Intra-frequency Cell Reselection

UTRA cells as well as inter-RAT cells, which can be UTRAN, GERAN or CDMA2000.

Parameters contained in the System Information Blocks (SIBs) control the cell reselection process.

SIB3 contains general reselection parameters that control how and when the UE searches for

neighboring cells. SIBs 4 and 5 contain information related to intra- and inter- E-UTRA cells

respectively whilst SIBs 6, 7 and 8 contain information related to UTRAN, GERAN and CDMA2000

neighbors respectively. In the case of non intra-frequency neighbors, LTE allows the definition of

priorities between neighboring cells of different networks. The relative priority of a detected cell then

impacts the manner in which it is assessed for reselection. Inter-RAT neighbors must always be either

higher or lower priority than the current camped LTE network.

3.1 Intra-frequency Cell Reselection

Introduction 3.1.1

For any cell to be considered for selection it must be suitable. A cell is considered suitable when:

Srxlev > 0

where

Srxlev = Qmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation

and

Pcompensation = max(PEMAX-PUMAX, 0)

where Qrxlevmin is the minimum required RSRP, Qrxlevminoffset is an offset that is applied uniquely to the

calculation of Srxlev when the UE searches for a higher priority PLMN while camped in a VPLMN.

Pcompensation is an offset associated with the maximum power that the UE will be allowed to

transmit such that PEMAX is the maximum allowed UE transmit power and PUMAX is the maximum UE

transmit power.

For intra-frequency cell reselection, the UE evaluates the relative ranking of the serving and neighbor

where Qmeas is the measured RSRP of the serving (s) or neighbor (n) cell, QHyst is the serving cell

hysteresis and Qoffset is the neighbor cell offset. The cell with the largest R value is considered to be

ranked the highest. Once a cell other than the serving cell in ranked highest, the timer Treselection is

started and continues whilst this condition is satisfied. Reselection to the new cell occurs when the

timer expires.

Figure 3-1 shows an example of how this mechanism works with a UE moving from A to B. The UE

is initially camped on the cell with Physical Cell ID 1. At point 1, SServingCell < Sintrasearch and the UE

begins to search for intra-frequency neighbors. At point 2, Cell 3 (Physical Cell ID 100)’s strength is

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higher than serving Cell 1 and the reselection timer for Cell 3 is started. At point 3, Cell 2 (Physical

Cell ID 50)’s strength is higher than serving Cell 1 too and the reselection timer for Cell 2 is started

too. However, before expiration of the reselection timer, cell 3 (Physical Cell ID 50) becomes worse

than serving Cell 1 at point 4 and the reselection timer for Cell 3 is stopped. At point 5 the reselection

timer for Cell 2 expires and the UE therefore now camps on cell 2.

Figure 3-1 Intra-Frequency Cell Reselection Example

< 1 sec

= 1 sec

Time S

QHyst = 2 dB

Qoffset = 0 dB

S

intrasearch

Cell 1 (Phy Cell ID: 1)

Cell 2 (Phy Cell ID: 50)

Cell 3 (Phy Cell ID: 100)

1 2 3 4

Qoffset = 1 dB

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Q

_rxlevmin

3.1.2

Definition: Minimum received signal level in terms of RSRP.

IE Value Engineering Units

Allowed Range Integer (-70…-22) -140…-44 dBm, in steps of 2 dB

Recommended -60 -120 dBm

Setting Tradeoff: If set too low, the UE may camp on a cell with low quality and may not be able to

receive reliable service. If set too high, the UE may prematurely declare a cell unsuitable and may

unnecessarily select another PLMN.

Dependencies/Constraints:

Traceability: TS 36.304 Sect. 5.2.3, TS 36.331 Sect. 6.3.4

RRC Message Structure: SIB1  CellSelectionInfo  q-RxLevMin,

SIB3  intraFreCellReselectionInfo  q-RxLevMin

Notes: The Qrxlevmin IE affects cell selection criteria. A necessary (but not sufficient) condition for

cell selection is that:

Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation > 0

With Pcompensation = max (PEMAX_H – PPowerClass, 0)

Where, Qrxlevmeas is the measured RSRP, Qrxlevminoffset is an offset applied when a UE is searching

for a higher priority PLMN when camped on a VPLM, PPowerClass is the maximum UE transmit power,

and PEMAX_H is the maximum allowed uplink transmit power.

It should be noted that the value defined in SIB1 will be used for initial acquisition and camping

whilst the value defined in SIB3 will be applied to assess the suitability of intra-frequency neighbors.

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S

intrasearch

3.1.3

Definition: Srxlev threshold to trigger intra-frequency measurements.

IE Value Engineering Units

Allowed Range Integer[0 … 31] 0 … 62 dB, in steps of 2 dB

Recommended 20 40 dB

Setting Tradeoff: If the parameter is too small, the UE will not perform intra-frequency

measurements and may miss the opportunity to perform cell reselection. If parameter is too large, the

UE will perform intra-frequency measurements, although the likelihood that the neighboring cells will

meet the cell reselection criteria is negligible, thereby wasting idle mode current and decreasing

stand-by time.

Traceability: TS36.304 Sect. 5.2.4.2, TS36.331 Sect. 6.3.1.

RRC Message Structure: SIB3  intraFreqCellReslectionInfo  s-IntraSearch

Notes:

As battery life is less of a concern for data cards, the device mix of the network can be considered

when setting this parameter.

The UE always searches if this parameter is not broadcast by the network.

Above recommendation assumes that Qrxlevmin = -120dBm

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Q

_Hyst

3.1.4

Definition: Hysteresis value for the serving cell used in the ranking criteria for cell reselection

IE Value Engineering Units

Allowed Range

dB0, dB1, dB2, dB3, dB4, dB5, dB6, dB8, dB10,dB12, dB14, dB16, dB18, dB20, dB22, dB24

0 dB, 1 dB, 2 dB … 20 dB, 22 dB 24 dB

Recommended dB4 4 dB

Setting Tradeoff: If parameter is too small, the UE may perform frequent cell reselection to cells

only marginally better than the current serving cell, which may lead to excessive battery consumption.

If parameter is too large, the UE may stay camped on a relatively weak cell even though a

significantly better cell is available.

Dependencies/Constraints: This IE should be set in conjunction with Qoffsetsince they jointly

affect cell ranking.

Traceability: TS36.304 Sect. 5.2.4.6, TS36.331 Sect. 6.3.1

RRC Message Structure: SIB3  CellReselectionInfoCommon  q-Hyst

Notes:

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Qoffset

_s,n

3.1.5

Definition: Offset applied to a neighbor cell in the cell ranking criteria for cell reselection.

IE Value Engineering Units

Allowed Range

dB-24, dB-22, dB-20, dB-18, dB-16, dB-14, dB-12, dB-10, dB-8, dB-6, dB-5, dB-4, dB-3,2, dB-1, dB0, dB1, dB2, dB3, dB4, dB5,dB6, dB8, dB10, dB12, dB14, dB16, dB18, dB20, dB22, dB24

24 dB, 22 dB, 20 dB, 18dB, -16dB … 20 dB, 22 dB, 24 dB

Recommended dB0 (Depends on deployment strategy)

0 dB (Depends on deployment strategy)

Setting Tradeoff: A positive value of Qoffset deters the UE from reselecting to that cell. If the

parameter is set too low, reselection may be triggered even though the neighboring cell is only

marginally better than the current cell. If parameter is set too high, the UE may not reselect to the cell

even though the cell significantly better than the current cell. The parameter should be set to avoid

potential ping-ponging whilst maintaining the UE in a cell with sufficient quality.

Dependencies/Constraints: Qoffsetshould be set in conjunction with QHyst since they jointly affect

cell ranking criterion.

Traceability: TS36.304 Sect. 5.2.4.6, TS36.331 Sect. 6.3.1

RRC Message Structure: SIB4  IntraFreqNeighCellList  IntraFreqNeighCelInfo 

q-11

OffsetCell

Notes:

A larger value of this parameter is recommended for cells situated at Tracking Area borders. This

increased reselection deterrent is a tradeoff between the increased signaling associated with the

Tracking Area Update procedure and staying camped on a cell that is not the best.

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Treselection

_EUTRAN

3.1.6

Definition: Intra-frequency cell reselection timer. This timer is started when the conditions required

for cell reselection have been met. Reselection takes places if the conditions remain throughout the

duration of the timer.

IE Value Engineering Units

Allowed Range Integer (0…7) 0 … 7 seconds

Recommended 2 2 seconds

Setting Tradeoff: If this parameter is too large, the UE may stay camped on a relatively weak cell

even though a significantly better cell is available. This has the potential to impact performance or

cause an outage. Is set too low, the UE may frequently and unnecessarily reselect another cell.

Dependencies/Constraints: Unique reselection thresholds are possible for each EUTRA carrier

frequency as well as for each other RAT that may be present.

Traceability: TS36.304 Sect. 5.2.4.7, TS36.331 Sect. 6.3.1.

RRC Message Structure: SIB3  intraFreqCellReselectionInfo  t-ReselectionEUTRA

Notes:

Cell Reselection is also prohibited if a previous reselection has occurred within 1 second.

This setting also depends on the defaultDRXCycle. The above recommended settings assume that

defaultDRXCycle = 1.28s.

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3.2 Inter-frequency and Inter-RAT Cell Reselection

Common Parameters

Reselection for inter-frequency and inter-RAT neighbors is controlled by a set of parameters and

priorities. Depending on whether the non-serving cell has equal, lower or higher priority will govern

the reselection behavior. Inter-frequency neighbor cells can be defined with equal, lower or higher

priority whilst inter-RAT cell can have only lower or higher priority.

Cell Reselection Priority 3.2.1

Definition: This parameter defines the absolute priority of a carrier frequency (or set of carrier

frequencies for GERAN) and is used to determine the cell reselection behaviour of the UE.

IE Value Engineering Units

Allowed Range Integer[0…7] 0...7

Recommended Depends on deployment strategy 0=lowest priority…7=highest priority

Setting Tradeoff: The setting of this parameter depends on deployment strategy. Various scenarios

exist governed by the number of LTE carriers an operator has and how he chooses to distribute UEs

in idle mode. For example, an operator could choose to prefer that all idle UEs remain on one specific

carrier frequency but be moved between different carriers in connected mode based on load

conditions or specific applications. Additionally, any other system of a different RAT belong to the

operator will also impact how priorities are defined.

Traceability: TS36.304 Sect 5.2.4.1, TS36.331 Sect 6.3.1

RRC Message Structure: SIB3  cellReselectionServingFreqInfo  cellReselectionPriority,

SIB5 interFreqCarrierFreqList  interFreqCarrierFreqInfo cellReselectionPriority,

SIB6 CarrierFreqListUTRA-FDD CarrierFreqUTRA-FDD cellReselectionPriority

SIB7 CarrierFreqInfoListGERAN carrierFreqs  commonInfoCellReselectionPriority

SIB8 CellReselctionParamtersCDMA2000  BandClassListCDAM2000 

CellReselectionPriority

Notes: A value of 0 corresponds to the lowest priority whilst a value of 7 corresponds to the highest

priority. Neighbor information defined in each of the SIBs that are broadcast by a network can have

different priorities.

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S

nonintrasearch

3.2.2

Definition: Srxlev threshold below which inter-frequency and inter-RAT measurements are

performed for equal or lower priority neighbors.

IE Value Engineering Units

Allowed Range Integer[0 … 31] 0 … 62 dB, in steps of 2 dB Recommended 5 (3 for inter-RAT) 10 dB (6 dB for inter-RAT) Setting Tradeoff: If the parameter is set too small, the UE will not perform inter-frequency (nor

inter-RAT) measurements and may miss the opportunity to perform reselection to another viable

frequency assignment or technology. If the parameter is too large, the UE will perform unnecessary

inter-frequency (and inter-RAT) measurements, most likely leading to inter-frequency and inter-RAT

reselection, even while the current cell is of acceptable quality, thereby decreasing stand-by time.

Dependencies/Constraints: Snonintrasearch needs to be chosen in accordance with Sintrasearch and to avoid

a situation where the UE performs an inter-frequency (and inter-RAT) search but no intra-frequency

search. Preferably the values are chosen such that Sintrasearch > Snonintrasearch.

Traceability: TS36.304 Sect. 5.2.4.2, TS36.331 Sect. 6.3.1

RRC Message Structure: SIB3  cellReselectionServingFreqInfo  s-NonIntraSearch

Notes: This parameter is common for all inter-frequency and inter-RAT neighbors

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Thresh

serving, low

3.2.3

Definition: Absolute threshold of Sxrlev of a serving cell below which one of the two conditions for

reselection to a lower priority inter-frequency (or inter-RAT) neighbor cell are met.

IE Value Engineering Units

Allowed Range Integer[0 … 31] 0 … 62 dB, in steps of 2 dB Recommended 2 (1 for inter-RAT) 4 dB (2 dB for inter-RAT)

Setting Tradeoff: A high value of this parameter causes reselection to a (frequency or

inter-4

RAT) cell of a lower priority when the current cell is of adequate signal strength. A low value of this

parameter may delay reselection from a weak cell to a stronger more suitable (inter-frequency or

inter-RAT) cell of a lower priority.

Dependencies/Constraints: Threshserving,low should be set in conjunction with Threshx,low and Qrxlevmin 8

of the inter-frequencyor inter-RAT neighbor.

Traceability: TS36.304 Sect. 5.2.4.6, TS36.331 Sect. 6.3.1

RRC Message Structure: SIB3  cellReselectionServingFreqInfo  threshServingLow

Notes: This is a common parameter that is used to assess during the reselection decision for both

inter-frequency and inter-RAT neighbors.

For pre-R8 UTRAN/GERAN, once UE reselects to UTRAN/GERAN it would reselect back to LTE

based only on OOS in UTRAN/GERAN. Set it to lower range (i.e., 2 dB) for pre-R8

UTRAN/GERAN.

For inter-frequency reselection, the bandwidth of source and target cells should be taken into

consideration.

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3.3 Inter-frequency Cell Reselection

Treselection

EUTRAN

3.3.1

Definition: Inter-frequency cell reselection timer. This timer is started when the conditions required

for cell reselection have been met. Reselection takes places if the conditions remain throughout the

duration of the timer.

IE Value Engineering Units

Allowed Range Integer (0…7) 0 … 7 seconds

Recommended 1 (2 for same priority neighbor) 1 second (2 seconds for same priority neighbor)

Setting Tradeoff: If this parameter is too large, the UE may stay camped on a relatively weak cell

even though a significantly better cell is available. This has the potential to impact performance or

cause an outage. Is set too low, the UE may frequently and unnecessarily reselect another cell.

Dependencies/Constraints: Unique reselection thresholds are possible for each EUTRA carrier

frequency as well as for each other RAT that may be present

Traceability: TS36.304 Sect. 5.2.4.7, TS36.331 Sect. 6.3.1.

RRC Message Structure: SIB5  interFreqCellReselectionInfo  t-ReselectionEUTRA

Notes: This timer applies regardless of the relative priorities of the serving and neighbor cells.

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Equal Priority Neighbor Inter-Frequency Reselection 3.3.2

For inter-frequency cell reselection to a frequency that has equal priority to the serving cell frequency,

the UE evaluates the relative ranking of the serving and neighbor cells Rs and Rn respectively such

where Qmeas is the measured RSRP of the serving (s) or neighbor (n) cell, QHyst is the serving cell

hysteresis and Qoffset is the neighbor cell offset (the sum of an individual cell Qoffsets,n plus the

Qoffsetfrequencythat applies equally to all cells of that frequency) The cell with the largest R value is

considered to be ranked the highest. Once a cell other than the serving cell in ranked highest, the

timer Treselection is started and continues whilst this condition is satisfied. Reselection to the new cell

occurs when the timer expires.

Figure 3-2 shows an example of a UE initially camped on the f1 cell with Physical Cell ID 1. The

only other neighbor defined is the f2 cell with Physical cell ID 50. The priorities of the two

frequencies f1 and f2 are defined as the same. At point 1, SServingcell < Snonintrasearch and the UE begins to

search for inter-frequency neighbor. The f2 cell is detected and is ranked the highest at Point 2 due to

the fact that its received level is (Qhyst+Qoffset) better than that of the serving f1 cell. The reselection

timer (2 sec) associated with f2 is started and as the cell remains the highest ranked for the period of

the timer, the UE reselects to this cell when the timer expires (Point 3).

Figure 3-2 Inter-Frequency Reselection Example (equal priority neighbor)

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Qoffsets,n

3.3.2.1

Definition: Offset applied to a neighbor cell in the cell ranking criteria for cell reselection.

IE Value Engineering Units

Allowed Range

dB-24, dB-22, dB-20, dB-18, dB-16, dB-14, dB-12, dB-10, dB-8, dB-6, dB-5, dB-4, dB-3,2, dB-1, dB0, dB1, dB2, dB3, dB4, dB5,dB6, dB8, dB10, dB12, dB14, dB16, dB18, dB20, dB22, dB24

24 dB, 22 dB, 20 dB, 18dB, -16dB … 20 dB, 22 dB, 24 dB

Recommended dB0 (Depends on deployment strategy)

0 dB (Depends on deployment strategy)

Setting Tradeoff: A positive value of Qoffset deters the UE from reselecting to that cell. If the

parameter is set too low, reselection may be triggered even though the neighboring cell is only

marginally better than the current cell. If parameter is set too high, the UE may not reselect to the cell

even though the cell significantly better than the current cell. The parameter should be set to avoid

potential ping-ponging whilst maintaining the UE on a cell with sufficient quality.

Dependencies/Constraints: Qoffsetshould be set in conjunction with Qhyst and Qoffsetfrequency since

they jointly affect cell ranking criterion. The total offset applied to a neighbor is the sum of Qoffset

and Qoffsetfrequency. Whereas Qoffset can be set on a per cell basis, Qoffsetfrequency applies equally to all

the neighbor cells of a defined frequency carrier.

Traceability: TS36.304 Sect. 5.2.4.6, TS36.331 Sect. 6.3.1

RRC Message Structure: SIB5  InterFreqNeighCellInfo  q-OffsetCell

Notes:

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Qoffsetfrequency

3.3.2.2

Definition: Offset applied to all cells of a specific frequency in the cell ranking criteria for cell

reselection.

IE Value Engineering Units

Allowed Range

dB-24, dB-22, dB-20, dB-18, dB-16, dB-14, dB-12, dB-10, dB-8, dB-6, dB-5, dB-4, dB-3,2, dB-1, dB0, dB1, dB2, dB3, dB4, dB5,dB6, dB8, dB10, dB12, dB14, dB16, dB18, dB20, dB22, dB24

24 dB, 22 dB, 20 dB, 18dB, -16dB … 20 dB, 22 dB, 24 dB

Recommended Depends on deployment strategy TBD

Setting Tradeoff: A positive value of Qoffset deters the UE from reselecting to that cell. If the

parameter is set too low, reselection may be triggered even though the neighboring cell is only

marginally better than the current cell. If parameter is set too high, the UE may not reselect to the cell

even though the cell significantly better than the current cell. The parameter should be set to avoid

potential ping-ponging whilst maintaining the UE on a cell with sufficient quality.

Dependencies/Constraints: Qoffsetfrequency should be set in conjunction with Qhyst and Qoffset since

they jointly affect cell ranking criterion. The total offset applied to a neighbor is the sum of Qoffset

and Qoffsetfrequency. Whereas Qoffset can be set on a per cell basis, Qoffsetfrequency applies equally to all

the neighbor cells of a frequency.

Traceability: TS36.304 Sect. 5.2.4.6, TS36.331 Sect. 6.3.1

RRC Message Structure: SIB5  InterFreqCarrierFreqInfo  q-OffsetFreq

Notes:

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Higher Priority Neighbor Inter-Frequency Reselection 3.3.3

Reselection from a lower priority serving cell to a higher priority cell does not consider the signal

strength or quality of the serving cell. If a higher priority EUTRAN frequency (or RAT) is specified

In document LTE Guidelines (Page 42-200)