lte idle mobility

8/11/2019 LTE Idle Mobility

1/17

2010 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.

LTE Idle Mobility

Vladimir Gusavac

RAN Performance Standards & Best Practices

Rev 0.2


2/17


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


3/17


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)


4/17

2010 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property.4 Rev 0.2

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)


5/17


LTE Cell selection and Intra-frequency Cell

reselection procedure


6/17


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


7/17


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


8/17


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


f C ll l i d (3/ )


9/17


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 I t f C ll R l ti d (4/4)


10/17


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.



11/17


LTE Inter-Frequency and Inter-RAT Cell Reselectionprocedure


12/17


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


13/17


Absolute priority levels

LTE Inter-Frequency and IRAT Cell Reselection procedure


14/17


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.



15/17


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.



16/17


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.



17/17


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

lte idle mobility

Documents