LTE: Cell Reselection

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 and algorithms 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 measurable 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 important thing in implementing and testing Cell Reselection.

Cell Reselection

Example for SIB3, taken from drive-test tool:

System Information Block 3
q-Hyst : dB4
s-NonIntraSearch : 2
threshServingLow : 1
cellReselectionPriority : 5
q-RxLevMin : -60
p-Max : 23
s-IntraSearch : 31
presenceAntennaPort1 : False
neighCellConfig
Binary string (Bin) : 01
[0 ] : 0
[1 ] : 1
t-ReselectionEUTRA : 1
t-ReselectionEUTRA-SF
sf-Medium : lDot0
sf-High : oDot75

UE in IDLE mode wakes up at the end of every DRx cycle to measure the signal of its Serving cell and calculate received signal strength (SrxLev) and received signal Quality to decide whether it should stay or move to another cell.

➢ If SrxLev is greater than specified threshold value, the UE stays in current serving cell, and if not, it triggers a "CELL RESELECTION" procedure.(The threshold value is specified through SIB 3).

There are 2 methods for selection of a cell:-

1. Cell Ranking. (For Intra-Freq cell reselection & Inter- Freq cell reselection with same cell priorities).

2. Priority based.

Following are the scenarios of "CELL RESELECTION":

1. LTE-LTE Intra Frequency Cell Reselection:

➢ If SrxLev > S-IntraSearchP [SIB 3] OR Squal > S-IntraSearchQ [SIB 3]-

If the serving cell's evaluation result does NOT meet above criteria, UE perform intra frequency measurement. If neither of s-IntraSearchP nor s-IntraSearchQ is specified, UE applies the default value (s-IntraSearchP = Infinity, s-IntraSearchQ = 0). It implies that if neither of s-IntraSearchP and s-IntraSearchQ are specified, UE always perform intra frequency measurement .In Intra-Frequency cell reselection, among the neighboring cells with SrxLev greater than threshold, the cell with the Highest SrxLev is selected by means of "Cell Ranking" method.

2. LTE to LTE Non Intra Frequency Cell Reselection:

➢ If SrxLev > S-NonIntraSearchP [SIB 3] OR Squal > S-NonIntraSearchQ [SIB 3]-

If the serving cell's evaluation result does not meet above criteria, UE perform non intra frequency measurement.

If neither of s-NonIntraSearchP nor s-NonIntraSearchQ is specified, UE applies the default value (s-NonIntraSearchP = Infinity, s-NonIntraSearchQ = 0). It implies that if neither of s-NonIntraSearchP and s-NonIntraSearchQ is specified, UE always perform non intra frequency measurement. In NonIntraFrequency cell reselection, among the neighboring cells with SrxLev greater than threshold, the cell with the Highest SrxLev is selected by means of "Cell Ranking" method only when priorities of both serving and target cell are same. And if incase of different priorities then Priority based method is used for reselection.

3. WCDMA to LTE Cell Reselection:

➢ 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 determine 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.

In following condition, detection measurements of lower priority LTE frequency are not required:

➢ Srxlev > s-PrioritySearch1 (SIB 19 of WCDMA)

➢ Squal > s-PrioritySearch2 (SIB 19 of WCDMA)

In following condition, UE should detect for both lower and higher priority LTE frequencies.

➢ Srxlev <= s-PrioritySearch1 (SIB 19 of WCDMA)

➢ Squal <= s-PrioritySearch2 (SIB 19 of WCDMA)

a) High Priority WCDMA cell to Low priority LTE cell:

If SrxLev < s-PrioritySearch1 and Squal < s-PrioritySearch2 then measurements of Low Priority LTE are done. And the reselection is performed when SrxLev (serving cell) < threshServingLow (threshServingLow is serving cell threshold when performing high priority to low priority cell reselection}.

Also SrxLev (Target cell) > threshXLow P (threshXLowP is target cell threshold when performing high priority to low priority cell reselection).

b) Low Priority WCDMA cell to High Priority LTE cell:

If LTE frequency is of higher priority, measurements are always performed and at a time 4 LTE frequencies can be measured. And the reselection is performed when SrxLev (serving cell) < threshServingHigh (threshServingHigh is serving cell threshold when performing low priority to high priority cell reselection).

Also SrxLev (Target cell) > threshXHighP

(ThreshXHighP is target cell threshold when performing low priority to high priority cell reselection).

4. LTE to WCDMA Cell Reselection:

a) High Priority LTE to Low 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 < sNonIntraSearchP (SIB3):

Srxlev = Qrxlevmeas - qRxLevMin (SIB1) Also, where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for camping.

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)

Srxlev = Qrxlevmeas - qRxLevMin (SIB1) where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping.

b) Low Priority LTE to High Priority WCDMA Cell:

When LTE cell has lower priority than WCDMA.The UE always have to perform measurements on WCDMA cell.

How often UE has to measure for WCDMA depends on condition:

-Srxlev of the serving cell < sNonIntraSearchP (SIB3)

If no parameter is set (meaning in default condition), detection of WCDMA cell should be performed at least every 60 seconds.

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):

Srxlev = Qrxlevmeas - qRxLevMin (SIB1), where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping.

sib4 {

                        IntraFreqNeighCellList [0] {

                           PhysCellId = 61

                           q_OffsetCell = dB0

                        }

                        csg_PhysCellIdRange {

                           Start = 1

                           Range = n4

                        }

                       IntraFreqNeighCellList [1] {

                           PhysCellId = 50

                           q_OffsetCell = dB0

                        }

                        csg_PhysCellIdRange {

                           Start = 1

                           Range = n4

                        }

                     }

• MIB– defines the most essential physical layer information of the cell required to receive further system information.
• SIB1– contains information relevant when evaluating if a UE is allowed to access a cell and defines the scheduling of other system information blocks.
• SIB2 – Contains common and shared channel information.
• SIB3 – Contains cell re-selection information.
• SIB4 – contains information about the serving neighboring frequencies and intra-frequency neighboring cells relevant for cell re-selection, covering both E UTRA and other RATs.
• SIB5 – contains information about other E UTRA frequencies and inter-frequency neighboring cells relevant for cell reselection.
• SIB6 – contains information about UTRA frequencies and UTRA neighboring cells relevant for cell re-selection.
• SIB7 – contains information about GERAN frequencies and GERAN neighboring cells relevant for cell re-selection/handover.
• SIB8 – contains information about CDMA2000 frequencies and CDMA2000 neighboring cells relevant for cell re-selection/handover.
• SIB9 – contains a home eNB identifier (HNBID)
• SIB10 – contains an ETWS primary notification
• SIB11 – contains an ETWS secondary notification

■ Cell Reselection Triggering

Serving Cell Measurement: UE, in Idle state, wakes up at the end of every DRX cycle to measure the signal of its serving cell (Q_rxlevmeas) and calculate the received signal level (Srxlev) of the serving cell to decide whether it should stay or move to another cell. Here, the UE’s transmission and reception conditions are reflected in the calculation, for example by applying minimum received signal level Q_rxlevmin, allowed maximum TX power level P_EMAX, etc., (see Section 2.2 for details).

Cell Reselection Triggering: If the received signal level of the serving cell (Srxlev) is greater than the specified threshold value (s-IntraSearch), the UE stays in the current serving cell. If not, it triggers a cell reselection procedure. The threshold value that works as triggering criterion is delivered through SIB 3, and defined as s-IntraSearch in Release 8 and as s-IntraSearchP and s-IntraSearchQ in Release 9.

UE in Idle state wakes up at the end of every DRX cycle to measure the received signal level of its serving cell (Srxlev) when it has stayed in the same location for a while. Let’s assume a UE that has camped on the same serving cell for a while because the received signal level of the cell has remained lower than the set threshold (s-IntraSearch). If it leaves the serving cell, the received signal level of the cell decreases gradually. Finally when the received signal level becomes lower than the threshold (s-IntraSearch), a cell reselection procedure is triggered. Then the UE begins to measure the signal strength of the neighbor cells (i.e. non-serving cells).  

■ Cell Reselection Criteria

Cell-Ranking Criterion: The UE ranks each cell (R_s, R_n) based on the measured signal strength of the serving cell (Q_meas,s) and neighbor cells (Q_meas,n). Parameters required for cell ranking are delivered through SIBs 3 and 4 (see Section 2.2). The serving cell is ranked using the hysteresis (q-Hyst) value stored in SIB 3 while the neighbor cells are ranked based on the offset (q-OffsetCell) value specified for each cell in SIB 4.

Cell Reselection: Once the serving cell and neighbor cells (non-serving cells) are ranked, the UE checks whether the cell reselection criterion is satisfied (R_n > R_s) or not. If there are neighbor cell(s) that satisfy the criterion, the UE selects the best satisfying cell, and then camps there. Cell reselection is performed only when the criterion is satisfied for a certain period of time (t-ReselectionEUTRA).

Mobile operators can prevent too frequent cell reselection and make sure reselection is performed in accordance with the cell status by controlling the UE’s dwelling time on the serving cell, based on the hysteresis and cell-specific offset values. In addition, they can control the q-Hyst and t-ReselectionEUTRA values by applying appropriate scaling factor (q-hystSF, t-ReselectionEUTRA-SF) depending on the traveling speed of the UE.

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: