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 andalgorithms 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 measureable 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 going on during the idle stage and in this page I will mostly deal
with < Cell Reselection Criteria >. Understanding this criteria is the most imporant thing in implementing
and testing Cell Reselection.
Following is the topics that I will deal with in this page.
How Cell Reselection Priority is determined ?
How to detect and reselect to another LTE cell while in LTE Cell (LTE to LTE Cell Reselction) ?
How to detect and reselect to LTE cell while in WCDMA (WCDMA to LTE Cell Reselction) ?
How to detect and reselect to WCDMA cell while in LTE (LTE to WCDMA Cell Reselction) ?
A little bit detailed criteria can be described as follows and you will see ever further details after this.
UE always have to measure frequecies and RAT with higher priority
UE has to have to measure frequecies and RAT with lower priority in the following fashion
o UE has to perform intra-frequency measurement only when SrxLev of the serving cell <=
SintraSearch
o UE has to perform lower or equal priority inter-frequency/InterRAT measurement only
when SrxLev of the serving cell <= SnonintraSearch
LTE cell reselection uses priority based levels. (These priorities can be specified in LTE
SIB4,5,6,7 and RRC Connection Release). Priority 0 indicate the lowest priority and 7 indicate
the highest priority. The priority cannot be same for WCDMA and LTE, meaning that it must be
set differently for different RAT.
Among the cell power and priority, which one plays more critical role in cell reselection ? Cell
Power defines the minimum condition. Once the minimum condition is met, Priority plays more
critical role. For example, let's assume that UE is now camped in WCDMA cell and detected a
neighbouring LTE cell with higher priority than the serving WCDMA cell, UE will perform cell
reselection to LTE cell as long as the LTE cell is above the minimum signal strength threshold. In
the same logic, if a UE camped on LTE cell detected a neighbouring WCDMA cell with higher
priority than the serving LTE cell, UE will perform cell reselection to WCDMA cell as long as
the WCDMA cell is above the minimum signal strength threshold.
WCDMA SIB19 specifies the LTE frequencies (EARFCN) and priorities. A maximum of 8
EARFCNs (4 FDD + 4 TDD) can be listed in SIB 19. For each EARFCN, you can configure a
max 16 blacklisted cells. The blacklisted cell is the cell UE should not make reselection to.
Using the eutraDetection parameter in SIB19, you can make UE to detect and display the
presence of lower priority LTE frequencies while it is in WCDMA cell.
LTE SIB6 specifies the WCDMA frequencies (DL ARFCN) and the parameters used for absolute
priority based cell reselection.
How Cell Reselection Priority is determined ?
There are roughly three different ways of informing UE of Reselection Priority as listed before : (refer to
36.304 5.2.4.1 Reselection priorities handling for details).
i) System Information
ii) RRC Connection Release message
iii) Inheriting from another RAT
Note 1 : if the priority is specified in both i) and ii), the one specified in ii) takes effect.
Note 2 : The UE shall only perform cell reselection evaluation for E-UTRAN frequencies and inter-RAT
frequencies that are given in system information and for which the UE has a priority provided.
Note 3 : The UE shall not consider any black listed cells as candidate for cell reselection
How to detect and reselect to another LTE cell while in LTE Cell (LTE to LTE Cell Reselction) ?
In order for UE to reselect from one LTE cell to another LTE Cell, the internal evaluation result in UE
side (SrxLev and Squal) should meet a certain criteria, otherwise it may not even monitor (measure)
neighboring cell and in result would not perform reselection. LTE SIB3 defines various parameter to
influence the evaluation result and the expected UE side behavior is described in 36.304 5.2.4.2
Measurement rules for cell re-selection.
If your device does not perform Cell Reselection, first Check if Cell Measurement Criteria is met and the
check if Cell Reselection Criteria is met and also check if following condition is met (Following
statement come from 36.304 5.2.4.1 Reselection Priority Handling. I saw some UE ignore this
requirement but some UE strictly follows this requirement.)
The UE shall only perform cell reselection evaluation for E-UTRAN frequencies and inter-RAT
frequencies that are give in system information and for which the UE has a priority provided.
If your UE strictly follow this requirement stated above, populate SIB4 for proper neighbour cells.
Following is the contents of SIB3 for your reference (The parameter shown here is just an example. You
have to set it properly based on your own test requirement)
sib-TypeAndInfo item: sib3 (1)
sib3
cellReselectionInfoCommon
q-Hyst: dB0 (0)
cellReselectionServingFreqInfo
s-NonIntraSearch: 62dB (31)
threshServingLow: 62dB (31)
cellReselectionPriority: 7
intraFreqCellReselectionInfo
q-RxLevMin: -140dBm (-70)
s-IntraSearch: 62dB (31)
.0.. .... presenceAntennaPort1: False
neighCellConfig: Not all neighbour cells
have the same MBSFN subframe allocation as serving cell (0)
t-ReselectionEUTRA: 0s
lateNonCriticalExtension: <MISSING>
s-IntraSearch-v920
s-IntraSearchP-r9: 62dB (31)
s-IntraSearchQ-r9: 0dB
s-NonIntraSearch-v920
s-NonIntraSearchP-r9: 62dB (31)
s-NonIntraSearchQ-r9: 0dB
q-QualMin-r9: -34dB
threshServingLowQ-r9: 0dB
< LTE to LTE Intra Frequency Measurement >
If the serving cell's evaluation result is as follows, UE may not perform intra frequency measurement.
SrxLev > S_IntraSearchP
Squal > S_IntraSearchQ
Note : If neither of S_IntraSearchP nor S_IntraSearchQ is specified, UE applies the default value
(S_IntraSearchP = Infinity, S_IntraSearchQ = 0 based on 36.331)
If the serving cell's evaluation result does NOT meet following criteria, UE perform intra frequency
measurement.
SrxLev > S_IntraSearchP
Squal > S_IntraSearchQ
Note : If neither of S_IntraSearchP nor S_IntraSearchQ is specified, UE applies the default value
(S_IntraSearchP = Infinity, S_IntraSearchQ = 0 based on 36.331)
< LTE to LTE NonIntra Frequency Measurement >
If the serving cell's evaluation result is as follows, UE may not perform intra frequency measurement.
SrxLev > S_NonIntraSearchP
Squal > S_NonIntraSearchQ
Note : If neither of S_IntraSearchP nor S_IntraSearchQ is specified, UE applies the default value
(S_IntraSearchP = Infinity, S_IntraSearchQ = 0 based on 36.331)
If the serving cell's evaluation result does NOT meet following criteria, UE perform intra frequency
measurement.
SrxLev > S_NonIntraSearchP
Squal > S_NonIntraSearchQ
Note : If neither of S_IntraSearchP nor S_IntraSearchQ is specified, UE applies the default value
(S_IntraSearchP = Infinity, S_IntraSearchQ = 0 based on 36.331)
How to detect and reselect to LTE cell while in WCDMA (WCDMA to LTE Cell Reselction) ?
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 determines 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.
The detection measurement of LTE frequencies should be done at least once every 60s for higher priority
LTE frequencies.
In following condition, detection measurements of lower priority LTE frequency is not required.
Srxlev > absPrioCellRes.sPrioritySearch1
Squal > absPrioCellRes.sPrioritySearch2
In following condition, UE should detect once every 30s for both lower and higher priority LTE
frequencies
Srxlev <= absPrioCellRes.sPrioritySearch1
Squal <= absPrioCellRes.sPrioritySearch2
The maximum number of LTE FDD Frequencies are 4. In this case, UE should have performed
measurement for detecting LTE cells on all 4 LTE frequencies once every 240 (4 x 60) s or 120(4 x 30) s
depending if UE measures above or below parameter threshold absPrioCellRes.sPrioritySearch1
How to detect and reselect to WCDMA cell while in LTE (LTE to WCDMA Cell Reselction)?
Measurement Criteria (From High Priority LTE Cell to Lower 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 < sNonIntraSearch (SIB3),
If we express this using dB/dBm, Srxlev of the serving cell < (2 x sNonIntraSearch (SIB3))
where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),
If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB3)) [dBm]
where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for camping
Measurement Criteria (From Lower Priority LTE Cell to Higher Priority WCDMA Cell): When LTE cell
has lower priority than WCDMA (WCDMA cell priority defined in SIB6 is higher than the serving cell
priority),The UE always have to perform measurements on WCDMA cell. How often UE has to measure
for WCDMA depends on whether Srxlev of the serving cell is greater or lower than
sNonIntraSearch(SIB3). If no parameter is set (meaning in default condition), detection of WCDMA cell
should be performed at least every 60 seconds.
Reselection Criteria (From High Priority LTE Cell to Lower Priority WCDMA Cell): 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),
where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),
where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for camping
Srxlev of WCDMA cell > threshXLowP (SIB6),
where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),
If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB3)) [dBm]
where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping
Note : If more than one WCDMA meet this condition, UE should select to the cell with highest Srxlev
Note : If these values are not specified in SIB and UE has to apply default values, UE has to perform
reselection when LTE RSRP is lower than -145 dBm and WCDMA cell is better than -119 dBm for at
least 2 seconds.
Reselection Criteria (From Lower Priority LTE Cell to Higher Priority WCDMA Cell): 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),
where Srxlev = Qrxlevmeas - qRxLevMin (SIB3),
where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping
Parameters related to Cell Reselection
These parameters are defined in 36.304 5.2.4.7 Cell reselection parameters in system information
broadcasts. Some of the important parameters and descriptions are as follows :
ThreshXHighP : The threshold of target cell Srxlev to perfrom reselection from a low priority to a high
priority cell. (i.e, a large ThreshXHigh value makes reselection harder)
ThreshXLowP : The threshold of target cell Srxlev to perfrom reselection from a high priority to a low
priority cell. (i.e, a large ThreshXLow value makes reselection harder)
ThreshXHighQ : The threshold of target cell Squal to perfrom reselection from a low priority to a high
priority cell. (i.e, a large ThreshXHigh value makes reselection harder)
ThreshXLowQ : The threshold of target cell Squal to perfrom reselection from a high priority to a low
priority cell. (i.e, a large ThreshXLow value makes reselection harder)
SIntraSearchP : The threshold of current cell Srxlev to perform intra-frequency. If the current cell Srxleve
is lower than this value, UE perform measurement for intra-frequency.
SIntraSearchQ : The threshold of current cell Squal to perform intra-frequency. If the current cell Squal is
lower than this value, UE perform measurement for intra-frequency.
SnonIntraSearchP : The threshold of current cell Srxlev to perform inter-frequency or interRAT
measurement. If the current cell Srxleve is lower than this value, UE perform measurement for inter-
frequency or interRAT cells.
SnonIntraSearchQ : The threshold of current cell Squal to perform inter-frequency or interRAT
measurement. If the current cell Srqual is lower than this value, UE perform measurement for inter-
frequency or interRAT cells.
