2G-3G Cell Reselection & Handover
ZTE university
Learning goals
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
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
Conditions for 2G-3G interworkingDefinition of UTRAN adjacent cells
Define cells out of BSCDefine adjacent cell for cell reselection/ handover
2Quater system message
MS reads system messageMeasurement 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
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)
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
Enhanced IUR-G-based 2G-3G handover flow (resource request)
SRNC BSCUE CN
MEASUREMENT REPORT
RADIO RESOURCE PREPARE
RADIO RESOURCE PREPARE READY
RELOCATION REQUIREDHANDOVER FROM UTRAN COMMAND
HANDOVER REQUEST
HANDOVER REQUEST ACKNOWLEDGE
RELOCATION COMMAND
HANDOVER COMPLETE
HANDOVER DETECT
HANDOVER COMPLETE
IU RELEASE COMMAND
IU RELEASE COMPLETE
NODE B
RADIO LINK DELETION REQUEST
RADIO LINK DELETION REPONSE
Conditions for 2G-3G interworking (extended functions of inter-working)
Selection of target cell of RAB redirection based on information of GSM
cell capacity and load SRNC BSCUE CNBSC
RAB ASSIGNMENT REQUEST
RNC Decides to perform Relocation
(Directed Retry)
RAB ASSIGNMENT RESPONSE
RELOCATION REQUIREDHANDOVER FROM UTRAN COMMAND
HANDOVER REQUEST
HANDOVER REQUEST ACKNOWLEDGE
RELOCATION COMMAND
HANDOVER COMPLETE
HANDOVER DETECT
HANDOVER COMPLETE
IU RELEASE COMMAND
IU RELEASE COMPLETE
RADIO RESOURCE PREPARE
RADIO RESOURCE PREPARE READY
Conditions for 2G-3G inter-working (extended functions of inter-working)
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
Reselection
2G-3G interworking strategies
Handover
PLMN selection/cell reselection2G-3G interworking occurs when UE is in idle mode
CS handover/PS handoverWhen 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)
2G-3G interworking strategies
Areas of high valueAreas of high valueKey areasKey areas
WCDMA : CS service/visual service/ medium-rate PS serviceWCDMA : CS service/visual service/ medium-rate PS service
Whole network coverageWhole 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.
2G-3G interworking strategiesIn initial phase of network, 3G subscribers should choose 3G network preferably.
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.
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
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
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.
2G-3G Reselection algorithms-cell selection/reselection
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 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.
Requirements for 2G network equipment
2G-3G Reselection algorithms-cell selection/reselection
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 principleRead broadcast
channelNormal residence
Keep searching
cells
PLMA selectionY
N
N
2G-3G Reselection algorithms-cell selection/reselection-S principle
Formula: Squal = Qqualmeas – QqualminSrxlev = 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.
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) threshold0 = - 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.
2G-3G Reselection algorithms-cell selection/reselection-reselection parameter 2
Cell reselection flow in idle mode & key parameters:
Parameter Meaning Value range
FDD_Qoffset
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
FDD_RSCPmin and FDD_Qmin_Offset are first added in version 3GPP R6, currently they are not supported in system.
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.
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.
这两个参数控制了跨系统的小区重选过程
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
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.
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
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
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.
Currently GSM-WCDMA and WCDMA-GSM PS handover is not supported in iBSC.
2G-3G handover algorithms-- ( 3G->GSM ) Signaling flow of 3G-GSM CS handover
2G MSC BSC3G MSCRNC
PREPARE HANDOVER
UE
RELOCATI ON REQUI RED
HANDOVER REQUEST
HANDOVER REQUESTACKPREPARE HANDOVER
RESPONSERELOCATI ON COMMAND
HANDOVER COMMANDHANDOVER DETECT
HANDOVER COMPLETE
HANDOVER COMPLETESEND END SI GNALREQUEST
RELEASE COMMAND
RELEASE COMPLETE SEND END SI GNALRESPONSE
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 cellsParameters of adjacent cell measurementHandover parameters
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_Initial
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 ) ;…
2G-3G handover algorithms- ( GSM ->3G HO parameters ) Signaling flow of GSM-3G CS handover
2G MSC
HANDOVER COMMAND
HANDOVER REQUI RED
BSC MS
HANDOVER COMMAND
RELEASE COMPLETE
RELEASE
3G MSCRNC
PREPARE HANDOVERREQUEST
PREPARE HANDOVERRESPONSE
RELOCATI ON REQUEST
RELOCATI ONRESPONSE ack.
I AMACM
PROCESS SI GNALLI NGRELOCATI ON DETECT
ANM
SEND END SI GNALRELOCATI ONCOMPLETE
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.
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
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.
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
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.
Load-balance handover algorithms- comparative analysis of handover algorithms
Methods Voice
histeresisSuccess
rateInfluence to 3G
networkApplication 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
Others – avoidance of problems
Solution A+B of TDD When GSM cells are configured with TD adjacent cells, halt or restart may
occur to MS due to flaws in some terminal chips. There are two solutions to the above problem:
Solution A : configure a GSM cell with an FDD adjacent cell and a TDD adjacent cell at the same time; when MS discovers there is FDD adjacent cell in SI2quater, it will not read the information of TDD adjacent cell, so that abnormal phenomena will not occur to MS when it receives information of TDD adjacent cell.
Solution B : set parameter NR_OF_TDD_CELLS=31 in SI2quater to prevent MS entering endless-loop when it reads SI2quater message, so as to avoid abnormal phenomena of MS.
Solution A and solution B are directed at MS chips with different flaws, but they can exist mutually, so they can be used together to solve chip problems. We call this “solution A+B”
Method: change “TDD handover allowed” to “Not allowed” at iBSC.