Iclterpop Section 5

Informa Telecoms & Media

Idle Mode Parameters

Idle Mode ParaMeters



Idle Mode ParaMeters

Defining RRC Idle Mode 4LTE IRAT Interactions 6Idle Mode Functions 8Cell Reselection 8PLMN Reselection 8Location Registration 8PLMN Selection 10Cell Selection 10Cell Reselection 10PLMN Selection Priorities 12Initial Cell Selection 14Femtocell Selection 16Overall Reselection Process 18LTE Measurement Rules 20Detailed Reselection Criteria from an LTE Cell 22Higher Priority Neighbours 22Equal Priority Neighbours 22Lower Priority Neighbours 22Scaling Modifications for Mobility 24Defining LTE Neighbours for UMTS 26UMTS Measurement Rules with Priorities 28Reselection from UMTS to LTE with Priorities 30Higher Priority Neighbours 30Equal Priority Neighbours 30Lower Priority Neighbours 30UMTS Measurement Rules without Priorities 32Reselection from UMTS to LTE without Priorities 34Defining LTE Neighbours for UMTS 36GSM Measurement Rules 38Reselection from GSM to LTE 40Higher Priority Neighbours 40Equal Priority Neighbours 40Lower Priority Neighbours 40PRACH Configuration Parameters 42Preamble Formats 42PRACH Resource Parameters 44PRACH Procedure Control Parameters 46PRACH Access Probe Procedure 48

4Idle Mode Parameters


defining rrC Idle Mode

The LTE RRC idle state is defined in TS 36.331 as shown in the diagram. This is essentially thesame description as would be used in any other digital terrestrial radio technology. AUEinidle mode is one that is switched on, has performed PLMN and cell selection andhasregistered successfully with a network. Once in this state it monitors broadcast signalling as required by cell configuration and monitors neighbour cells for potential cell reselection as determined by system information parameters.

A key characteristic of idle mode is that mobility is controlled by the UE. However, the UEs behaviour is driven by parameters and mode settings broadcast in system information that arepart of either manual or SON-based optimisation activities.

E-UTRA RRC IDLE

5 Informa Telecoms & Media

3GPP ts 36.331 defines idle mode with thefollowing: A UE-specific DRX may be configured by upper layers

UE-controlled mobility

The UE: monitors a Paging channel to detect incoming calls, system information change; for ETWS-capable UEs, ETWS notification; and for CMAS-capable UEs, CMAS notification performs neighbouring cell measurements and cell (re-)selection acquires system information performs logging of available measurements together

with location and time for logged measurement configured UEs

Fig. 1 e-Utra rrC Idle Mode



lte Irat Interactions

LTE idle mode corresponds directly with UMTS idle mode, and transitions between the two technologies for UEs in this mode are handled through cell reselection. All that is required forthis to function is the definition of appropriate neighbour information and measurement rulesfor each respective technology.

One minor complication is that there are sub-states within UMTS connected mode that have noequivalent in LTE; these being CELL_PCH and URA_PCH. UEs in this state are in connected mode but perform mobility procedures as if they are in idle mode. In this case, return to LTE ishandled through cell reselection controlled with idle mode parameters.

Interaction with GSM/GPRS is a little more complex. For normal GSM operation there may bedirect reselection between the two technologies. However, for interactions between LTE andGPRS there are a number of possibilities dependent on the specific activity in GPRS mode. GPRS Packet_Idle maps to LTE idle mode. Even when a GPRS mobile is in packet transfer mode, in some cases interaction may be by Cell Change Notification (CCN), which is a form ofcell reselection.

UMTS LTE GSM/GPRS

CELL_DCH

CELL_FACH

CELL_PCHURA_PCH

Idle E-UTRARRC Idle

GSM_Idle/GPRS

Packet_Idle

E-UTRA RRCConnected

GSMConnected

GPRS Packettransfer mode

Connectionestablishment/

release

Handover

Reselection Reselection

CCO,reselection


release


release

CCO withoptionalNACC

CCO,reselection

Reselection

Handover


Fig. 2 lte Irat Interactions



Idle Mode Functions

There are three main functions performed by the UE in idle mode: cell reselection, PLMN reselection and location registration. These are common in terms of definition with other 3GPPtechnologies. Release 8 of the technical specifications adds behaviour in connection withClosed Subscriber Groups (CSG), which relates to Home eNBs, i.e. femtocells.

Cell reselection

While in idle mode the UE will work to ensure that it is always camped on the cell most likely tooffer the most reliable service should a move to connected mode be required. This process involves evaluation in terms of radio signal quality assessment, but also service attributes such as consideration of CSG cell availability and access to the home PLMN as opposed to visited PLMNs.

PlMN reselection

When a UE is in idle mode on a visited network it is required periodically to search for its home network. The periodicity of this search is controlled by a timer value written onto the SIM card.

location registration

Location registration, known as a Tracking Area Update (TAU) in LTE, is of itself a connected mode procedure, but it is initiated as a result of idle mode cell reselection procedures.

AvailableCSG IDsto NAS

SelectedPLMN(optionalCSG ID)

Manualmode

Indicationto user

Locationregistrationresponse

CSG IDselected

PLMNsavailable

Registrationarea changes

Location registrationresponse

Radiomeasurements

Servicerequests

Automaticmode

PLMN selection

Location registrationCell selectionand reselection

NAScontrol

Support formanual CSGID selection


Fig. 3 Idle Mode Procedures

10



PlMN selection

PLMN selection is performed by the Non Access Stratum (NAS). The Access Stratum (AS) willscan according to a stored list or, if there is no stored information, it will scan the completeUE band/RAT capability. Each PLMN/RAT combination that is found is presented toNAS for consideration. The decision will be performed either in automatic mode or manuallyby the subscriber.

Cell selection

Once a PLMN has been selected by NAS and indication is given to the AS to scan for a suitablecell to camp on, AS scans for the strongest corresponding cell and tests it for suitability. Once the UE has camped on a suitable cell an indication is given to NAS, which will initiate aregistration process. This involves the use of connected mode, but, once complete, the UE drops into idle mode.

Cell reselection

Once in idle mode the UE performs neighbour cell measurements according to defined measurement rules. Measured cells are then evaluated using defined offsets and timers forreselection. Both measurement rules and neighbour cell evaluation parameters are subjectto optimisation activity.

PLMN selection

NASSelects PLMN/RATeither in automaticor manual modes

ASScans to

nd availablePLMN/RAT

NASIndicates selected PLMN/RAT to ASand requests a

cell for registration

ASScans to nd a

suitable cell on theselected PLMN/RAT

NASPerforms registrationin conjunction withAS then supplies

required informationfor idle mode

ASScans neighbours

and readsbroadcast signalling

as required

Cell selection Cell reselection

Camped NormalIdle Mode

Sys Info for idle modeSelection Suitable cellResults


Fig. 4 establishing Idle Mode

12



PlMN selection Priorities

PLMN selection may be performed in either automatic or manual mode. In automatic mode NAS makes the selection according to defined priorities. In manual mode NAS presents a listofavailable PLMN/RAT combinations to the users as a list ordered according to the same defined priorities.

Highest priority is the home PLMN. If the Home PLMN or an Equivalent PLMN is not available then the UE checks first the user-controlled PLMN selector file and then the operator-controlled PLMN selector file in the SIM. If a PLMN defined in this list is available it will be selected with highest priority given to the user-controlled list.

If no PLMNs appearing in the user- and operator-defined lists on the SIM are available or if the lists are not populated then the UE will randomly select a PLMN from those that are available and that fulfil the high-quality criterion. For LTE the high-quality criterion is defined as an RSRP greater than or equal to 110 dBm.

Only if there are no PLMNs meeting the high-quality criterion will the UE select a PLMN presenting the strongest signal level.

1. The HPLMN or the highest priority EHPLMN

2. Highest priority PLMN/RAT combination listed in the user controlled PLMN selector data file in the SIM

3. Highest priority PLMN/RAT combination listed in the operator controlled PLMN selector data file in the SIM

4. Other PLMN/RAT combinations meeting the high-quality criterion in random order

5. Other PLMN/RAT combinations in order of decreasing quality

High-Quality CriterionLTE RSRP 110 dBmUMTS FDD RSCP 95 dBmUMTS TDD RSCP 84 dBmGSM/GPRS RSSI 85 dBm

PLMN Priority List

eNB


Fig. 5 PlMN selection

14



Initial Cell selection

Once the NAS has made a PLMN/RAT selection it will request that the AS finds a suitable cell. This commences with a scan for the strongest cell measured in terms of RSRP. The strongest cell is then tested for suitability. The criteria for suitability is as shown in the diagram and involves checks for access rights and an assessment of radio usability. This latter part involves the calculation of the value Srxlev. A cell is suitable if Srxlev is greater than zero. Note that the outcome of Srxlev can be influenced by optimised parameters that are broadcast to the UE in system Information.

The main parameter influencing Srxlev is Qrxlevmin such that a higher value would make a cell lesslikely to be considered suitable. Qrxlevminoffset has no effect since it will be considered to bezero during initial cell selection.

At present there is only one UE power class defined for LTE (Class 3), which means that Pcompensation cannot be used to differentiate between different UE types.

The UE scans for the strongest cell first and then tests for suitability.

A cell is suitable if:

The cell is in the selected PLMN or Equivalent PLMN The cell is not barred The cell is in at least one TA that is not part of forbidden TAs for roaming For CSG, the cell is part of the UEs CSG white list Srxlev > 0 (dB)

where:

Srxlev = Qrxlevmeas (Qrxlevmin + Qrxlevminoffset) Pcompensation

Pcompensation = max(PEMAX_H PPowerClass, 0)

Qrxlevmeas = measured cell downlink RSRP in dBmQrxlevmin = minimum required cell downlink RSRP in dBmQrxlevminoffset = offset for Qrxlevmin in dB (set to 0 for initial cell selection)PEMAX_H = max permitted UL TX power in the cell in dBmPPowerClass = max UE TX power in dBm (from power class)


Fig. 6 Initial Cell selection

16



Femtocell selection

The NAS may additionally request that the AS scans for Closed Subscriber Group Identities (CSG ID). These are used to identify cells that are HeNBs (femtocells). In this case the AS will report the CSG ID to the NAS along with the HeNB name if present. If the NAS indicates that areported CSG ID is selected then the suitability test includes the requirement that the cell belongs to the selected CSG.

Home eNBeNB

CSG ID in SystemInformationBlockType1 HeNB name in SIB 9

eNB

eNB eNB

NASRequests AS toscan for CSG IDs

ASReports CSG ID +HeNB name andmodies suitabilitycriteria if selected

by NAS

ScanRequest

ScanResults


Fig. 7 Femtocell selection

18



overall reselection Process

The overall cell reselection process broadly follows the strategy first used by 3GPP for UMTS. AUE in idle mode will apply measurement rules to determine whether or not neighbour cell measurements should be taken. This involves the use of quality thresholds and a quality measurement taken only on the currently camped on cell. As for UMTS the application of measurement rules is not mandatory.

Cells that are measured after the application of measurement rules are evaluated. In effect this is a check for suitability as applied for initial cell selection. However, the parameter values used can be modified and, optionally, account can be taken of the UEs self-assessed mobility state. If layer priority levels are applied to frequency layers, reselection can be triggered directly for neighbours that are suitable if they are in a higher priority layer than the current selected cell. Ifpriority levels are not applied to frequency layers, or if an evaluated neighbour is not in a higher priority layer, then ranking is used to assess the need for reselection.

Ranking is used to compare the quality measure of an evaluated neighbour with that of the current selected cell. Offsets and hysteresis values are used to influence ranking, and these canbe layer-specific, cell-specific, or both. The UEs mobility state can also be accounted for.

Measurementrules Evaluation Ranking Reselection

Based on priority of RAT/Frequency layers Measurement thresholds can be set for each priority layer

Based on priority of RAT/Frequency layers Thresholds can be set globally, per priority layer or, in some cases, per neighbour cell Calculation can be modied according to the UE-determined mobility state (normal, medium, high)

Used only when neighbour cell is the same priority as the current serving cell Thresholds can be set per priority layer or per neighbour cell or both Calculation can be modied according to the UE-determined mobility state (normal, medium, high)

1 sec since last reselection Cell is suitable

Rs = Qmeas,s + QhystsRn = Qmeas,n + Qoffsets,n


Fig. 8 lte reselection overview

20



lte Measurement rules

Intra-Frequency rulesIntra-frequency measurements are controlled by the parameter Sintrasearch. If SServingCell isevaluated as higher than Sintrasearch then the UE is not required to make measurements onintra-frequency neighbour cells.

Inter-frequency and Irat rulesBoth inter-frequency and IRAT measurements are controlled by the same parameter Snonintrasearch. In this case account is also taken of the relative priority of the layer containingthe potential measured neighbour cells. In the case where neighbours are inahigherpriority layer, they must always be measured. In the case where they are in anequalorlower priority layer they may not be measured when SServingCell is evaluated ashigher than Snonintrasearch.

Parameter ValuesBoth Sintrasearch and Snonintrasearch are broadcast in SIB 3 and are variable in the range 031 dB.

ServingLTE cell(SServingCell= Srxlev)

IRATneighbourGSM/GPRSUMTS/HSPA1x/1xEV

Intra-frequency

LTE neighbour

Inter-frequency

LTE neighbour


Intra-frequency Measurement rulesIf SServingCell > Sintrasearch UE may not measure neighbours

If SServingCell Sintrasearch orSintrasearch not sent

UE must measure neighbours

Inter-frequency and Irat Measurement rulesHigher priority layer UE must measure neighbours

Equal or lower priority layer

If SServingCell > Snonintrasearch UE may not measure neighbours

If SServingCell Snonintrasearch orSnonintrasearch not sent


Fig. 9 lte Measurement rules

22



detailed reselection Criteria from an lte Cell

The criteria outlined in the diagram apply to any reselection carried out from a current selectedcell that is LTE. Thus it includes intra-frequency LTE reselections, inter-frequency LTEreselections and IRAT reselections. It assumes that priority levels have been set for frequency layers. If priority has not been set for a frequency layer then resection evaluation willnot be performed for that layer.

Higher Priority Neighbours

Higher priority neighbours are reselected if they exceed the defined quality threshold Threshx,high for TreselectionRAT seconds. This is done without ranking against the current selected cell. Notethat if more than one neighbour meets this criteria then ranking is used todetermine thebest neighbour.

equal Priority Neighbours

Equal priority neighbours are reselected if they meet the suitability criteria Srxlev > 0 and if they are ranked higher than the current selected cell for TreselectionRAT seconds. If this condition is true for more than one neighbour cell then ranking is also used to determine the best neighbour. Note that this case will never be applicable for IRAT neighbours because IRAT frequency layers cannot be allocated equal priority to LTE frequency layers.

lower Priority Neighbours

Lower priority neighbours are reselected only if no other type of suitable cell is available while the current serving cells quality measure (Srxlev) is below the quality threshold Threshserving,low forTreselectionRAT seconds. In this case a lower priority neighbour could be either LTE or IRAT as defined by priority allocations. However, to be reselected a neighbours quality measure still has to exceed the quality threshold Threshx,low for TreselectionRAT seconds.

Higher Priority Neighbour CellsCriteria 1: SnonServingCell,x (Srxlev) > Threshx,high for TreselectionRAT Camped on current selected cell for more than 1 second

Equal Priority Neighbour CellsCriteria S: SnonServingCell,x (Srxlev) > 0 Ranking criterion Rn > Rs for TreselectionRAT Camped on current selected cell for more than 1 second

Lower Priority Neighbour Cells No higher priority cell fulfils Criteria 1 No equal priority cell fulfils the Ranking Criteria SServingCell,x (Srxlev) < Threshserving,low for TreselectionRAT Lower priority n-cell SnonServingCell,x (Srxlev) > Threshx,low for TreselectionRAT Camped on current selected cell for more than 1 second


Fig. 10 lte reselection Criteria

24



scaling Modifications for Mobility

The UE determines its mobility state in terms of the frequency at which cell reselections arebeing performed. This assessment is based on the broadcast parameters TCR and NCR asshown in the diagram. These parameters are set through the optimisation process.

In the Normal mobility state no changes are made to any parameters relating to the standard cell reselection evaluation and ranking processes.

When a UE determines that it is the Medium mobility state then the value QHyst used in the ranking process and TreselectionRAT, used in both the evaluation and ranking processes are modified. QHyst is reduced through the addition of q-HystSF-Medium, which has a negative value (6, 4, 2, 0). TreselectionRAT is scaled through multiplication with TreselectionRAT-SF-Medium, which has a value between 0 and 1 (0.25, 0.5, 0.75, 1). The net effect of this is to speed up the reselection decision-making process.

When a UE determines that it is the High mobility state then the value QHyst used in the ranking process and TreselectionRAT, used in both the evaluation and ranking processes are modified. QHyst is reduced through the addition of q-HystSF-High, which has a negative value (6, 4, 2, 0). TreselectionRAT is scaled through multiplication with TreselectionRAT-SF-High, which has a value between 0 and 1 (0.25, 0.5, 0.75, 1). The net effect of this is to speed up the reselection decision making process.

Both q-HystSF-Medium/High and TreselectionRAT-SF-Medium/High are frequency-layer specific and as such are transmitted in the SIB relevant to the layer of interest.

Medium Mobility

Add q-HystSF-Medium to QHyst Multiply TreselectionRAT by TreselectionRAT-SF-Medium

High Mobility

Add q-HystSF-High to QHyst Multiply TreselectionRAT by TreselectionRAT-SF-High

Normal Mobility

No action

Number of reselections in TCRmax seconds > NCR_M and NCR_H

Return to Normal mobility state if not Medium and not High for TCRmaxHyst seconds

Number of reselections in TCRmax seconds > NCR_H

Number of reselections in TCRmax seconds NCR_M


Fig. 11 Modifications for Mobility state

26



defining lte Neighbours for UMts

The standard message in UMTS for the definition of neighbour lists is SIB 11. However, although this message does contain a field identified as IRAT neighbours, it is only specified for GSM/GPRS neighbour cell descriptions. Therefore, in order to define LTE neighbour relations a new message is defined, SIB 19. This message is only used for LTE neighbour descriptions.

It should be noted that SIB 19 does not make any provision for the description of specific LTE cells. Instead, the message simply identifies an LTE frequency layer along with its priority. There is, however, the option to include a field containing an black cell list, i.e. specific LTE cells that are not to be considered for reselection.

ServingUMTS cell

IRATneighbour

LTEInter-frequencyUMTS

neighbour

LTE frequency layer EARFCN Priority Reselection parameters Black cell list

IRATneighbourGSM/GPRSIntra-

frequencyUMTS

neighbour

Sys Info

Sys Info

SIB Type 11

SIB Type 19


Fig. 12 lte Neighbours from UMts

28



UMts Measurement rules with Priorities

When frequency layer priorities are used for neighbour cell definitions in UMTS the way in which measurement rules are applied is very similar to LTE.

For neighbour cells in higher priority frequency layers measurements must always be taken. Thus if an operator chooses to make LTE a higher priority than UMTS, the UE will always bescanning for and measuring LTE neighbours.

Alternatively, an operator may choose to define LTE frequency layers as a lower priority than UMTS. In this case neighbour cell measurements are not mandatory while quality measures of the current selected cell, defined in terms of Srxlev and Srxqual, are both above the respective thresholds Sprioritysearch1 and Sprioritysearch2.

Note that an LTE frequency layer cannot be defined as equal in priority to a UMTS frequency layer.

ServingUMTScell

LTEneighbour

Sys Info SIB Type 19


Irat to lte Measurement rulesHigher priority layer UE must measure neighbours

Lower priority layer

If SrxlevServingCell > Sprioritysearch1and if SrxqualServingCell > Sprioritysearch2

UE may not measure neighbours

If SrxlevServingCell Sprioritysearch1or if SrxqualServingCell Sprioritysearch2


SrxlevServingCell = SrxlevSrxqualServingCell = SrxqualSprioritysearch1 = 0-62 dB in steps of 2 } Sprioritysearch2 = 0-7 dB in steps of 1

Fig. 13 UMts Measurement rules (with absolute Priorities)

30



reselection from UMts to lte with Priorities

When frequency layer priorities are used for neighbour cell definitions in UMTS the way in which reselection criteria are applied is very similar to LTE.


Higher priority neighbours are reselected if they exceed the defined quality threshold Threshx,high for TreselectionRAT seconds. This is done without ranking against the current selected cell. Note that if more than one neighbour meets this criteria, ranking is used todetermine the bestneighbour.


This never applies to LTE neighbours because an LTE frequency layer cannot be defined asequal in priority to a UMTS frequency layer.


Lower priority neighbours are reselected only if either the current serving cells Srxlev quality measure is below the quality threshold Threshserving,low for TreselectionRAT seconds, orthecurrent serving cells Srxlev quality measure is below zero. However, to be reselected aneighbours quality measure still has to exceed the quality threshold Threshx,low for TreselectionRAT seconds.

Higher Priority Neighbour CellsCriteria 1: SrxlevnonServingCell,x (Srxlev) > Threshx,high for TreselectionRAT Camped on current selected cell for more than 1 second

Equal Priority Neighbour Cells (Never applies to LTE Neighbours)Criteria 2: SrxlevServingCell,x (Srxlev) > Threshserving,low for TreselectionRAT or SrxlevServingCell,x (Srxlev) > 0 for TreselectionRAT Lower priority n-cell SnonServingCell,x (Srxlev) > Threshx,low for TreselectionRAT Camped on current selected cell for more than 1 second

Lower Priority Neighbour CellsCriteria 3: SrxlevServingCell,x (Srxlev) < Threshserving,low for TreselectionRAT or SqualServingCell,x (Srxqual) < 0 for TreselectionRAT Lower priority n-cell SrxlevnonServingCell,x (Srxlev) > Threshx,low for TreselectionRAT Camped on current selected cell for more than 1 second

These parameters are carried in SIB Type 19


Fig. 14 UMts reselection Criteria (with absolute Priorities)

32



UMts Measurement rules without Priorities

If priorities are not applied for frequency layers then the standard UMTS measurement rules areapplied. There are two variants for this that are referred to as either with or without HSC rules. Since most operators use the version without HCS rules this is the variant described hereas an example.

In this case, separate thresholds are defined for each type of frequency layer, intra-frequency, inter-frequency and IRAT. Thus the thresholds that apply to LTE neighbours are SsearchRATm and SHCS,RATm. They are applied as indicated in the table on the left of the diagram.

It is important to note that most operators will also need to consider reselection from UMTS toGSM and measurement rules for this scenario also use SsearchRATm and SHCS,RATm. However, these parameters are set per RAT, so a separate pair of values for SsearchRATm and SHCS,RATm can bedefined for use with GSM neighbours.

ServingUMTS cell

Compare

IRATneighbour

LTE

Inter-frequencyUMTS

neighbour

IRATneighbourGSM/GPRS

Intra-frequencyUMTS

neighbour

CalculateSqual

(1 to 25)

Squal = Qqualmeas Qqualmin

Srxlev = Qrxlevmeas Qrxlevmin Pcompensation

Pcompensation = max(UE_TXPWR_MAX_RACH P_MAX, 0)

Qqualmeas(25 to 0)

Sys Info SIB Type 3

Intra UMTS and IRAT Measurement RulesSqual Sintrasearch UE must perform

intra-frequency measurements

Squal Sintersearch or Srxlev SsearchHCS

UE must perform inter-frequency measurements

Squal SsearchRATm or Srxlev SHCS,RATm

UE must perform inter-RAT measurements

Parameter ValueSintrasearch 321. to 20 in steps of 2

Sintersearch 321. to 20 in steps of 2

SsearchHCS 1051. to 91 in steps of 2

SsearchRATm2. 321. to 20 in steps of 2

SHCS,RATm2. 1051. to 91 in steps of 2

Qqualmin Ec/No (dB) 25 to 0 in steps of 1

Qrxlevmin RSCP (dBm) 115 to 25 in steps of 2

1. Negative values are considered to be 02. Parameter defined per RAT


Fig. 15 UMts Measurement rules (without absolute Priorities)

34



reselection from UMts to lte without Priorities

After the application of measurement rules those neighbour cells that are measured are submitted for evaluation. Although the standard process involves the calculation of both Srxlev and Srxqual, for LTE neighbours only Srxlev is calculated. The values of Qrxlevmin andP_MAX are specific to the LTE frequency layer and are broadcast in SIB19.

All the evaluated cells that are evaluated as meeting the cell selection criterion are submitted forranking. The value of Qoffset is specific to the LTE frequency layer and is broadcast in SIB19.

An LTE cell will be reselected if it is the highest ranked cell for Treselection seconds. The value of Treselection is specific to the LTE frequency layer and is broadcast in SIB19.

Squal = Qqualmeas QqualminSrxlev =rxlevmeas Qrxlevmin Pcompensation

Pcompensation = max(UE_TXPWR_MAX_RACH P_MAX, 0)

Measurementrules Evaluation Ranking Reselection

Based on measurement rules without priorities

Squal > 0 Srxlev > 0

Calculate Rs for the serving cell Calculate Rn for the considered neighbour cell

Rn must be greater than Rs for Treselection seconds 1 sec since last reselection Cell is suitable

Rs = Qmeas,s + QhystsRn = Qmeas,n + Qoffsets,n


Fig. 16 UMts reselection Criteria (without absolute Priorities)

36



defining lte Neighbours for UMts

LTE neighbour information is broadcast in System Information Type 2quater messages. Sincethe information field may be quite large and the overall size of a single system information message in GSM is restricted by the physical layer structure, the information field may be fragmented and mapped into more than one occurrence of the message. Thus an MS may need to read several copies of Type 2quater before it has stored all the required neighbour information, particularly where an operator has multiple GSM bands, UMTS and LTE. Fragmentation is indicated to the MS through the inclusion of Start/Stop bits in the information field.

It should be noted that System Information Type 2quater does not make any provision for the description of specific LTE cells; instead the message simply identifies an LTE frequency layer along with its priority and other reselection parameters. There is, however, the option to include a field containing not-allowed cells, i.e. specific LTE cells that are not to be considered for reselection.

Note that it is mandatory to use layer priorities to control the GSM to LTE reselection process.

ServingGSM/GPRS

cell

LTEneighbour

Reselection parametersv E-UTRAN Neighbour Cells EARFCN Measurement Bandwidth E-UTRAN_PRIORITY THRESH_E-UTRAN_high THRESH_E-UTRAN_low E-UTRAN_RXLEVMIN E-UTRAN Not Allowed Cells E-UTRAN PCID to TA Mapping

GERAN_PRIORITY THRESH_Priority_Search T_reselection H_PRIO THRESH_GSM_low

MS Calculates S_GSM = C1

S_non-serving_E-UTRAN = RSRP E-UTRAN_QRXLEVMIN

UMTSneighbour

Sys Info Type2quater

SelectedCell UMTS LTE


Fig. 17 lte Cell Information in GsM/GPrs

38



GsM Measurement rules

Normal idle mode behaviour is substantially modified for Release 8 and this new mode of operation is mandatory for reselection to LTE. Instead of continuous neighbour cell monitoring, the concept of measurement rules is introduced into GSM. This works in a very similar way toUMTS and LTE.

For neighbour cells in higher priority frequency layers, measurements must be taken of each higher priority layer at least once every 60 seconds. Thus if an operator chooses to make LTE a higher priority than GSM, then the UE will always be scanning for and measuring LTE neighbours.

Alternatively, an operator may choose to define LTE frequency layers as a lower priority than GSM. In this case neighbour cell measurements are not mandatory while quality measure of the current selected cell, defined in terms of RLA_C (RSSI), is above the threshold THRESH_Priority_Search.

Note that an LTE frequency layer cannot be defined as equal in priority to a GSM frequency layer.

ServingGSM/GPRS

cell

LTEneighbour


Irat to lte Measurement rulesHigher priority layer MS must measure at least once every 60 x Nhpf seconds

Lower priority layer

If RLA_C THRESH_Priority_Search

UE may not measure neighbours

If RLA_C < THRESH_Priority_Search


RLA_C = RSSITHRESH_Priority_Search = 98 56 dB in steps of 4 (value 15 = )Nhpf = the number of higher priority IRAT frequency layers

Fig. 18 GsM/GPrs Measurement rules

40



reselection from GsM to lte

Frequency layer priorities are mandatorily used for LTE neighbour cell definitions in GSM andtheway in which reselection criteria are applied is very similar to LTE. Note that once thisprocedure is applied for LTE neighbour cell reselection it will also be used for reselection ofUMTS cells from GSM.


Higher priority neighbours are reselected if they exceed the defined quality threshold THRESH_E-UTRAN for T_reselection seconds. This is done without ranking against the currentselected cell. Note that if more than one neighbour meets this criteria then ranking isused to determine the best neighbour.


This never applies to LTE neighbours because an LTE frequency layer cannot be defined asequal in priority to a GSM frequency layer.


The diagram shows the reselection criteria for reselection to cells in a lower priority layer. Ingeneral it will only occur if the quality of the current serving cell is poor, the quality of the target neighbour cell is good and there are no other acceptable GSM cells. This condition mustbe maintained for T_reselection seconds.

Higher Priority Neighbour Cells S_non-serving_E-UTRAN > THRESH_E-UTRAN_high for T_reselection

Lower Priority Neighbour Cells S_GSM < THRESH_GSM_low for T_reselection S_non-serving_E-UTRAN > THRESH_E-UTRAN_low for T_reselection S-GSM < C1 for all other measured GSM cells for T_reselectionElse if S_non-serving_E-UTRAN > S-GSM by H_PRIO for T_reselection S-GSM < C1 for all other measured GSM cells for T_reselection


Fig. 19 GsM/GPrs reselection Criteria

42



PraCH Configuration Parameters

The random access process must be used before any kind of transaction can be performed between the UE and the eNB. It may also be used while a UE is in RRC connectedmode for resource requests. It is therefore important that it works reliably and withminimal unnecessary interference contribution. In this regard there are a significant numberof PRACH-related parameters that may be part of the optimisation process.

Preamble Formats

There are four preamble formats that may be used. They differ in terms of the relative durations of the cyclic prefix, preamble and guard period. In total they may have a duration of either one, two or three slot periods, as shown. The preamble format to be used is indicated to the UE as part of the parameter prach_ConfigIndex, which is transmitted in SystemInformationBlockType2.

The choice of CP length will be based on expected cell time dispersion conditions, and the guard period is selected according to the expected maximum cell radius. Note that the maximum cell radius is 100 km.

Format1

Subframe 1ms

Set by prach_ConfigIndex found in SIB Type 2

CP21024 Ts

Preamble24576 Ts

Preamble49152 Ts

Preamble49152 Ts

CP15840 Ts

Format0

CP3168 Ts

GT2976 Ts

Preamble24576 Ts

Format2

CP6240 Ts

CP6048 Ts

Format3

CP21024 Ts

CP21984 Ts

3168 Ts 2976 Ts


Fig. 20 PraCH Physical layer Formats

44



PraCH resource Parameters

The single parameter prach_ConfigIndex controls three time domain aspects of PRACH resource configuration. The first of these is the preamble format. In addition it indicates what is referred toas the PRACH burst period. This is described in terms of a list of subframes in which PRACH may start and how frequently frames containing PRACH may occur. This is achieved by making the numerical value of prach_ConfigIndex (063) reference a table listing all possible combination in the standards (TS 36.211, 5.7.1). The example shown in the diagram is for value 9. The table indicates that this means preamble format 0, starting subframes 1, 4 and 7 and every frame available for PRACH use.

The frequency domain definition of the space allocated for PRACH transmission is provided by prach_FreqOffset. The parameter indicates the first RB to be used, but in all cases a total of six RBs are allocated. The example shows the allocation when the parameter is set to the value 6.

The range of preamble sequences available for use on a cell must also be defined. As shown there are three parameters with the information element prach-Config that influence this.

Define the set ofpreamble sequencesthat can be used

Index references a positionwithin a table that definesthe preamble format to beused and also the startingsubframes within the frame

E.g. prach-ConfigIndex = 9, prach-FreqOffset = 6

Frame

SF1

NRB

SF4 SF7 SF1 SF4 SF7

Frame

Sets lowest RB indexin set of six RBs

SIB Type 2

RadioResourceConfigSIB


prach-ConfigrootSequenceIndex 0837

prach-ConfigInfo

prach_ConfigIndex 063

High-speed-flag 1/0

zeroCorrelationZoneConfig 015

prach_FreqOffset 094

Fig. 21 PraCH resource Parameters

46



PraCH Procedure Control Parameters

The PRACH procedure itself is also subject to optimisation. The diagram shows the contents ofthe information element rach-Config. It can be seen that this contains the four key parameters that are used to control the transmission procedure. Between them they control power and power steps along with retransmission timings and numbers.

Note that this information element also contains more parameters that define the available preamble sequences. One aspect of this is the option to associate sub-ranges of preamble sequences (A and B) to different random access causes. A UE will use a preamble from Group B according to two thresholds. The first relates to the size of the layer 3 message that will be transmitted and the other relates to the power offset between the last successful preamble andthe message transmission itself.

Determines thespecific preamblefor the transmission

Determines thephysical layer preambletransmission procedure

Determines thebehaviour with respectto the subsequenttransmission of theLayer 3 message

SIB Type 2

RadioResourceConfigCommon


rach-ConfigpreambleInfo

numberOfRA-Preambles 4, 8, ...64 steps of 4

preambleGroupAConfig

sizeOfRAPreambleGroupA 4, 8, ...60 steps of 4

messageSizeGroupA 56, 144, 208, 256

messagePower OffsetGroupB

inf, 0, 5, 8, 10, 12, 15, 18 dB

powerRampingParameters

powerRampingStep 0, 2, 4, 6 dB steps of 2

preambleInitialReceived TargetPower

120, 118, ...90 dBm steps of 2

ra-SupervisionInfo

preambleTransMax 3, 4, 5, 6, 7, 8, 10, 20, 50, 100, 200

ra-responseWindowSize 2, 3, 4, 5, 6, 7, 8, 10 subframes

mac-ContentionresolutionTimer 8, 16, ...64 steps of 8 subframes

macHARQ-Msg3Tx 1, 2, ...8

Fig. 22 PraCH Procedure Control Parameters

48



PraCH access Probe Procedure

The probing process is controlled using four key parameters that may be part of the optimisation process. The first, preambleInitialReceivedTargetPower is used by the UE to calculate an initial power for the first preamble transmission attempt. Subsequent preamble transmissions are then stepped up in power by an amount determined by the setting of the parameter powerRampingStep.

Once a preamble has been transmitted the UE will wait for a response in the form of a resource allocation in PDCCH identified with a corresponding RA-RNTI. The wait time is determined by the parameter ra-responseWindowSize. Finally, the total number of preamble attempts before the procedure would be abandoned in the event that no response is received as set by the parameter preambleTransMax.

PPRACH = min{PCMAX, PREAMBLE_RECEIVED_TARGET_POWER + PL} dBmwhere:PL = Path loss estimated by the UEPCMAX = Max power allowed or power class of the UEPREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_TRANSMISSION_COUNTER 1)*powerRampingStepwhere:DELTA_PREAMBLE = 0 dB for formats 0 and 1, 3 dB for formats 2 and 3

RA-RNTI used in the RAR = 1 + t_id + 10*f_idwhere:t_id = the index of the first subframe usedf_id = the index of the RB used

preambleTransMax

powerRampingStep

PRACH

Pream

ble

powerRampingStep

ra-responseWindowSize

Wait forresponse(RAR

in DPCCH)

PRACH

Pream

ble

powerRampingStep

PPRACH

PRACH

Pream

ble


Fig. 23 PraCH access Probe Procedure

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