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Telecom protocol stack
Cell Reselection
Cell Reselection is a kind of mechanism to change cell after UE is camped on a cell. This is tolet UE get connected to cell which has the best condition among all the cells to which the UEis allowed to camp on. But UE does not change cells randomly, it uses a set of prettycomplicated criteria and algorithms for this reselection process. The details of these criteriaand algorithms will be described later but the high level guideline is as follows :
First Level Criteria : Absolute PrioritySecond Level Criteria : Radio Link QualityThird Level Criteria : Cell Accessibility
A little bit detailed criteria can be described as follows and you will see ever further detailsafter this.
UE always have to measure frequecies and RAT with higher priorityUE has to have to measure frequecies and RAT with lower priority in the followingfashion
UE has to perform intrafrequency measurement only when SrxLev of theserving cell <= SintraSearchUE has to perform lower or equal priority interfrequency/InterRATmeasurement only when SrxLev of the serving cell <= SnonintraSearch
LTE cell reselection uses priority based levels. (These priorities can be specified inLTE SIB4,5,6,7 and RRC Connection Release). Priority 0 indicate the lowest priorityand 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 cellreselection ? Cell Power defines the minimum condition. Once the minimumcondition is met, Priority plays more critical role. For example, let's assume that UEis now camped in WCDMA cell and detected a neighbouring LTE cell with higherpriority than the serving WCDMA cell, UE will perform cell reselection to LTE cell aslong as the LTE cell is above the minimum signal strength threshold. In the samelogic, if a UE camped on LTE cell detected a neighbouring WCDMA cell with higherpriority than the serving LTE cell, UE will perform cell reselection to WCDMA cell aslong as the WCDMA cell is above the minimum signal strength threshold.WCDMA SIB19 specifies the LTE frequencies (EARFCN) and priorities. A maximum of8 EARFCNs (4 FDD + 4 TDD) can be listed in SIB 19. For each EARFCN, you canconfigure a max 16 blacklisted cells. The blacklisted cell is the cell UE should notmake reselection to.Using the eutraDetection parameter in SIB19, you can make UE to detect anddisplay 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 forabsolute 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 performcell reselection to LTE.
UE measures two physical properties called for WCDMA signal. One is CPICH RSCPand CPICH EcNo. RSCP determines Srxlev and EcNo determines Squal. Srxlev = Qrxlevemeas qRxLevMin. Qrxlevemeas is RSCP level measured by UEand qRxLevMin is the value specified in SIB.Squal = Qqualmeas qQualMin. Qqualmeas is EcNo level measured by UE andqQualMin is the value specified in SIB.The detection measurement of LTE frequencies should be done at least once every60s for higher priority LTE frequencies.In following condition, detection measurements of lower priority LTE frequency isnot required.
Srxlev > absPrioCellRes.sPrioritySearch1Squal > absPrioCellRes.sPrioritySearch2
In following condition, UE should detect once every 30s for both lower and higherpriority LTE frequencies
Srxlev <= absPrioCellRes.sPrioritySearch1Squal <= absPrioCellRes.sPrioritySearch2
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The maximum number of LTE FDD Frequencies are 4. In this case, UE should haveperformed measurement for detecting LTE cells on all 4 LTE frequencies once every240 (4 x 60) s or 120(4 x 30) s depending if UE measures above or belowparameter 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 LTEcell has higher priority than WCDMA, it would stay in LTE cell but it performs measurementfor 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 xsNonIntraSearch (SIB3)) where Srxlev = Qrxlevmeas qRxLevMin (SIB3), If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] (2 xqRxLevMin (SIB3)) [dBm] where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRPlevel for camping
Measurement Criteria (From Lower Priority LTE Cell to Higher Priority WCDMA Cell): WhenLTE cell has lower priority than WCDMA (WCDMA cell priority defined in SIB6 is higher thanthe serving cell priority),The UE always have to perform measurements on WCDMA cell. Howoften UE has to measure for WCDMA depends on whether Srxlev of the serving cell is greateror 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 LTEcell is under the following condition with the duration longer than tReselectionUtra (SIB6), itshould reselect to WCDMA cell.
Srxlev of LTE cell (serving cell) < threshServingLow (SIB3), where Srxlev = Qrxlevmeas qRxLevMin (SIB3), where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRPlevel for camping
Srxlev of WCDMA cell > threshXLow (SIB6), where Srxlev = Qrxlevmeas qRxLevMin (SIB3), If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] (2 xqRxLevMin (SIB3)) [dBm] where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCPlevel for camping
Note : If more than one WCDMA meet this condition, UE should select to the cell with highestSrxlevNote : If these values are not specified in SIB and UE has to apply default values, UE has toperform reselection when LTE RSRP is lower than 145 dBm and WCDMA cell is better than119 dBm for at least 2 seconds.
Reselection Criteria (From Lower Priority LTE Cell to Higher Priority WCDMA Cell): If UE inLTE 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 RSCPlevel for camping
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1 comment:
Amar W 24 September 2013 at 04:40
Hi Folks...,You will get further more information in coming days on protocol stack...., if you need further clarificationplease comments , i will get back to you...:)
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