CELL RESELECTION LTE
Brief Overview:
Cell Re selection is a procedure used to change the cell after the UE is camped on a cell and stays in the Idle mode.This procedure is used to let the UE get camped on a cell which has the best conditions among all the other cells on which the UE is allowed to camp on .
Cell reselection is done by first checking the :
1.PRIORITY
2.RADIO LINK QUALITY
3.CELL ACCESSIBILITY
Let us see how priority triggers inter-frequency measurement
Example:
Serving cell priority(6)< Neighbor cell Priority(7)
Perform inter frequency measurement
CELL SELECTION
The role of Hysterisis can be illustrated as follows
Case 1:
Hysterisis=0
Measured value is fluctuating around the target value causing alot of unnecessary measurement report
Case 2:
Hysterisis=2dB
Even though the measured value is fluctuating around the target it does not trigger measurement report unless it fluctuates wider than hysterisis(Pink)
Brief Overview:
Cell Re selection is a procedure used to change the cell after the UE is camped on a cell and stays in the Idle mode.This procedure is used to let the UE get camped on a cell which has the best conditions among all the other cells on which the UE is allowed to camp on .
Cell reselection is done by first checking the :
1.PRIORITY
2.RADIO LINK QUALITY
3.CELL ACCESSIBILITY
| PRIORITY |
This is a parameter specifying the priority of EUTRA-FA(Evolved Universal Terrestrial Radio Access-Foreign Area) during idle reselection or mobility control information.
LTE cell reselection uses priority based levels (Priority 0 is Lowest and Priority 7 is highest)
This value is seen in SIB3 as cellReselectionPriority 7
LTE cell reselection uses priority based levels (Priority 0 is Lowest and Priority 7 is highest)
This value is seen in SIB3 as cellReselectionPriority 7
Sample Priority Assigned
(depending on the BW priorities can be interchanged)
(depending on the BW priorities can be interchanged)
Frequency
|
PRIORITY
|
2300(20MHz)
|
7
|
1800(5 MHz)
|
5
|
850(10MHz)
|
6
|
Let us see how priority triggers inter-frequency measurement
Example:
Serving cell priority(6)< Neighbor cell Priority(7)
Perform inter frequency measurement
CELL SELECTION
UE performs cell selection based on RSRP level according to 3GPP rel 8 or both RSRP and RSRQ level 3GPP Rel9).
The criteria for cell selection is that the Srxlev>0[Definition and details about Srxlev is available in the table below]
Hysterisis=0
Measured value is fluctuating around the target value causing alot of unnecessary measurement report
Case 2:
Hysterisis=2dB
Even though the measured value is fluctuating around the target it does not trigger measurement report unless it fluctuates wider than hysterisis(Pink)
CELL RESELECTION FOR INTRAFREQUENCY
S_INTRA_SEARCH
This parameter is the threshold value for intra-frequency measurement i.e it is used to trigger intra frequency measurementsActual
value in dB is obtained by multiplying by 2.
Corresponds to parameter s-IntraSearch specified
in SIB3
Measurement rules for intra frequency cell reselection
Serving_Srxlev>Sintrasearch -Do not perform intra frequency measurement
Example: If measured RSRP of the serving cell is -80 dBm
Then Srxlev=-80+128=48 [Srxlev=Qrxlevmeas-Qrxlevmin(-128)]
Srxlev is 48 which is >31 (Sintrasearch Value)
Do not perform intra frequency measurement
Serving_Srxlev<=Sintrasearch -Perform RSRP measurement
Example :If measured RSRP of the serving cell is -100 dBm
Then Srxlev=-100+128=28
Srxlev is 28 which is <31 (Sintrasearch Value)
Perform intra frequency RSRP measurement
Condition for reselection for Intrafrequency
RSRP Serving Cell+Qhyst < RSRP Neighbor cell -Qoffset during Treselection
UE needs to camp on the current serving cell over 1 sec(T_RESELECTION)
This specifies the Srxlev threshold (in dB) for E-UTRAN inter-frequency and inter-RAT measurements.Actual value of threshold is the value * 2 (dB).
Example
ServingCell<=Snonintrasearch(10*2=20)+Serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
Serving cell RSRP<=-128+20=-108dbm
UE performs target cell EUTRAN frequency measurement
THRESHX PARAMETERS
ThreshX parameters are used in LTE for idle mode cell reselection towards a different LTE Frequency .We have 2 parameters ThreshXlow and ThreshXHigh . The 2 parameters relate to the target cell.
The threshold used in the UE to reselect a frequency whose priority is higher than the current camped frequency.If the user has to move from a low priority LTE frequency band to higher priority LTE then the neighbor/ target LTE must have the Srxlev>ThreshXHigh.
Settings
12/14
Priority of the Target Cell is higher than the serving cell
Example 1:
Serving cell Priority (6)<Neighbor cell priority(7)
Measurement
Neighbor cell priority>Serving cell priority =Always perform inter frequencymeasurement
Condition for cell reselection
This specifies the Srxlev threshold (dB) used by the UE on the serving cell when reselecting towards a lower priority inter-RAT frequency (SIB3)
The threshold used to reselect low-priority frequency from high-priority frequency
This value is found in SIB 5 .If the user has to move from a high priority band to a low priority band then the target low priority band must have the Srxlev greater than Thresh_Xlow
Settings
10 [-108 dBm]when source is 2300 to any frequency
12[-104 dBm ]when source is 1800/850 to any frequency
1.UE always measures frequencies with higher priority.
2.UE has to measure frequencies with lower priority in the following cases
a>perform intrafrequency measurement (same priority)only when Srxlev of the serving cell<=Sintrasearch
Example: If measured RSRP of the serving cell is -80 dBm
Then Srxlev=-80+128=48 [Srxlev=Qrxlevmeas-Qrxlevmin(-128)]
Srxlev is 48 which is >31 (Sintrasearch Value)
Do not perform intra frequency measurement
Example :If measured RSRP of the serving cell is -100 dBm
Then Srxlev=-100+128=28
Srxlev is 28 which is <31 (Sintrasearch Value)
Perform intra frequency RSRP measurement
Condition for reselection for Intrafrequency
RSRP Serving Cell+Qhyst < RSRP Neighbor cell -Qoffset during Treselection
Qmeas,n is RSRP Neighboring cell
Qmeas,s is RSRP Serving cell
UE needs to camp on the current serving cell over 1 sec(T_RESELECTION)
CELL RESELECTION FOR INTER FREQUENCY
S_NON_INTRA_SEARCH
This specifies the Srxlev threshold (in dB) for E-UTRAN inter-frequency and inter-RAT measurements.Actual value of threshold is the value * 2 (dB).
Example
ServingCell<=Snonintrasearch(10*2=20)+Serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
Serving cell RSRP<=-128+20=-108dbm
UE performs target cell EUTRAN frequency measurement
THRESHX PARAMETERS
| THRESH_XHIGH |
The threshold used in the UE to reselect a frequency whose priority is higher than the current camped frequency.If the user has to move from a low priority LTE frequency band to higher priority LTE then the neighbor/ target LTE must have the Srxlev>ThreshXHigh.
Settings
12/14
Priority of the Target Cell is higher than the serving cell
Serving cell Priority (6)<Neighbor cell priority(7)
Measurement
Neighbor cell priority>Serving cell priority =Always perform inter frequencymeasurement
Condition for cell reselection
RSRP Neighbor /Target cell >target_qrxlevmin(SIB5,-64,-128 dBm) + threshXHigh(SIB5,14,28 dB)=-100dBm
| THRESH_SERVING_LOW |
This specifies the Srxlev threshold (dB) used by the UE on the serving cell when reselecting towards a lower priority inter-RAT frequency (SIB3)
| THRESH_XLOW |
The threshold used to reselect low-priority frequency from high-priority frequency
This value is found in SIB 5 .If the user has to move from a high priority band to a low priority band then the target low priority band must have the Srxlev greater than Thresh_Xlow
Settings
10 [-108 dBm]when source is 2300 to any frequency
12[-104 dBm ]when source is 1800/850 to any frequency
Priority of the Target Cell is lower than the serving cell
Current Cell priority (7)<Target Cell Priority(6)
Assumption:Serving Cell:2300MHz,Target Cell:850MHz
Measurement
Serving Cell<=Snonintrasearch(12*2=20)+Serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
Serving cell RSRP<=-128+20=-108dbm
UE performs target cell EUTRAN frequency measurement
Cell Re-Selection for interfrequency (when target cell is of lower priority)
Both conditions have to be met
Serving Cell's RSRP<THRESH_SERVING_LOW(10,10*2=20)+serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
&
Neighbor/Target cell RSRP> ThreshX,Low(10,10*2=20)+Target_qrxlevmin (SIB5,-64,-128 dBm)=-108dBm
Current Cell priority (7)<Target Cell Priority(6)
Assumption:Serving Cell:2300MHz,Target Cell:850MHz
Measurement
Serving Cell<=Snonintrasearch(12*2=20)+Serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
Serving cell RSRP<=-128+20=-108dbm
UE performs target cell EUTRAN frequency measurement
Cell Re-Selection for interfrequency (when target cell is of lower priority)
Both conditions have to be met
Serving Cell's RSRP<THRESH_SERVING_LOW(10,10*2=20)+serving_qrxlevmin(SIB3,-64,-128 dBm)=-108dBm
&
Neighbor/Target cell RSRP> ThreshX,Low(10,10*2=20)+Target_qrxlevmin (SIB5,-64,-128 dBm)=-108dBm
APPENDIX
2.UE has to measure frequencies with lower priority in the following cases
a>perform intrafrequency measurement (same priority)only when Srxlev of the serving cell<=Sintrasearch
b>perform lower priority inter-frequency measurement only when Srxlev of serving cell <=Snonintrasearch
3.LTE Cell Reselection uses priority based levels (Priority 0=Lowest,Priority 7=Highest)
3.LTE Cell Reselection uses priority based levels (Priority 0=Lowest,Priority 7=Highest)
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