LTE Procedures

Objectives• Describe the cell search procedure.• Describe the RRC UE states and state transitions.• Describe the system information broadcast procedure.• Describe UE and E-UTRA measurement types.• Describe the idle mode procedures.• Describe the random access procedure.• Describe the state characteristics of RRC.• Describe the NAS protocol states and state transitions.• Describe the network attach procedure.• Describe the LTE pooling relationships.• Describe the LTE mobile identities.• Describe the tracking area identities.• Describe the initial context setup procedures.• Describe the intra MME/S-GW handover.• Describe the inter MME/S-GW handover.

9-3

LTE

Version 1 Rev 2 Cell Search Procedure

Cell Search ProcedureCell search is the procedure by which a UE acquires its frequency reference, frame timing, and the FastFourier Transform (FFT) symbol timing with the (best) cell, and also to identify the cell ID.. A scalableoverall transmission bandwidth corresponding to 6 resource blocks and upwards is supported within theE-UTRA, therefore it doesn’t matter whether 20 MHz or 1.4 MHz is used as the 6 resource blocks canfit within any of the supported bandwidths.

Acquiring P-SCH and S-SCHOn power up the UE will scan the available LTE frequencies and detect the center frequencies of thebandwidth being used by the network(s). To be more specific the center 1.25 MHz bandwidth whichcorresponds to 72 subcarriers (or 6 PRBs) and there the UE will find the P-SCH, S-SCH and PBCH.There are 504 unique physical layer cell identities available, these are grouped into 168 unique physicallayer group identities with each group containing three unique orthogonal physical layer identities.Therefore a physical layer identity is identified by a number between 0–167 representing the cell identitygroup and a number in the range 0–2 representing the physical layer identity within the physical layeridentity groups.Two channels are used to achieve this, the P-SCH and S-SCH. The P-SCH uses the same OFDMwaveform for all cells over 72 subcarriers and this is used for SCH symbol timing and frequencyacquisition. A S-SCH is used to determine the cell group ID where the three cell IDs relevant tothat group can be detected using reference symbols for correlation detections, where the maximumcorrelation peak is obtained for the correct physical cell ID. This vastly reduces the amount of correlationdetections the UE has to do from 504 to 3.A one-to-one mapping between the 3 Cell IDs in each Cell ID group and downlink reference signals areapplied in the system. By processing the downlink reference signals, the cell ID (one out of 3) is derivedwithin the cell ID group.The physical cell id will give a unique combination of one orthogonal sequence and one pseudo-randomsequence. It is then possible to decode the PBCH and therefore decode the Master Information Block(MIB).3GPP TS 36.211 V8.1.0 (2007–11) — Physical Channels and Modulation

9-4

LTE

Cell Search Procedure Version 1 Rev 2

Cell Search Procedure

4. The three cell IDs relevant to that group can be detected using reference symbols from the downlink reference signals for correlation detections

2. P-SCH - SCH symbol timing and frequency acquisition

3. S-SCH - cell group ID

1. There are 504 unique physical layer cell identities available, these are groupedinto 168 unique physical layer group identities with each group containingthree unique physical layer identities.

9-5

LTE

Version 1 Rev 2 RRC UE States and State Transitions

RRC UE States and State TransitionsWhen the UE is first switched on it will acquire the physical cell ID of a cell. At this time it will haveno RRC connection (or any other type) with the eNB, the UE is then said to be in RRC_IDLE state. Ifthe UE establishes a connection with the cell it will do it via RRC messages and hence it will move intoRRC_CONNECTED state.

RRC_IDLE StateThe RRC_IDLE state can be characterised as follows:

• Transfer of broadcast/multicast data to the UE.• A UE specific DRX maybe configured by the upper layers.• UE controlled mobility.• The UE will:

– Monitor a paging channel to detect incoming calls.– Performs neighbour cell measurements and cell (re-) selections.– Acquires system information.

RRC_CONNECTED StateThe RRC_CONNECTED state can be characterised as follows:

• Transfer of unicast data to/from UE, transfer of broadcast/multicast data to the UE.• At the lower layers, the UE maybe configured with a UE specific DRX/DTX.• Network controlled mobility.• The UE will:

– Monitor control channels associated with the shared data channel to determine if data isscheduled for it.

– Provides channel quality and feedback information.– Performs neighbouring cell measurements and measurement reporting.– Acquires system information.

9-6

LTE

RRC UE States and State Transitions Version 1 Rev 2

RRC UE States and State Transitions

E-UTRA RRC IDLEE-UTRA

RRC IDLE

GPRS Packet transfer modeGPRS Packet transfer mode

GSM_CONNECTED

CELL_DCH

CELL_FACH

CELL_PCH URA_PCH

Connection establishment/release

CELL_DCH

CELL_FACHCELL_FACH

CELL_PCH URA_PCHCELL_PCH URA_PCH



UTRA_IDLEReselection

UTRA_IDLEUTRA_IDLEReselection

Reselection

E-UTRA RRC CONNECTED


E-UTRA RRC CONNECTEDE-UTRA RRC CONNECTED



GSM_IDLE/GPRS Packet_Idle

Reselection

CCO, Reselection



Reselection

CCO, Reselection

Handover Handover

CCO with NACC

CCO, Reselection

WCDMA 3G LTE GSM/GPRS 2G

9-7

LTE

Version 1 Rev 2 System Information (SI)

System Information (SI)System Information (SI) is an RRC message carrying a number of System Information Blocks (SIBs)that have the same scheduling requirements (i.e. periodicity). There may be more than one SI messagetransmitted with the same periodicity. Each SIB contains a set of related system information parameters.Several SIBs have been defined including the Master Information Block (MIB), that includes a limitednumber of most frequently transmitted parameters, and SIB type 1 containing the scheduling informationthat mainly indicates when the SI messages are transmitted, i.e. start times. SYSTEM INFORMATIONMASTER (SI-M) and SYSTEM INFORMATION 1 (SI-1) are special versions of an SI message onlycarrying a single SIB, namely the MIB and SIB type 1 respectively. The SI-M message is carried on BCHwhile all other SI messages are carried on DL-SCH.

System Information Blocks (SIBs)These SIBs relate to information contain in both RRC_IDLE and RRC_CONNECTED modes.

Master Information Block (MIB)The MIB defines the most essential physical layer information of the cell required to receive furthersystem information like downlink system bandwidth, number of transmit antennas and system framenumber. The MIB is transmitted on the BCH.

System Information Block Type 1 (SIB 1)The SIB 1 contains information relevant when evaluating if a UE is allowed to access a cell and definesthe scheduling of other SIBs.

System Information Block Type 2 (SIB 2)The SIB 2 contains common and shared channel information.

System Information Block Type 3 (SIB 3)The IE SIB 3 contains cell re-selection information, mainly related to the serving cell.

System Information Block Type 4 (SIB 4)The SIB 4 contains information about the serving neighbouring frequencies and intra-frequencyneighbouring cells relevant for cell re-selection, covering both E UTRA and other RATs. TheInformation Element (IE) includes cell re-selection parameters common for a frequency as well as cellspecific re-selection parameters.

System Information Block Type 5 (SIB 5)The SIB 5 contains information about other E UTRA frequencies and inter-frequency neighbouring cellsrelevant for cell re-selection. The IE includes cell re-selection parameters common for a frequency aswell as cell specific re-selection parameters.

System Information Block Type 6 (SIB 6)The SIB 6 contains information about UTRA frequencies and UTRA neighbouring cells relevant for cellre-selection. The IE includes cell re-selection parameters common for a frequency as well as cell specificre-selection parameters.

System Information Block Type 7 (SIB 7)The SIB 7 contains information about GERAN frequencies and GERAN neighbouring cells relevant forcell re-selection. The IE includes cell re-selection parameters common for a frequency as well as cellspecific re-selection parameters.

System Information Block Type 8 (SIB 8)The SIB 8 contains information about CDMA2000 frequencies and CDMA2000 neighbouring cellsrelevant for cell re-selection. The IE includes cell re-selection parameters common for a frequency aswell as cell specific re-selection parameters.3GPP TS 36.331 V8.1.0 (2008–03) Radio Resource Control

9-8

LTE

System Information (SI) Version 1 Rev 2

System Information (SI)

Master Information Block (MIB) on BCHMaster Information Block (MIB) on BCH

System Information Block Type 1 to 8 (SIB 1 to 8) on DL-SCHSystem Information Block Type 1 to 8 (SIB 1 to 8) on DL-SCH

MIB - Essential physical layer information

SIB 1 - If UE is allowed to access a cell and defines the scheduling of other SIBs

SIB 2 - Common and shared channel information

SIB 3 - Cell re-selection information, mainly related to the serving cell

SIB 4 – 8 Cell reselection parameters for different types of neighbours

9-9

LTE

Version 1 Rev 2 Physical layer Measurements

Physical layer MeasurementsWith the measurement specifications L1 provides measurement capabilities for the UE and E-UTRAN.These measurements can be classified in different reported measurement types: intra-frequency,inter-frequency, inter-system, traffic volume, quality and UE internal measurements.To initiate a specific measurement, the E-UTRAN transmits a ‘RRC connection reconfigurationmessage’ to the UE including a measurement ID and type, a command (setup, modify, release), themeasurement objects, the measurement quantity, the reporting quantities and the reporting criteria(periodical/event-triggered).When the reporting criteria are fulfilled the UE shall answer with a ’measurement report message’ to theE-UTRAN including the measurement ID and the results.

UE Measurement CapabilitiesThe following UE measurements can be initiated:

• Reference Signal Received Power (RSRP) — Is determined for a considered cell as thelinear average over the power contributions (in [W]) of the resource elements that carrycell-specific reference signals within the considered measurement frequency bandwidth. ForRSRP determination the cell-specific reference signals R0 and if available R1 (second antenna)accordingly can be used. If receiver diversity is in use by the UE, the reported value shall beequivalent to the linear average of the power values of all diversity branches. This is applicable for:– RRC_IDLE intra-frequency.– RRC_IDLE inter-frequency.– RRC_CONNECTED intra-frequency.– RRC_CONNECTED inter-frequency.

• E-UTRA Carrier Received Signal Strength Indicator (RSSI) — comprises the total receivedwideband power observed by the UE from all sources, including co-channel serving andnon-serving cells, adjacent channel interference, thermal noise etc.

• Reference Signal Received Quality (RSRQ) — is defined as the ratio N×RSRP/(E-UTRA carrierRSSI), where N is the number of RB’s of the E-UTRA carrier RSSI measurement bandwidth. Themeasurements in the numerator and denominator shall be made over the same set of resource.This is applicable for:– RRC_IDLE intra-frequency.– RRC_IDLE inter-frequency.– RRC_CONNECTED intra-frequency.– RRC_CONNECTED inter-frequency.

• UTRA FDD CPICH Received Signal Code Power (RSCP) — , the received power on one codemeasured on the Primary CPICH. This is applicable for:– RRC_IDLE inter-RAT.– RRC_CONNECTED inter-RAT.

• UTRA FDD carrier RSSI — The received wide band power, including thermal noise and noisegenerated in the receiver for 3G cells, . This is applicable for:– RRC_IDLE inter-RAT.– RRC_CONNECTED inter-RAT.

• UTRA FDD CPICH Ec/No — The received energy per chip divided by the power density in theband for 3G cells. This is applicable for:– RRC_IDLE inter-RAT.– RRC_CONNECTED inter-RAT.

• GSM carrier RSSI— Received Signal Strength Indicator, the wide-band received power within therelevant channel bandwidth. Measurement shall be performed on a GSM BCCH carrier. This isapplicable for:– RRC_IDLE inter-RAT.– RRC_CONNECTED inter-RAT.

• CDMA2000 1x RTT Pilot Strength — CDMA2000 1x RTT Pilot Strength measurement.• CDMA2000 HRPD Pilot Strength — CDMA2000 HRPD Pilot Strength Measurement.

9-10

LTE

Physical layer Measurements Version 1 Rev 2

Physical layer Measurements

RRC connection reconfiguration messageRRC connection reconfiguration message

Measurement report message

UE Serving and E-UTRA Neighbour Cell Measurements

• Reference Signal Received Power (RSRP)

• E-UTRA Carrier Received Signal Strength Indicator (RSSI)

• Reference Signal Received Quality (RSRQ)

E-UTRAN Measurements

• DL RS TX power

• Received Interference Power

• Thermal Noise Power

WCDMA 3G Neighbours• UTRA FDD CPICH Received Signal Code Power (RSCP)• UTRA FDD carrier RSSI• UTRA FDD CPICH Ec/No

GSM Neigbours• GSM carrier RSSI

CDMA2000 Neighbours• CDMA2000 1x RTT Pilot Strength• CDMA2000 HRPD Pilot Strength

E-UTRAN Measurement AbilitiesThe following E-UTRAN measurements can be initiated:

• DL RS TX power — Downlink reference signal transmit power is determined for a considered cellas the linear average over the power contributions (in [W]) of the resource elements that carrycell-specific reference signals which are transmitted by the eNode B within its operating systembandwidth. For DL RS TX power determination the cell-specific reference signals R0 and if availableR1.

• Received Interference Power — The uplink received interference power, including thermal noise,within one physical resource block’s bandwidth of resource elements.

• Thermal Noise Power — The uplink thermal noise power within the UL system bandwidth.

9-11

LTE

Version 1 Rev 2 Idle Mode Procedures

Idle Mode ProceduresWhen an UE is switched on, it attempts to make contact with a PLMN. The particular PLMN to becontacted may be selected either automatically or manually.The UE looks for a suitable cell of the chosen PLMN and chooses that cell to provide available services,and tunes to its control channel. This choosing is known as "camping on the cell". The MS will thenregister its presence in the registration area of the chosen cell if necessary, by means of a LocationRegistration (LR), GPRS attach or International Mobile Subscriber Identity (IMSI) attach procedure.If the UE loses coverage of a cell, or find a more suitable cell, it reselects onto the most suitable cellof the selected PLMN and camps on that cell. If the new cell is in a different registration area, an LRrequest is performed.If the UE loses coverage of a PLMN, either a new PLMN is selected automatically, or an indication ofwhich PLMNs are available is given to the user, so that a manual selection can be made.Registration is not performed by UEs only capable of services that need no registration.The purpose of camping on a cell in idle mode is fourfold:

• It enables the UE to receive system information from the PLMN.• If the UE wishes to initiate a call, it can do this by initially accessing the network on the control

channel of the cell on which it is camped.• If the PLMN receives a call for the UE, it knows (in most cases) the registration area of the cell in

which the MS is camped. It can then send a "paging" message for the UE on control channels of allthe cells in the registration area. The UE will then receive the paging message because it is tunedto the control channel of a cell in that registration area, and the UE can respond on that controlchannel.

• It enables the UE to receive cell broadcast messages.If the UE is unable to find a suitable cell to camp on, or the SIM is not inserted, or if it receives certainresponses to an LR request (e.g., "illegal MS"), it attempts to camp on a cell irrespective of the PLMNidentity, and enters a "limited service" state in which it can only attempt to make emergency calls.The idle mode tasks can therefore be subdivided into three processes:

• PLMN selection.• Cell selection and reselection.• Location registration.3GPP TS 23.122 V8.1.0 (2008–03) NAS functions in Idle Mode3GPP TS 36.304 V8.1.0 (2008–03) UE procedures in Idle Mode

9-12.

LTE

Idle Mode Procedures Version 1 Rev 2

Idle Mode Procedures

PLMN SelectionPLMN SelectionManual ModeManual Mode

Indication to userIndication to user

Automatic ModeAutomatic Mode

PLMNsavailable

Cell Selection and Reselection

PLMNsselected

Cell Selection and ReselectionCell Selection and Reselection

PLMNsselectedPLMNsselected

NAS Control

Radio measurements

NAS Control

Radio measurements

Location Registration

Registration area changes

Location RegistrationLocation Registration

Registration area changesRegistration area changes

Service requestsService requests

Location area response

Location area response

9-13

LTE

Version 1 Rev 2 Cell Selection and Reselection


Cell Selection ProcessThe UE uses one of the following two cell selection procedures:

1. Initial Cell SelectionThis procedure requires no prior knowledge of which RF channels are E-UTRA carriers. The UEscans all RF channels in the E-UTRA bands according to its capabilities to find a suitable cell. Oneach carrier frequency, the UE searches for the strongest cell. Once a suitable cell is found thiscell shall be selected.

2. Stored Information Cell SelectionThis procedure requires stored information of carrier frequencies and optionally also informationon cell parameters, from previously received measurement control information elements or frompreviously detected cells. Once the UE has found a suitable cell the UE shall select it. If no suitablecell is found the Initial Cell Selection procedure shall be started.

Service Types in Idle ModeThe action of camping on a cell is necessary to get access to some services. The levels of service aredefined for the UE:

• Limited service (emergency calls on an acceptable cell);.• Normal service (for public use on a suitable cell).• Operator service (for operators only on a reserved cell).

The cells themselves are then categorized according to which services they offer.

Acceptable CellAn "acceptable cell" is a cell on which the UE may camp to obtain limited service (originate emergencycalls). Such a cell shall fulfil the following requirements, which is the minimum set of requirements toinitiate an emergency call in a E-UTRAN network:

• The cell is not barred.• The cell selection criteria are fulfilled.

Suitable CellA "suitable cell" is a cell on which the UE may camp on to obtain normal service. Such a cell shall fulfilall the following requirements:

• The cell is part of either:– the selected PLMN, or;– the registered PLMN, or;– a PLMN of the Equivalent PLMN list.

according to the latest information provided by the NAS.• The cell is not barred.• The cell is part of at least one Tracking Area (TA) that is not part of the list of "forbidden LAs for

roaming", which belongs to a PLMN that fulfils the first bullet above.• The cell selection criteria are fulfilled.• For a cell identified as Closed Subscriber Group (CSG) cell by system information, the CSG ID

is part of the CSG white list.If more than one PLMN identity is broadcast in the cell, the cell is considered to be part of all TAs withTracking Area Identities (TAIs) constructed from the PLMN identities and the Tracking Area Code(TAC) broadcast in the cell.

Barred and Reserved CellsA cell is barred or reserved if it is so indicated in the system information3GPP TS 36.304 V8.1.0 (2008–03) UE procedures in Idle Mode

9-14

LTE

Cell Selection and Reselection Version 1 Rev 2


Stored information cell selection

Cell information stored for PLMN

Stored information cell selection

Cell information stored for PLMN

Go here whenever a new PLMN is selected

1



1

Initial cell selection

No cell information stored for PLMN

No suitable cell found Initial cell selection

No cell information stored for PLMN

No suitable cell found

Suitable cell found Suitable cell foundSuitable cell found Suitable cell found

Camped normally

2

Camped normallyCamped normally

22No suitable cell foundNo suitable cell found

Suitable cell found

Cell selection when leaving

connected mode

Return to idle mode

Connected mode

Leave idle mode


connected mode

Return to idle mode

Connected mode

Leave idle mode

Connected mode

Connected mode

Leave idle mode

Cell selection Evaluation

process

Trigger

Cell selection Evaluation

process

Trigger Suitable cell foundSuitable cell found


NAS indicates that registration on selected PLMN is rejected



NAS indicates that registration on selected PLMN is rejected


Any cell selectionAny cell selection

Go here when no USIM in the UE

Go here when no USIM in the UE

USIM inserted

1

USIM inserted

1

Camp on any cell

Acceptable cell found

Camp on any cell


Cell reselection Evaluation

process

Trigger Suitable cell found

Cell reselection Evaluation

process

Trigger Suitable cell found


connected mode

Return to idle mode


Connected mode

(emergency calls only)

Leave idle mode

Connected mode

(emergency calls only)

Leave idle mode

No acceptable cell foundNo acceptable cell found

No acceptable cell foundNo acceptable cell found

Suitable cell found 2Suitable cell foundSuitable cell found 2

9-15

LTE

Version 1 Rev 2 Cell Selection Criteria

Cell Selection CriteriaThe cell selection criterion S is fulfilled when:Srxlev > 0Where:Srxlev = Qrxlevmeas — Qrxlevmin — Pcompensation

Parameter Meaning

Srxlev Cell selection RX level value (dB)

Qrxlevmeas Measured cell RX level value (RSRP)

Qrxlevmin Minimum required RX level in the cell(dBm)

Pcompensation (FFS)

The Qrxlevmin parameter is found in the SIB Type 1 and is broadcast to the UE in its control channel.The parameter Pcompensation is labeled as For Further Study (FFS) in the current version in the 3GPPspecification listed below.3GPP TS 36.304 V8.1.0 (2008–03) UE procedures in Idle Mode

9-16.

LTE

Cell Selection Criteria Version 1 Rev 2

Cell Selection Criteria

SIB Type 1 on DL-SCH

Srxlev > 0 Where:

Srxlev = Qrxlevmeas – Qrxlevmin - Pcompensation

9-17

LTE

Version 1 Rev 2 Cell Reselection Criteria

Cell Reselection CriteriaOnce the UE performs selection onto a particular cell the UE will continue to take measurements on theserving cell. There are parameters to decide whether the UE should begin to take measurements ofother LTE intra or inter frequency neighbours or indeed other RATs. Of course the user/network maydecide to force the UE onto another cell and this is taken into consideration too.

Intra-frequency Measurement RulesThe following rules are used by the UE to limit needed measurements by the UE:

• If SServingCell > Sintrasearch, UE may choose to not perform intra-frequency measurements;• If SServingCell <= Sintrasearch, UE shall perform intra-frequency measurements.

Non Intra-frequency Measurement RulesThe UE applies the following rules for E-UTRAN inter-frequencies and inter-RAT frequencies which areindicated in system information and for which the UE has priority:

• For an E-UTRAN inter-frequency or inter-RAT frequency with a reselection priority higher than thereselection priority of the current E-UTRA frequency the UE shall perform measurements of higherpriority E-UTRAN inter-frequenc or inter-RAT frequencies.

• For an E-UTRAN inter-frequency with a equal or lower reselection priority than the reselectionpriority of the current E-UTRA frequency and for inter-RAT frequency with lower reselection prioritythan the reselection priority of the current E-UTRA frequency:– If SServingCell > Snonintrasearch UE may choose not to perform measurements of inter-RAT frequencies

or inter-frequencies of equal or lower priority.– If SServingCell <= Snonintrasearch the UE shall perform measurements of inter-RAT frequencies or

inter-frequencies cells of equal or lower priority.Where SServingCell is the S-value of the serving cell.

Mobility States of a UEThere are also parameters broadcast in system information to detect whether the UE is in:

• Normal mobility state.• Medium mobility state.• High mobility state.

If for instance the UE is detected in ’high mobility state’ then cells which have been designated as smallcells (micro and pico cells) shall be discouraged for reselection purposes by using speed dependantscaling rules. The exact implementation of this is FFS at the time of writing.3GPP TS 36.304 V8.1.0 (2008–03) UE procedures in Idle Mode

9-18.

LTE

Cell Reselection Criteria Version 1 Rev 2

Cell Reselection Criteria

Intra F requency Inter F requency or Inter R AT

System Information

Sintrasearch

Snonintrasearch

System Information

Sintrasearch

Snonintrasearch

SServingCell > Sintrasearch

SServingCell<= Sintrasearch

SServingCell > Sintrasearch

SServingCell<= Sintrasearch

SServingCell > Snonintrasearch

SServingCell<= Snonintrasearch

UE speed dependant rules also apply

9-19

LTE

Version 1 Rev 2 Intra-frequency Cell Reselection Criteria

Intra-frequency Cell Reselection CriteriaOnce the criteria has been met to measure intra-frequency neighbours then the UE will rank all cells thatmeet criterion S. It might be that the operator either wants to apply a hysteresis to the serving cell toprevent numerous unnecessary reselections. Or an offset might be applied to the neighbour to eithermake the neighbour more or less attractive.

Cell Ranking CriterionThe intra-frequency neighour cells are ranked according to the R criteria.Rs = Qmeas,s + Qhysts

Rn = Qmeas,n — QoffsetWhere:

• Qmeas — Averaged RSRP measurement quantity used in cell reselections.• Qoffset — Qoffsets,n + Qoffsetfrequency

Qoffsets,n — This specifies the offset between two cells.Qoffsetfrequency — Frequency specific offset for equal priority frequencies.The cell that is ranked highest by the R criterion will cause the UE to reselect if the following is true:

• The new cell is better ranked than the serving cell during a time interval Treselections.• Detect whether the UE is in high mobility state, if so apply scaling factor to this cell.• More than 1 second has elapsed since the UE camped on the current serving cell.3GPP TS 36.304 V8.1.0 (2008–03) UE procedures in Idle Mode

9-20

LTE

Intra-frequency Cell Reselection Criteria Version 1 Rev 2

Intra-frequency Cell Reselection Criteria

Intra F requency1. Rank cells by criterion S

2. Ranked by Rs < Rn for Treselection

3. UE has been camped on serving cell for > 1 sec

4. Speed taken into account

Rs = Qmeas,s + Qhysts

Rn Qmeas,n – Qoffset

Qoffset = Qoffsets,n + Qoffsetfrequency

System information – Treselection, Qhysts, Qoffsets,n, Qoffest frequency

9-21

LTE

Version 1 Rev 2 Inter-frequency and Inter-RAT Cell Reselection Criteria

Inter-frequency and Inter-RAT Cell Reselection CriteriaIt might be the case that the UE needs to move to a different LTE frequency or other RAT. The other RATmaybe WCDMA 3G, GSM or CDMA2000. The criteria to detect whether a reselection to these cells isdifferent to that of intra-frequency reselections and this procedure is explained here.

Cell Ranking CriterionIn this the inter-frequency and inter-RAT cell reselection may occur, but it has a dependency on whetherthe neighbour cell is:

• Higher priority RAT or frequency.• Lower priority RAT or frequency.• Equal priority RAT or frequency.

Higher Priority RAT or FrequencyIn this case the UE will reselect if:

• The SnonServingCell,x of a cell of a higher priority RAT or frequency is greater than Threshx, high during atime interval TreselectionRAT; and

• more than 1 second has elapsed since the UE camped on the current serving cell.

Lower Priority RAT or FrequencyIn this case the UE will reselect if:

• No cell of a higher priority RAT or frequency fulfills the criteria above; and• SServingCell < Threshserving, low and the SnonServingCell,x of a cell of a lower priority frequency RAT or frequency

is greater than Threshx, low during a time interval TreselectionRAT; and• more than 1 second has elapsed since the UE camped on the current serving cell.

Equal Priority RAT or FrequencyCell reselection to a cell on an equal priority frequency shall be based on ranking for Intra-frequency CellReselection.SnonServingCell,x is the S-value of a non-serving inter-RAT or inter-frequency cell. In all the above criteria thevalue of TreselectionRAT is scaled when the UE is in the medium or high mobility state.If more than one cell meets the above criteria, the UE shall reselect a cell ranked as the best cell on thehighest priority RAT among the cells meeting the criteria.3GPP TS 36.304 V8.1.0 (2008–03) UE procedures in Idle Mode

9-22

LTE

Inter-frequency and Inter-RAT Cell Reselection Criteria Version 1 Rev 2

Inter-frequency and Inter-RAT Cell Reselection Criteria

Inter F requency or Inter R ATInter F requency or Inter R AT

System information – TreselectionRAT, Threshx, high, Threshserving, low, Threshx, lowSystem information – TreselectionRAT, Threshx, high, Threshserving, low, Threshx, low

Higher Priority RAT or Frequency

1. SnonServingCell,x > Threshx, high for TreselectionRAT


Lower Priority RAT or Frequency

1. No cell of a higher priority RAT or frequency fulfills the criteria above; and

2. SServingCell < Threshserving, low and SnonServingCell,x > Threshx, low for TreselectionRAT


Equal Priority RAT or Frequency

Based on ranking for Intra-frequency Cell Reselection

9-23

LTE

Version 1 Rev 2 Random Access Procedure

Random Access ProcedureThe random access procedure is common between FDD and TDD and the size of the cell is irrespectiveto the procedure. The random access procedure is performed for the following five events:

• Initial access from RRC_IDLE.• Initial access after radio link failure (sending RRC connection re-establishment).• Handover requiring random access procedure.• DL data arrival during RRC_CONNECTED requiring random access procedure (UL sync status

“non-synchronized”).• UL data arrival during RRC_CONNECTED requiring random access procedure (UL sync status

“non-synchronized” or there are no PUCCH resources for scheduling resources available).Furthermore, the random access procedure takes two distinct forms:

• Contention based (applicable to all five events). In this case a signature is chosen by the UE thatmay also be chosen by other UEs in the network at the same time.

• Non-contention based (applicable to only handover and DL data arrival). In this case a signatureis assigned to the UE that is unique, so it cannot be duplicated by any other UE in that cell at thesame time.

This can be thought of in a different way: the random access procedure is utilized when ever UE is notuplink synchronized and it wants to exchange data with the eNode B. There are two distinct scenarios:

1. UE is not UL synchronized and not authenticated (RRC_IDLE).2. UE has lost UL synchronization, but it is authenticated (RRC_CONNECTED).Normal DL/UL transmission can take place after the random access procedure.

Initial Access from RRC_IDLEFor the purposes of this discussion we shall concentrate on the initial access from RRC_IDLE.This is a contention based procedure and is shown in the slide opposite and described in the fournumbered bullets below:

1. Random access preamble on the RACH in the uplink:– 5 bits random ID.– 1 bit to indicate the size of the scheduled transmission (from a group of signatures broadcast

on SI).2. Random access response generated by MAC on DL-SCH:

– Semi- synchronous with the random access preamble as it must arrive within a flexible window.– No HARQ.– It contains the random ID transmitted on random access preamble, timing alignment info,

initial uplink grant and temporary (could become permanent) Cell Radio Network TemporaryIdentifier (C-RNTI);This message could be delivered to multiple UEs in one DL-SCH message.

3. First scheduled transmission on UL-SCH. In this case it is a RRC Connection Request:– Uses HARQ.– Size of transport block depends on the UL grant conveyed in random access response.– Conveys NAS UE identifier (SAE- TMSI (S-TMSI) if there is one).

4. Contention resolution on DL-SCH: In this case it is the RRC Connection Setup.– Addressed to C-RNTI.– UE with correct C-RNTI responds with HARQ feedback.

3GPP TS 36.300 V8.4.0 (2008-03) — Overall Description

9-24

LTE

Random Access Procedure Version 1 Rev 2

Random Access Procedure

RRC_IDLE UE monitoring System Information and Paging

No uplink synchronization with network

Random access procedure used to synchronize with the eNB

UE monitoring System Information and Paging

No uplink synchronization with network

Random access procedure used to synchronize with the eNB

Random Access Preamble 1PRACH

Random Access Preamble 1Random Access Preamble 11PRACH

Random Access Response2DL-SCH

Random Access Response2 Random Access Response22DL-SCH

Scheduled transmission(RRC Connection Request)

3UL-SCH

Scheduled transmission(RRC Connection Request)

33UL-SCH

Contention resolution4(RRC Connection Setup)

DL-SCH

Contention resolution4 Contention resolution44(RRC Connection Setup)

DL-SCH

(NAS PDU)(RRC Connection Setup Complete)

55UL-SCH

9-25

LTE

Version 1 Rev 2 Physical Random Access Procedure

Physical Random Access ProcedureThe physical random access procedure is carried out within the transmission of the random accesspreamble and the random access response. The remaining messages shown on the previous slide aredelivered by the higher layers on the shared data channel and are not considered part of the L1 randomaccess procedure. However the L1 physical random access procedure must receive its parameters ofoperation from the higher layers to operate.

L1 Physical Random Access ParametersThe UE will discover the parameters by listening to downlink SI and radio resource control informationmessages. The content of these messages includes:

• Number of non-dedicated/dedicated random access preambles.• Size of the random access preambles groups.• Power ramping step.• The maximum number of preamble transmissions in an attempt.• Value of backoff timer.• Parameters to indicate the TTI window for random access response.

This information is then stored by the UE so when a random access procedure is required the MAC layercan use these parameters to initialize the random access procedure.

Random Access Procedure InitializationIf the Random Access Preamble and PRACH resource are explicitly signalled (non-contention based)the UE may proceed with the transmission. In the case where the UE is making initial access this willnot be the case and the random access preamble will be selected by the UE as follows:

• Select one of the two groups of random access preambles configured by RRC.• Randomly select a random access preamble within the selected group. The random function shall

be such that each of the allowed selections can be chosen with equal probability.

Random Access Preamble TransmissionThe random access procedure will be performed as follows:

• Set the initial access power using a open loop power control mechanism.• Determine the next available random access occasion.• Set the preamble transmission counter to zero.• Instruct the physical layer to transmit a preamble using the selected PRACH resource,

corresponding RA-RNTI, preamble index and transmit power.

Random Access Response ReceptionOnce the random access preamble is transmitted, the UE shall monitor the [PDCCH] in the TTI windowfor random access response(s). The UE may stop monitoring for random access response(s) aftersuccessful reception of a random access response corresponding to the random access preambletransmission.The random access response may contain an ’overload indication’ in which case the UE will start a’backoff’ timer that makes the UE wait before attempting another access.If no Random Access Response is received within the TTI window, or if all received random accessresponses contain random access preamble identifiers that do not match the transmitted random accesspreamble, the random access response reception is considered not successful and the UE shall:

• If the preamble transmission counter is less than maximum allowed transmissions, then incrementthe preamble transmission counter by 1.

• Increase the power of the UE by the power ramping step.• Retransmit the preamble.

If the preamble transmission counter is equal to the maximum allowed transmissions, then indicate tohigher layers that the access attempt has failed.

9-26.

LTE

Physical Random Access Procedure Version 1 Rev 2

Physical Random Access Procedure

StartStartStart

Obtain random access parameters from system

information


information


information

Select the random access preamble from the random

access group


access group


access group

Set initial power and determine the next available RACH

occasion


occasion


occasion

Set the preamble transmission counter to zero



Transmit preambleTransmit preambleTransmit preamble

Response from eNBResponse from eNB

Increment counter and increase the

power

N


power

N


power


power

N

Send NACK to L3

Y

Send NACK to L3

YY Process the received timing alignment value

If an UL grant was received, process the UL grant value

If the UE does not have a C-RNTI, a Temporary C-RNTI shall be set

Y

Process the received timing alignment value



Y

Process the received timing alignment value



Y

Counter equals max count?

N



N

If a successful access is made because the UE received the random access preamble identifiercorresponding to the transmitted random access preamble then the UE shall include these procedures:

• Process the received timing alignment value.• If an UL grant was received, process the UL grant value.• If the UE does not have a C-RNTI, a Temporary C-RNTI shall be set to the value received in the

random access response message no later than at the time of the first transmission correspondingto the UL grant provided in the random access response message.

3GPP TS 36.321 V8.1.0 (2008-03) — E-UTRA MAC Protocol Specification

9-27

LTE

Version 1 Rev 2 State Characteristics of the RRC

State Characteristics of the RRCThe physical random access procedure showed how the UE makes initial contact with the network andis able to send the RRC Connection Request message. The slide opposite summarises the states; idleand connected available within the RRC and what services and procedures can be accessed by the UEin both of these states.

RRC IdleThe following services and procedures can be accessed in RRC_IDLE:

• Paging.• Cell selection and reselection.• BCCH information read.

The following characteristics are applied to RRC_IDLE:

• No S1 connection.• No measurement control.• UE not known by the eNB.

RRC ConnectedThe following services and procedures can be accessed in RRC_CONNECTED:

• Data can be received and transmitted using the C-RNTI.• Unicast (and multicast) services.• Handovers.• Measurement of neighbour cells.

The following characteristics are applied to RRC_CONNECTED:

• S1 connected.• UE is known by C-RNTI.

What is required now is to be registered with the network and to be able to access services from thenetwork. This is achieved by the UE initiating an ’Attach Procedure’ when it sends the RRC ConnectionRequest message. The attach procedure and mobility management states for the NAS are describednext.

9-28.

LTE

State Characteristics of the RRC Version 1 Rev 2

State Characteristics of the RRC

UE not known in eNB

No Meas Control

No S1 Connection

BCCH Information

Cell (Re)selection

Paging

RRC_IDLE

UE is known by C-RNTI

S1 Connected

Meas of neighbour cells

Handovers

Unicast and multicast services

Data is rx/tx using C-RNTI

RRC_CONNECTED

RACH

RRC Messages

RRC NAS Messages

UE Power On

9-29

LTE

Version 1 Rev 2 NAS Protocol States and State Transitions

NAS Protocol States and State TransitionsThe NAS state model is based on a two-dimensional model which consists of EPS MobilityManagement (EMM) states describing the mobility management states that result from the mobilitymanagement procedures e.g. Attach and Tracking Area Update procedures, and of EPS ConnectionManagement (ECM) states describing the signalling connectivity between the UE and the EPC.The ECM and EMM states are independent of each other and when the UE is in EMM-CONNECTEDstate this does not imply that the user plane (radio and S1 bearers) is established.

NAS and AS State CharacteristicsThe relation between NAS and AS states is characterised by the following principles:

• EMM-DEREGISTERED & ECM-IDLE →RRC_IDLE:– In this condition the UE is not known by the network. It has to select a PLMN and signal to the

network for its IMSI to become registered. This is achieved during the attach procedure, untilthis occurs the location of the UE is unknown.

• EMM-REGISTERED andECM-IDLE →RRC_IDLE:– In this condition the UE has registered with the network, hence it has a SAE-Temporary Mobile

Subscriber Identity (S-TMSI) and its position is known at Tracking Area (TA) level so it hasa Tracking Area Identity (TAI) Code. An IP address will have been allocated to the UE by theP-GW selected by the MME.

– The UE will be able to perform cell reselections.• EMM-REGISTERED and ECM-CONNECTED with RBs established →RRC_CONNECTED:

– In this state the UE has a full context for data transmission and data reception. It is known bythe EPC on a cell level. In the case of an inter MME handover the EPC will be involved in thehandover.

9-30.

LTE

NAS Protocol States and State Transitions Version 1 Rev 2

NAS Protocol States and State Transitions

Registration

No data transfer

PLMN selection

UE unknown

IMSI identifier

No RRC or EPC Context

RRC: Null

EMM-DEREGISTERED &

ECM-IDLE

UE Power On

DRX on DL

TA Update

UE known at TA level

S-TMSI, TA-ID & IP-Addr

EPC Context

RRC: IDLE

EMM-REGISTERED &

ECM-IDLE

UL/DL data transfer

Handovers

UE known at Cell level

S-TMSI, TA-ID & IP-Addr

RRC & EPC Context

RRC: CONNECTED

EMM-REGISTERED &

ECM-CONNECTED

Service Request

TA-update paging etc

De-Registration

InactivityPeriodic TA-update timeout – out of area

9-31

LTE

Version 1 Rev 2 Network Attach Procedure

Network Attach ProcedureA UE/user needs to register with the network to receive services that require registration. This registrationis described as Network Attachment. The always-on IP connectivity for UE/users of the SAE system isenabled by establishing a basic IP bearer during Network Attachment.It is assumed that the random access procedure described on the previous page has already taken placeand the first message to be transmitted on the DCCH will be the UL information transfer containing theattach request message.The procedure indicated on the slide opposite shows the information flow for the attach procedure.

Network Attach Information FlowThe procedure indicated on the slide opposite shows the information flow for the attach procedure. Thesequence of steps may change because of solutions for key issues as the specifications evolve. Thesteps are describe below:

1. The UE discovers the SAE/LTE access system(s) and performs access system and networkselection. If network sharing is present, a shared network may be selected.

2. The UE sends an attach request to the MME/UPE, including its old registration information, e.g.temporary identity. If the UE has no old registration information it includes its permanent identity.In case network sharing is applied the attach request includes information for selecting network orMME/UPE. The Evolved RAN selects the MME/UPE. The attach request may include informationon Default IP Access Bearer (e.g. user preferred IP address and APN).

3. If old registration information was sent by the UE the MME/UPE tries to retrieve user informationfrom the old MME/UPE by sending the old registration information.

4. The old MME/UPE sends user information, e.g. the permanent user identity, to the MME/UPE.5. The user/UE is authenticated in the new MME/UPE.6. The MME/UPE registers itself as serving the UE in the HSS.7. The user/UE information in the old MME/UPE is deleted or the user/UE is marked as not present.8. The HSS confirms the registration of the new MME/UPE. Subscription data authorising the default

IP access bearer are transferred. Information for policy and charging control of the default IP accessbearer is sent to the MME/UPE.

9. An S-GW/P-GW is selected. The IP address configuration is determined by user preferencesreceived from the UE, by subscription data, or by HPLMN or VPLMN policies.

10. The S-GW/P-GW configures the IP layer with the determined user IP address. The user plane isestablished and the default policy and charging rules are applied. The user plane establishment isinitiated by the UE or by the MME/UPE.

11. The MME/UPE provides the E-UTRAN with QoS configurations for the default IP access bearer,e.g. the upper limits for transmission data rates.

12. The MME/UPE accepts the UE’s network attachment and allocates a temporary identity to the UE.Also the determined user IP address is transferred.

13. Roaming restrictions are checked and if violated the network attachment is rejected.14. The UE acknowledges the success of the network attachment.3GPP TR 23.882 V1.1415.0 (2008-0102) — 3GPP SAE Technical Options and Conclusions

9-32.

LTE

Network Attach Procedure Version 1 Rev 2

Network Attach Procedure

HSS Old MME/UPEUE eNB

Random AccessRandom Access

MME

DCCH UL-SCH UL INFORMATION TRANSFER

NAS: ATTACH REQUEST

Old S-TMSI, TAI (if none IMSI)

UE capabilities

IP version support

RRC_IDLE

RRC_CONNECTED

EMM Deregistered and ECM_IDLE

S1-AP INITIAL UE MESSAGE

NAS: ATTACH REQUEST

Old S-TMSI, TAI (if none IMSI)

UE capabilities

IP version support

S1-AP DL NAS TRANSPORT

NAS: Authentication Request

RAND, AUTH, KSIASME

DCCH DL-SCH DL INFORMATION TRANSFER

NAS: Authentication Request

RAND, AUTH, KSIASME

Calculate AUTN, RES &

keys

DCCH UL-SCH UL INFORMATION TRANSFER

NAS: Authentication response

RES

Authenticate network S1-AP UL NAS TRANSPORT

NAS: Authentication response

RES

Check RES

Select S-GW and P-GW

Collect old registration information

AUTHENTICATION DATE REQ

IMSI

MCC+MNC

Network Type

AUTHENTICATION DATE RES

MME Security context(s)

RAND, XRES, CK, IK, AUTH – (AVs)

Register MME

Delete registration

S-GW P-GWUE eNB MME

CREATE DEF. BEARER REQ

IMSI

MME context ID

P-GW address

Supported IP versions

CREATE DEF. BEARER REQ

S-GW address

S5 TEID

Supported IP versions

Allocate user IP

CREATE DEF. BEARER RESP

P-GW address

S5 TEID

User IP

CREATE DEF. BEARER RESP

S-GW address

S1 TEID

User IP

INITIAL CONTEXT SETUP REQ

NAS: Attach Accept

S-TMSI

UE capabilities

Security context

S-GW address

S1 TEID

Calculate E-UTRA

keys

RRC CONN. RECONFIG

NAS: Attach Accept

S-TMSI

Equivalent TA list

User IP

Security configuration

RRC CONN. RECONFIG COMPL

NAS: Attach CompleteINITIAL CONTEXT SETUP RESP

NAS: Attach Complete

S1 UE id

eNB address

S1 TEID

UPDATE BEARER REQ

eNB address

S1 TEIDUPDATE BEARER RESP

EMM Registered and ECM_CONNECTED

Default IP Connectivity

established i.e. ‘always on’ IP

connectivity

RRC_IDLE

EMM Registered and ECM_IDLE

9-33

LTE

Version 1 Rev 2 LTE Pooling Relationships

LTE Pooling RelationshipsA pool area is defined in LTE/SAE as an area within which a UE may roam without need to change theserving MME node. A pool area is served by one or more MMEs (“pool of MMEs) in parallel. All the cellscontrolled by a eNB belong to the same one (or more) pool area(s). Connectivity between eNBs andUPEs may follow the MME-pool/pool area configuration or be independent of it.

MMEs and eNodeBs within Pool AreasIn SAE/LTE the entity that controls roaming activities of a UE across tracking areas within a pool areaand across pool areas is the MME. Actually, a pool area can be regarded as an area consisting of a listof Tracking Areas and hence a list of eNBs that control cells within these tracking areas.The relation between eNBs and MMEs along the pool area definition in the context of the S1-flex conceptis realised by configuration that enables e.g. an eNB to contact the MME in case of initial access/attach,further it allows an MME to contact the relevant set of eNBs in case of paging.The slide opposite depicts the pool area concept. As shown, a Pool Area in LTE/SAE is served bya corresponding pool of MMEs. Pool areas might overlap, hence the RAN nodes (eNBs) within theoverlapping area may be served by more than one pool of MMEs.

UPEs and eNBs within Pool AreasSelection of the UPE will take place at initial attach (after the authentication procedure has beenperformed) or in course of an MME relocation with UPE involvement. In both cases, UPE selection hasto be regarded as a MME function, whereas the eNB is informed about the UPE to contact by meansof S1 signalling.The slide opposite depicts the pool area concept with UPEs. As shown, there might be different kinds ofconnectivity restrictions on S1 between eNBs and UPEs.

• Scenario 1 — These restrictions might follow the MME Pool / Pool Area configuration, i.e. a poolof MMEs constitutes a service area that corresponds to the service area of a pool of UPEs

• Scenario 2 There might be no restrictions at all, i.e. a pool of UPEs has full connectivity to all RANnodes and can be selected by all MMEs in a PLMN

• Scenario 3 Or the UPE pool configuration is de-coupled from the MME pool/pool area definition.In any case, it is the MME that should have knowledge about the UPE nodes that can be selectedand consequently the MME should be in the position to trigger a UPE relocation if needed.

9-34

LTE

LTE Pooling Relationships Version 1 Rev 2

LTE Pooling Relationships

eNB1

MME

eNB2 eNB3 eNB4 eNB5 eNB6 eNB7 eNB8 eNB9

MMEMME

MME

MMEMME

MMEMME

MME Pool A MME Pool B

Pool Area X Pool Area Y

eNB1

MME


MMEMME

MME

MMEMME

MMEMME



UPEUPE

UPE

UPE Pool 2UPE

UPEUPE

UPE Pool 3

UPEUPE

UPE

UPE Pool 1

Scenario 3

eNB1

MME


MMEMME

MME

MMEMME

MMEMME



UPEUPE

UPE

UPE Pool 1UPE

UPEUPE

UPE Pool 2

Scenario 1

eNB1

MME


MMEMME

MME

MMEMME

MMEMME



UPEUPE

UPE

UPE Pool 1

Scenario 2

9-35

LTE

Version 1 Rev 2 LTE Mobility Identities

LTE Mobility IdentitiesWhen the UE is in EMM-Registered state it will be identified by a S-TMSI within the MME and can bereached within a Tracking Area Identity (TAI).

Globally Unique Temporary UE IdentityThe MME shall allocate a Globally Unique Temporary Identity (GUTI) to the UE.The GUTI has two main components:

• one that uniquely identifies the MME which allocated the GUTI; and• one that uniquely identifies the UE within the MME that allocated the GUTI.

Within the MME, the mobile is identified by the M-TMSI.The Globally Unique MME Identifier (GUMMEI) is constructed from MCC, MNC and MME Identifier(MMEI).In turn the MMEI is constructed from an MME Group ID (MMEGI) and an MME Code (MMEC). TheMMGEI is the MME pool ID and the MMEC is the MME within that pool.The GUTI is constructed from the GUMMEI and the M-TMSI.The operator needs to ensure that the MMEC is unique within the MME pool area and, if overlappingpool areas are in use, unique within the area of overlapping MME pools.

SAE-Temporary Mobile Subscriber Identity (S-TMSI)For paging, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and theM-TMSI.The GUTI is used to support subscriber identity confidentiality, and, in the shortened S-TMSI form, toenable more efficient radio signalling procedures (e.g. paging and Service Request).

Tracking Area Identity (TAI)This is the identity used to identify tracking areas. The Tracking Area Identity is constructed from theMobile Country Code (MCC), Mobile Network Code (MNC) and Tracking Area Code (TAC).The TAI is assigned to a number of cells grouped together for paging purposes.

9-36

LTE

LTE Procedures

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