4G/LTE - Basic Procedures |
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Cell Reselection
Cell Reselection is a kind of mechanism to change cell after UE is camped on a cell and stay in IDLE mode. 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 andalgorithms will be described later but the high level guideline is as follows :
When you power on UE, usually it goes on with the following sequence. i) Cell Detection/Search ii) Cell Selection iii) RACH and Completion of Registration iv) < In IDLE > v) Keep measuring RSRP/RSRQ for the cell it registered vi) if RSRP/RSRQ is very poor or undetectable, go to step i) for finding other cell if RSRP/RSRQ is measureable at least, it evaluate < Cell Reselection Criteria > perform Cell Reselection if the criteria is met stay in the current cell if the criteria is not met Step vi) is the most complicated process during the idle stage and in this page I will mostly deal with < Cell Reselection Criteria >. Understanding this criteria is the most imporant thing in implementing and testing Cell Reselection.
Following is the topics that I will deal with in this page.
A little bit detailed criteria can be described as follows and you will see further details after this.
How Cell Reselection Priority is determined ?
There are roughly three different ways of informing UE of Reselection Priority as listed before : (refer to 36.304 5.2.4.1 Reselection priorities handling for details). i) System Information ii) RRC Connection Release message iii) Inheriting from another RAT Note 1 : if the priority is specified in both i) and ii), the one specified in ii) takes effect. Note 2 : The UE shall only perform cell reselection evaluation for E-UTRAN frequencies and inter-RAT frequencies that are given in system information and for which the UE has a priority provided. ==> This is very important thing you should consider. This mean that UE would try to check priorities only among those cells specified in SIB (SIB4 in case of intra frequency neighbour cell and SIB5 in case of inter frequency neighbour cell. If those cells are not specified in SIBs, UE would not try to find high priority cells even if you specify in Rrc Release). Note 3 : The UE shall not consider any black listed cells as candidate for cell reselection
Following is the configuration and procedure that I confirmed working with Amari Callbox and a commercial phone.
i) specified two cells in SIB5 as below. sib5: { interFreqCarrierFreqList { { dl-CarrierFreq 3350, q-RxLevMin -61, t-ReselectionEUTRA 1, threshX-High 0, threshX-Low 0, allowedMeasBandwidth mbw25, presenceAntennaPort1 FALSE, neighCellConfig '01'B }, { dl-CarrierFreq 1575, q-RxLevMin -61, t-ReselectionEUTRA 1, threshX-High 0, threshX-Low 0, allowedMeasBandwidth mbw25, presenceAntennaPort1 FALSE, neighCellConfig '01'B } ii) UE camped to Cell frequency 3350 iii) eNB send RrcRelease with following Priority Settings { message c1: rrcConnectionRelease: { rrc-TransactionIdentifier 0, criticalExtensions c1: rrcConnectionRelease-r8: { releaseCause other, idleModeMobilityControlInfo { freqPriorityListEUTRA { { carrierFreq 3350, cellReselectionPriority 3 }, { carrierFreq 1575, cellReselectionPriority 6 } } } } } } iv) UE reselect to the Cell Frequency 1575 if the cell is in detactable states (e.g, enough cell power)
Which SIBs involved in Cell Reselection process
There are many SIBs that are involved in Cell Reselection process and which SIBs are involved gets different depending on the specific cell reselection scenario. I will put down the examples of all of these SIBs for Cell Reselection as an examples (template). The SIBs shown here is just examples to show you which SIB carries what kind of parameters and you should change the values of each of the parameters (IE) depending on your own test cases. Blind copy of this example may not work for your specific scenario.
Following is the contents of SIB1 for your reference (The parameter shown here is just an example. You have to set it properly based on your own test requirement)
c1: systemInformationBlockType1 (1) systemInformationBlockType1 cellAccessRelatedInfo plmn-IdentityList: 1 item Item 0 PLMN-IdentityInfo plmn-Identity <...> cellReservedForOperatorUse: notReserved (1) trackingAreaCode: 0001 cellIdentity: 00000000 cellBarred: notBarred (1) intraFreqReselection: allowed (0) .... ..0. csg-Indication: False cellSelectionInfo q-RxLevMin: -130dBm (-65) p-Max: 23dBm freqBandIndicator: 4 schedulingInfoList: 3 items <...> si-WindowLength: ms20 (5) systemInfoValueTag: 0 nonCriticalExtension
Following is the contents of SIB3 for your reference (The parameter shown here is just an example. You have to set it properly based on your own test requirement) sib-TypeAndInfo item: sib3 (1) sib3 cellReselectionInfoCommon q-Hyst: dB0 (0) cellReselectionServingFreqInfo s-NonIntraSearch: 62dB (31) threshServingLow: 62dB (31) cellReselectionPriority: 7 intraFreqCellReselectionInfo q-RxLevMin: -140dBm (-70) s-IntraSearch: 62dB (31) .0.. .... presenceAntennaPort1: False neighCellConfig: Not all neighbour cells have the same MBSFN subframe allocation as serving cell (0) t-ReselectionEUTRA: 0s lateNonCriticalExtension: <MISSING> s-IntraSearch-v920 s-IntraSearchP-r9: 62dB (31) s-IntraSearchQ-r9: 0dB s-NonIntraSearch-v920 s-NonIntraSearchP-r9: 62dB (31) s-NonIntraSearchQ-r9: 0dB q-QualMin-r9: -34dB threshServingLowQ-r9: 0dB
Following is the contents of SIB4 for your reference (The parameter shown here is just an example. You have to set it properly based on your own test requirement). This gets involved in LTE-LTE Intrafrequency cell reselection sib-TypeAndInfo item: sib4 (2) sib4 intraFreqNeighCellList: 1 item Item 0 IntraFreqNeighCellInfo physCellId: 1 q-OffsetCell: dB3 (18)
Following is the contents of SIB5 for your reference (The parameter shown here is just an example. You have to set it properly based on your own test requirement). This gets involved in LTE-LTE Interfrequency cell reselection sib-TypeAndInfo item: sib5 (3) sib5 interFreqCarrierFreqList: 1 item Item 0 InterFreqCarrierFreqInfo dl-CarrierFreq: 5790 q-RxLevMin: -110dBm (-55) p-Max: 23dBm t-ReselectionEUTRA: 0s threshX-High: 4dB (2) threshX-Low: 2dB (1) allowedMeasBandwidth: mbw25 (2) .... .0.. presenceAntennaPort1: False cellReselectionPriority: 4 neighCellConfig: No MBSFN subframes are present in all neighbour cells (1) q-OffsetFreq: dB3 (18)
Following is the contents of SIB6 for your reference (The parameter shown here is just an example. You have to set it properly based on your own test requirement). This gets involved in LTE-WCDMA InterRAT/Interfrequency cell reselection sib-TypeAndInfo item: sib6 (4) sib6 carrierFreqListUTRA-FDD: 1 item Item 0 CarrierFreqUTRA-FDD carrierFreq: 10700 cellReselectionPriority: 3 threshX-High: 12dB (6) threshX-Low: 12dB (6) q-RxLevMin: -89dBm (-45) p-MaxUTRA: 24dBm q-QualMin: -24dB t-ReselectionUTRA: 0s
Following is the contents of SIB7 for your reference (The parameter shown here is just an example. You have to set it properly based on your own test requirement). This gets involved in LTE-GSM InterRAT/Interfrequency cell reselection
sib-TypeAndInfo item: sib7 (5) sib7 t-ReselectionGERAN: 0s carrierFreqsInfoList: 1 item Item 0 CarrierFreqsInfoGERAN carrierFreqs startingARFCN: 5 bandIndicator: dcs1800 (0) followingARFCNs: explicitListOfARFCNs (0) explicitListOfARFCNs: 0 items commonInfo cellReselectionPriority: 2 ncc-Permitted: 80 [bit length 8, 1000 0000 decimal value 128] q-RxLevMin: -79dBm (18) p-MaxGERAN: 27dBm threshX-High: 12dB (6) threshX-Low: 12dB (6)
How to detect and reselect to another LTE cell while in LTE Cell (LTE to LTE Cell Reselction) ?
In order for UE to reselect from one LTE cell to another LTE Cell, the internal evaluation result in UE side (SrxLev and Squal) should meet a certain criteria, otherwise it may not even monitor (measure) neighboring cell and in result would not perform reselection. LTE SIB1,SIB3 and SIB4 defines various parameter to influence the evaluation result and the expected UE side behavior is described in 36.304 5.2.4.2 Measurement rules for cell re-selection.
If your device does not perform Cell Reselection, first Check if Cell Measurement Criteria is met and then check if Cell Reselection Criteria is met and also check if following condition is met (Following statement come from 36.304 5.2.4.1 Reselection Priority Handling. I saw some UE ignore this requirement but some UE strictly follows this requirement.)
The UE shall only perform cell reselection evaluation for E-UTRAN frequencies and inter-RAT frequencies that are give in system information and for which the UE has a priority provided.
< LTE to LTE Intra Frequency Measurement >
If the serving cell's evaluation result is as follows, UE may not perform intra frequency measurement. SrxLev > s-IntraSearchP Squal > s-IntraSearchQ Note : If neither of S_IntraSearchP nor S_IntraSearchQ is specified, UE applies the default value (S_IntraSearchP = Infinity, S_IntraSearchQ = 0 based on 36.331)
If the serving cell's evaluation result does NOT meet following criteria, UE perform intra frequency measurement. SrxLev > s-IntraSearchP Squal > s-IntraSearchQ Note : If neither of s-IntraSearchP nor s-IntraSearchQ is specified, UE applies the default value (s-IntraSearchP = Infinity, s-IntraSearchQ = 0 based on 36.331). It implies that if neither of s-IntraSearchP and s-IntraSearchQ are specified, UE always perform intra frequency measurement.
< LTE to LTE NonIntra Frequency Measurement >
If the serving cell's evaluation result is as follows, UE may not perform nonIntra frequency measurement. SrxLev > s-NonIntraSearchP Squal > s-NonIntraSearchQ Note : If neither of s-NonIntraSearchP nor s-NonIntraSearchQ is specified, UE applies the default value (s-NonIntraSearchP = Infinity, s-NonIntraSearchQ = 0 based on 36.331)
If the serving cell's evaluation result does NOT meet following criteria, UE perform Nonintra frequency measurement. SrxLev > s-NonIntraSearchP Squal > s-NonIntraSearchQ Note : If neither of s-NonIntraSearchP nor s-NonIntraSearchQ is specified, UE applies the default value (s-NonIntraSearchP = Infinity, s-NonIntraSearchQ = 0 based on 36.331). It implies that if neither of s-NonIntraSearchP and s-NonIntraSearchQ are specified, UE always perform NonIntra frequency measurement.
How to detect and reselect to LTE cell while in WCDMA (WCDMA to LTE Cell Reselction) ?
UE must measure the LTE frequencies and detect the available LTE cell in order to perform cell reselection to LTE.
Note : I just put down a brief summary here about Reselection in WCDMA since this page is mainly for Reselection in LTE. For the details of Selection / Reselection in WCDMA, Refer to Cell Selection/Reselection in WCDMA page.
How to detect and reselect to WCDMA cell while in LTE (LTE to WCDMA Cell Reselction)?
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 (SIB1), If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB1)) [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 lower priority than 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 (SIB1), where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for camping Srxlev of WCDMA cell > threshXLowP (SIB6), where Srxlev = Qrxlevmeas - qRxLevMin (SIB1), If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB1)) [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 Srxlev Note : 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 > threshXHighP (SIB6), where Srxlev = Qrxlevmeas - qRxLevMin (SIB1), where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping
How to let UE reselect to Cell not specified in SIB neighbour cell configuration ?
What if I want to let UE reselct to those cells that are not listed in SIBs ? The simplest way would be to turn down (reduce the cell power under cell selection criteria) all the cells listed in SIB, then UE would perform PLMN search. But strictly speaking this would be considered as a cell selection rather than reselection,
Parameters related to Cell Reselection
These parameters are defined in 36.304 5.2.4.7 Cell reselection parameters in system information broadcasts. Some of the important parameters and descriptions are as follows :
Reference :
[1] TS 36.304 EUTRA; UE Procedures in idle mode [2] TS 36.331 EUTRA; RRC Protocol Specification [3] TS 25.304 UMTS; UE Procedures in idle mode and procedures for cell reselection in connected mode [4] TS 25.331 UMTS; RRC Protocol Specification
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