTRAN, just the CCO/NACC is particularly for GSM.

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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.

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LTE eRAN3.0 CS Fallback Feature

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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.

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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.

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

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The eNodeB’s configuration may affect following steps：

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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.

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2, Measurement phase 

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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.

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LTE eRAN3.0 CS Fallback Feature

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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.

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After receiving the CS Fallback Indicator, the eNodeB checks the blind-handover switch setting and UE capabilities for each RAT. It behaves as follows: 

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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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LTE eRAN3.0 CS Fallback Feature

ď Ź

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.

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LTE eRAN3.0 CS Fallback Feature

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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.

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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.

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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.

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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.

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LTE eRAN3.0 CS Fallback Feature

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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.

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

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LTE eRAN3.0 CS Fallback Feature

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MOD CSFALLBACKBLINDHOCFG

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

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LTE eRAN3.0 CS Fallback Feature

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MOD UTRANNCELL

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MOD GERANNCELL

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LTE eRAN3.0 CS Fallback Feature

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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.

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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.

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LTE eRAN3.0 CS Fallback Feature

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MOD CSFALLBACKHO

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The received signal code power (RSCP) threshold for CS Fallback to UTRAN is specified by CS FallbackHoUtranB1ThdRscp. 

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− 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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LTE eRAN3.0 CS Fallback Feature

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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

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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.

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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.

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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

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Hysteresis

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Reporting interval

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Maximum number of cells contained in one report

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Number of periodic reports

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LTE eRAN3.0 CS Fallback Feature

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LTE eRAN3.0 CS Fallback Feature

ď Ź

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

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.

BlindHoPriority

Indicates the RSSI threshold for event B1 that is used in CS fallback to GERAN. A UE sends a measurement CSFallBack CsfbHoGeranB report related to event B1 to the eNodeB when the RSSI Ho 1Thd in at least one GERAN cell exceeds this threshold and other triggering conditions are met. For details, see 3GPP TS 36.331.

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 CSFallBack CsfbHoGeranTi a measurement report to the eNodeB immediately.

Ho meToTrig 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.

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.

CSFallBack CsfbHoUtranB1 A UE sends a measurement report related to event B1 to Ho ThdEcn0 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.

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LTE eRAN3.0 CS Fallback Feature

ď Ź

Parameter description

MO

Parameter ID

Description

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.

CSFallBack CsfbHoUtranB1 This parameter indicates the RSCP requirement for the Ho ThdRscp 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.

CSFallBack CsfbHoUtranTi Ho meToTrig

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.

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

ENodeBAlg HoAlgoSwitch oSwitch

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.

ENodeBAlg HoModeSwitch oSwitch

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.

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

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LTE eRAN3.0 CS Fallback Feature

ď Ź

Parameter description

MO

Parameter ID

Description

Indicates the low-priority system to be considered in blind handovers. It is CDMA2000 by default. If this CSFallBack InterRatLowest parameter is set to NULL, the system with the low blind BlindHoCfg Pri handover priority is not specified. In this case, only blind handovers to the high- and medium-priority systems are applicable.

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

CSFallBack LocalCellId Ho

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

ENodeBAlg RimSwitch oSwitch

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 RANINFORMATION-REQUEST protocol data units (PDUs) to UTRAN cells to request multiple event-driven reports.

If this switch is turned off, the eNodeB cannot send RANINFORMATION-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 RANINFORMATION-REQUEST PDUs to CERAN cells to request multiple event-driven reports.

If this switch is turned off, the eNodeB cannot send RANINFORMATION-REQUEST PDUs to GERAN cells to request multiple event-driven reports.

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LTE eRAN3.0 CS Fallback Feature

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