lte idle mobility
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LTE Idle Mobility
Vladimir Gusavac
RAN Performance Standards & Best Practices
Rev 0.2
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LTE Idle Mode Behavior - Background
LTE Cell selection and Intra-frequency Cell reselection procedure
LTE Inter-Frequency and Inter-RAT Cell Reselection procedure
Summary
2 Rev 0.2
Agenda
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The idle mode algorithm ensures that the UE is camped on the best cell in idle
mode from where it can reach and be reached by the network
It involves the following processes:
Reading system information
PLMN selection and reselection
Cell selection and reselection
Tracking Area update
Paging procedure
The eNodeB broadcasts idle mode information that assists and controls the UEto select PLMN and best suitable LTE cell:
SIB1 - Access related parameters
SIB2 - Common and shared channel information
SIB3 - Cell reselection information
SIB4 - Intra frequency LTE neighbors
SIB5 - Inter frequency LTE neighbor information
SIB6 - IRAT cell reselection to UTRAN
SIB7 - IRAT cell reselection to GSM3 Rev 0.2
LTE Idle Mode Behavior Background (1/2)
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The UE performs a PLMN selection when switched on and for example when a new PLMN isfound at return from lack of coverage
Cell selection occurs after PLMN selection, at state change from connected to idle mode or
after a number of failed RRC connection requests
Cell selection procedures:
Initial cell selection - the UE scans all RF channels in the E-UTRA bands
Stored information cell selection - the UE uses stored carrier information to speed up the cell search,e.g. when returning to idle mode from RRC connected state.
Different cell selection/reselection criteria are used for evaluation of intra-frequency, inter-frequency and inter-RAT cells
UE speed dependent criteria can be applied for cell reselection
Black lists can be used to prevent the UE from selecting or reselecting specific intra andinter-frequency cells
When camped on a suitable cell, the UE enters Camped Normally state from where it canoriginate and receive calls. It will continuously evaluate signal quality criteria and reselect to abetter neighboring cell if found
If the UE cant find a suitable cell, it tries to find an acceptable cell. If found, it entersCamped on any cell state. The UE can in this state only originate emergency calls. In thisstate, the UE continuously scans LTE and other RAT frequencies to find a suitable cell.
LTE Idle Mode Behavior Background (2/2)
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LTE Cell selection and Intra-frequency Cell
reselection procedure
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The UE measures the RSRP (Reference Signal Received Power) of the serving cell and
evaluates the cell selection criterion (S-criterion)
A cell fulfills the cell selection criterion when:
Srxlev > 0, Srxlev = Qrxlevmeas q Rx Le v M i n Pcompensation
where,
Qrxlevmeas = Measured RSRP in the cell
q Rx Le v M in [ - 1 4 0 d Bm ] = Minimum required RSRP in the cell
Pcompensation = (p M a x Se r v in g Ce ll Pp ow e r Cla ss , 0 )
p M ax Se r v i n g Ce l l [ 1 0 0 0 ]= Max power the UE is allowed to use in the cell
Pp ow e r cla ss = Maximum UE power capability
When the UE is Camped Normally on a cell and it no longer fulfils the S-criterion, the UEinitiates measurements on all neighbor cells irrespectively of any idle mode measurementrules. If no suitable cell is found within 10 s, the UE is considered out of service and shall
attempt initial cell selection.
6 Rev 0.2
LTE Cell selection procedure
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LTE Intra-frequency Cell Reselection procedure (1/4)
1) Triggering of neighbor cell measurement
To minimize the impact on UE battery performance, it is possible to define a threshold abovewhich the UE may not measure intra-frequency neighbor cells. The UE must measure on intra-frequency neighbor cells if:
Srxlev < sI n t r a Se a r ch
where,
sI n t r aSe a r ch [62dB] - the threshold for intra-frequency
measurements
2) Neighboring cell qualification for cell reselection evaluation
Intra-frequency cell reselection process involves evaluation of the serving cell in relation toneighboring cells with the same frequency that fulfill the S-criterion. A neighboring cell fulfillsthe cell selection criterion when:
Srxlev(n) > 0, Srxlev(n) = Qrxlevmeas(n) qRxLevMin(n) Pcompensation(n)
where,
Qrxlevmeas(n) = measured RSRP in the neighboring cell
qRxLevMin(n) = Minimum required RSRP in the neighboring cells
Pcompensation(n) = (pMax PpowerClass, 0)
pMax = Max UE power allowed in the neighboring cell
Ppowerclass = Maximum UE power capability
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3) Ranking of serving and neighboring cells
The UE ranks the serving cell according to Rs criterion, and each qualified measuredneighboring cell according to the Rn criterion.
Rs = Qmeas,s + qHyst
Rn = Qmeas,n
where
Qmeas,s and Qmeas,n - the measured RSRP of the serving cell and neighboring cell(s) respectively
q H y st[4dB] - Hysteresis on serving cell in the intra-frequency cell ranking (R) criterion
The UE initiates a cell reselection to a better ranked neighbor cell when:
Rn > Rs for a duration of tReselectionEutrawhere
t Re se le ct ion Eu t r a [2sec] - minimum time an intra-frequency neighbor cell must be ranked better than the serving cellin order to trigger cell reselection
LTE Intra-frequency Cell Reselection procedure (2/4)
f C ll l i d (3/ )
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4) Speed dependent mobility states
The UE can apply faster cell reselection decision when cell reselections are performedfrequently
Depending on the UE speed, the UE may enter three different mobility states:
1) Normal mobility
2) Medium mobility
3) High mobility
The Medium and High states are specified by the parameters n Cel lCh an g eM e d iu m[16]and n Ce l l Ch a n g e H ig h[16] respectively defining number of cell reselections within the slidingtime window t Ev a lu at io n[240sec] that determines if the UE shall enter mobility states
Medium/High
LTE Intra-frequency Cell Reselection procedure (3/4)
LTE I t f C ll R l ti d (4/4)
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5) Speed impact on cell reselection criteria
In state Normal mobility, the UE applies normal hysteresis and time-to-trigger cellreselection criteria, i.e. tReselectionEutra and qHyst are used as is.
In state Medium or High mobility, it is possible to reduce the time-to-trigger and hysteresisfor cell reselection, thus making the UE more prone to reselect cell.
LTE Intra-frequency Cell Reselection procedure (4/4)
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LTE Inter-Frequency and Inter-RAT Cell Reselectionprocedure
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1) Triggering of Inter-frequency and IRAT cell search and measurements
The UE shall continuously search for inter-frequency and IRAT cells of higher AbsolutePriority (than the serving cell) at least every 60 x N seconds
where N = total number of E-UTRA, UTRA FDD, UTRA TDD, CDMA2000 1x and HRPD carrier frequencies thathave a higher absolute priority (increased by one if one or more groups of GSM frequencies is configured as ahigher priority)
The UE shall search for inter-frequency cells of higher, equal and lower Absolute Priority,and search for IRAT cells of higher and lower Absolute priority when:
serving cell Srxlev
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Absolute priority levels
LTE Inter-Frequency and IRAT Cell Reselection procedure
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2) Measurements
The UE measures the RSRP of the serving cell at least every DRX Cycle
The UE will measure inter-frequency and IRAT cells with a minimum periodicity as follows:
IF cells with higher priority, every 1-4 DRX cycle depending on DRX cycle length
IF cells of equal or lower priority, every 1-4 DRX cycle * number IF cells with lower orhigher priority (where 1-4 DRX cycle depends on DRX cycle length)
UTRAN cells with higher priority, every 3-16 DRX cycle (depending on DRX cycle length)
UTRAN cell with lower priority, every 3-6 DRX cycle * number if UTRAN carriers (where1-4 DRX cycle depends on DRX cycle length)
GERAN cell with higher priority, every 3-16 DRX cycle (depending on DRX cycle length)
GERAN cell with lower priority, every 3-16 DRX cycle (depending on DRX cycle length)
Note that 3GPP specifies that the UE does not need to continuously measure a higher priority
IF and IRAT carrier if determined that no cell reselection occurred after the cell was detected.
LTE Inter-Frequency and IRAT Cell Reselection procedure
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3) Triggering to equal prio IF cell and to higher prio IF/IRAT cell
An equal priority IF cell and higher priority IF or IRAT cell must meet the S-criterion to besubject for cell reselection
Triggering of cell reselection to IF cell of equal priority follows the same criteria as for Intra-frequency cell reselection
Cell reselection to a higher priority IF or IRAT cell (that fulfills the S-criterion) occurs when:
the IF or IRAT Srxlev > threshx,high for a duration of tReselectionRAT
where
t h r es h x ,h i g h[4dB] - the threshold used by the UE when reselecting towards a higher priority frequency from thecurrent serving frequency
t Res e le c t ion RAT[2sec] - Min. time a higher priority IF or IRAT cell needs to exceed the respectively threshHigh signalstrength threshold in order to trigger cell reselection from the serving cell to the higher priority IF or IRAT cell.
It is possible to apply a similar reduction of time-to-trigger tReselectionRAT as for intra-frequency cell reselection if the UE is in High or Medium mobility state.
LTE Inter-Frequency and IRAT Cell Reselection procedure
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4) Triggering to lower prio IF or IRAT cell
A lower priority IF or IRAT cell must meet the S-criterion to be subject for cell reselection.
Cell reselection to a lower priority IF or IRAT cell (that fulfills the S-criterion) occurs when:
the IF or IRAT cell Srxlev > threshx,low while the serving cell Srxlev < threshServingLow for aduration of tReselectionRAT
where
t h r es hx ,lo w [0dB] - the threshold used by the UE when reselecting towards a higher priority frequency from the currentserving frequency
t h r es h Se r v i n g Lo w [62dB]- signal threshold below which the serving cell must be in order to allow cell reselection to alower priority IF or IRAT cell
t Res e le c t ion RAT[2sec] - the min time a lower priority IF or IRAT cell must exceed threshLow in order to trigger cellreselection to a lower priority IF or IRAT cell
It is possible to apply a similar reduction of time-to-trigger tReselectionRAT as for intra-frequency cell reselection if the UE is in High or Medium mobility state.
LTE Inter-Frequency and IRAT Cell Reselection procedure
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Summary
Different cell selection/reselection criteria are used for evaluation of intra-frequency, inter-frequency and inter-RAT cells
UE speed dependent criteria can be applied for cell reselection
Black lists can be used to prevent the UE from selecting or reselecting specific intra andinter-frequency cells
We have been working with E///, Labs and Emerging technology teams to tune initial GSLTE Idle mode parameters