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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.
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.
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.
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.
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.
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.
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.
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
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
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
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)
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
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