Cell reselection is done by first checking the :
PRIORITY
RADIO LINK QUALITY
CELL ACCESSIBILITY
PRIORITY
This is a parameter specifying the priority of EUTRA-FA(Evolved Universal Terrestrial Radio
Access-Foreign Area) during idle re-selection or mobility control information.
LTE cell re-selection uses priority based levels starting from 0 to 7 as per 3GPP 36.331 where
Priority 0 is Lowest and Priority 7 is highest. This value can be seen in SIB3 as
cellReselectionPriority Information Element (IE).
An example is shown for a multi carrier vendor is shown as below. The operator has 3 carrier
2300 MHz, 1800 MHz and 850 MHz which different Bandwidth. The Priority here is decided
based on the channel bandwidth, highest channel bandwidth is given highest re-selection priority.
Frequency PRIORITY
2300(20MHz) 7
1800(5 MHz) 5
850(10MHz) 6
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
radio 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 re-selection or mobility control information.
LTE cell re-selection uses priority based levels starting from 0 to 7 as per 3GPP 36.331 where
Priority 0 is Lowest and Priority 7 is highest. This value can be seen in SIB3 as
cellReselectionPriority Information Element (IE).
An example is shown for a multi carrier vendor is shown as below. The operator has 3 carrier 2300
MHz, 1800 MHz and 850 MHz which different Bandwidth. The Priority here is decided based on
the channel bandwidth, highest channel bandwidth is given highest re-selection priority.
Frequency
PRIORITY
2300(20MHz)
7
1800(5 MHz)
5
850(10MHz)
6
CELL SELECTION
UE performs cell selection based on RSRP level as per 3GPP Rel 8 or consider both RSRP and
RSRQ level as per 3GPP Rel9.
http://www.techplayon.com/wp-content/uploads/2019/03/Reselection.png
The criteria for cell selection is that the Srxlev > 0 [Definition and details about Srxlev is available
in the table below].
Parameter
Information
Settings
Q_RX_LEV_MIN
This parameter is minimum RX level
required in a cell that is operating as
EUTRA-FA and its unit is dBm
(measured RSRP). Value is found in
SIB 1 , SIB3 and is
Calculated as Value in dBm/0.5
-128 dBm ( IE value in SIB 1 is -64 )
Q_RX_LEV_MEAS
Measured cell’s RX Level
value(RSRP)
SRXLEV
Cell selection RX level value (in dB)
measured by UE
Srxlev=Qrxlevmeas-Qrxlevmin(128) Example Cell selection
Criterion:
Srxlev>0
RSRP(Qrxlevmeas)>Qrxlevmin(128)
This parameter is the interval
(timer) of reselection execution.
1 sec
T_RESELECTION
Q offset
Offset applicable to a specific
neighboring cell used for evaluating
the cell as a candidate for cell reselection.
QHyst
The parameter QHyst is introduced
to prevent from ping pong reselections
2dB
The role of Hysteresis can be illustrated as follows
Case 1:
Hysteresis=0 dB, Measured value is fluctuating around the target value causing a lot of
unnecessary measurement report
http://www.techplayon.com/wp-content/uploads/2019/03/Hst-1.png
Case 2:
Hysteresis= 2dB, Even though the measured value is fluctuating around the target it does not
trigger measurement report unless it fluctuates wider than hysteresis(Pink).
http://www.techplayon.com/wp-content/uploads/2019/03/HST2.png
Cell Re-selection for Intra Frequency Parameters
 S_INTRA_SEARCH:nThis parameter is the threshold value for intra-frequency
measurement i.e it is used to trigger intra frequency measurements Actual value in dB is
obtained by multiplying by 2.Corresponds to parameter s-IntraSearch specified in
SIB3 Settings:31 ,62dB
Measurement rules for intra frequency cell re-selection
 Serving_Srxlev > Sintrasearch – Do not perform intra frequency measurement for
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 for 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 and Condition for reselection:
RSRP Serving Cell+Qhyst< RSRP Neighbor cell -Qoffset during Treselection
http://www.techplayon.com/wp-content/uploads/2019/03/resel_1.png
Here 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)
http://www.techplayon.com/wp-content/uploads/2019/03/resel_2.png
Cell Reselection for Inter Frequency
 S_NON_INTRA_SEARCHThis specifies the Srxlev threshold (in dB) for E-UTRAN interfrequency and inter-RAT measurements.Actual value of threshold is the value * 2 (dB).
Frequency
settings(IE
value)
Formula
Calculated Value
2300
10
Serving_Qrxlevmin -64 (SIB3) X 2
+
Snonintrasearch 10 (SIB3) X 2
-128+20=-108
dBm
1800
12
Serving_Qrxlevmin -64 (SIB3) X 2
+
Snonintrasearch 12 (SIB3) X 2
-128+24=-104
dBm
850
12
Serving_Qrxlevmin -64 (SIB3) X 2
+
Snonintrasearch 12 (SIB3) X 2
-128+24=-104
dBm
Example

ServingCell<=Snonintrasearch (10*2=20) +Serving_qrxlevmin (SIB3,-64,-128 dBm) =-108
dBm
 Serving cell RSRP<=-128+20=-108 dbm
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.
 ThreshXHigh: 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 frequency
measurement
 Condition for cell reselection: RSRP Neighbor /Target cell > target_qrxlevmin (SIB5,64,-128 dBm) + threshXHigh (SIB5,14,28 dB) =-100 dBm
http://www.techplayon.com/wp-content/uploads/2019/03/resel_3.png

Thresh_Xlow:This specifies the Srxlev threshold (dB) used by the UE on the serving cell
when re-selecting towards a lower priority inter-RAT frequency (SIB3)
Example Settings
Frequency
Example IE value
Formula
Calculated Value
2300
10
Serving_Qrxlevmin -64 (SIB3) X 2 +
ThreshServingLow 10 (SIB3) X 2
-128+20=-108 dBm
1800
12
Serving_Qrxlevmin -64 (SIB3) X 2 +
ThreshServingLow 12(SIB3) X 2
-128+24=-104 dBm
850
12
Serving_Qrxlevmin -64 (SIB3) X 2 +
ThreshServingLow 12 (SIB3) X 2
-128+24=-104 dBm
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 theSrxlev greater than Thresh_Xlow. The threshold
used to re-select low-priority frequency from high-priority 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 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
http://www.techplayon.com/wp-content/uploads/2019/03/resel_4.png