2G-3G Cell Selection & Re-Selection ZTE

1. 2G-3G Cell Reselection & Handover
ZTE university

2. Objectives
 To know interworking between 2G-3G networks
 To master strategies of reselection & handover
between 2G-3G networks
 To know algorithms of cell reselection & handover
between 2G-3G networks

3. Contents
 Conditions for 2G-3G interworking
 2G-3G reselection & handover strategies
 2G-3G reselection algorithms
 2G-3G handover algorithms
 2G-3G load-balance handover algorithms

4. Conditions for 2G-3G interworking
Definition of UTRAN
adjacent cells
Define cells out of BSC
Define adjacent cell for
cell reselection/ handover
2Quater system
message
MS reads system
message
Measurement of UTRAN
systems
Setting of 2G-3G
interworking parameters
Handover & reselection
parameters
2G covered
area
3G covered
area
Make
measurement of
3G network at
the its edge area
MS moves to 2G
network after
originating a
call in 3G
network

5. Conditions for 2G-3G interworking
 2quater message

The precondition for system to send message is that data
configuration must meet one of the following criteria:

The cell is configured with 3G adjacent cells;

The cell supports NC2 function (network controls cell
reselection);
 2quater content

Description of FDD(TDD) at UTRAN side

Description of 3G measurement parameters

Description of GPRS 3G measurement parameters

Network control measurement parameters

Information of cells adjacent to UTRAN FDD(TDD)

6. Conditions for 2G-3G interworking (extended
functions of inter-working)
 IUR-G interface

To realize wireless integration of 2G and 3G networks

To reach inter-system balance based on information of GSM
cell capacity and load

To select handover target cell based on information of GSM
cell capacity and load

To select target cell of RAB redirection based on information of
GSM cell capacity and load

7. Conditions for 2G-3G inter-working (extended
functions of inter-working)
 Selection of target cell of RAB redirection based on
information of GSM cell capacity and load

8. Contents
 Conditions for 2G-3G interworking
 2G-3G reselection & handover strategies
 2G-3G reselection algorithms
 2G-3G handover algorithms
 2G-3G load-balance handover algorithms

9. 2G-3G interworking strategies
 Reselection
 PLMN selection/cell reselection
 2G-3G interworking occurs when UE is in idle mode
 Handover
 CS handover/PS handover

When subscriber originates service, 2G-3G interworking occurs
during the service connection between UE and mobile
networks. The service falls into two types:
– CS handover （ CS ）
– PS handover （ PS ）

10. 2G-3G interworking strategies
Areas of high value
Key areas
WCDMA ： CS service/visual service/ medium-rate PS service
Whole network coverage
GSM/GPRS ： CS service/ low-rate PS service
capacitycapacity
coveragecoverage GSM covers the
whole area
WCDMA covers key areas
Actual
capacity
Constructed
capacity
Actual
capacity
Constructed
capacity
GSM faces heavy
capacity load
WCDMA is idle; need to
explore subscribers
Relation between GSM
and WCDMA at the initial
state:
WCDMA focuses on
coverage and tries to
form complete coverage
with the help of GSM;
GSM reduce its load
pressure with the help
of WCDMA.

11. 2G-3G interworking strategies
 2G-3G interworking strategies:
 Only when UE leaves 3G-covered area , will it perform
3G-2G cell reselection; once it returns to 3G-covered
area, it will start 2G-3G cell reselection.
 In order to ensure the continuity of speech, 3G-2G
handover should be supported.
 CS service which is being performed in 2G network
doesn’t need handover to 3G network, so as to reduce
call drops due to UTRAN handover (hard handover).
 In order to guarantee PS subscribers’ satisfaction, bi-
directional handover of PS service is recommended.

12. Contents
 Conditions for 2G-3G interworking
 2G-3G reselection & handover strategies
 2G-3G reselection algorithms
 2G-3G handover algorithms
 2G-3G load-balance handover algorithms

13. 2G-3G Reselection algorithms-PLMN
selection/reselection
 UE can select and roam between 2G and 3G networks through
selecting PLMN （ MCC+MNC ） , and there is no need to upgrade
2G network.
 Whether 3G and 2G adopt the same PLMN-ID, the strategy of
‘preferably selecting 3G network’ can be realized.
Set 3G network to be home
network （ HPLMN ） ,UE
searches and makes
reselection periodically after
it switches to 2G
network （ VPLMN ） .
3G and 2G adopt different
PLMN-ID
Set 3G network with the
preferred radio access
technology （ Access
Technology ） , then UE will
preferably adopt 3G to make radio
access when it selects PLMN.
3G and 2G adopt the
same PLMN-ID

14. 2G-3G Reselection algorithms-PLMN
selection/reselection
 PLMN selection
& reselection
3G and 2G networks
are configured with
different IDs during
network planning;
HPLMN (home
network) of USIM is set
to be 3G when an
account is opened;
Set USIM PLMN
reselection interval when
an account is opened;
note there is an
“hysteresis of 6mins”.
PLMN selection
RPLMN
HPLMN
“Operator Controlled PLMN Selector
with Access Technology” in USIM
PLMN with good signal quality (in random
order)
Other PLMN [in order of signal quality (high-low)]
PLMN selection after
UE is powered
Change of
PLMN （ ie.
reselection,
which occurs
during cell
reselection ）
+
“User Controlled PLMN Selector with
Access Technology” in USIM
Histeresis of 6mins ： after UE resides in GSM network, it has to wait for at least
6mins before it can return to 3G network.

15. 2G-3G Reselection algorithms-cell
selection/reselection
 Requirements for 2G network equipment
 Cell reselection is dependent on subscribers’ satisfaction; as long
as the UE is within 3G network coverage, it can access 3G network
immediately.
 This algorithm requires 2G-3G cell reselection in the whole
network, so 2G network needs to be upgraded to some extent.
 Wireless controllers at 2G sites
 They should support the reselection
threshold parameters of cells configured
with broadcast UTRAN system.
 They should support configuration of 3G
adjacent cells.
Requirements for 2G network equipment

16.  Cell selection: select the cell with the best signal in current network;
 Cell reselection: as signals of the serving cell (in which UE resides)
and its adjacent cells change/deteriorate, UE needs to select a new
cell with better signal to reside.
Reside in
original cell
Reside in cell at random
（ call barring ）
Cell is non-HPLMN
Confirm primary
scrambling code of
cell
S principle not satisfied, keep
searching cells
Search cell S principle
Read broadcast
channel
Normal
residence
Keep
searching
cells
PLMA selection
Y
N
N
2G-3G Reselection algorithms-cell
selection/reselection

17. 2G-3G Reselection algorithms-cell
selection/reselection-S principle
Formula:
Squal = Qqualmeas – Qqualmin
Srxlev = Qrxlevmeas – Qrxlevmin – Compansation
If
For FDD: Squal > 0 & Srxlev > 0
For TDD: Srxlev > 0
then UE will take the cell as a suitable one and reside in it.
S-principle is used to judge whether a cell’s signal RxLev
and RxQual are suitable for UE to redside in the cell.

18. 2G-3G Reselection algorithms-cell selection/reselection
-reselection parameter 1
Cell reselection flow in idle mode & key parameters:
Parameter Meaning Value
range
Qsearch_I Search for 3G cells if signal level is below (0 7) or above‑
(8 15) threshold‑
0 = - 98 dBm, 1 = - 94 dBm, … ,
6 = - 74 dBm, 7 = ∞ (always)
8 = - 78 dBm, 9 = - 74 dBm, … ,
14 = - 54 dBm, 15 = ∞ (never).
Default value = ∞ (never).
0-15
Location: 3G adjacent cell list and cell reselection parameters are described in system
broadcast message SI2quater （ and SI2ter ） which is sent on BCCH or extended
BCCH.
Function: to control the adjacent cell measurement and reselection process under UE
idle mode.
Location: 3G adjacent cell list and cell reselection parameters are described in system
broadcast message SI2quater （ and SI2ter ） which is sent on BCCH or extended
BCCH.
Function: to control the adjacent cell measurement and reselection process under UE
idle mode.

19. 2G-3G Reselection algorithms-cell
selection/reselection-reselection parameter 2
 Cell reselection flow in idle mode & key
parameters:
Parameter Meaning Value
range
FDD_Qoffse
t
Applies an offset to RLA_C for cell re selection to access‑
technology/mode XXX (one or more) ： 0 = - ∞ (always select a
cell if acceptable), 1 = -28 dB, 2 = -24 dB, … , 15 = 28 dB.Default
value = 0 dB.
0-15
FDD_Qmin A minimum threshold for Ec/No for UTRAN FDD cell re-
selection,0= -20dB,1=-6dB, 2= -18dB, 3= -8dB, 4= -16dB, 5=
-10dB, 6= -14dB, 7= -12dB.Default
value= -12dB.
0-7

20. 2G-3G Reselection algorithms-cell selection/reselection
-reselection parameter 2
Application of FDD_Qoffset and FDD_Qmin
这两个参数控制了跨系统的小区重选过程
These two parameters control inter-system cell reselection process. When
a UE satisfies all the following conditions, it can make reselection to
UTRAN adjacent cells.
RSCP >= FDD_Qoffset + RLA_C
RSCP >= FDD_Qoffset + RLA_n
Ec/No >= FDD_Qmin-FDD_Qmin_Offset
RSCP>= FDD_RSCP_threshold
Notes:
2G-3G cell reselection is not allowed within 5s after 3G-2G reselection; if
more than one 3G cell satisfies the conditions, UE shall select the cell with
the largest RSCP value.
Notes:
2G-3G cell reselection is not allowed within 5s after 3G-2G reselection; if
more than one 3G cell satisfies the conditions, UE shall select the cell with
the largest RSCP value.

21. 2G-3G Reselection algorithms-suggestions
 Principle of field strength
 Only when the field strength of 3G network is guaranteed, will
reselection to 3G occur. Normally, in 3G covered areas with weak
field strength and the marginal areas overlapped by 3G and GSM
networks,this principle shall be considered first.
 Principle of time efficiency
 On condition that the principle of field strength is satisfied, selection
to WCDMA should be as soon as possible, so as to provide
subscribers with high-end services. In areas continuouly covered
by WCDMA network, this principle shall be considered first.
 Principle of electricity efficiency
 Dual-mode terminal consumes more power in performing inter-
system signal measurement, so this principle shall be fully
considered when we deploy networks with overlapped coverage
areas.

22. 2G-3G Reselection algorithms-cell reselection
（ PS ）
 PBCCH supported in BSS
 PSI3quarter sends parameters related to inter-system cell reselection (3G
adjacent cell information, measurement and control parameters of cell
reselection) to MS.
 PBCCH not supported in BSS / PBCCH not configured
 Packet Cell Change Order or Packet Measurement Order send parameters
related to inter-system cell reselection (3G adjacent cell information,
measurement and control parameters of cell reselection) to MS; these
parameters will be used in PS cell reselection process.
parameter Meaning Value range
Qsearch_P Search for 3G cells if signal level below threshold (0-7): - 98,
- 94, … , - 74 dBm, ∞ (always) or above threshold (8-15):-
78, - 74, … , - 54 dBm, ∞ (never). Default value = ∞ (never).
0-15
FDD_
GPRS_
Qoffset
Applies an offset to RLA_P for cell re selection to access‑
technology/mode XXX (one or more), 0 = - ∞ (always select
a cell if acceptable), 1 = -28 dB, 2 = -24 dB, … , 15 = 28 dB.
Default value = 0 dBm.
0-15

23. 2G-3G Reselection algorithms -NCCR
 Explanation of （ NC2/NCCR ）
 It supports different cell reselection parameters in PS service to realize cell
reselection of “packet idle mode” and “packet transmission mode”.
 Advantage: it makes full use of available network information to make
reasonable judgment and realize optimized distribution of network
services; it reduces the useless cell reselection performed by MS, and
improves TBF transmission efficiency.
Network_Control-
Order
Meaning of
value
MS operations
NC0 Controlled by
normal MS
MS executes autonomous cell reselection
NC1 Controlled by
MS which
reports MRs
MS sends MRs to network; MS continues cell reselection.
NC2 Controlled by
network
MS sends MRs to network, and realizes cell reselection only
based on the reselection order from network. Only when cell
reselection is triggered by failure of sending downlink signalling or
random access, will MS execute autonomous cell reselection.

24. 2G-3G Reselection algorithms -NACC
 Explanation of （ NC2/NCCR ）
 3GPP R5 stipulates NACC’s ability to support inter-BSC and UTRAN
services.
 Realization of （ NC2/NCCR ）
 BSC and RNC can collect information of external cells through Gb (lur-g)
interface and RIM process. (Currently we can only realize external NACC
through Gb interface and RIM process.)
Master BSC enables external cells to start RAN Information Send process through
RAN Information Request, so as to transmit the system information of external
cells to the serving cell.
BSC/RNC 1 BSC/RNC 2CN
RAN Information Request

25. Contents
 Conditions for 2G-3G inter-working
 2G-3G reselection & handover strategies
 2G-3G reselection algorithms
 2G-3G handover algorithms
 2G-3G load-balance handover algorithms

26. 2G-3G handover algorithms- （ 3G->GSM ）
 For 3G-GSM CS handover, main requirements stay with
3G network; for GSM network, attention should be paid to
the following:
 BSSAP signaling Handover Request at A interface should at least
support R99 (version), which means it should adapt to the changes
of coding mode of Cell Identifier (Serving) which carries 3G serving
cell ID;
 Ignore the RF value (6 , 7) in classmark2 which is carried in
BSSAP signaling Handover Request;
 For the encrypted algorithm sent by GSM MSC, even the
encryption function is not enabled in BSS, it can also ignore the
encrypted algorithm and perform the following handover process.
 When BSS sends HANDOVER REQUEST ACK to GSM MSC,
Chosen Encryption Algorithm IE must contain the specific
encryption mode, whether encryption function is enabled or not.

27. 2G-3G handover algorithms-- （ 3G->GSM ）
 Signaling flow of 3G-GSM CS handover

28. 2G-3G handover algorithms- （ GSM ->3G ）
 GSM cell collects information of 3G adjacent cells and measurement
parameters.
 BSC broadcasts SI 2quater on BCCH or extended BCCH;
 MS may receive Measurement Information in downlink SACCH sent by
BSC during a call.
3G initial
stage
3G expansion
stage
3G mature
stage
HO due to
coverage
HO due to
coverage
HO due to
coverage
HO due to
coverage
HO due to
coverage
HO due to
coverage
HO due to serviceHO due to serviceHO due to serviceHO due to service
Network resource
optimization
Network resource
optimization
Network resource
optimization
Network resource
optimization
Messages to be collected
by MS
Description of 3G
adjacent cell parameters
Configuration of 3G
adjacent cells
Parameters of adjacent
cell measurement
Handover parameters

29. 2G-3G handover algorithms- （ GSM ->3G HO
parameters ）
Parameter Meaning Value range
Qsearch_C Search for 3G cells if signal level below threshold (0-7):
- 98, - 94, … , - 74 dBm, ∞ (always)
or above threshold (8-15):
- 78, - 74, … , - 54 dBm, ∞ (never)
0-15
Qsearch_C_In
itial
Indicates the Qsearch value to be used in connected mode before
Qsearch_C is received,
0 = use Qsearch_I, 1 = ∞ (always).
Default value = use Qsearch_I.
0/1
3G_SEARCH_
PRIO
Indicates if 3G cells may be searched when BSIC decoding is
required,
0 = no, 1 = yes
Default value = yes
0/1
Besides the important parameters listed above, some other parameters are also sent to MS in the system
messages, such as:
FDD_REP_QUANT （ measurement of quantity, RSCP or Ec/No ）；
REPORT_TYPE （ type of MR, MR or EMR ） ;…

30. 2G-3G handover algorithms- （ GSM ->3G
HO parameters ）
 Signaling flow of GSM-3G CS handover

31. 2G-3G handover algorithms-strategy of network
selection priority
The strategy is to decide which network shall be prioritized for handover.
 To decide target cell through processing of MRs and the strategy of network
selection priority :
 MSC controls strategy of network selection priority
HANDOVER REQUEST carries “Service Handover” field, which is used to indicate
handover strategy for BSC in the following handover process.
 BSC controls the strategy of network selection priority
Based on the configuration principles set in OMCR, BSC decides which network shall
be prioritized for handover according to the strategy of network selection priority,
which can be set in OMCR as well.
 Both ways can be adopted:
 Prioritize WCDMA cells ；
 Prioritize GSM cells
 What’s more, if both WCDMA and TD-SCDMA adjacent cells are available at
the same time, the priority of WCDMA and TD-SCDMA can be decided
through parameter “FDD/TDD handover” （ FDDTDDPrio ） at iBSC side.

32. Contents
 Conditions for 2G-3G interworking
 2G-3G reselection & handover strategies
 2G-3G reselection algorithms
 2G-3G handover algorithms
 2G-3G load-balance handover algorithms

33. Load-balance handover algorithms- directed-
retry handover process
UE UTRAN
Signaling connection establishment request
Signaling connection establishment reject
Signaling establishment fails due to lack of
resource in 3G network; 3G is redirected to be
2G network; UE starts access again in 2G
network without target cell defined.
To support directed-retry, the switch DrInd shall be open at iBSC OMCR.

34. Load-balance handover algorithms- traffic
handover process
For 3G-GSM CS handover ， requirements stay with WCDMA network.
CN UTRAN
Assignment request
Success response to
assignment request
When RNC receives RAB assignment message, if CN
requires handover of corresponding services to 2G
system, UTRAN handover process will be started, which is
3G-2G handover, thus 3G network performance will be
optimized.
Handover request

35. Load-balance handover algorithms- handover
due to serving cell load
CNUTRAN
Assignment request
Assignment failure
Service establishment fails due to lack of resource in 3G
network; if it’s 2G-supported CS service or low-rate PS
service, 3G will appoint suitable 2G cell, and CN will start
handover to the appointed 2G cell.
iBSC sends LOAD INDICATION （ BSSAP ） to MSC periodically.

36. Load-balance handover algorithms-
comparative analysis of handover algorithms
Methods
Voice
hysteresis
Success
rate
Influence to 3G
network
Application scenario
Directed-retry Long Low Light
Signalling establishment
failure
Service
handover
Short High Serious
CN requests handover of
service to 2G network after
service establishment
success.
Handover for
load balance
Short High Serious Cell with light congestion