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 Cell Reselection  Cell Reselection is a kind of mechanism to change cell after UE is camped on a cell. This is to let UE get connected to cell which has the best condition among all the cells to which the UE is allowed to camp on. But UE does not change cells randomly, it uses a set of pretty complicated criteria and algorithms for this reselection process. The details of these criteria and algorithms will be described later but the high level guideline is as follows :

First Level Criteria :  Absolute Priority Second Level Criteria : Radio Link Quality Third Level Criteria : Cell Accessibility

A little bit detailed criteria can be described as follows and you will see ever further details after this. UE always have to measure frequecies and RAT with higher priority UE has to have to measure frequecies and RAT with lower priority in the following fashion

o UE has to perform intra-frequency measurement only when SrxLev of the serving cell <= SintraSearcho UE has to perform lower or equal priority inter-frequency/InterRAT measurement only when SrxLev

the serving cell <= SnonintraSearch LTE cell reselection uses priority based levels. (These priorities can be specified in LTE SIB4,5,6,7 and RRC

Connection Release). Priority 0 indicate the lowest priority and 7 indicate the highest priority. The priority cannot be same for WCDMA and LTE, meaning that it must be set differently for different RAT.

Among the cell power and priority, which one plays more critical role in cell reselection ? Cell Power defines the minimum condition. Once the minimum condition is met, Priority plays more critical role. For example, let's assume that UE is now camped in WCDMA cell and detected a neighbouring LTE cell with higher priority than the serving WCDMA cell, UE will perform cell reselection to LTE cell as long as the LTE cell is above the minimum signal strength threshold. In the same logic, if a UE camped on LTE cell detected a neighbouring WCDMA cell with higher priority than the serving LTE cell, UE will perform cell reselection to WCDMAlong as the WCDMA cell is above the minimum signal strength threshold.

WCDMA SIB19 specifies the LTE frequencies (EARFCN) and priorities. A maximum of 8 EARFCNs (4 FDD + 4 TDD) can be listed in SIB 19. For each EARFCN, you can configure a max 16 blacklisted cells. The blacklisted cell is the cell UE should not make reselection to.

Using the eutraDetection parameter in SIB19, you can make UE to detect and display the presence of lower priority LTE frequencies while it is in WCDMA cell.

LTE SIB6 specifies the WCDMA frequencies (DL ARFCN) and the parameters used for absolute priority based cell reselection.

  How to detect and reselect to LTE cell while in WCDMA ? UE must measure the LTE frequencies and detect the available LTE cell in order to perform cell reselection to LTE. 

UE measures two physical properties called for WCDMA signal. One is CPICH RSCP and CPICH EcNo. RSCP determines Srxlev and EcNo determines Squal.  

Srxlev = Qrxlevemeas - qRxLevMin. Qrxlevemeas is RSCP level measured by UE and qRxLevMin is the value specified in SIB.

Squal = Qqualmeas - qQualMin. Qqualmeas is EcNo level measured by UE and qQualMin is the value specified in SIB.

The detection measurement of LTE frequencies should be done at least once every 60s for higher priority LTE frequencies.

In following condition, detection measurements of lower priority LTE frequency is not required.Srxlev > absPrioCellRes.sPrioritySearch1Squal > absPrioCellRes.sPrioritySearch2

In following condition, UE should detect once every 30s for both lower and higher priority LTE frequencies

Srxlev <= absPrioCellRes.sPrioritySearch1Squal <= absPrioCellRes.sPrioritySearch2

The maximum number of LTE FDD Frequencies are 4. In this case, UE should have performed measurement for detecting LTE cells on all 4 LTE frequencies once every 240 (4 x 60) s or 120(4 x 30) s depending if UE measures above or below parameter threshold absPrioCellRes.sPrioritySearch1

  How to detect and reselect to WCDMA cell while in LTE ? Measurement Criteria (From High Priority LTE Cell to Lower Priority WCDMA Cell): When LTE cell has higher priority than WCDMA, it would stay in LTE cell but it performs measurement for the low priority WCDMA if UE is under the following condition :

Srxlev of the serving cell < sNonIntraSearch (SIB3),                If we express this using dB/dBm, Srxlev of the serving cell < (2 x sNonIntraSearch (SIB3))      where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),                If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB3)) [dBm]             where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for camping

 Measurement Criteria (From Lower Priority LTE Cell to Higher Priority WCDMA Cell): When LTE cell has lowerthan WCDMA (WCDMA cell priority defined in SIB6 is higher than the serving cell priority),The UE always have to perform measurements on WCDMA cell. How often UE has to measure for WCDMA depends on whether Srxlev of the serving cell is greater or lower than sNonIntraSearch(SIB3). If no parameter is set (meaning in default condition), detection of WCDMA cell should be performed at least every 60 seconds. Reselection Criteria  (From High Priority LTE Cell to Lower Priority WCDMA Cell): If UE in LTE cell is under the following condition with the duration longer than tReselectionUtra (SIB6), it should reselect to WCDMA cell.

Srxlev of LTE cell (serving cell)  < threshServingLow (SIB3),      where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),             where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for campingSrxlev of WCDMA cell  > threshXLow (SIB6),      where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),                If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB3)) [dBm]             where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping

Note : If more than one WCDMA meet this condition, UE should select to the cell with highest SrxlevNote : If these values are not specified in SIB and UE has to apply default values, UE has to perform reselection when LTE RSRP is lower than -145 dBm and WCDMA cell is better than -119 dBm for at least 2 seconds. Reselection Criteria  (From Lower Priority LTE Cell to Higher Priority WCDMA Cell): If UE in LTE cell is under the following condition with the duration longer than tReselectionUtra (SIB6), it should reselect to WCDMA cell.

Srxlev of WCDMA cell  > threshXHigh (SIB6),      where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),             where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping

  

RRC Course Content

RRC (L3 -   Access Stratum)

(1)RRC Idle Mode - Initial Operation RRC-Idle State Transition Idle Mode Tasks Overall Idle Mode Process Initial Synchronization  SIB Information List Cell Categories - Service Types PLMN Selection TA Update (Registration)  TAU Purpose  TAU initiation Triggers  TAU within LTE and GERAN/UTRAN Measurements  Measurement Architecture  Measurement Procedures  Meas. Objects, ID, Report Cell-Reselection

(2)RRC Idle Cell-Reselection Cell Type Definitions S & R Criteria  Speed Factors Ranking Access Restrictions CSG Considerations SIB3 Processing Cell State Analysis Log Measurements 

(3)Paging Procedure Purpose & Initiation Paging through E-UTRAN using S-TMSI Paging through E-UTRAN using IMSI Paging for CS fallback to A/Gb/Iu mode Paging for SMS RRC Paging Procedure Paging Occasion & Paging Frames Paging Message & Structure

(4)System Information Reception

Frequency Scan of LTE Bands Synchronisation with PSS, SS & RS MIB and SIBs  SI Change Notification & Modification Window MIB/SIB Examples MIB, BCCH (DL-SCH) Message Structure SI - Scheduling Information SI-Window and Periodicity SI Acquisition & Timing SI Aquisition Failure Scenarios Reception of GSM, UMTS and CDMA System Information

(5)RRC Connection Control RRC Connection Control Connection Context Setup/Modify/Release E-RAB Setup/Modify/Release DL CCCH and DCCH Messages UL CCCH and DCCH Messages Connection Control Procedures   Paging

RRC connection establishment Request/Setup/Complete - Reject and Accept

Access Barring Check Initial security activation RRC connection reconfiguration Mobility Parameters Counter check RRC connection re-establishment RRC connection release RRC connection release requested by upper layers Radio resource configuration DRB Add/Modify/Release SCell Add/Modify/Release Radio link failure related actions UE actions upon leaving RRC_CONNECTED UE actions upon PUCCH/ SRS release request Proximity indication

(6)RRC Intra-RAT Handover Mobility Idle Mode Mobility Connected Mode mobility Handover Signalling with X2/ with S1

HO Call Flow HO Preparation Resource Allocation, HO Notification HO Cancellation RRCConnectionReconfig-MobilityControlInfo Path Switch Handover Notify HandoverCancellation

Handover Failure C-Plane and H-Plane HO Processing PDCP Data Forwarding on HO

Mobility Robustness Optimization Too Late/Early/Wrong Handover RLF Indication Handover Report CQI and Congestion management Load Reporting among Two Cells

(7)RRC Inter-RAT Handovers Inter-RAT Concept Unnecessary HO to another RAT DL/ULInformationTransfer Mobility from LTE to other RAT MobilityFromEUTRACommand Success and Failure case from EUTRA HandoverfromE-UTRAPreparationRequest(CDMA2000) CSFBParameterRequestCDMA2000 ULHandoverPreparationTransfer Handover from GERAN/UTRA to E-UTRA Inter-RAT Cell Change Order

(8)RRC Measurements Measurement configuration Measurement Structures and Parameters Measurement report triggering Measurement reporting Meas. Id Add/Delete/Modify Measurement related actions Performing measurements Inter-frequency RSTD measurement indication L3 Filtering A1-A6 and B1-B2 Events Speed Dependent scaling Logged Measurement Configuration Release of Logged Measurement Configuration Measurements logging

(9)RRM Timings and Tolerance Cell-Reselection, Intra and Inter-Freq Timing S&R Criteria, Paging Interruption Inter/Intra FDD/TDD Handover Requirement RRC Re-establishmentDelay Requirement Random Access Requirement UE Transmit Timing & TA Cell Phase Synchronization Requirement LTE and CDMA/HRPD Synchronization Radio Link Monitoring (Q-in, Q-Out) Requirement

(10)MBMS and MBSFN MBMS Introduction MBMS Scheduling MBMS Structure and Parameters Information Validity and Change Notification

MBMS Info acquisition MBMS Bearer(MRB) Configuration Counting Procedure

If you intend to cover any topic in details, please contact us.

Question on trigger for Measurement_Control and Handover DescisionsPosted on 25. Apr, 2010 by karthikc in LteWorld Forum

Could somebody point me to the relevant docs that describe how the eNB initiates a

measurement_control message?

Specifically, I'm looking for information that describes the measurement process leading upto a

handoff decision. Does the eNB initiate a measuremen_control when the serving cell RSRP falls

below a particular value etc?

Also, how often does the UEi submit serving cell RSRP reports?

Thanks,

Karthik

‹ RE: AOA Estimation Measurements in E-UTRAN ›

Add new comment jaiMon, 04/26/2010 - 12:28

See if follwoing helps

See if following helps, I would suggest more reading on refered sections

==========================================

3GPPi - 36.300: http://www.3gpp.org/ftp/Specs/html-info/36300.htm

10.1 Intra E-UTRANi

In E-UTRAN RRCi_CONNECTED state, network-controlled UEi-assisted handovers are performed

and various DRX cycles are supported.

10.1.2.1 Handoveri

Below is a more detailed description of the intra-MMEi/Serving Gateway HO procedure:

0 The UE context within the source eNB contains information regarding roaming restrictions which

where provided either at connection establishment or at the last TA update.

1 The source eNB configures the UE measurement procedures according to the area restriction

information. Measurements provided by the source eNB may assist the function controlling the UE's

connection mobility.

2 UE is triggered to send MEASUREMENT REPORT by the rules set by i.e. system information,

specification etc.

3 Source eNB makes decision based on MEASUREMENT REPORT and RRM information to hand

off UE.

10.1.3 - Measurements to be performed by a UE for intra/inter-frequency mobility can be controlled

by E-UTRAN, using broadcast or dedicated control. In RRC_IDLE state, a UE shall follow the

measurement parameters defined for cell reselection specified by the E-UTRAN broadcast. The use

of dedicated measurement control for RRC_IDLE state is possible through the provision of UE

specific priorities (see sub-clause 10.2.4). In RRC_CONNECTED state, a UE shall follow the

measurement configurations specified by RRC directed from the E-UTRAN (e.g. as in UTRAN

MEASUREMENT_CONTROL).

10.2.3.1 Inter-RATi handovers from E-UTRAN

Measurements to be performed by a UE for inter-RAT mobility can be controlled by E-UTRAN, using

broadcast or dedicated control. In RRC_CONNECTED state, a UE shall follow the measurement

parameters specified by RRC directed from the E-UTRAN (e.g. as in UTRAN

MEASUREMENT_CONTROL).

UE performs inter-RAT neighbour cell measurements during DL/UL idle periods that are provided by

the network through suitable DRX/DTX period or packet scheduling if necessary.

===========================================================

3GPP - 36331 (RRC) - http://www.3gpp.org/ftp/Specs/html-info/36331.htm

5.5 Measurements

The UE reports measurement information in accordance with the measurement configuration as

provided by E-UTRAN. E-UTRAN provides the measurement configuration applicable for a UE in

RRC_CONNECTED by means of dedicated signalling, i.e. using the

RRCConnectionReconfiguration message.

The UE reports measurement information in accordance with the measurement configuration as

provided by E-UTRAN. E-UTRAN provides the measurement configuration applicable for a UE in

RRC_CONNECTED by means of dedicated signalling, i.e. using the

RRCConnectionReconfiguration message.

5.5.2 Measurement configuration

The UE shall:

1> if the received measConfig includes the s-Measure:

2> set the parameter s-Measure within VarMeasConfig to the lowest value of the RSRP ranges

indicated by the received value of s-Measure;

5.5.3 Performing measurements

5.5.4 Measurement report triggering

IRAT reselection LTE to 2GThis question is answered

Posted by nailwal_kailash 

on 28 Jun 2011 5:41 PM

Hi,

 

i am working on a scenario in which UE needs to reselect from LTE to 2G. I am ginving q-RxLevmin value as 0-45 in

SIB 7 and LTE cell is at -85dbm/15khz. when i am using q-RxLevmin  as 0 value which comes out to be as -115 (0*2-

115) which is less than -85 so UE will not reselect to the 2G cell which is activated at -70dbm. But if i use q-

RxLevmin  as 45 value then   value to trigger will be -25 (45*2-115) which is greater than -85 so UE should reselect to

2G cell.

Can someone please comment if the understanding is correct on this scenario.

thanks in advance.

regards

kailash

 

Posted by Don 

on 30 Jun 2011 9:49 AM

Verified Answer

Verified by nailwal_kailash

Sorry, my mistake: I meant SIB 3, not SIB 2. SIB 3 contains the parameters for cell reselection. The formula you're

referring to is the check to ensure that the proposed target (GSM) cell has a strong enough measurement before a

reselection is attempted. The SIB 3 parameters I was mentioning determine when the UE should take the

measurements in the first place; if the current LTE cell is strong enough, the UE won't bother looking at GSM at all.

Posted by Don 

on 29 Jun 2011 3:52 PM

The Qrxlevmin value in SIB 7 determines the minimum required signal in the GSM cell before the UE will reselect to it

(an RSSI value). The Qrxlevmin value in SIB 2 (and RSRP measurement) is used to trigger the reselection

measurements; more accurately, a UE will scan for 2G cells if the current RSRP measurement is less than Qrxlevmin

(SIB 2) + Snonintrasearch (SIB 2). It will actually move to the GSM cell if (RSSI - Qrxlevmin (SIB 7)) > (RSRP -

Qrxlevmin (SIB 2) + Qhyst (SIB 2)).

Posted by nailwal_kailash 

on 29 Jun 2011 9:23 PM

HI Don,

Thanks for the information.

Then how we can relate it to the below formula given in 3GPP

If in SIB7,  q-RxLevMin = 25 and threshX-High = 0

=>

The actual value of Qrxlevmin in dBm = (IE value * 2)-115 = -65

Srxlev = Qrxlevmeas - Qrxlevmin = -71 - (-65) = -6

The actual value of Threshx,high = IE value * 2 = 0

so Srxlev <0 and hence the Criteria1 of cell reselection is not fulfilled which is Srxlev > Threshx,high.

Hence UE should not reselect but if Srxlev comes out to be >threshx-high the  UE will reselct the 2G cell.

But where we are mentioning the Qrxlevmin in SIB 2 as you are suggesting. Can you please eloborate it a bit more.

Posted by Don 

on 30 Jun 2011 9:49 AM

Verified Answer

Verified by nailwal_kailash

Sorry, my mistake: I meant SIB 3, not SIB 2. SIB 3 contains the parameters for cell reselection. The formula you're

referring to is the check to ensure that the proposed target (GSM) cell has a strong enough measurement before a

reselection is attempted. The SIB 3 parameters I was mentioning determine when the UE should take the

measurements in the first place; if the current LTE cell is strong enough, the UE won't bother looking at GSM at all.

Posted by nailwal_kailash 

on 6 Jul 2011 8:15 PM

thanks.

Posted by sureshh 

on 13 Feb 2013 9:39 PM

Hi,

Quick question, in the equation "Qrxlevmin in dBm = (IE value * 2)-115 = -65"

why do you use the value -115 ?

Posted by Don 

on 14 Feb 2013 4:50 PM

The correct conversion for Qrxlevmin is IE * 2 - 140; a Qrxlevmin value of 0 equates to -140 dBm, not -115 dBm, and

25 would be -90 dBm.

Below is the List and How Intra RAT HO events in LTE.

Event A1 : Serving becomes better than threshold

Event A2 : Serving becomes worse than threshold

Event A3 : Neighbor becomes offset better than serving

Event A4 : Neighbor becomes better than threshold

Event A5 : Serving becomes worse than threshold1 and neighbor becomes better than threshold2

Event A1 (Serving becomes better than threshold)Ms − Hys > Thresh (Entering condition)Ms + Hys < Thresh (Leaving condition)Event A2 (Serving becomes worse than threshold)Ms + Hys < Thresh (Entering condition)Ms − Hys > Thresh Leaving conditionEvent A3 (Neighbor becomes offset better than serving)Mn +Ofn +Ocn − Hys > Ms +Ofs +Ocs +Off (Entering condition)Mn +Ofn +Ocn + Hys < Ms +Ofs +Ocs +Off (Leaving condition)Event A4 (Neighbor becomes better than threshold)Mn +Ofn +Ocn− Hys > Thresh (Entering condition)Mn +Ofn +Ocn + Hys < Thresh (Leaving condition)Event A5 (Serving becomes worse than threshold1 and neighbor becomes better than threshold2)Ms + Hys < Thresh1 AND Mn +Ofn +Ocn − Hys > Thresh2 (Entering condition)Ms − Hys > Thresh1 OR Mn +Ofn +Ocn + Hys < Thresh2 (Leaving condition)Ms (dBm, dB) is the measurement result of the serving cell, not taking into account any cell individual offset.Hys (dB) is the hysteresis parameter for this event (i.e. hysteresis as defined within the VarMeasurementConfiguration for this event)Thresh (dBm, dB) is the threshold parameter for this event (i.e. a1/a2/a4-Threshold as defined within the VarMeasurementConfiguration for this event)Mn (dBm, dB) is the measurement result of the neighboring cell.Ofn is the frequency specific offset of the frequency of the neighbor cell (equals Ofs for intra-frequency measurements and is included in MeasObjectEUTRA corresponding to the inter frequency as offsetFreq for inter-frequency measurements)Ocn is the cell specific offset of the neighbor cell. If not configured zero offset shall be applied (included in MeasObjectEUTRA of the serving frequency as parameter cellIndividualOffset for intra-f measurements and included in MeasObjectEUTRA corresponding to the inter frequency as parameter cellIndividualOffset for interfrequency measurements).

Ofs (dB) is the frequency specific offset of the serving frequency (i.e. offsetFreq within the MeasObjectEUTRA corresponding to the serving frequency)Ocs (dB) is the cell specific offset of the serving cell (included in MeasObjectEUTRA of the serving frequency as parameter cellIndividualOffset)Off (dB) is the offset parameter for this event (i.e. a3-Offset as defined within the VarMeasurementConfiguration for this event)Thresh1 is the threshold parameter for this event (i.e. a5-Threshold1 as defined within the VarMeasurementConfiguration for this event)Thresh2 is the threshold parameter for this event (i.e. a5-Threshold2 as defined within the VarMeasurementConfiguration for this event)

Read more http://www.teletopix.org/4g-lte/intra-rat-ho-events-in-lte