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5G Idle Mode Procedure |
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The term 'Cell Selection Criterion' may be a vague expression, since there can be many different criteria from many different perspective. In broad sense, cell selection would be influenced by following factors.
i) Is the cell transmitting power strong enough to be recognized/detected by the UE ? (Signal Strength/Quality Criteria) ii) Is the PLMN of the cell acceptable to the UE ? (PLMN selection criteria) iii) Is the service type of the cell acceptable to the UE ? (Service Type criteria)
But in most of the situation when we say "Cell Selection Criteria", it is likely to say the first criteria (Signal Strength/Quality Criteria). This signal quality criterion as descrbed in 38.304 - 5.2.3.2 as follows.
According to this criterion, UE would not start registration even though it sucessfully detected a cell and even decoded MIB and SIBs unless the Srxleve > 0 and Squal > 0. So if a device does not even initiate the PRACH process even when it successfully decoded all the MIB and SIBs, checking on this criteria would be a good first step for the troubleshooting. (Of course, this is not the only issues for this case. there may be USIM issue and Band Indicator Issue, PLMN issues etc).
Out of the variables used in the equation, only Qrxlevmeas and Qqualmeas is the value UE really measures when it turns on and most of other parameters are determined by a specific SIB (SIB1 in LTE case) or calculated by some other predefined values.
Following is the part of LTE SIB1 which is related to Cell Selection Criterion and Cell Selection Procedure. Following is overall information and functionality of SIB1 information element.
SIB1 ::= SEQUENCE { cellSelectionInfo SEQUENCE { q-RxLevMin Q-RxLevMin, q-RxLevMinOffset INTEGER (1..8) OPTIONAL, q-RxLevMinSUL Q-RxLevMin OPTIONAL, q-QualMin Q-QualMin OPTIONAL, q-QualMinOffset INTEGER (1..8) OPTIONAL } OPTIONAL, -- Need S cellAccessRelatedInfo CellAccessRelatedInfo, connEstFailureControl ConnEstFailureControl OPTIONAL, si-SchedulingInfo SI-SchedulingInfo OPTIONAL, servingCellConfigCommon ServingCellConfigCommonSIB OPTIONAL, -- Need R ims-EmergencySupport ENUMERATED {true} OPTIONAL, -- Need R eCallOverIMS-Support ENUMERATED {true} OPTIONAL, -- Cond Absent ue-TimersAndConstants UE-TimersAndConstants OPTIONAL, -- Need R
uac-BarringInfo SEQUENCE { uac-BarringForCommon UAC-BarringPerCatList OPTIONAL, -- Need S uac-BarringPerPLMN-List UAC-BarringPerPLMN-List OPTIONAL, -- Need S uac-BarringInfoSetList UAC-BarringInfoSetList, uac-AccessCategory1-SelectionAssistanceInfo CHOICE { plmnCommon UAC-AccessCategory1-SelectionAssistanceInfo, individualPLMNList SEQUENCE (SIZE (2..maxPLMN)) OF UAC-AccessCategory1-SelectionAssistanceInfo } OPTIONAL } OPTIONAL, -- Need R
useFullResumeID ENUMERATED {true} OPTIONAL, -- Need N lateNonCriticalExtension OCTET STRING OPTIONAL, nonCriticalExtension SEQUENCE{} OPTIONAL }
ConnEstFailureControl ::= SEQUENCE { connEstFailCount ENUMERATED {n1, n2, n3, n4}, connEstFailOffsetValidity ENUMERATED {s30, s60, s120, s240, s300, s420, s600, s900}, connEstFailOffset INTEGER (0..15) OPTIONAL }
Now you may have a couple of questions of q-RxLevMin. The first question of what kind of power this represents ? Is it RSSI or RSRP or RSRQ ? How the value of this IE maps to real power value (dBm) ? You can get the answers to these two questions at once from 36.331.
38.331 has a description as follows.
q-RxLevMin
The IE Q-RxLevMin is used to indicate for cell selection/ re-selection the required minimum received RSRP level in the (NR) cell. Corresponds to parameter Qrxlevmin in TS38.304. Actual value Qrxlevmin = field value * 2 [dBm].
q-RxLevMinOffset
Parameter "Qrxlevminoffset" in TS 38.304. Actual value Qrxlevminoffset = field value * 2 [dB]. If absent, the UE applies the (default) value of 0 dB for Qrxlevminoffset. Affects the minimum required Rx level in the cell.
q-QualMin
Parameter "Qqualmin" in TS 38.304, applicable for serving cell. If the field is not present, the UE applies the (default) value of negative infinity for Qqualmin.
q-QualMinOffset
Parameter "Qqualminoffset" in TS 38.304. Actual value Qqualminoffset = field value [dB]. If the field is not present, the UE applies the (default) value of 0 dB for Qqualminoffset. Affects the minimum required quality level in the cell.
connEstFailOffset
Parameter "Qoffsettemp" in TS 38.304. If the field is not present the value of infinity shall be used for "Qoffsettemp".
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