Cell Selection Re Selection Explained
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Transcript of Cell Selection Re Selection Explained
UMTSCell Selection & ReselectionUMTS02 v2.1Wireless Network Engineering
April 03
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 2
Cell Selection & Reselection
• PLMN Selection– Slot Synchronization
– Frame Synchronization
– Scrambling Code Identification
– System Information decoding
• Cell Selection• Cell Reselection (idle&Cell_Fach)• Location Registration
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 3
Mobility & Cell Selection
• Network Selection
– Mobile in idle mode acquires the best received UMTS cell and identifies its scrambling code
– Mobile in idle mode selects the PLMN
• Cell Selection
– Mobile in idle mode selects the cell to camp on according to basic criteria
• Cell Reselection
– Mobile in idle mode selects a new cell according to parameters broadcasted on the current cell
• Location Registration
– The mobile in idle mode informs the network about a change of location area
• Handover
– Mobile in RRC connected mode in one cell is handed over another cell
4 steps:
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 4
Mobile Supposed Behaviour
Mobile is switched ONMobile is switched ON
Does the UE have in memory the frequency used
previously ?
Mobile is searching for P-SCH on this frequency
Mobile is searching for P-SCH on this frequency
YesYes NoNo
Mobile is scanning the band from the lower UMTS Frequency
Mobile is scanning the band from the lower UMTS Frequency
P-SCH FoundP-SCH Found No P-SCHNo P-SCH
Scanning next frequency
Scanning next frequency
P-SCH FoundP-SCH Found
No P-SCHNo P-SCH
Cell Selection Cell Selection
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 5
Slot SynchronizationP-SCH1
P-SCH3
P-SCH2
P-SCH1S-SCH1 P-CCPCH P-CCPCH
P-SCH2S-SCH2 P-CCPCH P-CCPCH
P-SCH3S-SCH3 P-CCPCH P-CCPCHP-CCPCH
1 Slot = 667s
UE synchronizes on the strongest correlation peak
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 6
Frame Synchronization
……..
2560 chips
acp
Slot # ?
P-SCH acp
Slot #?
16 11S-SCH
acp
Slot #?
2 Group 4Slot 12,13,14
slot number Scrambling Code Group #0 #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14
Group 0 1 1 2 8 9 10 15 8 10 16 2 7 15 7 16
Group 1 1 1 5 16 7 3 14 16 3 10 5 12 14 12 10
Group 2 1 2 1 15 5 5 12 16 6 11 2 16 11 15 12
Group 3 1 2 3 1 8 6 5 2 5 8 4 4 6 3 7
Group 4 1 2 16 6 6 11 15 5 12 1 15 12 16 11 2
…
Group 61 9 10 13 10 11 15 15 9 16 12 14 13 16 14 11
Group 62 9 11 12 15 12 9 13 13 11 14 10 16 15 14 16
Group 63 9 12 10 15 13 14 9 14 15 11 11 13 12 16 10
256 chips
S-SCH
P-SCH
512 Primary Scrambling Codes divided into 64 groups512 Primary Scrambling Codes divided into 64 groups
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 7
Scrambling Code IdentificationP-SCH
S-SCH
P-CPICH
P-CPICH: Predefined sequence (20 bits) spread with Cch,256,0 scrambled with the primary SC
x OVSFCch,256,0
x SC #iSymbol by Symbol
Correlation ?
Primary DL Scrambling Code Known (best correlation peak)
For i = 1 to 8
YesNo
Mapping between group and Scrambling codes defined in TS 25.331
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 8
Cell Information
P-SCH: Coverage indication, Slot Synchronization
S-SCH: Frame Synchronization, Group identification
P-CPICH: Scrambling Code Identification
P-CCPCH: System Information Broadcast
Logical Channel
BCCH
Transport Channel
BCH
Physical Channel
P-CCPCH
OVSF Cch,256,1
Primary Scrambling CodeTransmitted during 9/10th slot
Bit Rate: 12.3 kbps RLC Mode: transparentMac-B: transparent
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 9
FDDCell Parameters
Parameter Object Range Default Value Recommended Value Class
bchPowerRelativetoPcpich FDDCell Float (dB)
Min= -35, max= 15
Step = 0.1
-5 -2 C0
dlFrequencyNumber FDDCell Int [10551..10999]
(UARFCN)
N.A. N.A. C0
pcpichPower FDDCell Float (dBm)
Min=-10 max = 50
Step = 0.1
35 1 carrier : 35
2 carriers: 32
C0
primaryScramblingCode FDDCell Int [0..511] N.A. N.A. C0
pschPowerRelativeToPcpich FDDCell Float (dB)
Min = -35, max = 15
Step = 0.1
-5 -5 C0
sschPowerRelativeToPcpich FDDCell Float (dB)
Min = -35, max = 15
Step = 0.1
-5 -5 C0
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 10
PLMN Selection
Parameter Object Range Default Value Recommended Value Class
MobileCountryCode Network BcdString length = 3 N.A. N.A. C0
MobileCountryCode RNC BcdString length = 3 N.A. N.A. C0
MobileNetworkCode Network BcdString length = 3 N.A. N.A. C0
MobileNetworkCode RNC BcdString length = 3 N.A. N.A. C0
MCC = Mobile Country Code
MNC = Mobile Network Code
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 11
Synchronization of SCH at BTS levelSo as to prevent overlapping of SCH channels from differents cells belonging to the same NodeB are shifted by a certain number of 1/10th of time slot, according to the parameter Tcell, representing the timing delay relative to BFN used for defining start of SCH, CPICH and Donwlink csrambling code for a cellCell #1
Cell #2
Cell #3
Cell #2
Cell #1
Cell #3
STSR3 Case
1 Slot (2560 chips, 0.667 ms)
TCell = 0
TCell = 3
TCell = 6
1 Slot (2560 chips, 0.667 ms)
Reference Slots (form BTS Frame Number Counter)
SCH
SCH
SCH
SCH
SCH
SCH
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 12
Tcell Parameter
Parameter
Object Range Default Value Recommended Value
Class
TCell FDDCell Int [0..9] 0 See rule C0
In case of multi-sectorial site, Tcell parameter has to be different for each sector, so as to prevent problems during handover procedures.
The gap (for Tcell value) has to be as big as possible for cells belonging to the same site.
Example:
1 cell Tcell = 0
2 cells Tcell = 0 and 5
3 cells Tcell = 0, 3 and 6
In case of more than one frequency layer used, Tcell can take the same value for cells belonging to different layers:
Example
Frequency layer 1 Tcell = 0, 3 and 6 for three cells
Frequency layer 2 Tcell = 0, 3 and 6 for three cells
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 13
Cell Selection ProcedureDefinitions:
• Acceptable Cell:
Cell on which UE can obtain limited services (emergency calls)
• Suitable Cell:
Cell on which the UE can obtain a normal service
• UE States for cell Selection / Reselection:
• Camped on any cell:
UE monitors System Information but has chosen a cell irrespective of PLMN identity
• Camped on a cell:
UE monitors System Information and paging information
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 14
Cell Selection Criteria
Suitable Cell Conditions:
Squal > 0 AND SRXLev > 0
Suitable Cell Conditions:
CPICH_Ec/No > qQualmin
AND
CPICH_RSCP > qRxLevMin + Pcompensation
Suitable Cell Conditions:
CPICH_Ec/No > qQualmin
AND
CPICH_RSCP > qRxLevMin + Pcompensation
With:
Squal= Qqualmeas – qQualMin = CPICH_Ec/No – qQualMin
SrxLev= QrxLevMeas – qRxLevMin – Pcompensation = CPICH_RSCP – qRxLevMin – Pcompensation
Pcompensation= MAX (SibMaxAllowedUlTxPowerOnRach – p_MAX, 0)
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 15
Cell Selection Parameters
Parameter Object Range Default Value Recommended Value
Class
qQualMin CellSelectionInfo Int [-24..0]
(dB)
-10 -16 C2
qRxLevMin CellSelectionInfo Int [-115..-25]
Step = 2 (dBm)
-45 -115 C2
SibMaxAllowedUlTxPowerOnRach PowerConfClass Int [-50..33]
(dBm)
33 33 C2
P_Max = maximum UE output power (dBm) according to its class
Power Class Maximum Output Power (dBm)
1 33
2 27
3 24
4 21
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 17
Cell Reselection Procedure
?
2G2G
3GFreq. 2
3GFreq. 2
3GFreq. 1
3GFreq. 1
General Description:
• On a cell, the UE listens to system information and performs radio measurements
• The network controls what the UE shall measure and sends the system information data concerning the neighboring cell
• The UE then uses an algorithm to select a better cell
• The Inter-Frequency algorithm uses a set of broadcast parameters :
• a criteria for searching Inter-frequency cells and performing measurements
• a criteria S to assess whether the cells are eligible
• a criteria R for ranking eligible cells and determining the best one
3G can be favoured compared to 2G by playing on engineering parameters
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 18
Cell Reselection ProcedureSqual
SintraSearch
SinterSearch
SinterRATMeasurement on same frequency Measurement on
other frequencies Measurement on other RAT
If Squal = CPICH_Ec/No – qQualMin < Threshold
Associated measurements are performed
Thresholds are broadcasted in SIB 11In UMTS02, 2 types of measurements are done: Intra frequency and inter RAT
Thr
esho
lds
give
n as
exa
mpl
e
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 19
Cell Reselection Thresholds
Parameter Object Range Default Value Recommended Value Class
sInterSearch CellSelectionInfo Int [-32..20] (dB)
Step=2
-32 6 C2
sIntraSearch CellSelectionInfo Int [-32..20] (dB)
Step=2
-32 6 C2
sSearchGSM CellSelectionInfo Int [-32..20] (dB)
Step=2
-32 6 C2
csDrxCynLngCoef FDDCell Int [6..9] 6 TBD C0
psDrxCynLngCoef FDDCell Int [6..9] 6 TBD C0
Without HCS (Hierarchical Cell Structures, not implemented in UMTS02), for FDD cells, Squal is compared to different threshold broadcast in the System Information to determine which kind of measurement (intra-frequency, inter-frequency and inter-RAT) the UE shall do.
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 20
Cell Reselection: Ranking ?
First step: Eligible Cells Selection:
Squal > 0 and SrxLev > 0 for FDD cells
SrxLev > 0 for GSM Cells
In UMTS02 & 03 qualMeas = CPICH Ec/No (qualMeas determines the cell selection quality measure)
qOffset1sn is a static parameter for FDD neighboring cells, its value is 0
Second Step: 1st Ranking, considering CPICH_RSCP:
Serving cell: Rs = CPICH_RSCP + qHyst1
Neighbor FDD cell: Rn = CPICH_RSCP - qOffset1sn
GSM Cell: RnGSM = RxLev - qOffset1sn
Third Step: 2nd Ranking, considering CPICH_Ec/No
(Only applied if an FDDcell is ranked as the best cell, and qualMeas = CPICH Ec/No (always the case in UMTS02):
Serving Cell: Rs = CPICH_EcNo + qHyst2
Neighbor FDD Cell: Rn = CPICH_EcNo – qOffset2sn
2G2G
3G3G
qHystx and qOffsetxsn are Hysteresis (to prevent Ping-Pong Effect) and Offset (to favoritize a neighbor cell) values for
serving and neighbor cells
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 21
Cell Reselection Measurements
Measurements:
• DRX Cycle
• csDrxCynLngCoef int [6..9]
• psDrxCynLngCoef int [6..9]
These two parameters determine the measurement frequency in idle mode , as defined in TS 25.133
DRX Parameter DRX Cycle Length (s) Nserv (nb of cycles) TmeasureFDD (s) TevaluateFDD (s) TmeasureGSM (s)
6 0.64 4 1.28 5.12 5.12
7 1.28 2 1.28 6.4 6.4
8 2.56 2 2.56 7.68 7.68
9 5.12 1 5.12 10.24 10.24
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 22
Cell Reselection Conditions
• Squal > 0 and SrxLev > 0 for the new cell
• New cell is better ranked than serving cell during tReselection
• More than 1 second has elapsed since the UE camped on the current serving cell
Squal > Threshold
Squal =< Threshold Measurements
Tmeasure
Tevaluate
Cell RankingNew cell choice
tReselection
Cell ReselectionFDD Meas. FDD Meas. FDD Meas.
FDD Meas.
+
GSM Meas.
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 23
Cell Reselection Parameters
Parameter Object Range Default Value Recommended Value
Class
qHyst1 CellSelectionInfo Int [0..40] (dBm)
Step = 2
10 4 C2
qHyst2 CellSelectionInfo Int [0..40] (dB)
Step = 2
4 6 C2
qOffset1sn GSMCell Int [-50..50] (dB) 0 TBD C0
qOffset2sn UMTSFDDNeighbouring
Int [-50..50] (dB) 0 TBD C0
qualMeas CPICH_EcNo or CPICH_RSCP
CPICH_EcNo N.A. Static
tReselection CellSelectionInfo Int [0..31] (s) 31 6 C2
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 25
Mobility in Idle Mode Strategy
Target: Staying in UMTS as much as possibleUMTS cells
GSM cells
Squal
No Intra frequency measurementsIntra frequency measurements performed
No Inter frequency measurementsInter frequency measurements performed
No Inter System measurementsInter system measurements performed
Sintrasearch
Sintersearch
SsearchRAT
Measurement Trigger:
Sintrasearch > SsearchRAT
For the serving cell• Rs = Qmeas,s + qHyst
For the neighboring cells • Rn = Qmeas,n - qOffset
Cell Ranking CriteriaQoffset (3G cell) < Qoffset (2G cells)Qhyst set to a “high value”qRxLevMin setting “not too low” to avoid too many 2G eligible cells
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 27
Cell Access Restrictions
Parameter Object Range Default Value Recommended Value
Class
accessClassBared FDDCell List 0..15
Barred/notbarred
N.A. N.A. C0
barredOrNot FDDCell Barred
notBarred
notBarred notBarred C0
cellReservedForOperatorUse
FDDCell Reserved
notReserved
notReserved notReserved C0
cellRservationExtension FDDCell Reserved
notReserved
notReserved notReserved C0
intraFreqCellReselectInd FDDCell Allowed
notAllowed
Allowed Allowed C0
tBarred FDDCell barredS10, barredS20, barredS40, barredS80, barredS160, barredS320, barredS640, barredS1280
BarredS160 TBD C0
The following parameters allow to restrict access to some cells. They prevent to send the initial access message, but there is no impact on cell selection/reselection
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 28
BarredOrNot
intraFreqCellReselectInd CellReservedForOperatorUse
CellReservationExtension
wait
tBarred
barred notbarred
notReserved
UE tries to select another intra-freq cell
allowed notAllowed
Only UEs Class 11 and 15
are accepted
Reserved
All the
other UEs
UE tries to select another
frequency
If no other Freq
Retry on the same cell
all UE are accepted
notReserved
Reserved
Initial Cell Selection, SIB decoding
waitMax_tBarred(barredS1280)
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 29
Location Registration ProcedureUE selects the PLMN according to the network identifiers broadcasted in SIB and stored in the SIM card
Parameter Object Range Default Value Recommended Value
locationAreaCode FDDCell Int [0..65535] N.A. N.A.
routingAreaCode FDDCell Int [0..255] N.A. N.A.
mobileCountryCode GSMCell
Network
RNC
String
Length = 3
N.A. N.A.
mobileNetworkCode GSMCell
Network
RNC
String
Length = 3
N.A. N.A.
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 30
UE RNCNode B New MSC/VLR
Connection Request
Connection Confirm
Initial UE Message
(MM: Location updating Req.)
Radio Link Setup Response
Radio Link Setup Request
RACH / RRC Connection Request (Cause)
FACH / RRC Connection Setup (U-RNTI)
The RRC Connection Request message initiated by the UE contains the establishment cause
The NBAP Radio Link configuration contains hte TFC Used for the DCCH that will be used in the early call setup phase
RRCRRC
RRC
RRC RRC
NBAP
NBAPNBAP
NBAP
RRCDCH / RRC Connection Complete
Measurement ControlRRC RRC
The RRC Connection Setup message contains the signaling bearer TFC. A UTRAN Radio Network Temporary Identity is also allocated to the UE
RRCRRCInitial Direct Transfer (Location Updating Request)
SCCP SCCP
SCCPSCCP
RANAP RANAP
SCCP Connection Request may be encapsulated in the RANAP Initial UE
Message:
RANAP RNC MSC with SCCP Conn.Req.
SCCP RNC MSC Connection Conf.
Old MSC/VLR
Location Update Dataflow (1/2)
Cell_DCH
MAP MAP
MAPMAP
Send Indentification (TMSI)
Send Indentification Ack (IMSI)
UL Synchro
DL SynchroDCH_FP
DCH_FPDCH_FP
DCH_FP
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 31
UE RNCNode B New MSC/VLR Old MSC/VLR
HLR
Security Mode Complete
Security Mode CommandRANAPRANAP
RRCRRC
RRC RRC
RANAP RANAP
The ciphering and integrity procedures between UE and RNC are activated by the MSC
Security Mode Command
Security Mode Complete
MAP MAP
MAP MAP
MAP MAP
MAP MAP
MAP MAP
MAP MAP
TMSI Reallocation Complete
Location Updating Accept (new TMSI, new LAI)MMMM
MMMM
Update Location
(IMSI, new VLR)
Cancel Location
(IMSI)
Cancel Location Ack
Insert Subscriber Data
Insert Subscriber Data Ack
Update Location Ack
Location Update Dataflow (2/2)
Cell_DCH
The UE is authenticated by the MSCAuthentication & Ciphering Response
Service Accept + Authentication & Ciphering RequestMMMM
MMMM
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 32
Periodic Location Area Updtae
It is possible to configure an periodic location area update through LocUpdatePeriod parameter.
This can be used in case of LAC Reconfiguration or MSC Problem (then VLR will have to re-record subscribers).
Parameter Object Range Default Value Recommended Value
locUpdatePeriod RadioAccessService
Int [0..65535]
Unit = deciHour (=6 min.)
0 Between 20 and 60
(2 to 6 hours)
Note: This parameter is also known as T3212 Timer
The periodic Routing Area Update is also possible, but by configuring it at the SGSN level (timer T3312)
WNE / UMTS / Cell Selection & Reselection / April 2003 / v02.02 - 33
Available Documents
• UMT/SYS/DD/0054 : UMTS Radio Mobility
• 3GPP TS 25.304 : UE Procedures in Idle and Procedures for Cell Reselection in Connected Mode
• 3GPP TS 25.301: Radio Interface Protocol Architecture
• 3GPP TS 25.303 : Interlayer Procedures in Connected Mode
• 3GPP TS 25.331 : RRC Protocol Specification
• 3GPP TS 23.122 : NAS functions related to MS in Idle Mode
• 3GPP TS 25.133 : Requirements for Support of Radio Resource Management (FDD)
• 3GPP TS 25.922 : Radio Resource Management Strategies