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Cell Selection/ReSelection

 

Note : Before you think of Cell Selection - Cell Selection is the procedure that happens when Cell Detection/Search procedure were successful and complete. So when you try to test or troubleshooting Cell Selection step, you have to prove from UE side log that Cell Detection/Search procedure were successful and complete. The cell power criteria described in this page (in 3GPP as well) is determined only by P-CPICH power, you have to think of another signal power (PSS, SCC power and quality) which UE has to detect even before detecting P-CPICH. This is not all. In purely based on 3GPP criteria, P-CPICH detection may be good enough for cell selection, but when it comes to testing if Cell Selection is successful or not, we normally use Registration (or at least PRACH transmission/Reception) as a success criteria. So if you think of this factor, additional channel power/quality (P-CCPCH, S-CCPCH etc) should be considered as well. Therefore, I would like to say the completion of cell selection in terms of testing would involve much more factors than those described in this page.

 

Followings are the topics in this page.

 

 

Type of Cell Selection

 

There are two types of Cell Selections. "Initial Cell Selection" and "Stored Information Cell Selection". Definition of these two methods are described as follows in  25.304 5.2.3 Cell Selection Process.

 

a) Initial Cell Selection

This procedure requires no prior knowledge of which RF channels are UTRA carriers. The UE shall scan all RF channels in the UTRA bands according to its capabilities to find a suitable cell. On each carrier, the UE need only search for the strongest cell. Once a suitable cell is found this cell shall be selected.

 

b) Stored Information Cell Selection

This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, e.g. scrambling codes, from previously received measurement control information elements.

Once the UE has found a suitable cell the UE shall select it. If no suitable cell is found the Initial cell selection procedure shall be started.

 

 

Cell Selection Criteria

 

For a cell to be selected, they should meet a certain criteria as follows.

 

When UE is in HPLMN

    Squal = Qqualmeas Qqualmin > 0

    Srxlev = Qrxlevmeas Qrxlevmin + QrxlevminOffset Pcompensation > 0

 

When UE is in VPLMN

    Squal = Qqualmeas (Qqualmin + QqualminOffset) > 0

    Srxlev = Qrxlevmeas (Qrxlevmin + QrxlevminOffset) Pcompensation > 0

< 25.304 - 5.2.3.1.2 Criteria >

 

(In FDD) For a cell to be selected, both Squal and Srxlev should be greater than 0 and these two values are calculated as follows.

 

 

 

< Example 1 >

 

Assumption : It is assumed that there is only one Cell is available and cell selection is determined only by S_rxlev and S_qual. If there is any other competing cells around, you have to consider a lot of other factors like absolute measured cell power, priority and various cell search/reselection criteria as well.

 

CPICH_RSCP = -110 dBm, Qrxlevmin:-58, UE Power Class:3(P_MAX = 23)

UE_TXPWR_MAX_CCH (maxAllowedUL-Tx-Power in SIB3) : 24

CPICH_Ec/N0: -10, Qqualmin:-24

 

Srxlev = -110 -(-58 x 2) - max(24-23,0)

       = -110 - (-116) - max(1,0)

       = -110 + 116 -1

       = 5

Squal=(-10)-(-24)

     = 14

 

Squal > 0, Srclev > 0. So The UE will select this cell.

 

 

< Example 2 >

 

Assumption : It is assumed that there is only one Cell is available and cell selection is determined only by S_rxlev and S_qual. If there is any other competing cells around, you have to consider a lot of other factors like absolute measured cell power, priority and various cell search/reselection criteria as well.

 

CPICH_RSCP = -110 dBm, Qrxlevmin:-58, UE Power Class:3(P_MAX = 23)

UE_TXPWR_MAX_CCH (maxAllowedUL-Tx-Power in SIB3) : 0

CPICH_Ec/N0: -10, Qqualmin:-24

 

Srxlev = -110 -(-58 x 2) - max(0-23,0)

       = -110 - (-116) - max(-23,0)

       = -110 + 116 - 0

       = 6

Squal=(-10)-(-24)

     = 14

 

Squal > 0, Srclev > 0. So The UE will select this cell.

 

 

< Example 3 >

 

Assumption : It is assumed that there is only one Cell is available and cell selection is determined only by S_rxlev and S_qual. If there is any other competing cells around, you have to consider a lot of other factors like absolute measured cell power, priority and various cell search/reselection criteria as well.

 

CPICH_RSCP = -110 dBm, Qrxlevmin:-58, UE Power Class:3(P_MAX = 23)

UE_TXPWR_MAX_CCH (maxAllowedUL-Tx-Power in SIB3) : 33

CPICH_Ec/N0: -10, Qqualmin:-24

 

Srxlev = -110 -(-58 x 2) - max(33-23,0)

       = -110 - (-116) - max(10,0)

       = -110 + 116 - 10

       = -4

Squal=(-10)-(-24)

     = 14

 

Squal > 0, Srxlev < 0. So The UE will not select this cell.

 

 

Intra Search Criteria

 

When the following condition meets :

 

When UE is in HPLMN

    (Squal = Qqualmeas Qqualmin) <= s-Intrasearch

    or

    s-Intrasearch is not configured

 

When UE is in VPLMN

    (Squal = Qqualmeas (Qqualmin + QqualminOffset)) <= s-Intrasearch

    or

    s-Intrasearch is not configured

     

If you want to let UE to perform Intracell search all the time, the easiest way would be to set as follows.

    i) set s-Intrasearch to be MAX value

    ii) set Qualmin to be MAX value

 

 

SIBs for Cell Selection / Reselection

 

Followings are System Information Block messages that are related to Cell Selection/Reselection. I enabled IEs as much as possible just to show you all the usable IEs. You may not see many of these IEs in real deployment. Which IE and values are used are completely up to each network operators.

 

< SIB 3 >

 

SysInfoType3

    .0.. .... sib4indicator: False

    cellIdentity: 00000000

    cellSelectReselectInfo

        cellSelectQualityMeasure: cpich-RSCP (1)

            cpich-RSCP: NULL

        modeSpecificInfo: fdd (0)

            fdd

                rat-List: 2 items

                    Item 0

                        RAT-FDD-Info

                            rat-Identifier: gsm (0)

                            s-SearchRAT: 10

                            s-HCS-RAT: -53

                            s-Limit-SearchRAT: 0

                    Item 1

                        RAT-FDD-Info

                            rat-Identifier: cdma2000 (1)

                            s-SearchRAT: -16

                            s-HCS-RAT: -53

                            s-Limit-SearchRAT: -16

                q-QualMin: -24

                q-RxlevMin: -45

        q-Hyst-l-S: 0

        t-Reselection-S: 0

        maxAllowedUL-TX-Power: 24

    cellAccessRestriction

        cellBarred: notBarred (1)

            notBarred: NULL

        cellReservedForOperatorUse: notReserved (1)

        cellReservationExtension: notReserved (1)

    v4b0NonCriticalExtensions

        sysInfoType3-v4b0ext

        v590NonCriticalExtension

            sysInfoType3-v590ext

                cellSelectReselectInfo-v590ext

            v5c0NoncriticalExtension

                sysInfoType3-v5c0ext

                    cellSelectReselectInfoTreselectionScaling-v5c0ext

                        non-HCS-t-CR-Max: notUsed (0)

                            notUsed: NULL

                        speedDependentScalingFactor: 0

                        interFrequencyTreselectionScalingFactor: 4

                        interRATTreselectionScalingFactor: 4

 

 

< SIB 11 >

 

SysInfoType11

    ..0. .... sib12indicator: False

    measurementControlSysInfo

        use-of-HCS: hcs-not-used (0)

            hcs-not-used

                cellSelectQualityMeasure: cpich-RSCP (0)

                    cpich-RSCP

                        intraFreqMeasurementSysInfo

                            intraFreqMeasurementID: 1

                            intraFreqCellInfoSI-List

                                removedIntraFreqCellList: removeNoIntraFreqCells (2)

                                    removeNoIntraFreqCells: NULL

                                newIntraFreqCellList: 1 item

                                    Item 0

                                        NewIntraFreqCellSI-RSCP

                                            intraFreqCellID: 0

                                            cellInfo

                                                cellIndividualOffset: -20

                                                referenceTimeDifferenceToCell: accuracy40 (0)

                                                    accuracy40: 0

                                                modeSpecificInfo: fdd (0)

                                                    fdd

                                                        primaryCPICH-Info

                                                            primaryScramblingCode: 9

                                                        primaryCPICH-TX-Power: 28

                                                        .1.. .... readSFN-Indicator: True

                                                        ..0. .... tx-DiversityIndicator: False

                                                cellSelectionReselectionInfo

                                                    q-OffsetS-N: -50

                                                    maxAllowedUL-TX-Power: 24

                                                    modeSpecificInfo: fdd (0)

                                                        fdd

                                                            q-QualMin: -24

                                                            q-RxlevMin: -45

                            intraFreqMeasQuantity

                                filterCoefficient: fc2 (2)

                                modeSpecificInfo: fdd (0)

                                    fdd

                                        intraFreqMeasQuantity-FDD: cpich-RSCP (1)

                            intraFreqReportingQuantityForRACH

                                sfn-SFN-OTD-Type: noReport (0)

                                modeSpecificInfo: fdd (0)

                                    fdd

                                        intraFreqRepQuantityRACH-FDD: cpich-EcN0 (0)

                            maxReportedCellsOnRACH: currentCell (1)

                            reportingInfoForCellDCH

                                intraFreqReportingQuantity

                                    activeSetReportingQuantities

                                        dummy: noReport (0)

                                        .... .0.. cellIdentity-reportingIndicator: False

                                        .... ..0. cellSynchronisationInfoReportingIndicator: False

                                        modeSpecificInfo: fdd (0)

                                            fdd

                                                1... .... cpich-Ec-N0-reportingIndicator: True

                                                .0.. .... cpich-RSCP-reportingIndicator: False

                                                ..0. .... pathloss-reportingIndicator: False

                                    monitoredSetReportingQuantities

                                        dummy: noReport (0)

                                        .... .0.. cellIdentity-reportingIndicator: False

                                        .... ..1. cellSynchronisationInfoReportingIndicator: True

                                        modeSpecificInfo: fdd (0)

                                            fdd

                                                1... .... cpich-Ec-N0-reportingIndicator: True

                                                .0.. .... cpich-RSCP-reportingIndicator: False

                                                ..0. .... pathloss-reportingIndicator: False

                                measurementReportingMode

                                    measurementReportTransferMode: acknowledgedModeRLC (0)

                                    periodicalOrEventTrigger: eventTrigger (1)

                                reportCriteria: intraFreqReportingCriteria (0)

                                    intraFreqReportingCriteria

                        interFreqMeasurementSysInfo

                            interFreqCellInfoSI-List

                                newInterFreqCellList: 1 item

                                    Item 0

                                        NewInterFreqCellSI-RSCP

                                            interFreqCellID: 0

                                            frequencyInfo

                                                modeSpecificInfo: fdd (0)

                                                    fdd

                                                        uarfcn-DL: 9800

                                            cellInfo

                                                modeSpecificInfo: fdd (0)

                                                    fdd

                                                        primaryCPICH-Info

                                                            primaryScramblingCode: 11

                                                        primaryCPICH-TX-Power: 28

                                                        ...0 .... readSFN-Indicator: False

                                                        .... 0... tx-DiversityIndicator: False

                                                cellSelectionReselectionInfo

                                                    q-OffsetS-N: 3

                                                    maxAllowedUL-TX-Power: 24

                                                    modeSpecificInfo: fdd (0)

                                                        fdd

                                                            q-QualMin: -24

                                                            q-RxlevMin: -45

                interRATMeasurementSysInfo

                    interRATCellInfoList

                        removedInterRATCellList: removeAllInterRATCells (0)

                            removeAllInterRATCells: NULL

                        newInterRATCellList: 1 item

                            Item 0

                                NewInterRATCell-B

                                    interRATCellID: 0

                                    technologySpecificInfo: gsm (0)

                                        gsm

                                            cellSelectionReselectionInfo

                                                q-Offset1S-N: -50

                                                q-Offset2S-N: -50

                                                maxAllowedUL-TX-Power: -50

                                                hcs-NeighbouringCellInformation-RSCP

                                                    hcs-CellReselectInformation

                                                        penaltyTime: notUsed (0)

                                                            notUsed: NULL

                                                modeSpecificInfo: fdd (0)

                                                    fdd

                                            interRATCellIndividualOffset: -50

                                            bsic

                                                ncc: 0

                                                bcc: 0

                                            frequency-band: dcs1800BandUsed (0)

                                            bcch-ARFCN: 0

 

 

< SIB 19 >

 

SysInfoType19

    utra-PriorityInfoList

        utra-ServingCell

            priority: 3

            s-PrioritySearch1: 0

            s-PrioritySearch2: 0

            threshServingLow: 0

    gsm-PriorityInfoList: 1 item

        Item 0

            GSM-PriorityInfo

                gsmCellGroup

                    startingARFCN: 0

                    bandIndicator: dcs1800 (0)

                    followingARFCNs: explicitListOfARFCNs (0)

                        explicitListOfARFCNs: 0 items

                priority: 0

                qRxLevMinGSM: -58

                threshXhigh: 0

                threshXlow: 0

    eutra-FrequencyAndPriorityInfoList: 1 item

        Item 0

            EUTRA-FrequencyAndPriorityInfo

                earfcn: 300

                measurementBandwidth: mbw25 (2)

                priority: 4

                qRxLevMinEUTRA: -55

                threshXhigh: 9

                threshXlow: 9

                eutra-blackListedCellList: 2 items

                    Item 0

                        EUTRA-BlacklistedCell

                            physicalCellIdentity: 100

                    Item 1

                        EUTRA-BlacklistedCell

                            physicalCellIdentity: 102

                1... .... eutraDetection: True

    v920NonCriticalExtensions

        sysInfoType19-v920ext

            utra-PriorityInfoList-v920ext

                threshServingLow2: 0

            eutra-FrequencyAndPriorityInfoList-v920ext: 1 item

                Item 0

                    EUTRA-FrequencyAndPriorityInfo-v920ext

                        qqualMinEUTRA: -34

                        threshXhigh2: 0

                        threshXlow2: 0

        va80NonCriticalExtensions

            sysInfoType19-va80ext

 

 

Inter Search Criteria

 

When the following condition

 

When UE is in HPLMN

    (Squal = Qqualmeas Qqualmin) <= s-Intersearch

    or

    s-Intrasearch is not configured

 

When UE is in VPLMN

    (Squal = Qqualmeas (Qqualmin + QqualminOffset)) <= s-Intersearch

    or

    s-Intrasearch is not configured

     

If you want to let UE to perform Intracell search all the time, the easiest way would be to set as follows.

    i) set s-Intersearch to be MAX value

    ii) set Qualmin to be MAX value

 

 

Troubleshoot for Cell Selection

 

You may think Cell Selection (or Reselection) would be relatively easy to troubleshoot, but in reality it is not simple at all. First, not so many people has clear understanding of the cell selection criteria itself. Even though you have clear understanding of all these 3GPP details, there are technical issues that make this troubleshooting difficult. The first step for the troubleshooting is to guarantee / verify the following two factors.

    i) Signal Quality from the network (or UE Test Equipment) is good enough in whole power range of the test

    ii) Signal detection/decoding capability of UE is good enough in whole power range of the test

Item ii) is relatively easy comparing to item i) if you have UE side logging tool and skill to analyze the log very in detail.

The problem is item ii). Usually Cell Selection test is done in very low power. One of the most accurate way to verify the signal quality of any transmitter is to use Vector Signal Analyzer and measure EVM or CDP(Code Domain Power. See some example ). But the problem is that none of the vector signal analyzer (as far as I know) can decode/analyze the modulated signal at such a lower power level (e.g, under -100 dBm). Technically, if you have very good quality of amplifier with high dynamic range, you can use it to amplify the signal from the network and then put it into spectrum analyzer. But in reality it would be difficult to use such a complicated setup for day-to-day troubleshooting.

Therefore, in real life of UE testing.. if you don't have detailed UE logging and the skills to analyze them, the troubleshooting process would be almost completely like 'matter of luck'.

 

If you have UE side log, you may check following items one by one. (In this step, I assume that the signal quality from NodeB is good enough)

 

  • i) Check P-CPICH RSCP and Ec/Io (If UE log fail to print this value, you may check PSS, SSS detection.. but usually we take it as failure without further checking. But technically, you have to check PSS, SSS detection before you give up)
  • ii) Check if UE log successfully print out PSC (Primary Scrambling Code) and see it matches the PSC of the NodeB that you want the UE to select.
  • iii) Check if UE successfully decode MIB
  • iv) Check if UE sucessfully decode all the SIBs scheduled in MIB
  • v) Check P-CPICH power and quality meets the cell selection criteria (To do this, UE needs to successfully decode SIB3 at least.) Usually, most of UE log would print out Squal and Srxlev value if you set proper filter in the logging tool so you don't have to calculate them manually.
  • vi) If both Squal and Srxlev is greater than 0, then check if UE successfully decoded all the scheduled SIBs (or at least SIB 1, 3, 5)
  • vii) If UE successfully decoded the necessary SIBs, it should send PRACH. If UE does not send PRACH even at this condition, it is UE side protocol issue or sometimes I saw some UE manufacturer put some hard limiter in such a way that UE does not send PRACH under a certain value regardless of cell selection criteria.
  • vii) If UE send PRACH, check if the PRACH power is big enough for network reception range. (The proper PRACH power is also determined by a complicated process called 'Open Loop Power Control')
  • viii) If UE has sent PRACH with proper power and NodeB (or Test equipment) fail to detect it, then network side debugging should be done.

 

 

Reference :

 

[1] TS 25.304 UMTS; UE Procedures in idle mode and procedures for cell reselection in connected mode

[2] TS 25.331 UMTS; RRC Protocol Specification

[3] Inter-System Cell Reselection Parameter Optimization in UMTS by Qualcomm