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


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:


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


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


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


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


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


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


PLMN Selection


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


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


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


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


Cell Selection Criteria

Suitable Cell Conditions:

Squal > 0 AND SRXLev > 0


CPICH_Ec/No > qQualmin

AND

CPICH_RSCP > qRxLevMin + Pcompensation


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)


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


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


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


Cell Reselection Thresholds


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.


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


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


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.


Cell Reselection Parameters


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


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


Cell Access Restrictions


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


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)


Location Registration ProcedureUE selects the PLMN according to the network identifiers broadcasted in SIB and stored in the SIM card


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.


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


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


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).


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)


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