Multi Carrier Cell Re-selection in LTE

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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.

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)
      T_RESELECTION  This parameter is the interval (timer) of reselection execution. 1 sec
Q offset Offset applicable to a specific neighboring cell used for evaluating the cell as a candidate for cell re-selection.
QHyst The parameter QHyst is introduced to prevent from ping pong re-selections 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

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).

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

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)

Cell Reselection for Inter Frequency

  • S_NON_INTRA_SEARCHThis specifies the Srxlev threshold (in dB) for E-UTRAN inter-frequency 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

  • 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

Article is submitted by  Prajna Poojari  (Senior LTE & VoLTE RNPO Expert)

About  Prajna Poojari  
Prajna Poojari  is a Senior LTE & VoLTE RNPO Expert and has successfully delivered LTE RNPO Projects in different regions across India.