LTE Paging

Paging procedure is applicable to UE in both in Idle and Connected mode, it can be initiated by either MME and eNB, below are the scenarios in which paging is initiated by eNB and MME respectively. Paging procedure can be used for below mentioned scenarios:

• Initiated to inform UE to re-acquire system information.
• Update on ETWS notification.
• Update on CMAS notification.
• Informing about incoming PS data connection
• Informing about incoming SMS
• To initiate an MT CS fallback

Paging procedure

• To receive paging, UEs in Idle mode monitors PDCCH channel for P-RNTI(FFFE)
• UE monitors the PDCCH at certain occasions (specific subframes and radio frames) as provided in SIB

• IF UE finds P-RNTI within PDCCH then it proceeds to decode the resource allocation information to find the resource block allocation in PDSCH for paging message.
• UE decodes the RRC paging message from the PDSCH resource blocks and check UE-Identity in the paging records.
• If UE finds its identity within a paging record, then it triggers RACH procedure to establish RRC connection.
• In connected mode, UE do not have any specific paging occasions which it needs to monitor for paging messages.
• However UEs check for paging in similar fashion as in case of Idle mode.

What are those specific Subframe and Frame at which UE monitors PDCCH for?

E-UTRAN configures which of the radio frames and subframes are used for paging which is configured using SIB2, The UE needs to calculate the radio frame (Paging Frame PF) ad subframe with that PF( Paging Occasion PO), When DRX is used the UE needs only to monitor one PO per DRX cycle, One thing to note is here is paging DRX is optional and if UE is using UE specific DRX cycle length the UE can specify the DRX cycle length in attach request or tracking area update request.

Paging Frame PF: Radio Frame, which may contain one or multiple Paging Occasion(s).

Paging Occasion PO: subframe where there may be P-RNTI transmitted on PDCCH addressing the paging message).

PF and PO is determined by following formulae using the DRX parameters provided in System Information:

PF is given by following equation:

SFN mod T = (T div N)*(UE_ID mod N)

The following Parameters are used for the calculation of the PF

• T: DRX cycle of the UE. T is determined by the shortest of the UE specific DRX value, if allocated by upper layers, and a default DRX value broadcast in system information. If UE specific DRX is not configured by upper layers, the default value is applied.
• nB: 4T, 2T, T, T/2, T/4, T/8, T/16, T/32
• N: min(T,nB)
• UE_ID: IMSI mod 1024.

One thing to note here is that T can be derived two different sources, one from the system information (SIB2, IE defaultPagingCycle) and the one from upper layer (DRX). If upper layer sends the DRX value, then it is given priority else UE must use the value from SIB2. And nB is also derived from SIB2.

Index i_s pointing to PO from subframe pattern defined in below table will be derived from following calculation:

i_s = floor(UE_ID/N) mod Ns

The following Parameters are used for the calculation of the i_s:

• Ns: max(1,nB/T)
• N: min(T,nB)
• UE_ID: IMSI mod 1024.

Looking at above table we can see that there will 1 PO if Ns is 1, 2 PO if Ns is 2 and 4 PO if Ns is 4