One of the important activities performed by UE during RRC Idle mode is listening to paging message. The UE should listen to the paging message to know about incoming calls, system information change, and ETWS (Earthquake and Tsunami Warning Service) notification for ETWS capable UEs. In LTE, the UE needs to monitor PDCCH continuously (every 1ms) for P-RNTI to listen to paging message and this drains UE battery power heavily. Instead, if the UE is allowed to monitor P-RNTI in PDCCH, only at predetermined period say every 60ms or 100ms, it will reduce UE’s power consumption. The process of monitoring PDCCH discontinuously for P-RNTI to listen to paging message during RRC idle state is known as Discontinuous Reception (DRX) in RRC_Idle.

What is Idle mode for LTE UE?

  • No NAS Signaling connection exists between UE and core network
  • At least one PDN Connection exists
  • UE is registered with MME & EMM context exists at MME
  • UE location is known to the MME with an accuracy of a list of tracking areas
  • No RRC connection exists between UE and eNB
  • The eNB has no context information

Paging in Idle Mode

Paging is a procedure to transmit paging message to the UE in RRC Idle mode or EMM registered state. Paging message may be triggered either by MME in core network or eNB in Radio Access Network.

Discontinuous Reception for Paging

Paging information for the UE is being carried on the PDSCH in the resource blocks indicated by the PDCCH. Paging indication on the PDCCH is single fixed indicator (FFFE) called the Paging RNTI (P-RNTI). Different group of UEs monitor different sub frames for their paging messages.

UEs in idle mode monitor the PDCCH channel for P-RNTI used for paging to receive paging message from eNB. The UE needs to monitor the PDCCH only at certain UE specific paging occasion, i.e. only at specific subframe in specific radio frame. At other times the UE may go for sleep mode to save battery power.

Let us discuss how can UE find out paging frame and paging occasion for its paging message.

Paging Frame (PF): One Paging Frame (PF) is one Radio Frame in which the UE is looking for paging message. One Paging Frame may contain one or multiple Paging Occasion(s).

Paging Occasion (PO): It is a subframe where there may be P-RNTI transmitted on PDCCH addressing the paging message for the UE. There is always only one paging occasion for each UE in a DRX cycle.

To find out PF (Paging Frame) and PO (Paging Occasion), the UE uses DRX parameters broadcasted in the system information SIB2.

  

Significance of DRX Parameters:

DRX cycle: Indicates the number of radio frames in the paging cycle. Larger the value of this parameter, lesser the UE battery power consumption. Whereas smaller the value of DRX cycle, increase in the UE battery power consumption. DRX cycle may be cell specific or UE specific parameter. DRX cycle is cell specific when it is configured at eNB and broadcasts to all UEs in a cell in system information SIB2 as default paging cycle. DRX cycle is UE specific when MME may configure DRX cycle and provide it to UE in NAS signaling as’ UE specific DRX cycle’ and to eNB in PAGING S1 AP message as ‘Paging DRX’ for MME initiated paging message. Both UE and eNB use minimum of default and dedicated DRX cycle.

  

An UE with DRX cycle of 128 radio frames needs to wake up every 1.28 sec and look for paging message.

nB: This cell specific parameter indicates number of Paging occasions in a cell specific DRX cycle. Configuration of nB value depends on paging capacity required in a cell. The larger the value of nB is configured to, the larger the paging capacity. Likewise, the smaller the paging capacity when the value of nB is smaller.

  

With this we can infer that a maximum of 1 radio frame is available per DRX cycle of the UE for paging. Also, the minimum of 1 sub frame per radio frame (nB ≤ 1T) and maximum of 4 sub frames (for nB=4T) are available for paging message for the UE.

Calculation of Paging Frame Number

Both the eNB and the UE calculate the Paging Frame number by the following relation:

  

The eNB will receive UE_ID from the MME in PAGING S1AP message as “UE Identity Index Value” for MME initiated paging message.

The UE will find out the UE_ID by using the relation IMSI mod 1024 (Ref: TS 36.304 section 7.1)

The IMSI is given as sequence of digits of type Integer (0..9). The IMSI shall, in the formulae above, be interpreted as a decimal integer number, where the first digit given in the sequence represents the highest order digit.

For example: IMSI = 12 (digit1=1, digit2=2). In the calculations, this shall be interpreted as the decimal integer "12", not "1x16+2 = 18".

Calculation of Paging Occasion (Paging Sub frame Number in a Paging Frame)

  

  

Paging Occasion for the UE is obtained from the predefined subframe pattern (36.304 section 7.2, shown below for quick reference) based on the value of Ns, i_s. For example, if Ns=2 and i_s = 1, UE interprets that sub frames 4 and 9 are configured for paging in the cell and subframe 9 is paging occasion for it.

Subframe Patterns (Ref: TS 36.304 section 7.2)

FDD

  

TDD (All DL/UL Patterns)