- This document proposes concepts for supporting circuit-switched fallback (CSFB) in 3GPP2 specifications. - It outlines CSFB architecture, call flows for pre-registration, mobile origination, and mobile termination between E-UTRAN, MME, 1xRTT CS access network and MSC. - A new protocol "Circuit Switch Tunneling Data Protocol" is proposed to encapsulate and tunnel 1xRTT messages over the LTE network. Additional changes are needed to 3GPP2 specifications to support mobility parameters in TS 36.331 for CSFB.

1. Support for CSFBSupport for CSFB
Tony Lee aslee@via-telecom.comTony Lee aslee@via-telecom.com
David WangDavid Wang davidwang@via-telecom.comdavidwang@via-telecom.com
June/15/2009
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2. IntroductionIntroduction
• This contribution list out some stage two
concepts that needs to be incorporate into
C.P0087 v2.0 to support CSFB

3. CSFB backgroundCSFB background
TS 23.272 CSFB in EPS stage 2, it contains the
requirements for E-UTRAN to 1xRTT CSFB,
which includes:
• Architecture
• UE
• E-UTRAN
• MME

4. ArchitectureArchitecture
E-UTRAN
MME
Serving/PDN
GW
SGi
1xRTT CS
Access
1xRTT
MSC
1xCS IWS
S102
S11S1-MME
S1-U
A1
A1
Tunnelled 1xRTT messages
1xCS
CSFB
UE
1xCS
CSFB
UE

5. CSFB call flow: Pre-registrationCSFB call flow: Pre-registration

6. Pre-registrationPre-registration
1. UE attaches to E-UTRAN as specified in TS 23.401 [2];
2. Based on a radio layer trigger (e.g. an indication from the E-UTRAN when the UE is
in connected state or an indication over the broadcast channel), the UE decides to
register with the 1xRTT CS domain;
3. UE generates a 1xRTT CS registration request.
3a. The 1xRTT CS message is transferred from the UE to E-UTRAN.
3b. E-UTRAN forwards the 1xRTT CS message to the MME including the Reference
CellID.
3c. MME selects a 1xCS IWS node based on Reference CellID. The IMSI is used to
distinguish S102 signalling transactions belonging to different UEs. The MME sends a
S102 Direct Transfer message (IMSI, 1xCS message) to the 1xCS IWS node.
4. 1xRTT CS registration is then performed by the 1xCS IWS node based on 3GPP2
A.S0008 [16].
5a. 1xRTT CS registration response is tunnelled back to the MME in a S102 Direct
Transfer message (IMSI, 1xCS message).
5b. MME forwards the 1xRTT CS message to the E-UTRAN.
5c. E-UTRAN forwards the 1xRTT CS message to the UE.
If the triggers for 1xCS registration change over time, the UE (both in idle or active state),
uses this information to update the 1xCS registration via the tunnel.

7. CSFB call flow: Mobile OriginationCSFB call flow: Mobile Origination

8. Mobile OriginationMobile Origination
1. UE is E UTRAN attached and registered with 1xRTT CS as defined in clause B.2.1.1.‑
2. UE makes a decision to perform a mobile originated CS call.
3. UE sends a Service Request (CS Fallback Indicator) to the MME.
4. MME sends S1-AP: UE Context Modification (UE capabilities, CS Fallback Indicator) to indicate
the E UTRAN to move the UE to 1xRTT.‑
5. E-UTRAN may optionally solicit a measurement report from the UE to determine the target
1xRTT cell to which the CS Fallback will be performed.
6. E-UTRAN triggers RRC connection release with redirection to 1xCS or inter-RAT Cell Change
Order to a 1xRTT neighbour cell.
7. E-UTRAN sends an S1 UE Context Release Request (Cause) message to the MME. Cause
indicates that the S1 UE Context Release was caused by CS fallback to 1xRTT.
8. MME sets the UE context to suspended status and sends to the S-GW a Suspend Request
(IMSI) message that requests the suspension of EPS bearers for the UE.
9. S-GW acknowledges the Suspend Request message and marks the UE as suspended. When a
downlink data arrives at the S-GW, the S-GW should not send a downlink data notification
message to the MME if the UE is marked as suspended.
10. S1 UE Context in the E-UTRAN is released as specified in TS 23.401 [2].
11. UE moves to 1xRTT and performs the procedure for mobile originating call as specified in
3GPP2 A.S0013 [18].
Once CS service ends in the 1xCS domain, the UE returns to E UTRAN by performing reselection.‑
The EPS service is resumed as described in clause 6.5.

9. Mobile Termination

10. Mobile Termination
1. UE is E-UTRAN attached and pre-registered with 1xRTT CS as defined in clause B.2.1.1.
2. 1xMSC sends a paging request to the 1xCS IWS node.
3. 1xCS IWS node forwards the 1x RTT CS paging request via the S102 tunnel to the MME.
4. If the UE is in idle state, the MME performs the network initiated Service Request procedure in order to bring
the UE to active state prior to tunnelling of the 1x RTT CS paging request toward the UE.
5. MME forwards the 1xRTT CS paging request to the UE.
6. UE sends a Service Request (CS Fallback Indicator) to the MME.
7. MME sends S1-AP: UE Context Modification (UE capabilities, CS Fallback Indicator) to indicate the E UTRAN
to move the UE to 1xRTT.
8. E-UTRAN may optionally solicit a measurement report from the UE to determine the target 1xRTT cell to which
the CS Fallback will be performed.
9. E-UTRAN triggers RRC connection release with redirection to 1xCS or Inter-RAT Cell Change Order to a
1xRTT neighbour cell.
10. E-UTRAN sends an S1 UE Context Release Request (Cause) message to the MME. Cause indicates that the
S1 UE Context Release was caused by CS fallback to 1xRTT.
11. MME sets the UE context to suspended status and sends to the S-GW a Suspend Request (IMSI) message
that requests the suspension of EPS bearers for the UE.
12. S-GW acknowledges the Suspend Request message and marks the UE as suspended. When a downlink data
arrives at the S-GW, the S-GW should not send a downlink data notification message to the MME if the UE is
marked as suspended.
13. S1 UE Context in the E-UTRAN is released as specified in TS 23.401 [2].
14. UE tunes to 1xRTT and acknowledges the page by transmitting a 1xRTT Paging Response message over the
1x Access Channel.
15. Subsequently UE performs the procedure for mobile terminated call establishment as specified in 3GPP2
A.S0013 [18].

11. CSFB backgroundCSFB background
TS 36.331 Radio Resource Control, it contains
requirement for:
• Messages and parameters to set up 1x registration
• SIB 8
• System time
• Long Code State
• CSFB registration parameters

12. Requirements for 1xRequirements for 1x
Synchronize with base station using information
provided by SIB 8
Need to send LAC encapsulated message over E-
UTRAN tunnel
Perform pilot measurement during E-UTRAN gap
Track registration status
Perform origination or page response with
minimum delay

13. Proposal Overview
MAC MAC
RLC RLC
PDCP PDCP
RRC CSTCP
CSTDP
LAC
L3
CSFB Parameters
Measurement Request
1x Signaling Message
PHY PHY
LTE
cdma2000 1x
Data Plane
Control Plane

14. Proposal: Data plane view
1x Layer 3
1x LAC
CSTDP
LTE Air
Interface
LTE Air
Interface
S1-MME
Lower Layer
S1-MME
Lower Layer
S102
Lower Layer
CSTDP
S102
Lower Layer
1x Layer 3
1x LAC
Lower Layer
A1
Out of scope
1x CSFB UE eNB MME IWS
To 1x MSC

15. Proposal details
CSTDP
 Circuit Switch Tunneling Data Protocol
 Encapsulate LAC messages with parameters similar to CSNA:
 ARQ parameters
 Logical channel
 Protocol revision number
CSTCP
 Interface between E-UTRAN and 1x
 Base station: generate necessary 1x overhead parameters
 Mobile: process 1x parameter and pass this on to L3

16. Proposal details
Changes to 1x mobile
 Bypass pilot acquisition
 Use SIB 8 information to synchronize with base
station
 No need to monitor control channel, this includes
paging and BCCH
 No need to start any supervision timer
 Obtain parameters from CSTCP

17. Additional changes for C.S0087
Need to add MobilityParametersCDMA2000 in
C.S0087 to support TS 36.331
In TS 36.331 RRC document has two messages
related to CSFB
 CSFBParametersRequestCDMA2000 ( UE to eNB)
 CSFBParametersResponseCDMA2000
 MobilityParametersCDMA2000 is defined, but refers to
C.S0087 for details. The parameters are necessary to
generate the registration message

18. THANK YOU!