Introduction and scope of the document LTE/EPC

Radio Optimisation Guidelines:
CSFB guidelines
Jacek DOBRZYŃSKI, Marcin AUGUSTYNIAK, Rafał SZPILA
Orange Labs Networks and Service Platforms/Radio Access Systems Agency
21 November 2013

List of Contents
1. Introduction and scope of the document
2. CSFB overview
a. Architecture
b. Procedures
c. Main issues
3. Strategy and recommendations
4. Conclusions
5. Backup slides

Introduction and scope of the document
LTE/EPC systems as next generation of mobile systems are optimised
for high speed data transmission and do not support CS voice.
In fact in the target architecture, Voice service in LTE/EPC networks will
be provided using VoIMS.
Until then, CSFB feature will be used to provide voice service to
LTE/3G/2G capable smartphones.
− CSFB enables mobile originating and terminating voice services by
redirecting or handing over users from LTE to GERAN/UTRAN network
domain.
− With CSFB, LTE terminals will camp on LTE network and fall back to
3G or 2G to initiate or receive voice calls
 To provide voice services in early deployments of LTE, it is recommended
to switch on CSFB feature in the network.

CSFB overview: architecture
To implement CSFB to UTRAN or GERAN, an SGs interface was introduced
between the Mobility Management Entity (MME) and the Mobile Switching
Center (MSC).
Figure below shows the architecture needed for CSFB to UTRAN/GERAN.
UE E-UTRAN MME
LTE-Uu S1-MME
GERAN
UTRAN
Um
Uu
SGSN
MSC
Server
SGs
Gs
A
Iu-cs
Gb
Iu-ps
S3
SGs ” an interface between MME and MSC (defined in 3GPP Release 8). It is used for:
• mobility management (combined attach / combined tracking area update, initiates a location
area update in the MSC/VLR while user remains under LTE coverage, so that voice calls and
SMS can be received from 2G/3G CS domain)
• paging procedures between EPS and CS domain

CSFB overview: Several procedures for CSFB operation
Target IRAT Solution Release Description
CSFB to UTRAN
RRC Connection Release
with Redirection without
System Information (SI)
Release 8
The eNodeB receives a CS Fallback Indicator, and then it sends an RRC Connection
Release message to release the UE. The message contains information about a target
UTRAN frequency.
with Redirection with
deferred measurements
SIB11/SIB12 reading
Release 8
Terminal reads only mandatory SIBs like SIB 1, 3, 5, 7 and skips the rest of them before
accessing the target cell. System Information Block type 11/12 (neighbor relations) is
known from measurement reports just after UE is connected to the target cell.
Release 9
SIBs are tunnelled from the target cell in target RAT through core network. This
information is included in redirection message which is sent to the terminal, accelerating
thus the redirection process to the target cell.
PS Handover Release 8
The UE is transferred from E-UTRAN to UTRAN by performing a PS handover. It then
initiates a CS service once under UTRAN.
CSFB to GERAN
with Redirection without
Release 8
The eNodeB receives a CS Fallback Indicator, and then it sends an RRC Connection
Release message to release the UE. The message contains information about a target
GERAN carrier frequency group.
Release 9
SIs are tunneled from the target GERAN cell to LTE through core network. This information
is included in redirection message which is sent to the UE.
Cell Change Order (COO)
with Network Assist Call
Control (NACC)
Release 8
The eNodeB receives a CS Fallback Indicator from the MME, and then 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 a target cell based on the information received, and then
performs initial access to the cell to initiate a CS service.
If the GERAN or the UEs do not support DTM, the ongoing PS services must be
suspended before the CS service can be established.
PS Handover Release 8
The UE is transferred from the E-UTRAN to the GERAN by performing a PS handover. It
then initiates a CS service in the GERAN. If the GERAN or the UEs do not support DTM,
the ongoing PS services must be suspended before the CS service can be established.

CSFB overview: Main issues (1/4)
Call setup time
The weakest point of CSFB is probably the extra time needed to make or receive
calls. From Quality of experience point of view, this is a crucial factor.
Depending on the CS fallback scenario, different extra call setup time occurs from
about 1s up to several seconds compared to legacy 2G/3G network voice call
setup time:
−Towards UTRAN with RRC release and Redirect without SI (Release8)
− MOC (called UE in 3G) : ~ +1.1 s
− MTC (calling UE in 3G): ~ +1.2 s
− MTM (both UEs In LTE) : ~ +2.4 s
−Towards GERAN with RRC release and Redirect without SI (Release8)
− MOC (called UE in 2G) : ~ +5.3 s
− MTC (calling UE in 2G): ~ +4.0 s
− MTM (both UEs in LTE) :~ +6.0 s
To improve the user Quality of Experience, it is recommended to favor either the RRC
Release with redirection with SI (Release 9) or The PS Handover mechanisms for CSFB
procedure

Dropped connection and disruption time for data session
During CSFB procedure from LTE to 2G, data session has to be suspended when
no DTM is supported. If there is no CCO or NACC activated in the network, the
data session can be dropped.
The data session drop probability increases when UEs have problems with return
back to LTE domain, i.e. too high return time or redirection back to a no/poor LTE
coverage
Data interruption time, during LTE to 3G mobility during CSFB procedure, is:
− Around 5s with a Release and Redirection procedure
− Around 500ms with packet handover procedure
Nevertheless the data interruption when falling back to 3G is less user noticeable
than on 2G because of the lack of DTM in most networks and overall worse data
performance on GSM.
To avoid both higher data session dropped rate and interruption time it is recommended to
implement CSFB based on PS HO to 3G network. This improves KPIs related to data
sessions when CSFB to 3G is performed.

TAC/LAC misalignment
Figure below shows a simple example for misaligned LAs/TAs: UEs on TA3 of LTE,
could fall back either on LA3 or LA2 of 3G/2G network.
If over LA2, then the call will be handled by the wrong MSC (MSC1).
 Potentially drop of the call (notably in case of Mobile Terminating Call)
2G/3G
EUTRAN
RNC1
MSC1
MSC Pool Area 1
MSC Pool Area 2
RNC2
MSC2
MME1 MME2
LA1 LA3
TA1 TA2 TA3
LA2
SGs SGs
UE
To avoid LAC/TAC misalignment and high call dropped rate in the border between different
MSCs, it is recommended:
−To configure more precisely the TAs at pool area boundaries such that they do not
extend beyond the coverage of the corresponding LAs,
− and to create MSC pooling, i.e. to declare MSC in pool

Bad or absence of 2G/3G coverage
The initial LTE coverage will be quite small due to the usage of higher frequencies (1.8,2.6 GHz) but this will
improve with time. In this case, one can easily suppose that under a LTE service area, there will always be a
2G/3G coverage.
But, when it comes to LTE800 MHz, a substantially higher coverage frequency, some problems may appear
regarding two scenarios:
1. Blind redirection to 3G performed while no 3G coverage but with 2G coverage
The network will perform a CSFB to a UTRAN frequency that has no coverage in the current UE position but
with a 2G cell in range.
 The behavior of the UE is completely vendor and implementation specific as it is not standardized
Upon not being possible to connect to 3G, the UE may search for other voice capable RATs and eventually
camp on a 2G cell to make the call (but with a very long connection time, up to 12s)
2. Blind redirection to 3G, with no 2G/3G coverage -
Upon the redirection triggered by CSFB, the UE goes to a frequency that has no coverage in this area, and
there is no other voice capable RAT available.
 In this case, it has been measured that the UE tried to find a suitable carrier for over 30 seconds and
after that comes back to LTE.
This corresponds to a worst case scenario
• The CSFB will fail (no possibility to do a voice call)
• The UE is unreachable for more than half a minute for both voice calls and data connection
See recommendations slides (next section) for details on ways to tackle this issue

It is recommended that:
 Both CSFB towards 3G and 2G should be activated on each site (with parameterization corresponding to
operator strategy) in order to support voice service continuity everywhere it is possible.
 WCDMA network should have higher priority than GERAN network for the CSFB procedure due to shorter
call setup time and possibility redirecting data session without long disruption time.
 If not possible to configure at the same time CSFB to 3G and to 2G (some vendors restriction), then it is
recommended, when 3G coverage is sufficient, to configure CSFB to 3G.
2G
3G
LTE
CSFB to 3G:
- RRC Release with Redirection Rel 8 (present)
- RRC Release with SI Rel.9 (future)
- PS Handover (present)
Handover to 2G
CSFB return
from 2G:
- cell reselection
- Fast Return
CSFB return from 3G:
- cell reselection
- Fast Return
CSFB to 2G:
- CCO + NACC
Strategy – Recommendations (1/5)
Case 1: Both 2G AND 3G coverage available under LTE

3G
LTE CSFB to 3G:
- PS Handover (present)
CSFB return to LTE:
- cell reselection
- Fast Return
It is recommended:
 To adapt the configuration to the available underlying technology:
 CSFB to 2G or CSFB to 3G (as depicted below)
2G
LTE
CSFB to 2G:
- RRC Release with Redirection Rel 8(present)
- CCO w/o NACC (future)
CSFB return to LTE:
- cell reselection
- Fast Return
Case 2: only 2G OR 3G coverage available under LTE

It is recommended:
 To configure CSFB to 2G to reduce as much as possible CSFB failure rate, even if CSFB call
setup time will not be optimal
 Feature improving CSFB LTE to 2G performances is under study (first tests planned for end of 2013).
Case 3: 3G coverage available but not complete under LTE
but sufficient 2G coverage

The following table summarizes the CSFB configuration depending on the deployment hypothesis in 3G & 2G:
In case 3G coverage available and similar coverage for LTE and 3G
 it is recommended to switch on CSFB based on PS Handover (PS HO)
 If PS HO is not available in current E2E configuration it is recommended to switch on CSFB Release with
Redirection Rel.8 (consider CSFB based PS HO as a target configuration).
 Fast Return from 3G to LTE (network level) for all vendors should be switched on
Only for Huawei, it is recommended to switch on service Packet Redirection from 3G to LTE.
In case there is no or insufficient 3G coverage and 2G and LTE coverage are supposed similar
 it is recommended to switch on CSFB Release with Redirection Rel. 8 and to parameterize Idle Reselection
from 2G to LTE for all vendors.
 As next step, it is foreseen to migrate to CSFB Release with Redirection Rel. 9 with Idle Reselection 2G to
LTE and also Fast Return from 2G to LTE (when this latter will be improved*)
Frequencies 3G 2100 3G 900 No 3G
LTE 2600 CSFB to 3G CSFB to 3G CSFB to 2G
LTE 1800
Depends on
coverage map
CSFB to 3G CSFB to 2G
LTE 800
Depends on
coverage map
CSFB to 3G CSFB to 2G
Mobility features recommended for CSFB to 3G and 2G
*Currently Fast Return in Ericsson and Alcatel-Lucent doesn’t work properly, Huawei and NSN are not yet assessed.
Moreover there is no clear recommendation to activate Fast Return as a device feature.

 Activation recommendation of fast return on 3G according to RAN vendors maturity:
 On Ericsson and ALU 3G radio, Fast return 3G to LTE shall be activated in areas where
a good LTE coverage is present. If not, mobile can be unreachable on LTE for a duration
up to 20s (lab tests)
 On Huawei 3G radio, both features, ‚Service PS redirection‛ and ‚Fast return‛ shall be
activated and depending on the type of mobile and type of call, one of the procedures will
apply (‚fast return‛ will take precedence over ‚service PS redirection‛ when both are
applicable).
 Activation recommendation of fast return on 2G according to RAN vendors maturity:
 On Ericsson and ALU 2G radio, it is NOT recommended to activate on the network ‚fast
return 2G to LTE‛ network feature since
• return to LTE is performed in blind mode.
• after the call ends, every voice call can force return to LTE (not only CSFB calls),
even if LTE coverage is not present or insufficient as soon as the mobile is LTE
capable.
 On Huawei and NSN 2G radio: not evaluated yet.
Note: when this feature is deactivated, and if 2G RAN is compatible, an LTE cell reselection shall occur once
the mobile is in idle mode (once the voice call ended, some data traffic is usually observed in 2G, so idle
mode is not usually reached in a very short time).
Fast return activation as RAN feature

 CSFB must be activated in order to ensure voice call services for LTE clients everywhere 3G or 2G coverage
is available
 As a first phase, it is recommended to switch on CSFB RRC Release with Redirection Rel. 8 or CSFB based
on PS Handover. It gives voice call service continuity and quite good setup times for MOC/MTC/M2M.
” The further step as a natural continuity of the first one is implementation of CSFB RRC Release with Redirection Rel.9
(especially recommended for CSFB to 2G in areas where no 3G coverage -> significantly lower call setup time).
 CSFB RRC Connection Release with Redirection Rel. 9 gives the best results ” of call setup time which is
supposed to be only slightly longer than call setup in legacy network 3G. It implies having a well configured
core network. At this moment there are no commercial implementations of this solution.
 CSFB based on PS Handover significantly improves data interruption time.
 Very good TAC/LAC mapping should be done
” it is recommended to plan more TAC in one LAC. The inverse situation can lead to much higher problems with
dropped calls.
” To avoid problems with call setup on border of different MSCs, MSC in Pool solution is recommended and tuning
relevant timer on MSC which is responsible for waiting time of paging procedures on SGs interface
 To avoid ping-pong effects after CSFB call in WCDMA and GERAN it is recommended to verify mobility
settings between different RATs
 At this moment the most common commercial implementation is CSFB RRC Connection Release with
Redirection Rel. 8 as the simplest and the cheapest solution provided by all vendors. There are only few
implementation of PS Handover in the group at this moment since this feature is unavailable in some
vendors’ solutions.
Main recommendations

 Huawei -> CSFB Red. Rel8/9 and PS HO are available
o Huawei supports both CSFB Redirection Rel.9 (eRAN 2.2/RAN13/GBSS13) and CSFB based on PS
Handover (eRAN 2.0/RAN12/GBSS13). There are commercial implementations of CSFB PS HO in Orange
Group.
o Fast Return 3G to LTE is available in Huawei from RAN14. There are two features, „Service PS Redirection‛
and „Fast Return‛ offered by Huawei. Both solutions shall be activated with priority for „Fast Return‛.
o Fast Return 2G to LTE is available but not tested yet
o Huawei supports to activate CSFB to both 3G and 2G with a priority settings between these two RATs
 Ericsson -> CSFB based on PS HO is not available
o Ericsson doesn’t support CSFB PS Handover to 3G both in RAN and Core Network part. Feature is planned
in W14B SW release.
o Ericsson supports CSFB Red. Rel.9 (CSFB with RIM) to 2G and 3G (L12B/W12B/B12A)
o Fast Return 3G to LTE is available from W12B SW release. It can be activated per NodeB.
o Fast Return 2G to LTE configured per cell is available from G12B. It is not recommended to switch on this
feature if 2G coverage is poor -> return is triggered after each voice call is ended
o There is no possiibility to define CSFB to both 2G and 3G at the same time
Conclusions on suppliers’ implementation 1/2

 ALU -> CSFB Red. Rel8/9 and PS HO are available
o Alcatel-Lucent in RAN supports CSFB Red. Rel.9 (LA4/UA8.1GBSS13) and CSFB PS Handover
(LA4/UA8.1.4 ” not tested)
o Fast Return from 2G is available (B11) but from the same reasons as for Ericsson it is not recommended to
switch it on.
o Fast Return 3G to LTE is available from UA8.1.5.
o As CSFB is based on blind redirection and only considers the highest priority, it is completely dedicated to
UTRAN or GERAN and not on both
 NSN -> CSFB Red. Rel9 is not available
o NSN supports CSFB based on PS HO from RL40
o CSFB Red. Rel9 is planned in RL60.
o Fast Return 3G to LTE ‚Smart LTE Layering‛ is supported from RU40.
o Fast Return 2G to LTE „RG301854: Fast return to LTE‛ is supported from RG30EP1
o NSN supports to activate CSFB to both 3G and 2G with a priority settings between these two RATs
Conclusions on suppliers’ implementation 2/2

System software requirements for CSFB solutions
(*) No terminal available
HUAWEI ERICSSON ALCATEL NSN
CSFB Mode
Blind / With
measurements
E-UTRAN UTRAN GERAN E-UTRAN UTRAN GERAN E-UTRAN UTRAN GERAN E-UTRAN UTRAN GERAN
CSFB with
RRCConnectionRelease with
Redirection (R8)
Blind
eRAN2.0 No Impact
No Impact
L11B
W12B – from
Q2.2012
G10
LA3/LA4 -
from
Q1.2012
UA7
RL20 (to
WCDMA)
RU20 RG20
With
measurements
No Impact (*)
LA4- from
Q1.2012
RL30 (to
GSM) –
from
Q1.2012
RU50 –
from
Q4.2013
CSFB with
RRCConnectionRelease + SIB
(R9)
Blind
eRAN2.2
RAN14.0 -
Q2.2012
GBSS13.0
L12B
(GSM+WC
DMA)
W12B/W13B(
New CS
Fallback
detection
mechanism,
i.e. 3GPP rel. 9
Information
Element CS
Fallback) Q2-
Q4.2013
LA4- from
Q1.2012
UA8.1.4 -
from
Q3.2012
RL30– from
Q1.2012
Future
(>RU50,
Q4.2014)
With
measurements
(GBSS13 for trial) G12A
RL60 (for
GSM) –
from
Q1.2014
RG30 EP1
CSFB with inter-RAT PS HO to
UTRAN or GERAN
Blind
eRAN2.0 RAN12.0 GBSS13.0(*)
L14B –
from Q1
2014
Future
(SRVCC
(CS+PS) ”
W14B)
Future
LA4 - from
Q1.2012
UA8.1.4 -
from
Q3.2012
RL40 –
from
Q4.2012
RU30 –
from
Q1.2012
future
With
measurements
 Actually the most frequent implemented solution is CSFB RRC Release with Redirection Relase 8 without
measurement.
 Due to unavailability of CSFB with PS HO for most vendors, there are only few CSFB based on PS HO
implementation in Orange Group (e.g. Moldova).

TP secret - INTERNAL
Thank you

TP secret - INTERNAL
Backup slides

Vendor specific
solution for CSFB

Huawei solution 1/4
CSFB Mode
Blind / With
measurements
E-UTRAN UTRAN GERAN
CSFB with
RRCConnectionRele
ase with Redirection
(R8)
Blind
eRAN2.0
No
Impact
No Impact
With
measurements
No Impact(**)
CSFB with
RRCConnectionRele
ase + SIB (R9)
Blind
eRAN2.2(*)
RAN14.0
(*)
GBSS13.0(*)
(GBSS12 for
trial)
With
measurements
CSFB with inter-RAT
PS HO to UTRAN or
GERAN
Blind
eRAN2.0 RAN12.0 GBSS13.0(**)
With
measurements
(*) FOA only
(**) No terminal available
Huawei RAN minimum soft release to support CSFB
LTE eRAN 3.0 CSFB required features:
 LOFD-001033 CS Fallback to UTRAN
 LOFD-001034 CS Fallback to GERAN
 LOFD-001035 CS Fallback to CDMA2000 1xRTT
 LOFD-001052 Flash CS Fallback to UTRAN
 LOFD-001053 Flash CS Fallback to GERAN
 LOFD-001068 CS Fallback with LAI to UTRAN
 LOFD-001069 CS Fallback with LAI to GERAN
CSFB to UTRAN : supported mechanisms
 CSFB based on PS handover
 CSFB based on PS redirection (R8)
 Flash CSFB (R9)
CSFB to GERAN : supported mechanisms
 CSFB based on PS handover
 CSFB based on PS redirection (R8)
 Flash CSFB (R9)
 CSFB based on CCO/NACC

Huawei solution 2/4
CSFB to GERAN
Fallback Mechanism
Impact on the
Networks
Impact on the
UEs
CS Service
Access Delay
PS Service
Interruption Time
CSFB based on PS
handover
Complex Complex Short Short
CSFB based on PS
redirection
Simple Simple Long Long
Flash CSFB Medium Medium Short Medium
Fallback Mechanism
Impact on the
Networks
Impact on the
UEs
CS Service
Access Delay
PS Service
Interruption Time
CSFB based on PS
redirection
Simple Simple Long Long
CSFB based on PS
handover
Complex Complex Short Short
CSFB based on
CCO/NACC Medium Medium Medium Medium
Flash CSFB Medium Medium Short Medium
Comparison of CS fallback mechanisms according to Huawei eRAN 3.0
feature description
CSFB to UTRAN

Huawei solution 3/4
CSFB related features on UTRAN (RAN14)
 WRFD-020129 Service-Based PS Service Redirection from UMTS to LTE
 WRFD-140218 Service-Based PS Handover from UMTS to LTE
 WRFD-140226 UMTS-to-LTE Fast Return enables a UE that moves from an LTE cell to a UMTS cell
through CSFB to quickly return to the LTE network after the call is terminated. This feature works as follows:
 The RNC first determines that a UMTS/LTE UE is a CSFB UE when the UE meets either of the following
conditions:
 The UE moves from an LTE cell to a UMTS cell through a PS handover. The RELOCATION
REQUEST message contains a "cause" information element (IE) whose value is "CS Fallback
triggered (268)" or a "CSFB Information" IE whose value is "CSFB" or "CSFB High Priority".
 The UE complies with 3GPP Release 9.4.0 or later. The first service that the UE processes after
moving from an LTE cell to a UMTS cell is a CS service.
 After the UE finishes its CS service in the UMTS cell, the RNC includes the information about the
neighboring LTE cells whose frequencies have higher absolute priorities than the frequency of the UMTS
cell in an RRC CONNECTION RELEASE message. Upon receiving the message, the UE selects a target
cell based on the information and attempts to camp on this cell.
CSFB related features on GERAN (GBSS14)
 GBFD-511313 CSFB It is recommended that the CSFB be used together with the GBFD-511301 Cell
Reselection Between GSM and LTE and the GBFD-511312 Fast LTE Reselection at 2G CS Call Release.
 GBFD-511301 Cell Reselection Between GSM and LTE
 GBFD-511312 Fast LTE Reselection at 2G CS Call Release

Huawei solution 4/4
Implementation in Orange Moldova
 According to information and experience gathered in OMD during acceptance tests and commercial
implementation no problems were found with Huawei CSFB implementation.
 During tests both RRC Redirection and PS HO modes were tested (with eRAN 2.1). Currently in
commercial network CSFB is based on PS HO is implemented (with eRAN3.0 and RAN14).
 Generally default parameters were kept with some threshold optimization (radio optimization done
case by case).
 UMTS to LTE fast return feature has not been tested yet.
CSFB Performance in Orange Moldova
For LTE to 3G call
 1.9 s difference between reference call set up time in 3G and CSFB call set up time via Redirection
 2.6 s difference between reference call set up time in 3G and CSFB call set up time via PS HO
 0.4 s difference between CSFB R8 w-o meas and R8 w meas
 0.7 s difference between the R8 w meas and PS HO w meas
 0.7s difference between CSFB PS HO w meas and PS HO w-o meas

Ericsson solution 1/3
CS FallBack Rel. 8 in Ericsson solution is implemented as ‚FAJ 121 0856 - CS Fallback to GERAN and UTRAN‛.
Requirements:
SW rel. L11B
GERAN SW. Rel. G10 and higher
UTRAN SW. Rel. W12B
 FAJ 121 0493 WCDMA Session Continuity, Coverage-Triggered must be activated
FAJ 121 0495 GERAN Session Continuity, Coverage-Triggered must be activated
There must be at least one defined frequency relation for mobility. The GeranFreqGroupRelation and UtranFreqRelation MOs
represent frequency relations.
A license for the CS Fallback to GERAN and UTRAN feature is activated. The CsfbToGeranUtran Managed Object (MO)
holds the licensing parameters.
In the LTE area should be good 2G/3G coverage
It is recommended to switch on SIB6 in case LTE CSFB to WCDMA and SIB7 in case LTE CSFB to GERAN, which contain
information and parameters required by the UE to perform idle mode reselection to the 2G/3G networks.
ERICSSON
CSFB Mode E-UTRAN UTRAN GERAN
CSFB RRC RwR Release 8
FAJ 121 0856 - CS Fallback to
GERAN and UTRAN
FAJ 121 0493, WCDMA Session Continuity,
Coverage-Triggered
FAJ 121 1610 - CS Voice Fallback from LTE
FAJ 121 0495, GERAN Session Continuity,
Coverage-Triggered
FAJ 121 0876, Redirect with System
Information
FAJ 121 2179: RAN Information Management(RIM)
support for System Information transfer to LTE -
CSFB with PS HO - FAJ 121 1611, Packet Switched Handover from LTE -
Return from 3G/2G->LTE
- FAJ 121 2174: Release with Redirect to LTE within
the WCDMA Radio Access Network (RAN)
FAJ 121 1873, Fast Return to LTE after Call
Release
FAJ 123 155 - GSM-LTE Cell Reselection

Features realted to CSFB
FAJ 121 1873, Fast Return to LTE after Call Release For users that are in LTE and place a call with the CS Fallback
procedure, this feature ensure that the user can quickly return to LTE after the call has ended and continue with the data
session. It reduces time spent in GSM after a call release, ensuring that the user can quickly benefit from LTE connectivity
and end-user performance is improved since the user faster can re-establish the data session in LTE.
FAJ 121 2179: RAN Information Management (RIM) support for System Information transfer to LTE RIM support for system
Information transfer to LTE facilitates faster call setup in WCDMA by updating an eNB with information about external UTRAN
cells. The support for RIM procedures that the feature introduces makes it possible for an eNB to receive information for a
specific UTRAN cell or to create and maintain relationships with UTRAN cells. Such relationships are referred to as RIM
associations. The main benefit of using this feature is to shorten call setup time for CSFB by delivering SI to UE through core
network.
FAJ 121 2174: Release with Redirect to LTE within the WCDMA Radio Access Network (RAN) This feature provides basic
mobility for an LTE-capable UE from 3G network to LTE. Whole process is controlled by network and it can be initiated at
several trigger points. Each of them can be individually switched on/off. This feature is switched on per each UTRAN cell and
redirection to LTE can be initiated from the active states CELL_DCH, CELL_FACH or URA_PCH.
FAJ 121 0876, Redirect with System Information (in case when RRC Release with Redirection Rel. 9 is implemented) This
feature extends the other features by including system information for cells belonging to target Radio Access Technology
(RAT) when performing Radio Resource Control (RRC) connection release with redirect to GSM or WCDMA. The system
information enables the UE to connect to target GSM or WCDMA cell faster.

CS: ~1.5s (UTRAN) / ~2.5s (GERAN) higher call setup time
PS: stop in non-DTM GERAN or RAU to establish PS bearers (5-6s)
CSFB Performance in Ericsson CSFB Release with Redirection Rel. 8
CSFB Performance in Ericsson CSFB Release with Redirection Rel. 9
CS: ~0.5s higher call setup time
PS: stop in non-DTM GERAN or RAU to establish PS bearers (5-6s)
CS: ~0.5s higher call setup time
PS: ~0.2s PS interruption in WCDMA
CSFB Performance in Ericsson CSFB Release based on PS HO
* all times comapring to call setup time in 3G

Alcatel-Lucent solution 1/4
In Alcatel-Lucent eUTRAN portfolio, since version LA3.0, three CS fallback features are supported in
eNodeB [AluLpug]:
FRS-92025: CS Fallback to UTRA for Voice Calls,
FRS-92026: CS Fallback to GERAN for Voice Calls and
FRS-116051: Support of enhanced RRC Releases redirection for CSFB to UTRAN
” System information (SIB), of up to 16 UTRAN cells manually provisioned by operator are provided to UE in
release/redirection to reduce the need for UE to read the SIBs after reselection to UMTS and reduce the overall
call setup time.
In LA4.0, a new enhanced redirection procedure is supported for CS fallback to either UTRAN or GERAN:
FRS-114190.1: CSFB Enhancement to UTRAN/GERAN ” Enhanced Redirection and PSHO
” For all LTE to UTRAN/GERAN redirections including CS fallback redirection and non-CS fallback redirection,
except for the case when redirection is triggered by event A2 ‘Below-Serving-Floor’. The eNB is able to
automatically retrieve system information from the neighbour UTRAN/GERAN cells through RIM (RAN Information
Management) procedures.
Along with following counters related to CSFB available in NPO:
” VS_CsFallbackRequest_ActiveUENonEmergency(LC12853_1)(EVENT)
” VS_CsFallbackRequest_IdleUEEmergency(LC12853_2)(EVENT)
” VS_CsFallbackRequest_IdleUENonEmergency(LC12853_0)(EVENT)
” VS_RedirectionToUtraFdd_CsFallbackTriggered(LC12715_6)(EVENT)
” VS_RedirectionToUtraFdd_CsFallbackTriggered(LC12715_6)(EVENT)

CSFB configuration in eUTRAN
Feature activation is done at the eNodeB level through eNB NE instance / Components / Activation Service /
Activation Service ID 0 [AluVal]:
– isCsFallbackToUtraAllowed = true
– isMobilityToUtranAllowed = true
– isCsFallbackToGeranAllowed = true
– isMobilityToGeranAllowed = true
If apropriate flags are set the CS fallback will be triggered when eNodeB receives CSFallbackIndicator.
At the LTE Cell level, the target RAT/carrier to be used by CS fallback is configured.
The MobilityPriorityTable::defaultConnectedPriorityOfFreq and ServiceTypePriority::eMctaPriority associated
with each RAT/carrier neighbors need to be configured.
CSFB R8 redirection feature’s activation consists only of one flag that needs to be set to true.
When CS Fallback will be triggered on UE, the UTRAN FDD freq for which the defaultConnecterPriorityOfFreq is
the highest will be indicated as the UTRAN FDD target for redirection in the RrcConnectionRelease message. UE
will scan this 3G frequency and select a 3G cell to initiate its CS Call.
Besides, from LTE side, it is not necessary to declare 3G & 2G neighboring cells as the mechanism that will be
triggered to perform CS Fallback is Blind Redirection. Nevertheless, the TAC-LAC neighboring has to be declared
on MME side to avoid misalignment.

Engineer guidelines
It is recommend using Redirection R8 (blind) scenario for Alcatel-Lucent implementations. This option was
thoroughly tested during the ALU LTE RAN validation and proved to be robust and well working. R9 (RIM - RAN
Information Management; or enhanced redirection as in LPUG) based Redirection is a very interesting
enhancement but not supported on EPC during validation. It should be the target release when fully supported
and tested in lab environment because there should be noticeable enhancements in voice call connection times.
PS Handover needs to be enabled on ALU in order to enable measurements before blind redirection.
Feature CS Fallback to UTRA for Voice Calls (FRS-92025) over the LA04.0.3 EL3 eNodeB software can be
recommended. It is valid for CS Fallback to UTRAN Release 8 redirection with blind redirection in accordance with
Alcatel-Lucent’s implementation.
During tests of the abovementioned scenario no protocol or behavior issues have been observed, CSFB (mobile
originating, mobile terminating, mobile to mobile and emergency call) is successfully completed and blind
redirection toward the highest priority carrier defined in eMCTA is well respected.
CS Fallback to UTRAN Release 9 redirection requires on UTRAN side the UA8.1.4 release. The lab validation is
planned in Q4 2013 due to late availability of this feature on core network.
Measurement based CSFB requires the support of PS Handover which is not supported by the 2012A Ericsson
SGSN release.
The feature CS Fallback to GERAN for Voice Calls (FRS-92026) is working fine but not recommended due to long
transition times leading to long connection time values.

Features realted to CSFB - eMCTA
eMCTA is a proprietary ALU mobility management framework in the eNodeB used for allocating the traffic efficiently for
LTE sessions across multiple RAT and multiple LTE RF carriers based on enhanced triggers.
As the CSFB mobility procedure is based on blind redirection, only one best carrier is selected, which should be the one
with highest priority assigned for each Service Type.
For each service type a different UTRAN FDD carrier can be designed as highest priority and CSFB blind redirection is
triggered toward the appropriate one when conditions of the service type are fulfilled.
As CSFB is based on blind redirection and only considers the highest priority, it is completely dedicated to UTRAN or
GERAN and not on both. No mechanism was implemented on the RAN to consider GERAN if there is no coverage on
UTRAN.
Features realted to CSFB - RAN Information Management (RIM) procedure for UTRAN
RAN Information Management (RIM) procedure is used to retrieve the system information of a neighbor UTRAN cell to
be included in enhanced redirection (R9).
To support the RIM procedure, operator should configure one or multiple instances of RncAccess that control the UTRAN
neighbor cells (pointed to by UtranFddNeighboringCellRelation::rncAccessId) and indicate whether RIM for UTRAN SI
feature is supported by the RNC or not (RNCAccess::rimForUtraSiEnabled). Operator should also configure one or
multiple instances of UtraSiTimersConf used by RIM procedure to retrieve system information from the RNC (pointed to
by RncAccess::utraSiTimersConfId).

NSN solution 1/4
CS FallBack Rel. 8 in NSN solution is implemented as “CS Fallback to UTRAN or GSM via Redirect (LTE562)”.
System requirements for CSFB:
System release – RL20
eNodeB - LBTS2.0
MME - NS20
S-GW - NG20
To activate on CSFB feature (LTE562) in NSN it should be done following steps:
 In the LNBTS MOC set the parameter Activate CS Fallback Via Redirection from drop-down list to Enabled. (This will set
actCSFBRedir flag to Enabled).
 It is required to provide REDRT MOC instances related to target carriers used by UE which are: redirection process and priorities
values between different target RATs.
Fast return feature should be implemented in very good LTE coverage area
NSN
CSFB Mode E-UTRAN UTRAN GERAN
RL20: LTE562: CSFB to UTRAN or GSM via redirect
RL40:LTE1073:Measurement based redirect to UTRAN RU20 RAN2067 -
RL40: LTE984 GSM Redirect with System Information
RL60:LTE498RIM for GSM
- RG301854 EP1: Fast return to LTE
CSFB with PS HO
RL40:LTE736:CSFB to UTRAN with PS HO
LTE 56 IRAT HO WCDMA
RL60: LTE57: Inter RAT HO from UTRAN RU20:RAN2176LTE PS HO -
Return from 3G/2G->LTE - RU40:RAN2717Smart LTE Layering RG301854 EP1: Fast return to LTE

NSN solution 2/4
Fast return to LTE (RG301854). The feature RG301854 enables fast return UE from 2G network to LTE
once call ends. It is recommended to activate this feature in area where both LTE and GSM network
coverage is good. In normal case, on return path to LTE, UE measures LTE quality, performs LA/RA update
in GSM and then makes cell reselection to LTE with TA update in LTE domain. It takes approximately 32s.
During this time the service break is observed. Fast return to LTE enables UE come back to LTE almost
immediately ” it needs approx. 0.45s because target LTE frequency and other parameters are delivered by
BSC to mobile in Channel Release Command. In Figure 38 below simple schema how this feature works is
presented.
Smart LTE Layering. This feature is planned in RU40. It enables faster return from 3G network to LTE.
Generally while the feature is activated, when CSFB call ends in 3G network in Call Release command UE is
triggered to make cell reselection to LTE domain.
NSN plans introduce some new features to improve/extend CSFB feature in all RATs:
“ Smart LTE Layering in RU40 (RAN2717)
“ LTE 1073 Measurement Based to Redirect UTRAN (RL40)
“ Smart LTE Layering in RU40 (RAN2717)
“ Smart LTE Handover in RU50 (RAN2264)
“ Inter System NCCR for LTE in RG30 (RG301737)
“ Fast return to LTE in RG30EP1 (RG301754)
“CS Fallback with Redirection with Sys Info (RIM) ” not planned in WCDMA

NSN solution 3/4
RAN2067: LTE Interworking should be activated ” it enables cell reselection from 3G to LTE. Feature RAN2067 LTE
interworking is involved in cell re-selection process from UTRAN to LTE. This feature is responsible for cell re-selection in
situations where UE lacks LTE coverage and camp on 3G site.
BSS21529: ‚LTE System Information w/ MML support‛ is the first step towards interworking between the 2G and LTE
networks. This feature supports cell reselection from 2G to LTE network. System Information message type 2quater (SI2q),
which is broadcasted on BCCH, carries information about LTE cells.
LTE762: Idle Mode Mobility from LTE to WCDMA, GERAN. This feature is available from RL10 and it is responsible for
mobility from LTE to WCDMA in IDLE state. LTE762 makes easier operator to apply strategy to keep idle mode UEs in LTE or
in WCDMA.
LTE56: Inter RAT Handover to WCDMA . This optional feature is available in RL30 and it is coverage based inter-RAT Packet
switched (PS) handover from LTE to WCDMA. Typically scenario when PS HO will be observed is poor coverage of LTE
network.
LTE442: Network Assisted Cell Change to GSM. This feature is available in RL30 software release and it enables service
continuity for data service when moving from LTE to GSM.
LTE984: GSM Redirect with System Information. This feature is planned in RL40 software release and needs LTE562 feature
activated. It enables the CS fallback with redirect to GSM as well as the normal redirect to GSM but it is enhanced by system
information. In this situation call setup time is improved in comparison to basic redirect solution.
LTE736: CS Fallback to UTRAN CSFB to UTRAN (LTE736) feature will be available in RL40 software release. PS handover
based CS fallback from LTE to UTRAN (WCDMA/TDSCDMA) for mobile originated and mobile terminated call setup is
supported. The target cell selection is based on fast measurement solicitation.

NSN solution 4/4
CS: ~1.5s (UTRAN) / ~2.5s (GERAN) higher call setup time
PS: nd
CSFB Performance in NSN CSFB Release with Redirection Rel. 8
CSFB Performance in NSN CSFB Release with Redirection Rel. 9
CS: ~0.7- 1.2s higher call setup time
PS: nd
CS: ~0.6 ” 1.1s higher call setup time
PS: ~0.2s PS interruption in WCDMA
CSFB Performance in NSN CSFB Release based on PS HO
* comapring to call setup time in 3G

RRC states and tranistions
Handover
CELL_PCH
URA_PCH
CELL_DCH
UTRA_Idle
E-UTRA
RRC_CONNECTED
E-UTRA
RRC_IDLE
GSM_Idle/GPRS
Packet_Idle
GPRS Packet
transfer mode
GSM_Connected
Handover
Reselection Reselection
Reselection
Connection
establishment/release
Connection
Connection
CCO,
Reselection
CCO with
optional
NACC
CELL_FACH
CCO, Reselection

REFERENCES
• CSFB and fast return recommendations_01082013 v2.doc
• CSFB and fast return recommendations_exec summary_01082013 v2.doc
• 3GPP TS 23.272 v8.12.0 (2012-06) Circuit Switched (CS) fallback in Evolved Packet System (EPS);
Stage 2 Release 8
• 3GPP TS 23.272 v9.13.0 (2012-12) Circuit Switched (CS) fallback in Evolved Packet System (EPS);
Stage 2 Release 9
• Alcatel LTE Ran LA4 CSFB R8 validation report-Final, 2012
• Alcatel FDD eNodeB LTE Parameters User Guide, LPUG LA4, 16.03.2012
• 3GPP TS 23.401 V9.15.0 (03-2013), General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access
• 3GPP TS 23.216, Single Radio Voice Call Continuity (SRVCC);
• 3GPP TS 23.292, IP Multimedia Subsystem (IMS) centralized services
• 3GPP TS 23.060 V9.14.0 (03-2013), General Packet Radio Service (GPRS); Service description;
Stage 2
• 3GPP TS 36.331 V9.15.0 (06-2013) Radio Resource Control (RRC); Protocol specification (Release 9)
• ‚Circuit-Switched fallback. The first phase of voice evolution for mobile LTE devices.‛, white paper,
Qualcomm 2012
• Ericsson documentation {feature description}, ALEX for LTE L12B
• CSFB in Ericsson for Orange radio Part A2_OLP.pdf, July 2013
• CSFB_Lab&Field_tests_results_PDF_OFR.pdf, 06.12.2012
• ServicemobilityRAN_12.182_CRIP Ericsson, CSFB experimentation report phase 2.pptx, 31.10.2012
• CSFB+Performance+customer.pdf, version 2.1, February 2013,
• NSN documentation RU20, RU30, RL20, RL30, overall system description, feature’s description.
• VoLTE in eNB.pdf, May, 2013 ” Interworking functionalities LTE-WCDMA-GSM
• eRAN 3.0 Feature Documentation
• RAN14 Feature Documentation
• GBSS14 Feature Documentation
• Radio Optimization Guidelines: Mobility strategy and parameters settings for LTE deployment

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