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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 :

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

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.

  • 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
    • UE has to perform intra-frequency measurement only when SrxLev of the serving cell <= SintraSearch
    • UE has to perform lower or equal priority inter-frequency/InterRAT measurement only when SrxLev of 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 and then detected a neighbouring WCDMA cell with higher priority than the serving LTE cell, UE will perform cell reselection to WCDMA cell as long 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 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.

Note 3 : The UE shall not consider any black listed cells as candidate for cell reselection

 

 

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.

  • 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 (25.304-5.2.6.1.2a).
    • Srxlev > s-PrioritySearch1 (SIB19)

      Squal > s-PrioritySearch2 (SIB19)

  • In following condition, UE should detect for both lower and higher priority LTE frequencies(25.304-5.2.6.1.2a)
    • Srxlev <= s-PrioritySearch1 (SIB19)

      Squal <= s-PrioritySearch2 (SIB19)

  • 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 s-PrioritySearch1

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

 

 

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 :

 

ThreshXHighP : The threshold of target cell Srxlev to perfrom reselection from a low priority to a high priority cell. (i.e, a large ThreshXHigh value makes reselection harder)

 

ThreshXLowP : The threshold of target cell Srxlev to perfrom reselection from a high priority to a low priority cell. (i.e, a large ThreshXLow value makes reselection harder)

 

ThreshXHighQ : The threshold of target cell Squal to perfrom reselection from a low priority to a high priority cell. (i.e, a large ThreshXHigh value makes reselection harder)

 

ThreshXLowQ : The threshold of target cell Squal to perfrom reselection from a high priority to a low priority cell. (i.e, a large ThreshXLow value makes reselection harder)

 

SIntraSearchP : The threshold of current cell Srxlev to perform intra-frequency. If the current cell Srxleve is lower than this value, UE perform measurement for intra-frequency.

 

SIntraSearchQ : The threshold of current cell Squal to perform intra-frequency. If the current cell Squal is lower than this value, UE perform measurement for intra-frequency.

 

SnonIntraSearchP : The threshold of current cell Srxlev to perform inter-frequency or interRAT measurement. If the current cell Srxleve is lower than this value, UE perform measurement for inter-frequency or interRAT cells.

 

SnonIntraSearchQ : The threshold of current cell Squal to perform inter-frequency or interRAT measurement. If the current cell Srqual is lower than this value, UE perform measurement for inter-frequency or interRAT cells.

 

 

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