CSFB Features - Radio View

CSFB CS FB Fe Feat atur ures es - Ra Radi dio o Vie iew w

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Seamless Voice Service Continuity • Two scenarios need to be differentiated…

• Transfer of voice calls to other RAT during call set up phase – service continuity then maintained via well-established 2G/3Gmechanisms

• Service continuity for an established voice call, when leaving LTE coverage area

CSFB

SRVCC

…in combination with potential voice evolution steps in LTE 2

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CS Fall Back

CSFB options defined in 3GPP Source:TS 36.300, Table 10.2.5-1: CS fallback options Target RAT CS fallback to UMTS

Solutions RRC Connection Release with Redirection without Sys Info RL20 RRC Connection Release with Redirection with Sys Info RL30 (RIM in later release) PS handover with DRB(s)

Release Rel-8

UE Capability (NOTE 1)

Rel-9

Mandatory for UEs supporting CS fallback to UMTS (NOTE 1)

FGI Index

e-Redirection UTRA Rel-8

RL40

(NOTE 1) Mandatory for UEs supporting CS fallback to UMTS

FGI8 (EUTRA RRC_CONNECTED to UTRA CELL_DCH PS handover) FGI22 (UTRAN measurements, reporting and measurement reporting event B2 in E-UTRA connected mode)

CS fallback to GSM

RRC Connection Release with Redirection without Sys Info

Rel-8

RL20 RRC Connection Release with Redirection with Sys Info

Rel-9

(NOTE 2) Mandatory for UEs supporting CS fallback to GSM

RL40 Cell change order without NACC


Rel-8


Cell change order with NACC

Rel-8


PS handover NOTE 1: NOTE 2: NOTE 3:

3

Rel-8

FGI10 (EUTRA RRC_CONNECTED to GERAN (Packet_) Idle by Cell Change Order) FGI10 (EUTRA RRC_CONNECTED to GERAN (Packet_) Idle by Cell Change Order with NACC (Network Assisted Cell Change)

(NOTE 2)

interRAT-PS-HO-ToGERAN All CS fallback to UMTS capable UE shall indicate that it supports UTRA FDD or TDD and supported band list in the UE capability. All CS fallback to GSM capable UE shall indicate that it supports GERAN and supported band list in the UE capability. The measurement may be performed before any of the above CS fallback solution is triggered to select the target cell or frequency layer more accurately based on eNB decision. eNB may trigger any of above CS fallback solutions blindly. FGI = feature group indicator (descriptions from TS 36.331, Annex B)

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CSFB features in LTE radio •

CS Fallback related features in RL20

– CS Fallback to UTRAN or GSM via Redirect (LTE562) – introduces service-based redirection from LTE to UTRAN or GERAN whenever CS voice call is to be set-up

– Emergency Call Handling (LTE22) – introduces means for special handling of emergency calls in the context of CS Fallback procedure

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– CS Fallback to UTRAN with System Information – CSFB towards UTRAN via redirection has been enhanced with provisioning of System Information of potential target cells (only towards 3GPP R9 compliant UEs)

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– CS Fallback to UTRAN via PS HO (LTE736) – CSFB towards UTRAN realized via PS HO mechanism – GSM Redirect with System Information (LTE984) – CS Fallback to GERAN via redirection will be enhanced with provisioning of System Information of potential target cells (only towards 3GPP R9 compliant UEs) 

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this enhancement applies also to RRC Connection Release with Redirection (LTE423)

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CSFB features in LTE radio •


– RAN Information Management for GSM (LTE498) – Retrieval of SI1, SI3, SI13 from BSC – Used for eNACC and CSFB to GSM with SI

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CS Fallback to UTRAN and GSM via redirect • The CS fallback function allows for a service based redirection from LTE to UTRAN or to GSM during the call setup.

Benefits: Voice call establishment on 2G/3G, utilizing existing CS network

• Both mobile originated and mobile terminated call setup is supported.

• Operator configurable priorities for the target

SMS

Voice calls LTE

frequency bands per cell

WCDMA or GSM

The terminal stays in LTE where the SMS is received or sent

The CS core network investments can be reused in the initial phase of LTE – CAPEX savings 6

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CS Fallback to UTRAN or GSM via Redirect -

CS Fallback requires CS-fallback-capable UE 3GPP Rel.8 compliant UE that supports access to E-UTRAN/EPC as well as to the CS domain over GERAN and/or UTRAN such terminals must also support combined EPS/IMSI attach/detach as well as combined TA/LA update procedures UE informs the network about its CSFB capability by requesting combined EPS/IMSI attach ( E P S A t t a c h T y p e IE) within A t t a c h R e q u e s t message

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CS Fallback may be applied to CSFB-capable UEs being camped on E-UTRAN and being simultaneously IMSI attached (registered to MSC for CS service via combined EPS/IMSI attach or combined TA/LA update with IMSI attach) CS Fallback may be triggered by both mobile originated and mobile terminated calls and is supported in both ECM-IDLE and ECM-CONNECTED modes

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CS Fallback to UTRAN or GSM via Redirect is based on RRC Connection Release w ith Redirect feature redirection is triggered by MO or MT CS call BTS triggers a RRC connection release with redirect based on source cell downlink RSRP measurements (event A2). UEs will be redirected to other frequency layers in case of bad radio conditions. redirection is performed by transmission R R C C o n n e c t i o n R e l ea s e message with a R e d i r e c t e d C a r r i e r I n f o IE (information on target RAT) towards the UE RedirectedCarrierInfo enforces the UE to switch to ECM-IDLE state and start searching for a cell w ithin the indicated target layer (UTRAN or GERAN) priorities of particular target layers for CSFB are controlled with dedicated parameter ( c s F a l l B P r i o ) and may be different than for normal RRC Connection Release with Redirect

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Redirect CSFB - UE in RRC idle

starting from RL30, 3GPP R9 UE may be provided also with System Information of potential UTRAN target cells so that call setup time is shortened

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Based on UE capabilities and operator’s settings eNB selects a CSFB target layer. eNB selects the highest priority layer supported by UE

Redirect CSFB – UE in RRC connected

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Key radio parameters actCSFBRedir

Activate CS Fallback Via Redirection

object:

LNBTS

unit:

-

This parameter controls the status of the CS Fallback to UTRAN or GSM via Redirect feature in a BTS.

range:

enabled, disabled

Default value of the parameter disables feature in a BTS.

step:

-

Feature activation prerequisites:

default:

disabled

multiplicity:

1

- feature is license controlled and hence license has to be provided - at least one UTRAN or GSM redirection target (REDRT object) has to be defined with all necessary attributes and

csFallBPrio

csFallBPrio parameter must be set to value different that “not used”.

Redirection Priority for CS Fallback to UTRAN or GSM via Redirect

object:

REDRT

unit:

-

range:

1..6, 0 (not used)

This parameter defines the priority for target RAT selection for CS Fallback to UTRAN or GSM via Redirect (LTE562) Value 1 means the highest priority, while value 6 means the lowest priority (see note for parameter).

redirectPrio

step:

-

default:

-

Value “not used” means that certain target is not considered in case of CS Fallback to UTRAN or GSM via Redirect. Only UTRAN and GERAN targets may have values different than “not used”.

multiplicity:

1

Rule: up to 6 REDRT instances may be created and each REDRT instance must have unique priority

(csFallBPrio), except of the value “not used”. The same REDRT object can be assigned to several redirection types simultaneously and several REDRT objects may be created for the same RAT. systemInforma tionListWC DMA object:

LNBTS

unit:

System Information

-

This parameter defines WCDMA System Information that is to be included in the RRC Connection Release message (3GPP name: utra-BCCH-Container-r9 ).

range:

4..2050 characters

System information for up to 16 W CDMA neighbour cells may be configured.

step:

-

Rule: this parameter must be added as character string in hexadecimal notation with leading '0x', where

default:

-

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for each octet two character must be configured, with a minimum length of one octet. The octet string has to be defined according to TS25.331.

Seamless Voice & Data service continuity

CS Fallback to UTRAN, based on PS Handover Benefits:

• The CS fallback function, based on PS HO allows for a faster service based handover from LTE to UTRAN, when an ongoing PS connection exists in parallel

• Both mobile originated and mobile terminated call

Voice call establishment on 2G/3G, ut ilizing existing CS network – shorter call setup time for an ongoing PS connection

SMS

Voice calls

setup is supported.

LTE

• Measurement based, operator configurable handover threshold (event B1)

• Blacklists configurable

WCDMA

The terminal stays in LTE where the SMS is received or sent

• CSFB service can also be applied from LTE-idle state (incoming voice call) Enhanced end user experience due to shorter call setup time when changing from LTE to UTRAN 11

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LTE736 CS Fallback to UTRAN enhancement •

CS Fallback to UTRAN (LTE736) introduces an enhanced way of realizing the CS fallback so that EPSattached UEs can be provided with CS voice service with even better user experience – This feature introduces service-based handover of multimode and PS-HO-capable UE from E-UTRAN to UTRAN whenever CS voice call is to be established 

E-UTRAN coverage must be overlapped by UTRAN coverage

– having terminated CS call, the UE may either stay in current RAT or go back to E -UTRAN using existing mechanisms

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EPC must support CS inter-working for mobility management and paging – SGs interface between MME and MSC server is needed – interworking between SGSN and MME and PDN-GW is realized via pre-Rel.8 Gn interface or via S3/S4 interfaces

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CS domain must support CS Fallback, i.e. MSC must support SGs interface to MME

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LTE 736 require also PS Handover to UTRAN capable UEs which simultaneously supports B1 measurements for UTRAN

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CS Fallback to UTRAN (LTE736) is based on InterRAT Handover (LTE56) functionality introduced in RL30/25TD release

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Contrary to the CS Fallback via Redirection feature where UE is forced to switch to ECM-IDLE state and start searching for a cell w ithin the indicated target layer (UTRAN or GERAN), in LTE736 UE is switched to RRCCONNECTED state and further handed over to UTRAN cell in which CS call setup is established

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Signaling procedures - UE in RRC_IDLE (1/4) •

Upon receiving Extended Service Request from UE, MME triggers CSFB towards eNodeB by sending Initial Context Setup Request message with CS Fallback Indicator – Prior to establishment of measurement and DRB

Selection of CSFB type

setup, eNB decide on CSFB type 

The decision is based on the IE:UE-EUTRA Capabilities which provides information whether UE supports B1 measurement in UTRAN or not ( then CSFB via redirect is triggered) => bit 15 (event B1 support) and bit 22 (UTRAN measurement report) must be set to “1” in the IE: F e at u r e G r o u p I n d i c a t o r s

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Once the CSFB type is selected, eNB configures B1 measurements for the UE and setup Data Radio Bearer – If UE Capabilities are not present in the Initial Context Setup Request they are retrieved via RRC UE Capability Transfer procedure

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RRC UE Capability Transfer (optionally)

Signaling procedures - UE in RRC_IDLE (2/4) • eNB sends S1-AP Initial Cont ext Setup R e s p o n s e to MME when UE acknowledges:

– activation of user plane security (with RRC Security Mode Complete ) and…

– … establishment of data radio bearer ( RRC Connection Reconfiguration Comp lete ) 

After reception of the R R C C o n n e c t i o n R e c o n f i g u r a t i o n C o m p l e t e eNB starts timer b1SupervisionTimer

• After sending S1-AP: Initial Context Setup R e s p o n s e eNB requests the UE for its

UTRAN capabilities – The procedure of retrieving UTRAN capabilities is similar to the E-UTRAN capabilities i.e. eNB initiates UE Capability Transfer procedure by sending RRC UE Capability En quir y with RAT type set to UTRAN >> IE: RAT-Type=utran

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b1SupervisionTimer started

Signaling procedures - UE in RRC_IDLE (3/4) • Upon receiving measurement results eNB decides whether CS Fallback will be performed via PS HO or Redirection – if TCL is not empty eNB will trigger CSFB via PS HO. Otherwise CSFB via Redirection will be triggered

• eNB initiates Handover Preparation phase>> by

Decision to perform PS HO

sending S1-AP:Hando ver Requir ed message – S 1 -A P : H a n d o v e r R e q u i r e d transfers information needed by MME for creating a request for radio resource allocation on WCDMA side

– HO Preparation ends with the reception of S1 - A P : H an d o v e r C o m m a n d by the eNB

• Upon receiving S 1 -A P : H a n d o v e r C o m m a n d eNB starts Handover Execution Phase>> in which: – the UE moves to a specific UTRAN cell – eNB stops radio link monitoring

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Handover Preparation

Signaling procedures - UE in RRC_IDLE (4/4) • The last part of the CSFB via PS HO procedure at the EUTRAN side is the Handover Completion phase>> – Upon successful handover procedure MME is informed by the target RAT that the handover was succeeded and then UE Context Release procedure is performed 

During UE Context Release procedure UEassociated S1-connection and all E-UTRAN resources are released

• In case of MOC, UE tries to establish connection in E- UTRAN via Initial Access procedure with Extended Service Request message with Service Type set to “mobile originating CS fallback” without any trigger from the network side – the subsequent part of the procedure is the same as in case of MTC

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Signaling procedures - UE in RRC_Connected • HO procedures are similar as in RRC-IDLE state. • Having successfully completed PS HO in UTRAN, UE may start CS RT service establishment via standard procedures leading to CS voice bearer

• The CN initiates establishment of the Radio

UE

NB

RNC

UE is in Cell_DCH state with PS service Signalling Connection Setup with CS domain and UE/CN NAS-signalling for AMR-call setup in Cell_DCH state

Access Bearer using the RAB Assignment Request message

RANAP: RAB Assignment Request (Setup)

• Upon that RNC sends NBAP: Radio Link Setup Request which includes UL/DL configuration data for the radio link

• Node B allocates resources for new DRB and send information to the RNC

• RNC sends the RB Setup message to add the new DCH

• After the activation the UE will respond w ith complete message

• At the end of the procedure UE is in Cell_DCH state with PS and CS services 17

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MSC / MSC Server

Resource request for SRB and RT RB to cell-specific PS NBAP: Radio Link Setup Request NBAP: Radio Link Setup Response

RRC: Radio Bearer Setup RRC: Radio Bearer Setup Complete

RANAP: RAB Assignment Response

RRC: Measurement Control

UE is in Cell_DCH state with CS and PS RB

Measurement in CS Fallback based on PS HO • In case of LTE736 blind handover i.e. w/o measurements, is not supported. For this reason eNB will trigger Measurements prior to the CS Fallback to UTRAN

– In case of LTE736 eNB configure B1 event based measurements for UTRAN – Event B1 used for CS Fallback to UTRAN indicate that radio conditions of InterRAT neighbor becomes better than threshold

• For “pure” InterRAT Handover to WCDMA (LTE56) Event B2 is used i.e. serving becomes worse than threshold1 and inter RAT neighbor becomes better than threshold2 however…

• … LTE radio conditions are meaningless in case of CS Fallback to UTRAN as UE must leave E-UTRAN anyway (regardless of the signal strength). For this reason Event B1 is used which consider only other RAT radio conditions

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CSFB Features - Radio View

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