Day 2 4. Cell Selection Reselection and Signalling

1 © 2005 Nokia V1-Filename.ppt / yyyy-mm-dd / Initials

Company Confidential

WCDMA Cell Selection Reselection Deepak Yadav15-17 January 2007



Contents

• Cell Search Procedure

• Cell Selection Criteria

• Cell Reselection Procedure and Settings

• WCDMA Signalling• AMR Call Signalling

• PS Data Call Signalling

• CS ISHO Signalling

• PS ISHO Signalling

• WCDMA Handovers• Soft/Softer HO

• Inter Frequency HO

• Inter System HO



Call Setup Performance

• Key parameter settings• Cell reselection triggering time (Treselection = 0s)

• Reselection takes place immediately when the UE notices that there is difference between the cells’ Ec/No values (in worst case scenario there can be up to 3dB + Qhyst difference based on the measurement accuracy requirement)

• Cell reselection hysteresis 2 (Qhyst2 = 4dB)• This will add 4dB hysteresis to the neighboring cell evaluation (target for

the cell reselection)

• Note that Qhyst1 is used only in case the cell selection and re-selection quality measure is set to CPICH RSCP (default is CPICH Ec/No so Qhyst 1 is not used in intra-FDD reselection)

• S intrasearch (Sintrasearch = 4dB)• This parameter is used by the UE to calculate the threshold (CPICH Ec/No)

to start intra frequency (SHO) measurements (Sintrasearch above QqualMin value)

• Minimum required quality level in the cell (QqualMin = -20dB)



Cell Reselection problem• Several UEs cannot perform cell reselection during RRC Connection Setup phase

Cell A Cell B

CPICH Ec/NoCPICH RSCP


-15dB

UE Sends First RRC

Connection Request Message

RF Conditions OK and CPICH

Ec/No is –10dB

UE Sends First RRC

Connection Request Message

RF Conditions OK and CPICH

Ec/No is –10dB

RNC responds with RRC Connection Setup message

which is not heard by the UE due to CPICH Ec/No falls

below –15dB

RNC responds with RRC Connection Setup message

which is not heard by the UE due to CPICH Ec/No falls

below –15dB

The UE sends all the rest of the RRC Connection Request –messages to the old cell, Cell A without changing the serving cell to Cell B (Kenny can perform this reselection and

send next RRC Connection Request –message to Cell B) and the call setup fails

The UE sends all the rest of the RRC Connection Request –messages to the old cell, Cell A without changing the serving cell to Cell B (Kenny can perform this reselection and

send next RRC Connection Request –message to Cell B) and the call setup fails

-100dB

Time

-10dB

UE moving from Cell A towards Cell B• The time when the source cell CPICH Ec/No is too low as well as the gap between new and old cell is too large i.e. the UEs that cannot make cell reselection during RRC Connection Setup phase will have failed call attempts



Call Setup Performance

• There is big chance that the UEs with limited capability to perform proper cell reselection will have failed call attempts seen as consecutive RRC Connection Request messages sent by the UE (seen from the UE log) but no response from RNC or RRC Connection Setup received but then no actions after RRC Connection Setup Complete message (i.e. RRC Connection Setup Complete message is not heard by the BTS)


TimeQqualMin = -20dB

QqualMin+Sintrasearch=-20dB + 4dB = -16dB

QqualMin+Sintrasearch+Qhyst2= -16dB + 4dB = -12dBCalls setups failures

Calls setups difficult

Calls setups ok

Calls setups very successful

Recommended area



UE starts the neighboring cell

measurements here and it takes some time

to perform the reselection


measurements here and it takes some time

to perform the reselection

Call Setup Performance-Idle Mode Cell Reselection-

• Default parameter settings

Cell A Cell B



QqualMin = -20dB

UE “hangs” in CELL A until

QqualMin+Sintrasearch = -16dB threshold is

reached.However at this level the RRC Connection Setup is

probably not heard by the UE any more or the BTS does not hear the RRC Connection Request

UE “hangs” in CELL A until

QqualMin+Sintrasearch = -16dB threshold is

reached.However at this level the RRC Connection Setup is

probably not heard by the UE any more or the BTS does not hear the RRC Connection Request

Time


Qhyst2= -16dB + 4dB = -12dB

Cell Reselection

• The time when the source cell CPICH Ec/No is too low as well as the gap between new and old cell is too large i.e. the UEs that cannot make (all except Kenny) make cell reselection during RRC Connection Setup phase will have failed call attempts



Call Setup Performance-Idle Mode Cell Reselection-

• New set Case 2: Sintrasearch = 12 dB, Treselection = 0 sec and Qhyst2 =4 dB

Cell A Cell B



QqualMin = -20dB

Time


Qhyst2 = -8dB + 4dB = -4dB

• 2 second hysteresis give UE especially at LA, RA border to be more stable than ping pong cell reselection. This also reduces signaling and call failure at LA/RA border due to LA/RA update

• Qqualmin shall not be changed as it will greatly affect cell coverage especailly when traffic is higher because CPICH Ec/No will lower and creating a coverage hole

More time to select and monitor a better

cell

More time to select and monitor a better

cell

Cell reselection happens here where cell B is about –8 dB

and Cell A is about –12 dB

Cell reselection happens here where cell B is about –8 dB

and Cell A is about –12 dB

Worst case scenario


measurements near earlier than at cell

edge


measurements near earlier than at cell

edge



WCDMA Handovers



Handover Types• Intra-Frequency Handovers

• Softer Handover• Handover between sectors of the same Node B (handled by BTS)• No extra transmissions across Iub interface• Maximum Ratio Combining (MRC) is occurring in both the UL and DL

• Soft Handover• MS simultaneously connected to multiple cells (from different Node Bs)• Extra transmission across Iub, more channel cards are needed (compared to non-SHO)• Mobile Evaluated Handover (MEHO)• DL/UE: MRC & UL/RNC: Frame selection combining

• Hard Handover• Arises when inter-RNC SHO is not possible (Iur not supported or Iur congestion)• Decision procedure is the same as SHO (MEHO and RNC controlled)• Causes temporary disconnection of the (RT) user

• Inter-Frequency Handover• Can be intra-BS, intra-RNC, inter-RNC • Network Evaluated Handover (NEHO)• Decision algorithm located in RNC

• Inter-RAT Handover • Handovers between GSM and WCDMA (NEHO)



Maximum number of neighbours

• The maximum number of neighbours that can be defined in RNC database is

• ADJS: 31• 31 neighbours + serving cell = 32 cells to measure

• ADJI: 32/carrier, total 48

• ADJG: 32



SHO: Measurement Averaging

• The Handover measurement is based on CPICH Ec/Io

• The accuracy of pilot Ec/Io measurements is important for Handover performance

• Measurement accuracy depends on the filtering length and mobile speed.• When a MS moves slowly or is stationary, filtering length (in ms) has to be just long enough

to avoid fast fades leading to errors and causing unnecessary Handovers within a short period.

• Long filtering length will cause Handover delays to fast moving MS

• UE can perform averaging on EC/IO measurement prior to the event reporting process

• EcNoFilterCoefficient (FMCS) defines the filtering period for intra-frequency CPICH EC/IO measurements used by UE (default = 200ms)

• Similar filter than in Higher Layer Filtering of common and dedicated BTS measurements

• RNC averages CPICH Ec/Io measurements using EcNoAveragingWindow (HOPS)• defines the number of event triggered or periodic intra-frequency measurement reports

used by RNC to calculate averaged CPICH EC/IO (default = 8 Measurements)

• Used for intra-frequency hard handovers



Soft Handover

• HC supports the following measurement reporting events and features:• Event 1A: A primary CPICH enters the reporting range (Ncell addition)

• Event 1B: A primary CPICH leaves the reporting range (Ncell deletion)

• Event 1C: A non-active CPICH becomes better than an active primary CPICH (Ncell replacement)

• Cell individual offsets for modifying measurement reporting behaviour

• Mechanism for forbidding a neighboring cell to affect the reporting range

• Handover decision performed by RNC based on measurements and available resources

• Admission Control can reject the branch addition in case the maximum load is achieved in DL (threshold + offset), valid both for RT and NRT bitrates.

• Hard blocking may prevent branch addition



SHO_e1a

UE BTS RNC

Handover tresholds fulfilled (MEHO)- Event 1A is triggered

RRC: Active Set Update Request

RRC: Measurement Report

NBAP: Radio Link Addition Response

NBAP: Radio Link Addition Request

RRC: Active Set Update Complete

RRC: Measurement Control



4

1A: A Primary CPICH Enters the Reporting Range

Strongest CPICH in AS

time

Ec/Io

P CPICH 3

P CPICH 1

P CPICH 2

1

2

3

AdditionWindow

AdditionTime

AdditionReportingInterval

RNC

MeasurementReport

To theAS?

no

ActiveSetWeightingCoefficient



SHO_e1b

UE BTS RNC

Handover tresholds fulfilled (MEHO)- Event 1B is triggered


RRC Measurement Report


NBAP: Radio Link Deletion Request

NBAP: Radio Link Deletion Response




Strongest CPICH in AS

time

Ec/Io

P CPICH 3

P CPICH 1

P CPICH 2

1

2

3

DropWindow

DropTime

MeasurementReport

1B: A Primary CPICH Leaves the Reporting Range

Remove the

reported cell from the AS



SHO_e1c

UE BTS RNC

Handover tresholds fulfilled (MEHO)- Event 1C is triggered



NBAP:Radio Link Addition Response

NBAP: Radio Link Addition Request


NBAP: Radio Link Deletion Request

NBAP: Radio Link Deletion Response




time

weakest CPICH3 in AS

Ec/Io

P CPICH 3

P CPICH 1

P CPICH 2

P CPICH 4

AS has 3 cells

ReplacementReportingInterval3

1

2

ReplacementWindow

ReplacementTime

MeasurementReport

RNC

ASupdate?

no

1C: A non-active CPICH becomes better than an active primary CPICH



IFHO/ISHO Process Overview

HO Triggering Thresholds set in RNCHO Triggering Thresholds set in RNC

Event Triggered Coverage/Capacitybased HO fulfilled in RNC

Event Triggered Coverage/Capacitybased HO fulfilled in RNC

RNC commands selected UE(s) to startIF/IS measurements

RNC commands selected UE(s) to startIF/IS measurements

Measurements are done in Compressed Mode (CM)

Measurements are done in Compressed Mode (CM)

UE reports GSM cells withstrongest RSSI signals to RNC

UE reports GSM cells withstrongest RSSI signals to RNC

RNC makes HO decision andcommands UE to target cell

RNC makes HO decision andcommands UE to target cell

RSSI measurements and BSICverification for GSM cells

RSSI measurements and BSICverification for GSM cells

Different decision methods for IF HOOnly one decision method for IS HO

5 Coverage/Capacity HO Reasons

About 25 HO parameters

Reporting cells are active set cells (max 3) + max 6 IFHO, max 6 ISHO neighb.

Max 32 neighbours could be measured

(31 Intra-Freq neighbours)48 Inter-Freq neighbours 32 Inter-System neighbours can be measured



IF/IS Handover Measurement Triggering Reasons

4. DL DPCH approaches itsmaximum allowed powerFMCI: IFHOcauseTxPwrDLFMCG: GSMcauseTxPwrDL

5. Quality deterioration report from UL outer loop PCFMCI: IFHOcauseUplinkQualityFMCG: GSMcauseUplinkQuality

3. UE Tx power approaches itsmaximum allowed power, event 6A/6DFMCI: IFHOcauseTxPwrULFMCG: GSMcauseTxPwrUL

2 . Low measured absoluteCPICH RSCP, events 1E/1FFMCI: IFHOcauseCPICHrscp, FMCG: GSMcauseCPICHrscp

1. Low measured absolute CPICH Ec/No, event 1E/1F FMCI: IFHOcauseCPICHEcNoFMCG: GSMcauseCPICHEcNo

Measurement

Trigger

6 . Others- Load and Service based HO- IMSI based HO- Emergency ISHO

Frequency Measuring Control for Inter-Frequency = FMCIFrequency Measuring Control for Inter-System (GSM) = FMCG



1. Measurement trigger CPICH Ec/No

• RNC starts IF/IS measurement when event 1F occurs for all cells in the active set: A Primary CPICH becomes less than an absolute threshold

• RNC stops IF/IS measurement when event 1E occurs for at least one cell of the active set : A Primary CPICH becomes better than an absolute threshold

• Note:IF/IS measurements can be stopped if event 1Fs are cancelled by events 1E only when IFHO/ISHO was not successful and only inside the time between CM measurements, specified by the time InterFreqMinMeasInterval GsmMinMeasInterval/default 10s, recommendation 2s .

• Filtering applied before event evaluation in the UE: • FMCS: EcNoFilterCoefficient/0= 200ms filtering period



1. Measurement trigger CPICH Ec/NoReporting event: 1E: A P-CPICH exceeds an absolute threshold

1F: A P-CPICH falls below an absolute threshold (HO triggered if all)

time

Cell 1 Cell 2

Cell 3

absolutethreshold

e.g

. P

-CP

ICH

Ec/N

o

1E: HHoEcNoCancel

1E: HHoEcNoCancelTime

1F: HHoEcNoThreshold

1F: HHoEcNoTimeHysteresis



• BSIC verification always performed for AMR calls – no interrupt in voice call

CNUE Node B

RNC



NBAP: Radio Link Reconfiguration PrepareNBAP: Radio Link Reconfiguration

ReadyNBAP: Radio Link Reconfiguration CommitRRC: Physical Channel

ReconfigurationRRC: Physical Channel Reconfiguration Complete NBAP: Compressed Mode

Command



NBAP: Compressed Mode Command



RRC: Handover from UTRAN Command

GSM BSIC Identification

GSM RSSI Measurement

ISHO triggering (3 reasons are possible)

Initial Compressed Mode Configuration

RANAP: Relocation Required

RANAP: Relocation Command

RANAP: IU Release Command

RANAP: IU Release Complete

ISHO 3G -> 2G - AMR Signalling Flow



• In most cases BSIC verification is not required (data interrupt as UE moves to 2G)

• PS makes use of RRC: CELL CHANGE ORDER FROM UTRAN message

UE

Node B

RNC



NBAP: Radio Link Reconfiguration Prepare

NBAP: Radio Link Reconfiguration Ready

NBAP: Radio Link Reconfiguration Commit

RRC: Physical Channel Reconfiguration

RRC: Physical Channel Reconfiguration Complete

NBAP: Compressed Mode Command


RRC: Measurement Control GSM RSSI Measurement

ISHO triggering (5 reasons are possible)

Initial Compressed Mode Configuration

CN

RANAP: SRNS Context Request

RANAP: SRNS Context Response

RANAP: IU Release Command

RANAP: IU Release Complete

RRC: Cell Change Order from UTRAN

RANAP: SRNS Data Forward Command

ISHO 3G -> 2G - PS Signalling Flow

Day 2 4. Cell Selection Reselection and Signalling

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