LTE Paging: Procedure, Mechanism, and Call Flow

Paging in LTE is a critical process that ensures efficient communication between the network and User Equipment (UE) in idle mode. It allows the network to notify a UE about incoming calls, messages, or data when it’s not actively connected to a specific cell. The LTE paging procedure minimizes network signaling overhead and UE power consumption, making it a vital feature in cellular communication.

In this article, we’ll explore the key aspects of paging, including its mechanism and procedures. We’ll also delve into the LTE paging call flow diagram, outlining the detailed steps and interactions between network elements such as the eNodeB, MME, and UE, to understand how the paging message is delivered effectively. We will discover fields used in LTE S1AP paging message and RRC paging message etc.

LTE Paging Call Flow

The LTE paging procedure can be used for the following:

• To initiate a mobile-terminated PS call.
• To initiate a mobile-terminated CS fallback call.
• To trigger an LTE UE to re-acquire system information.
• To provide an Earthquake and Tsunami Warning System (ETWS) indication.

Image: LTE paging procedure

As shown in the diagram, the MME is responsible for initiating the LTE paging procedure. The MME accomplishes this by forwarding an S1AP paging message to one or more eNodeBs. The contents and structure of the S1AP paging message are detailed below in Table 1.

Procedure and Mechanism of Paging in LTE

The LTE paging procedure applies to UEs in ECM IDLE state. UEs in this state are in RRC IDLE mode and don’t have S1 connectivity with the MME. The location of a UE in ECM IDLE state is known by the MME on a per-tracking area basis. The MME must forward the S1AP paging message to all eNodeBs within the relevant tracking area.

The MME forwards the paging message to multiple eNodeBs because a UE can be registered with more than one tracking area.

• As mentioned in the figure, the MME starts timer T3413 after sending the S1AP paging message for a PS data call, and the LTE UE is addressed by S-TMSI instead of IMSI.
• The eNodeB receives the S1AP paging message from the MME and constructs the RRC paging message. A single RRC message can carry information from multiple S1AP messages. A paging message can include multiple paging records to page multiple UEs. Table 2 below details the contents and structure of the RRC paging message.
• A UE in RRC IDLE mode checks for paging once every DRX cycle. The paging occasion within the paging frame defines the specific subframe during which an LTE UE checks for a paging message.
• The UE searches for P-RNTI within the PDCCH of the subframe belonging to the paging occasion. The P-RNTI has a value of FFFE and indicates that the UE may have a paging message on the PDSCH.
• If the UE finds the P-RNTI in the PDCCH, it will decode the resource allocation information.
• This information directs the UE to the PDSCH RBs where the paging message has been sent.
• The UE decodes the RRC message from the PDSCH RBs and checks the UE identity in all the records. If the UE doesn’t find its identity in the paging record, it will return to check the PDCCH for P-RNTI at each of the paging occasions.
• If the UE finds its identity, it will trigger the random access procedure to establish an RRC connection.
• The UE sends an RRC connection request message, and the eNodeB responds with an RRC connection setup message.
• If the LTE paging procedure is for a PS data call, the UE includes a service request NAS message within the RRC connection setup complete message.
• If the paging procedure is for a CS fallback call, the UE includes an extended service request NAS message within the RRC connection setup complete message.
• The eNodeB forwards the NAS message to the MME, which will stop T3413 if it is running and will proceed to establish a connection with the UE.
• A paging retransmission will be triggered if T3413 expires before the MME receives a NAS message from the UE.
• The UE checks for the RRC paging message for the SI modification flag and the ETWS flag. If the former is present, the UE reacquires BCCH SI. If the latter is present, the UE reads ETWS notifications in SIB10 and/or SIB11.

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

LTE S1AP Paging Message

Table 1 below lists the S1AP paging message contents.

The UE identity index is used at the eNodeB to calculate the paging frame and is defined as follows:

$UE\ identity\ index = UE\ IMSI\ mod\ 1024$

RRC Paging Message

Table 2 below lists the RRC paging message contents. It’s transferred using the PCCH logical channel, the PCH transport channel, and the PDSCH physical channel.

Paging Occasions in LTE

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

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

The paging frame occurs when the following equation is satisfied:

$SFN\ mod\ T = (T/N) \times (UE\_ID\ mod\ N)$

Where:

• $T$ = DRX cycle length in radio frames
• $N = Min(T, nB)$
• $nB$ is broadcast within SIB2 and can have values of ${4T, 2T, T, T/2, T/4, T/8, T/16, T/32}$
• Therefore, N can have values of ${T, T/2, T/4, T/8, T/16, T/32}$
• $UE\_ID = IMSI\ mod\ 1024$

The UE doesn’t need to check for all 1ms subframes within its paging frames. A UE only needs to check the subframe identified by its paging occasion.

Conclusion

Paging in LTE plays a pivotal role in optimizing network resources and ensuring smooth communication with idle UEs. By understanding the LTE paging mechanism and examining the call flow diagram, we gain valuable insights into how LTE networks achieve efficient signaling and connectivity. The process reduces unnecessary power consumption for UEs and network elements, making it a cornerstone of modern LTE technology.