INTRODUCTION
To support intra-LTE cell reselection, eNB broadcasts the System Information Block type 3 (SIB3), System Information Block type 4 (SIB4), and System Information Block type 5(SIB5). The UE shall monitor E-UTRAN BCCH during idle mode to retrieve these SIBs for the preparation of intra-LTE cell reselection.
Then, UE makes measurements on neighbouring cells based on the criteria and performs cell reselection to intra-/ inter-frequency neighbouring cells when needed.
The parameters for intra-LTE cell reselection broadcasted in SIB3, SIB4, and SIB5 are as follows:
SIB3 conveys the common information for intra-frequency, inter-frequency and/
or inter-RAT cell reselection.
SIB3 also conveys the specific information for intra-frequency cell reselection.
SIB4 conveys the intra-frequency neighbouring cell related information, that is, intra-frequency neighbour cell list and blacklisted cells.
SIB5 conveys the specific information for inter-frequency cell reselection.
BENEFIT
The operator can provide idle mobility to its subscribers within E-UTRAN.
The LTE users in idle state can be moving within E-UTRAN.
DEPENDENCY AND LIMITATION
N/A
FEATURE DESCRIPTION
PLMN Selection
When LTE UE is switched ON, it will start a process to find Public Land Mobile
Based on the request from NAS layer of UE, if a required PLMN is already associated with LTE, UE shall scan LTE carriers based on UE stored information.
The UE shall search for the strongest PLMN cell and tune to the Physical
Downlink Shared Channel (PDSCH to read SIB1(s), where PLMN information is delivered. The PLMN which is reported to NAS shall have its measured RSRP value. Once PLMN (high quality or otherwise) is selected, UE access stratum will be instructed to measure reference signal and read the PDSCH for SIB1. This process occurs again to initiate cell selection using the S-Criteria (based on Q_RX_LEV_MIN). At this stage if the S-criteria is not met, UE will go into limited service (for emergency calls) or will find an equivalent PLMN.
The following figure shows the idle mode state procedure:
Selected PLMN available/unavailable: The UE scans all RF channels in EUTRAN band according to its capabilities to find available PLMNs.
Not camped: No suitable cell found.
Camped normally: The UE obtains normal service and performs the following tasks:
oSelect and monitor the PCH of the cell.
oPerforms system information monitoring.
oPerform necessary measurements for the cell reselection evaluation procedure.
oExecute the cell reselection evaluation procedure.
Camped on any cell: The UE obtains limited service and periodically searches for a suitable cell in the selected PLMN, if UE supports.
Cell selection: The UE selects a suitable cell and the radio access mode based on idle mode measurements and cell selection criteria.
Cell reselection: If after cell reselection evaluation process a better cell is found, the cell reselection is performed. If no suitable cell is found, UE enters to the next state 'Any cell selection'.
Any cell selection: The UE searches an acceptable cell of any PLMN to camp on The following table shows the parameters for PLMN selection:
Parameter Name Description
Q_RX_LEV_MIN Minimum required RX level in the cell (dBm) (SIB1)
PLMN MCC and MNC (SIB1)
Cell Selection
Initial Cell Selection
The following figure shows initial cell selection procedures:
The UE scans all RF channels in E-UTRAN bands to its capability to find acceptable cells, which are not barred and measure RSRP value greater than or equal to -110 dBm. To read PLMN identity and decide the availability of the cell, UE shall detect Primary/Secondary synchronization signals (PSS/SSS) and decode cell specific reference signal (CRS) and read at least MIB and SIB1. PCID should not be overlapped between adjacent cells for successful detecting and decoding of the signals. The PLMN reading will be reported to NAS layer, and the search for PLMNs may be stopped on request of NAS.
Once UE has selected the PLMN, the cell selection procedure shall be performed to select a suitable cell of that PLMN to camp on to access available services, as described in TS36.304. If UE has stored information of carrier frequencies and also (optionally) information on cell parameters from previously received measurement, UE can use this information to speed up the selection procedure.
The suitable cell should satisfy that:
The cell is not barred
The cell is part of the selected PLMN or the registered PLMN or a PLMN of an equivalent PLMN list
The cell is part of at least one TA that is nor port of the list of 'forbidden tracking
Cell Barring
The LTE E-UTRAN cells broadcast cell selection information through SIB1 and SIB2 (AC-Barring).
SIB1 has two fields for cell status indication:
cellBarred
cellReservedForOperatorUse
cellBarred is common for all PLMNs and cellReservedForOperatorUse is specific per PLMN.
When cell status is indicated as 'not barred' and 'not reserved' for operator use, all UEs shall treat this cell as candidate during the cell selection and cell reselection procedures.
When cell status is indicated as 'not barred' and 'reserved' for operator use for any PLMN,
The UEs assigned to Access Class 11 or 15 operating in their HPLMN/EHPLMN shall treat this cell as candidate during the cell selection and reselection procedures if the field cellReservedForOperatorUse for that PLMN set to 'reserved'.
The UEs assigned to an Access Class in the range of 0 to 9, 12 to 14 shall behave as if the cell status is 'barred' in case the cell is 'reserved for operator use' for the registered PLMN or the selected PLMN.
When cell status 'barred' is indicated or to be treated as if the cell status is 'barred', UE is not permitted to select/reselect this cell, not even for emergency calls.
Cell Selection Criteria
The cell selection is performed on the detected cell with RX signal and decoded MIB and SIBs.
Cell selection criteria:
Srxlev > 0 AND Squal > 0
Where, Srxlev = Qrxlevmeas - (Q_RX_LEV_MIN + Q_RXLEV_MIN_OFFSET) - Pcompensation, Squal = Qqualmeas - (Q_QUAL_MIN + Q_QUAL_MIN_OFFSET) The following table shows the parameters of cell selection criteria:
Parameter Name Description
Srxlev Cell selection RX level value (dB)
Squal Cell selection quality value (dB)
Qrxlevmeas Measured cell RX level value (RSRP)
Qqualmeas Measured cell quality value (RSRQ)
Q_RX_LEV_MIN Minimum required RX level in the cell (dBm) (SIB1) Q_QUAL_MIN Minimum required quality level in the cell (dB) (SIB1)
Q_RXLEV_MIN_OFFSET Offset to the signalled Q_RX_LEV_MIN taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN (SIB1)
Q_QUAL_MIN_OFFSET Offset to the signalled Q_QUAL_MIN taken into account in the Squal
Parameter Name Description
evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN (SIB1)
Pcompensation Max (PEMAX -PPowerClass, 0) (dB)
P_MAX Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) defined as P_MAX in [TS 36.101] (SIB1)
PPowerClass Maximum RF output power of the UE (dBm) according to the UE power class
as defined in [TS 36.101]
Since Q_QUAL_MIN and Q_QUAL_MIN_OFFSET are not provided in network, devices will test Srxlev only.
If q-QualMinWB (in SIB1/SIB3/SIB5) is present, UE shall, when performing RSRQ measurement, use a wider bandwidth.
Cell Reselection
The following figure shows initial cell reselection procedures:
When a cell reselection condition is met, UE in idle mode shall attempt to detect, synchronize, and read system information of candidate frequencies. The UE shall only perform cell reselection evaluation for E-UTRAN frequencies and inter-RAT frequencies that are given in system information and for which UE has a priority provided.
The cell reselection procedures are triggered when one of the following conditions is met:
1 The serving cell does not fulfill Srxlev > S_INTRA_SEARCH_P and Squal >
S_INTRA_SEARCH_Q. In this case, UE performs intra-frequency cell reselection procedures.
3 The service cell does not fulfil Srxlev > S_NON_INTRA_SEARCH_P and Squal
> S_NON_INTRA_SEARCH_Q. In this case, UE performs inter-RAT cell reselection procedures for E-UTRA inter-frequency or UTRA frequency with an equal or lower reselection priority than the reselection priority of the current E-UTRA frequency.
Since RSRQ related parameters are not provided in network, devices will test Srxlev only. The device will use S_INTRA_SEARCH and
S_NON_INTRA_SEARCH instead of S_INTRA_SEARCH_P and S_NON_INTRA_SEARCH_P respectively.
The following table shows the parameters that trigger cell reselection procedures:
Parameter Name Description
Srxlev Cell selection RX level value (in dB) measured by UE
Squal Cell selection quality value (in dB) measured by UE
S_INTRA_SEARCH This specifies the Srxlev threshold (in dB) for intra-frequency measurements.
This parameter is used by Rel-8 device (SIB3).
S_INTRA_SEARCH_P This specifies the Srxlev threshold (in dB) for intra-frequency measurements.
This parameter is used by Rel-9 device (SIB3).
S_INTRA_SEARCH_Q This specifies the Squal threshold (in dB) for intra-frequency measurements.
This parameter is used by Rel-9 device (SIB3).
S_NON_INTRA_SEARCH This specifies the Srxlev threshold (in dB) for E-UTRAN inter-frequency and inter-RAT measurements. This parameter is used by Rel-8 device (SIB3).
S_NON_INTRA_SEARCH_P This specifies the Srxlev threshold (in dB) for E-UTRAN inter-frequency and inter-RAT measurements. This parameter is used by Rel-9 device (SIB3).
S_NON_INTRA_SEARCH_Q This specifies the Squal threshold (in dB) for E-UTRAN inter-frequency and inter-RAT measurements. This parameter is used by Rel-9 device. (SIB3).
Q_RX_LEV_MIN This specifies the minimum required Rx level in the cell in dBm (SIB3).
Q_QUAL_MIN_REL9 This specifies the minimum required quality level in the cell in dB. This parameter is used by Rel-9 device (SIB3).
Thresholds and Priority Design
In network, cell reselection triggering thresholds and priority shall be configured.
Dur to that, UEs can select LTE network as a primary network in the presence of an acceptable LTE signal. In network, RSRP is used as a measurement triggering criteria because RSRQ can vary even in the center of the serving cell from -3 dB to -10 dB depending on traffic load from the serving cell. S_INTRA_SEARCH shall be greater than S_NON_INTRA_SEARCH so that LTE capable UEs can select LTE frequency as long as it moves under LTE coverage.
The following figure shows the thresholds for cell reselection:
The UE triggers measurement of intra-frequency when the RSRP signal strength from LTE serving cell decreases below the threshold calculated as follows:
RSRP Strength from Serving Cell =< S_INTRA_SEARCH + Q_RX_LEV_MIN + Q_RXLEV_MIN_OFFSET + Pcompensation
Where, Pcompensation is max (PEMAX -PPowerClass, 0) (dB). PEMAX is defined as PMAX in 3GPP TS36.101, and PPowerClass is 23 dBm as per 3GPP TS36.101. (118
dBm).Therefore, Pcompensation is usually assumed to be 0.
The UE triggers measurement of UTRA frequency when the RSRP signal strength from LTE serving cell decreases below the threshold calculated as follows:
RSRP Strength from Serving Cell =< S_NON_INTRA_SEARCH + Q_RX_LEV_MIN + Q_RXLEV_MIN_OFFSET + Pcompensation
The UE will start measurements of LTE frequency when the measured RSRP is less than -64 dBm [Q_RX_LEV_MIN=-63(-126 dBm),
Q_RXLEV_MIN_OFFSET=0, S_INTRA_SEARCH=31(62 dB), Pcompensation=0], and start the measurements of UTRA frequency when the measured RSRP is less than -112 dBm [Q_RX_LEV_MIN=-63(-126 dBm),
Q_RXLEV_MIN_OFFSET=0, S_NON_INTRA_SEARCH=7(14 dB), Pcompensation=0).
In order for UEs to select primarily LTE frequency when UEs end a CSFB call or when UEs come back into LTE coverage, LTE frequency priority must be greater than UTRA frequency.
The priority of each frequency is broadcasted in SIB3 (E-UTRA frequency).
Intra-Frequency Cell Reselection
Rn = Qmeas,n - Q_OFFSET_FREQ
The following table shows the description of above mentioned parameters:
Parameter Name Description
Rs Rs is for serving cell.
Rn Rn is for neighbour cell.
Qmeas RSRP measurement quantity used in cell reselections.
Q_HYST This parameter (in dB) is to reduce Ping-Pong effects between serving and neighbor cells.(SIB3)
Q_OFFSET_FREQ In case of intra-frequency: Equals to Qoffsets,n, if Qoffsets,n is valid, otherwise this equals to zero.
T_RESELECTION This specifies the reselection timer value for EUTRAN (SIB3).
The UE shall perform ranking of all cells that fulfill the cell selection criterion S.
The cells shall be ranked according to the R criteria specified above, deriving Qmeas,n and Qmeas,s and calculating the R values using averaged RSRP results. If a cell is ranked as the best cell, UE shall perform cell reselection to that
corresponding cell.
The UE shall reselect the new cell, only if the following conditions are met:
The new cell is better ranked than the serving cell during a time interval T_RESELECTION.
More than 1 second has elapsed since UE camped on the current serving cell.
Initial Attach
When UE camps on a suitable cell, if new cell does not belongs to at least tracking areas to which UE is registered previously, the UE will register to the network by sending a Tracking Area Update message.
The following figure shows the initial attach procedures:
1~4) Step 2~4 completes RRC connection establishing of a SRB. The attach procedure starts with RRC connection establishment procedure. The Attach Request message included in RRCConnectionSetupComplete is transparently delivered to MME in INITIAL UE MESSAGE.
15~16) The eNB sends RRCConnectionReconfiguration message to data radio bearer. After eNB receives CONTEXT SETUP REQUEST message from MME, it creates a default radio bearer by sending RRCConnectionReconfiguration message to UE. When UE receives RRCConnectionReconfiguration message, it can
transmit packets in uplink and eNB can deliver the packets toward S-GW.
17) The eNB sends Initial Context Setup Response message to MME and completes the establishment of S1 bearer.
18~19) The UE sends ULInformationTransfer message to eNB, which includes Attach Complete message. This message is transparently delivered to MME in UPLINK NAS TRANSPORT message.
20~21) The MME sends Modify Bearer Request message to S-GW, to provide downlink tunnel information of eNB. After S-GW receives the Modify Bearer Request message, it can transmit packets in downlink.
If both DRB and SRB carry no packets in downlink and uplink for a certain time period, eNB releases the RRC connection and S1 bearer. The operator can configure INTERNAL_SIGNALING_INACTIVITY for a signalling bearer and INTERNAL_USER_INACTIVITY for a data bearer at eNB level.
When both inactivity timers expire, eNB sends UE CONTEXT RELEASE message to MME and releases the S1 connection for UE. Also, this message shall indicate the cause value 'User Inactivity'.
The following figure shows the connection release procedure by the inactivity timer triggered:
Combined EPS and IMSI Attach
When supporting Combined EPS/IMSI Attach Request, MME selects IWF (MSC/VLR) based on TA and LA mapping and sends the location update request with new LAI, IMSI and MME name to IWF. On receiving the request of the respective VLR will create an association for SGs interworking with MME. In response, VLR will provide VLR TMSI to MME.
The following figure shows the combined EPS/IMSI attach call flow:
The combined EPS/IMSI Attach procedures are:
1~5) The UE sends an Attach Request to MME with Attach Type as „Combined EPS/IMSI‟, UE capability as „CSFB‟ and data APN name. The APN name is depending on the subscriber type. The UE may include any of the Internet APN.
6) The MME sends authentication information request message to HSS. After
9~10) Since UE has requested for combined EPS/IMSI attach, after the default bearer establishment MME updates UE‟s location in 3G network by sending the location update message with new LAI, IMSI and MME name to IWF
(MSC/VLR). After accepting the attach request by the network, default bearer will be established. IWF updates UE‟s CS location in HLR.
11~14) The eNB acquires UECapabilityInformation and reports it to MME.
15~16) The eNB sends the integrity-protected AS Security Mode Command message to UE. Then, UE starts control plane signalling integrity.
17~18) The eNB sends RRCConnectionReconfiguration message to data radio bearer. After eNB receives CONTEXT SETUP REQUEST message from MME, it creates a default radio bearer by sending RRCConnectionReconfiguration message to UE. When UE receives RRCConnectionReconfiguration message, it can
transmit packets in uplink and eNB can deliver the packets toward S-GW.
19) The eNB sends Initial Context Setup Response message to MME and completes the establishment of S1 bearer.
20~21) The UE sends ULInformationTransfer message to eNB, which includes Attach Complete message. This message is transparently delivered to MME in UPLINK NAS TRANSPORT message.
22~23) The MME sends Modify Bearer Request message to S-GW, to provide downlink tunnel information of eNB. After S-GW receives the Modify Bearer Request message, it can transmit packets in downlink.
Combined EPS and IMSI Detach
To detach the combined EPS/IMSI attached UE, the UE is required to be detached from both EPS and CS domain.
The following figure shows the combined EPS/IMSI detach call flow:
The combined EPS/IMSI Detach procedures are:
1~2) The UE sends Detach Request to MME.
3) The MME sends Delete Session Request message to S-GW for deactivating the default bearer for UE. The S-GW forwards the Delete Session Request message to P-GW.
4) The IMSI Detach Indication message from MME to IWF (MSC/VLR) so as to remove the SGs association with regarding to UE IMSI.
5) The P-GW replies with Delete Session Response to MME.
6~8) The MME sends Detach Accept to UE and releases S1-MME signalling connection.
Related SIB Messages
SIB2 contains radio resource configuration information that is common for all UEs.
The following table shows the SIB2 message:
ac-BarringInfo ac-BarringForEmergency
ac-BarringForMO-Signalling (TAU, Attach/Detach message)
ac-BarringforMO-Data (Service Request, Extended Service Request messages)
freqInfo ul-CarrierFreq
ul-Bandwidth
additionalSpectrumEmission
radioResourceConfigCommonSIB rach-config, bcch-config, pcch-config, prach-config, pdsch-config, pusch-config, and pucch-config
UL-CyclicPrefixLength
uplinkPowerControlCommon ue-TimersAndConstants
timeAlignmentTimerCommon (to control how long UE is considered uplink time aligned) mbsfn-SubframeConfigLit
SIB3 contains cell re-selection information common for intra-frequency, inter-frequency and/ or inter-RAT cell re-selection.
The following table shows the SIB3 message:
cellReselectionInfoCommon q-Hyst
speedStateReselectionPars (Q-hysteresis scaling factor depending on UE speed)
cellReselectionServingFreqInfo s-NonIntraSearch
threshServingLow
cellReselectionPriority intraFreqCellReselectionInfo q-RxLevMin
t-ReselectionEUTRA-SF (speed dependent scale factor)
SIB4 contains neighbouring cell related information relevant only for intra-frequency cell re-selection. Also, it includes cells with specific re-selection parameters as well as blacklisted cells.
The following table shows the SIB4 message:
intraFreqNeighbCellList (List of intra-frequency neighbouring cells with specific cell re-selection parameters, up to 16)
physCellId
q-OffsetCell (Qoffsets,n, the offset between the two cells)
intraFreqBlackCellList (List of blacklisted intra-frequency neighbouring cells, up to 16)
SIB5 contains information for inter-frequency cell re-selection.
The following table shows the SIB5 message:
InterFreqCarrierFreqInfo (list of frequency information up to 8)
dl-CarrierFreq
q-RxLevMin
p-Max
t-ReselectionEUTRA
t-ReselectionEUTRA-SF
threshX-High (cell reselection to a cell on a higher priority E-UTRAN frequency or inter-RAT frequency than the serving frequency if a cell of a higher priority RAT/ frequency fulfils Srxlev
> ThreshX, HighP during a time interval TreselectionRAT)
threshX-Low (cell reselection to a cell on a lower priority E-UTRAN frequency or inter-RAT frequency than the serving frequency if the serving cell fulfils Srxlev < ThreshServing, LowP and a cell of a lower priority RAT/ frequency fulfils Srxlev > ThreshX, LowP during a time interval TreselectionRAT)
allowedMeasBandwidth
presenceAntennaPort1 (to indicate whether all the neighbouring cells use Antenna Port 1)
cellReselectionPriority
neighCellConfig (MBSFN and TDD related information)
q-OffsetFreq (Qoffsetfrequency, Frequency specific offset for equal priority E-UTRAN frequencies)
interFreqNeighCellList (up to 16)
physCellId
q-OffsetCell (Qoffsets,n , the offset between the two cells)
interFreqBlackCellList (up to 16)
SYSTEM OPERATION
How to Activate
The Idle Mobility Support is a collective feature with which UE in Idle State
The Idle Mobility Support is a collective feature with which UE in Idle State