LTE Paging Procedure | S1AP, RRC paging in LTE call flow

This page describes LTE paging procedure and paging call flow. It covers paging in LTE including S1AP paging message, RRC paging message etc.

The LTE paging procedure can be used for the following:
•  To initiate mobile terminated PS call
•  To initiate mobile terminated CS fallback call
•  To trigger LTE UE to re-acquire system informations
•  To provide an Earthquake and Tsunami Warning System(ETWS) indication

LTE paging procedure

As shown in the figure, MME is responsible for the initiation of LTE paging procedure. MME does this by forwarding S1AP paging message to one or more eNodeB. The contents and structure of S1AP paging message is mentioned below in the table-1.

LTE Paging procedure

The LTE paging procedure is applicable to UE in ECM IDLE State. UE in this state are in RRC IDLE mode and do not have S1 connectivity with MME.

The location of a UE in ECM IDLE state is known by MME on a per tracking area basis. The MME has to forward S1AP paging message to all eNodeB within the relevant tracking area.

MME forward paging message to multiple eNodeBs as UE can be registered with more than a single tracking area.

•  As mentioned in the figure, MME starts timer T3413 after sending S1AP paging message for PS data call and LTE UE is addressed by S-TMSI instead of IMSI.

•  eNodeB receives S1AP paging message from MME and constructs RRC paging message. Single RRC can carry information from multiple S1AP. Paging message can include multiple paging records to page multiple UE. Table-2 below mentions contents and structure of RRC paging message.

•  UE in RRC IDLE mode checks for paging once every DRX cycle. paging occasion within the paging frame defines specific subframe during which a LTE UE checks for paging message.

•  UE searches forP-RNTI within PDCCH of subframe belong to paging occasion. P-RNTI has value of FFFE and indicates that UE may have a paging message on PDSCH.

•  UE finds P-RNTU in PDCCH, it will decode resource allocation information.

•  This information directs UE to PDSCH RB where in paging message has been sent.

•  UE decodes RRC message from PDSCH RBs and checks UE identity in all the records. If UE do not find its identity in paging record then it will return to check PDCCH for P-RNTI at each of the paging occasions.

•  If the UE find its identity, it will trigger random access procedure to establish RRC connection.

•  UE sends RRC connection request message and eNodeB responds with RRC connection setup message.

•  If the LTE paging procedure is for PS data call, UE includes service request NAS message within RRC connection setup complete message.

•  If the paging procedure is for CS fallback call, UE includes extended service request NAS message within RRC connection setup complete message.

•   The eNodeB forwards NAS message to MME which will stop T3413 if it is running and will proceed to establish connection with UE.

•  A paging retransmission will be triggered if T3413 gets expire prior to MME receiving a NAS message from UE.

•  UE checks for RRC paging message for SI modification flag and ETWS flag. If the former is present UE reacquires BCCH SI. If the later is present, UE reads ETWS notifications in SIB10 and/or SIB11.

REFERENCES: 3GPP TS 36.304, TS36.331, TS24.301

S1AP paging message in LTE

The table-1 below mentions S1AP paging message contents. UE identity index is used at eNodeB to calculate paging frame and is defined as follows:
UE identity index = UE IMSI mod 1024

Information Elements Value
UE Identity Index 0 to 1023
UE Paging Identity S-TMSI or IMSI
Paging DRX 32, 64, 128, 256
Core Network Domain PS or CS
List of Tracking Area Identities (TAI) 1 to256 instances
List of Closed Subscriber Group(CSG) Identities 0 to 256 instances

RRC LTE paging message

The table-2 below mentions RRC paging message contents. It is transferred using PCCH logical channel. PCH transport channel and PDSCH physical channel.

Information Elements Value
Paging Record List Paging Record( 1 to 16 instances) UE Identity(S-TMSI or IMSI)
Core Network Domain(CS or PS)
System Information Modification TRUE or FALSE
ETWS Indication TRUE or FALSE

Paging Occasions in LTE

LTE UE in RRC IDLE mode use DRX to reduce the power consumption. DRX cycle will determine how frequently UE will check for paging messages. DRX cycle is broadcast within SIB2. It can have values of 32, 64, 128 or 256 radio frames. These corresponds to time intervals of 320, 640, 1280 and 2560ms.

The UE can also propose its own DRX cycle length within ATTACH REQUEST and TRACKING AREA UPDATE REQUEST messages. The set of allowed values are same as used in SIB2.

Paging frame occurs when following equation satisfies:
SFN mod T = (T/N) x (UE_ID mod N)
T = DRX cycle length in radio frames
N = Min(T,nB)
nB is broadcast within SIB2 and can have values of {4T,2T,T,T/2,T/4,T/8,T/16,T/32} N can have values of {T,T/2,T/4,T/8,T/16,T/32}
UE_ID = IMSI mod 1024

UE do not need to check for all 1ms subframes within its paging frames. A UE only needs to check the subframe identified by its paging occasion.

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