LTE Paging Procedures

·      The paging procedure can be used to

·      initiate a mobile terminating PS data connection

·      initiate a mobile terminating SMS connection

·      initiate a mobile terminating CS fallback connection

·      trigger a UE to re-acquire system information

·      provide an Earthquake and Tsunami Warning System (ETWS) notification

·      provide a Commercial Mobile Alert Service (CMAS) notification

·      The paging procedure is applicable to UE in both:-

·      RRC Idle mode

·      RRC Connected mode.

·      For example :-

·      UE in RRC Idle mode could be paged to initiate a mobile terminating CS fallback connection.

·      while a UE in RRC Connected mode could be paged to signal a CMAS notification.

Paging procedure

·      Paging procedure can be initiated by either the MME or the eNode B.

·      MME can use the paging procedure to initiate a mobile terminating PS data connection

·       The eNode B could use the paging procedure to trigger a UE to re-acquire the system information

·      The MME initiated paging procedure also known as the Sl paging procedure

·      The MME starts the procedure by sending an Sl AP Msg.

·      Paging message to each eNode B with cells belonging to the relevant Tracking Area(s).

·       If the paging procedure is being used to establish a PS data connection and the UE is being addressed by its S-TMSI then the MME starts timer T3413 when sending the SlAP: Paging message.

·       Timer T3413 is not used when paging a UE for a CS fallback connection nor when paging an IMSI attached UE for an incoming SMS. Timer T3413 is also not used when addressing a UE by its IMSI (it is usual to address a registered UE by its S-TMSI. A UE may be paged by its IMSI during a network error recovery situation)

·      T3413 is a supervision timer for the paging procedure. Its expiry time is implementation dependent and is not specified by 3GPP.

·      The MME can re-attempt the paging procedure ifT3413 expires before a response is received

·      The content of the SlAP:

·      Infonnation Elements

·        UE Identity Index (0 to I 023)

·        UE Paging Identity (S-TMSI or IMSI)

·        Paging DRX (32, 64, 128, 256)

·        Core Network Domain (PS orCS)

·        List of Tracking Area Identities (T AI)( 1 to 256 instances)

·        List of Closed Subscriber Group (CSG) Identities

·        TAl CSGid(0 to 256 instances)

·        Paging Priority

·      The eNode B receives the SlAP:

·      Paging message from the MME and constructs an RRC: Paging message.

·      A single RRC: Paging message can include information from multipleS lAP: Paging messages, i.e. a single RRC:

·      Paging message can include multiple paging records to page multiple UE.

·      The RRC: Paging message is transferred using the PCCH logical channel, the PCH transport channel and the PDSCH physical channel. It uses transparent mode at the RLC layer.

·      Infonnation Elements

·        Paging Record

  List I Paging Record (1 to 16 instances)

  UE Identity (S-TMSI or IMSI)

  Core Network Domain (CS or PS)

·        System Infonnation Modification (True, False)

·        ETWS Indication (True, False)

·        CMAS Indication (True, False)

·      UE in RRC Idle mode check for paging messages once every Discontinuous Receive (DRX) cycle.

·      The Paging Occasion within the Paging Frame (PF) defines the specific subframe during which a UE checks for a paging message.

·      UE in RRC Connected mode do not have specific Paging Occasions during which they need to check for paging messages.

·      UE in RRC Idle mode search for the P-RNTI within the PDCCH of the subframe belonging to the Paging Occasion.

·      The P-RNTI has a single fixed value ofFFFE and serves as a flag to indicate that the UE may have a paging message on the PDSCH.

·      UE in RRC Connected mode also search for the P-RNTI within the PDCCH when checking for paging messages.

·      If a UE finds the P-RNTI within the PDCCH then it proceeds to decode the resource allocation information from within the PDCCH.

·      The UE decodes the RRC: Paging message from the PDSCH Resource Blocks and checks the UE identity within each of the paging records.

·      If the UE does not find its identity within a paging record then it returns to checking the PDCCH for the P-RNTI at each Paging Occasion

·      If the UE finds its identity within a paging record then it triggers the Random Access procedure to establish an RRCconnection.

·      The UE sends an RRC Connection Request message while the eNode B responds with an RRC Connection Setup message.

·      If the paging procedure is for a PS data connection, the UE includes a 'Service Request' Non-Access Stratum (NAS) message within the RRC Connection Setup Complete message

·      If the paging procedure is for a CS fallback connection, the UE includes an 'Extended Service Request' NAS message within the RRC Connection Setup Complete message.

·      The eNode B forwards the NAS message to the MME which then stops T3413 if it was running, and proceeds to establish a connection with the UE

·      Paging retransmission can be triggered if T3413 expires prior to the MME receiving a NAS message from the UE.

3GPP references: TS 36.304, TS 36.331, TS 24.301

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