Discontinuous Reception (DRX) in RRC IDLE

By | May 9, 2014

First Question is What is DRx?

DRx is the Discontinuous Reception in LTE.Even while there is no traffic between the network and UE, UE has to keep listening to Network. At least it should be ready to decode PDCCH. It means UE has to be “ON” all the time even when there is no traffic. But being ON all the time would drain the battery.

In LTE, DRX mode can be enabled in both RRC_IDLE and RRC_CONNECTED states.DRX is used to reduce 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 Signalling 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

Applicable when UE is in RRC_IDLE state.

  • The UE is registered with the evolved packet system mobility management (EMM_REGISTERED) but does not have an active session (ECM_IDLE).
  • In this state the UE can be paged.
  • UE monitors paging messages using idle mode DRX configuration.
  • Idle mode DRX configuration is broadcast within System Information Block 2(SIB2).
  • Idle mode DRX configuration is used to calculate Paging Frame(PF) and Paging Occation(PO).
  • One Paging Occasion (PO) is a subframe where there may be P-RNTI transmitted on PDCCH addressing the paging message.
  • One Paging Frame (PF) is one Radio Frame, which may contain one or multiple Paging Occasion(s).
  • When DRX is used the UE needs only to monitor one PO per DRX cycle.

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.

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