LTE Idle Mobility
Vladimir Gusavac RAN Performance Standards & Best Practices
© 2010 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property. Rev 0.2
Agenda
LTE Idle Mode Behavior - Background
LTE Cell selection and Intra-frequency Cell reselection procedure
LTE Inter-Frequency and Inter-RAT Cell Reselection procedure
Summary
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LTE Idle Mode Behavior – Background (1/2) 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 UE to select PLMN and best suitable LTE cell: 3
SIB1 SIB2 SIB3 SIB4 SIB5 SIB6 SIB7
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Access related parameters Common and shared channel information Cell reselection information Intra frequency LTE neighbors Inter frequency LTE neighbor information IRAT cell reselection to UTRAN IRAT cell reselection to GSM
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LTE Idle Mode Behavior – Background (2/2) The UE performs a PLMN selection when switched on and for example when a new PLMN is found 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, interfrequency 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 and inter-frequency cells
When camped on a suitable cell, the UE enters Camped Normally state from where it can originate and receive calls. It will continuously evaluate signal quality criteria and reselect to a better neighboring cell if found
If the UE can’t find a suitable cell, it tries to find an acceptable cell. If found, it enters Camped on any cell state. The UE can in this state only originate emergency calls. In this state, the UE continuously scans LTE and other RAT frequencies to find a suitable cell .
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LTE Cell selection and Intra-frequency Cell reselection procedure
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LTE Cell selection procedure 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 R x L e v M i n – Pcompensation
where, Qrxlevmeas = Measured RSRP in the cell q R x Le v M i n [ - 1 4 0 d B m ] = Minimum required RSRP in the cell
Pcompensation = (p M a x Se r v in g Ce ll – Pp o w 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 P p o w e r c l a s s = Maximum UE power capability
When the UE is Camped Normally on a cell and it no longer fulfils the S-criterion, the UE initiates measurements on all neighbor cells irrespectively of any idle mode measurement rules. If no suitable cell is found within 10 s, the UE is considered “out of service” and shall attempt initial cell selection.
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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 above which the UE may not measure intra-frequency neighbor cells. The UE must measure on intrafrequency neighbor cells if:
Srxlev < sI n t r a Se a r ch where, sI n t r a Se 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 to neighboring cells with the same frequency that fulfill the S-criterion. A neighboring cell fulfills the 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 7
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LTE Intra-frequency Cell Reselection procedure (2/4) 3) Ranking of serving and neighboring cells The UE ranks the serving cell according to Rs criterion, and each qualified measured neighboring 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 s t [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 tReselectionEutra
where t R e s e l e c t i o n E u t r a [2sec] - minimum time an intra-frequency neighbor cell must be ranked better than the serving cell
in order to trigger cell reselection
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LTE Intra-frequency Cell Reselection procedure (3/4) 4) Speed dependent mobility states The UE can apply faster cell reselection decision when cell reselections are performed frequently
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 C e l l C h a n g e M e d i u m [16] and n C e l l C h a n g e H i g h [16] respectively defining number of cell reselections within the sliding time window t E v a l u a t i o n [240sec] that determines if the UE shall enter mobility states Medium/High
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LTE Intra-frequency Cell Reselection procedure (4/4) 5) Speed impact on cell reselection criteria
In state Normal mobility, the UE applies normal hysteresis and time-to-trigger cell reselection 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 hysteresis for cell reselection, thus making the UE more prone to reselect cell.
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LTE Inter-Frequency and Inter-RAT Cell Reselection procedure
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LTE Inter-Frequency and IRAT Cell Reselection procedure 1) Triggering of Inter-frequency and IRAT cell search and measurements The UE shall continuously search for inter-frequency and IRAT cells of higher Absolute Priority (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 that have a higher absolute priority (increased by one if one or more groups of GSM frequencies is configured as a higher 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 <= sNonIntraSearch Where s N o n I n t r a Se a r ch [0dB] - Threshold for measurements of inter-frequency of equal or lower priority than serving cell, and threshold for measurements of inter-RAT frequencies of lower priority than the serving cell.
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LTE Inter-Frequency and IRAT Cell Reselection procedure
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Absolute priority levels
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LTE Inter-Frequency and IRAT Cell Reselection procedure 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 or higher 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 (where 1-4 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.
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LTE Inter-Frequency and IRAT Cell Reselection procedure 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 be subject for cell reselection
Triggering of cell reselection to IF cell of equal priority follows the same criteria as for Intrafrequency 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 the
current serving frequency t R e s e l e c t i o n R A T [2sec] - Min. time a higher priority IF or IRAT cell needs to exceed the respectively threshHigh signal
strength 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 intrafrequency cell reselection if the UE is in High or Medium mobility state.
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LTE Inter-Frequency and IRAT Cell Reselection procedure 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 a duration of tReselectionRAT where x ,lo w [0dB] - the threshold used by the UE when reselecting towards a higher priority frequency from the current t h r es h
serving 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 a
lower priority IF or IRAT cell t R e s e l e c t i o n R A T [2sec] - the min time a lower priority IF or IRAT cell must exceed threshLow in order to trigger cell
reselection to a lower priority IF or IRAT cell
It is possible to apply a similar reduction of time-to-trigger tReselectionRAT as for intrafrequency cell reselection if the UE is in High or Medium mobility state.
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© 2010 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property. Rev 0.2
Summary Different cell selection/reselection criteria are used for evaluation of intra-frequency, interfrequency 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 and inter-frequency cells
We have been working with E///, Labs and Emerging technology teams to tune initial GS LTE Idle mode parameters
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