LTE_Handovers & Events

LTE RRC/RRM May 2012 Bong Youl (Brian) Cho, [email protected]

Contents • LTE RRC/RRM • UE State Model • Call Handling and Bearer Management • Handover

TTA LTE/MIMO Standards/Technology Training

Contents • LTE RRC/RRM • UE State Model • Call Handling and Bearer Management • Handover


LTE RRC/RRM



Time Scale of RRM Functions


RRC Functions • • • • • • • • •

System Information Broadcast (eNB) Radio Resource Management (eNB) Connection Management (eNB) Measurement Configuration and Mobility Control (Handover) (eNB) Paging (MT-Access, System Info Modification, ETWS) (eNB) AS Security (eNB) Transport NAS messaging (UE and eNB) System Selection (UE) Cell Reselection (UE)


System Information • MIB defines the most essential physical layer information of the cell required to receive further system information –

Cell bandwidth, SFN, etc

• SIB1 contains information relevant when evaluating if a UE is allowed to access a cell and defines the scheduling of other system information blocks –

plmn-IdentityList, cellReservedForOperatorUse, trackingAreaCode, cellBarred, csg-Indication, si-Periodicity, sib-MappingInfo, si-WindowLength, etc

• SIB2 contains common and shared channel information –

Lower layers (including RACH) configuration information: RACH parameters, preamble information, and UE timers

• SIB3 contains cell re-selection information, mainly related to the serving cell • SIB4 contains information about the serving frequency and intra-frequency neighbouring cells relevant for cell re-selection (including cell re-selection parameters common for a frequency as well as cell specific re-selection parameters)

• SIB5 contains information about other E-UTRA frequencies and interfrequency neighbouring cells relevant for cell re-selection (including cell reselection parameters common for a frequency as well as cell specific reselection parameters) TTA LTE/MIMO Standards/Technology Training

System Information

– cont’d

• SIB6 contains information about UTRA frequencies and UTRA neighbouring cells relevant for cell re-selection (including cell re-selection parameters common for a frequency as well as cell specific re-selection parameters)

• SIB7 contains information about GERAN frequencies relevant for cell reselection (including cell re-selection parameters for each frequency)

• SIB8 contains information about CDMA2000 frequencies and CDMA2000 neighbouring cells relevant for cell re-selection (including cell re-selection parameters common for a frequency as well as cell specific re-selection parameters)

• • • • •

SIB9 contains a home eNB identifier (HNBID) SIB10 contains an ETWS primary notification SIB11 contains an ETWS secondary notification SIB12 contains an CMAS notification SIB13 contains MBMS related information


System Selection/Registration Information on USIM HPLMN on RAT basis CSG Identities list Forbidden PLMN list RPLMN etc…

Information on ME UE Category, RAT support Frequency band Forbidden PLMNs/TAs/Las MRU Acquisition information Barred Cells/Barred Frequencies etc…

Information written to USIM Registered PLMN Forbidden PLMN list


* Qualcomm

Overall UE Camping Procedure (1) PLMN selection Read USIM Read stored info on ME Select Band, PLMN, etc

NAS

RRC

(2) Trigger System Acquisition

(4) Schedule Broadcast Control Channel read

AS

PHY

(3) Acquisition Scan Band/Freq


(10) Service Obtained (Camped)

(6) Process SIB1 Check PLMN Is Cell reserved? Is CSG Id valid? Cell belong to Forbidden TA? Cell barred? If fail, go back to (3). If ok, go to (7).

(5) Read MIB/SIB1 Using SI-RNTI

(7) SIB2 and other SIBs

(8) All SIBs obtained

(9) Cell is selected and UE camps

Cells and Services Categories of Cell • • • • •

Acceptable*: May “camp” to obtain Limited service Suitable*: Can “camp” to obtain Normal service Reserved: UEs with AC 11 & 15 are allowed to “camp” in HPLMN Barred: Not available for “camping” CSG: Only UE of Closed Subscriber Group can “camp”

Requirements for cell selection on a “Suitable Cell” • • • • •

Part of Selected, Registered, or Equivalent PLMN Not barred Park of Tracking Area that is not Forbidden CSG ID mush be from the allowed CSG list Must satisfy the cell selection criteria

Types of Services • Normal: Receive Paging and can transition to Connected state • Limited**: Emergency calls and ETWS • Operator: For operators only on reserved cell * Cell that is “Acceptable” to one UE can be a “Suitable” for another UE and vice versa. ** UE in “Limited Service” periodically scans system to obtain “Normal Service”


Cell Selection Criteria Srxlev > 0 AND Squal > 0

where: Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation Squal = Qqualmeas – (Qqualmin + Qqualminoffset) Srxlev Squal Qrxlevmeas Qqualmeas Qrxlevmin Qqualmin Qrxlevminoffset

Qqualminoffset

Pcompensation PEMAX_H PPowerClass

Cell selection RX level value (dB) Cell selection quality value (dB) Measured cell RX level value (RSRP) Measured cell quality value (RSRQ) Minimum required RX level in the cell (dBm), obtained in SIB1 Minimum required quality level in the cell (dB), obtained in SIB1 Offset to the signalled Qrxlevmin 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, obtained in SIB1 Offset to the signalled Qqualmin taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN, obtained in SIB1 max(PEMAX_H –PPowerClass , 0) (dB) Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) defined as PEMAX_H in [TS 36.101], obtained in SIB1 Maximum RF output power of the UE (dBm) according to the UE power class as defined in [TS 36.101]


UE State Model


EMM state model • EPS mobility management state model – The UE enters the EMM-REGISTERED state by a successful registration procedure which is either an Attach procedure or a Tracking Area Update procedure


ECM state model • EPS connection management state model – For a UE in ECM-CONNECTED state, a signaling connection exists between the UE and the MME. This signaling connection consists of two parts:  RRC connection (in UE)

 UE-associated signaling connection across the S1_MME called UE-associated logical S1-

connection (in MME)


RRC Connection RRC Connection established

RRC-Idle

RRC-connected RRC Connection released

RRC-Idle • • •

A UE specific DRX may be configured by upper layers. UE controlled mobility; (Cell selection/reselection, TA update) 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.

RRC-Connected • • • •

Transfer of unicast data to/from UE. At lower layers, the UE may be configured with a UE specific DRX. Network controlled mobility, i.e. handover; The UE: – Monitors a Paging channel and/ or System Information Block Type 1 contents to detect system information change, for ETWS capable UEs, ETWS notification, and for CMAS capable UEs, CMAS notification; – Monitors control channels associated with the shared data channel to determine if data is scheduled for it; – Provides channel quality and feedback information; – Performs neighbouring cell measurements and measurement reporting; – Acquires system information.


Call Handling and Bearer Mgmt


Connection Establish and Release Procedure


E-RAB Setup


E-RAB Release


UE Context Modification


Handover


LTE Handover • LTE uses UE-assisted network controlled handover – UE reports measurements; network decides when handover and to which Cell – For search and measurement of inter-frequency neighboring cells only carrier frequency need to be indicated

• X2 interface used for HO preparation and forwarding of user data – Target eNB prepares handover by sending required information to UE transparently through source eNB as part of the Handover Request ACK message – Buffered and new data is transferred from source to target eNB until path switch  preventing data loss – UE uses contention-free random access to accelerate handover


Mobility Measurement Metrics • Metrics within Events tell UE what to measure • E-UTRA metrics – Reference signal received power (RSRP) – Reference signal received quality (RSRQ)

• UTRA metrics – UTRA FDD CPICH RSCP – UTRA FDD carrier RSSI – UTRA FDD CPICH Ec/No

• GSM metrics – GSM carrier RSSI

• CDMA2000 metrics – CDMA2000 1xRTT pilot strength – CDMA2000 HRPD pilot strength


Mobility Measurement Reporting Event Reporting Event

Purpose

A1

Serving becomes better than threshold

A2

Serving becomes worse than threshold

A3

Neighbor becomes offset better than serving cell + extra margin

A4

Neighbor becomes better than threshold

A5

Serving becomes worse than threshold1 AND neighbor becomes better than threshold2

B1

Inter RAT neighbor becomes better than threshold

B2

Serving becomes worse than threshold1 AND inter RAT neighbor becomes better than threshold2

Periodical Reporting • Report strong cells periodically regardless, if configured by network TTA LTE/MIMO Standards/Technology Training

Inter-eNB Handover


Inter-eNB Handover (X2-based) 1. The source eNodeB makes the decision to handover the UE to the target eNodeB based on the MEASUREMENT REPORT of the UE and RRM information.

2. The source eNodeB issues a HANDOVER REQUEST message via the X2 interface to the target eNodeB which passes necessary information to prepare the handover at the target side. This message includes signalling references, transport layer addresses and tunnel endpoint identifiers to enable the target eNodeB to communicate with the source eNodeB and the EPC nodes, as well as QoS information for the UE's bearers and RRM information.

3. Admission Control is performed by the target eNodeB dependent on the received radio bearer QoS information and S1 connectivity to increase the likelihood of a successful handover. If the resources can be granted by the target eNodeB, it configures the required resources according to the received UE context information, and reserves a C-RNTI (cell radio network temporary identifier) and a dedicated preamble for the UE.

4. The target eNodeB prepares the handover regarding layer 1 and layer 2 and sends a HANDOVER REQUEST ACKNOWLEDGE message via X2 to the source eNodeB. The HANDOVER REQUEST ACKNOWLEDGE message includes a transparent container to be sent to the UE later as part of the CONNECTION RECONFIGURATION message. The container includes the new C-RNTI and the value of the dedicated preamble to be used by the UE to synchronise with the target cell as well as other parameters required by the UE.


Inter-eNB Handover (X2-based) 5. The source eNodeB sends a CONNECTION RECONFIGURATION message towards the UE, which includes the transparent container (of the previous step) received from the target eNodeB.

6. The SN STATUS TRANSFER message is sent from the source to the target eNodeB. Thereby PDCP layer information is transferred to ensure uplink and downlink PDCP SN continuity for every bearer that requires PDCP status preservation.

7. Some time after sending the CONNECTION RECONFIGURATION message to the UE ( and possibly before sending the SN STATUS TRANSFER message to t he target eNodeB), the source eNodeB begins forwarding user data in the form of PDCP SDUs using the resources set up previously and continues as long as packets are received at the source eNB from the EPC.

8. When the UE receives the CONNECTION RECONFIGURATION message with the necessary parameters (i.e. new C-RNTI, dedicated preamble, target cell ID etc.) it is commanded by the source eNodeB to perform the handover immediately to the target cell. The UE then performs the non-contention based random access procedure.

9. The random access response conveys timing alignment information and initial uplink grant for handover.

10. When the UE has successfully accessed the target cell, it sends the CONNECTION RECONFIGURATION COMPLETE message (containing its new C-RNTI) to the target eNodeB to indicate that the handover procedure is completed for the UE. TTA LTE/MIMO Standards/Technology Training

Inter-eNB Handover (X2-based) 11. If a new “Measurement Configuration” is to be sent to the UE, it is sent in a separate CONNECTION RECONFIGURATION message.

12. The target eNodeB sends a PATH SWITCH REQUEST message to the MME to inform it that the UE has been handed over to another eNodeB.

13. The MME sends a USER PLANE UPDATE REQUEST message to the S-GW, which includes the target eNodeB's TEID(s) received before to enable the user data path to be switched from the source to the target eNodeB.

14. The S-GW switches the downlink data path to the target eNodeB. Before the S-GW can release any U-plane/TNL resources towards the source eNodeB, it sends one or more “end marker” packet(s) to the source eNodeB as an indication that the downlink data path has been switched.

15. The S-GW sends a USER PLANE UPDATE RESPONSE message to the MME to confirm that it has switched the downlink data path.

16. The MME confirms the PATH SWITCH REQUEST message with the PATH SWITCH REQUEST ACK message.

17. By sending a UE CONTEXT RELEASE message, the target eNodeB informs the source eNodeB of the success of the handover and triggers the release of resources. The target eNodeB does not release its data forwarding tunnels from the source eNodeB until it has received an “end marker” packet.

18. Upon reception of the UE CONTEXT RELEASE message, the source eNodeB may forward any remaining PDCP SDUs TTA LTE/MIMO Standards/Technology Training

LTE_Handovers & Events

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