DRX (Discontinuous Reception)

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What is DRX and DRX active period? What channels does it monitor in active period?

Data transmission between UE and their base stations(eNodeB) uses radio frames with10ms duration, where in each radio frame consists of 10 1ms( 1 frame=10ms duration=10 subframe) subframes. Physical Downlink Control Channel (PDCCH) at the beginning of each subframe indicates whether there is data for UE to receive or not.
Devices has to monitor these PDCCHs in each subframe so that it can find out whether they carry any data to receive. Because devices do not receive data in every subframe, this monitoring process naturally leads to high battery consumption. 

What is the purpose of DRX?

The main purpose of DRX is to lower battery consumption when there is no UL/DL data. means, when there is no traffic(DL/UL) that time devices enter into sleep mode (with RF module turned off) for some period of time as configured by operator. When there is traffic, the UEs wake up for data reception/transmission. 

Where can you find information regarding DRX?

The network delivers this information to UE  through upper layer control message RRC reconfiguration message (during handover)  or System Information Block Type 2 (SIB2) broadcast-ed by eNodeB (During initial attach).

What are the different types of DRX?

There are two type of DRX process that can be used in either RRC-Idle or RRC-Connected state. When DRX is used in Idle state it is called Idle mode DRX and when in Connected state it is called Connected mode DRX (C-DRX).
Idle mode DRX: it is used for Paging DRX cycle.
Connected mode DRX (C-DRX):  There are two types in connected mode - Short DRX cycle, Long DRX cycle.

       When the UE is in RRC Connected state, it keeps monitoring PDCCH. During this period if  any DL Grant or downlink data for that UE is received then that time  DRX inactivity timer and the main RRC Inactivity timer are restarted
       If there is UL grant for UE with DL Grant that time both DRX and RRC inactivity timers are restarted and after 4 ms UE sends data in uplink.
       The DRX Inactivity timer gets expired when there are no further grants for uplink or downlink. Although UE was constantly monitoring PDCCH, UE now enters in the short DRX cycle. If operator has only configured long DRX then UE directly enters in Long DRX, but in the above example both are configured. The current consumption drops and battery is conserved.
        When the DRX short cycle timer expires, UE end up  short DRX cycle and goes into  the long DRX cycle

       When there is no activity in UL or DL for the duration of RRC Inactivity timer, the timer expires and the UE enters into RRC IDLE state. In this state, UE uses paging DRX cycle.

How eNodeB maximizes resource utilization using DRX?

The UE would be transmitting frequent periodic CSI or SRS (if configured by operator) without DRX.

With DRX, during OFF periods, the UE is not allowed to transmit Periodic CSI or SRS, so to maximize resource utilization, the eNB can assign these resources to the other UEs.

What things are needed to configure for DRX? 

DRX is controlled by RRC by configuring the following timers:

  1. onDurationTimer
  2. drx-InactivityTimer
  3. drx-RetransmissionTimer (one per DL HARQ process except for the broadcast            process),
  4.  drx-ULRetransmissionTimer (one per asynchronous UL HARQ process)
  5. the longDRX-Cycle
  6. drxStartOffset
  7. drxShortCycleTimer(optional)
  8. shortDRX-Cycle(optional)

  (3GPP TS 36.321 version 13.3.0 Release 13)
        onDurationTimer: the number of consecutive PDCCH-subframes to monitor at the beginning of each DRX Cycle (DRX ON). Range: PDCCH subframe 1 to PDCCH subframe 200.
        drx-InactivityTimer: The number of consecutive PDCCH subframe(s) to monitor after successfully decoding a PDCCH indicating scheduling info. Range: PDCCH subframe 1 to PDCCH subframe  2560.
        drx-RetransmissionTimer: The maximum number of consecutive PDCCH-subframe(s) till a DL retransmission is received.
Its Range: PDCCH subframe 1 to PDCCH subframe 33.
        drxstartoffset: the subframe where the DRX cycle starts.
        drxShortCycleTimer: number of time short DRX cycle is repeated. Range(1 to 16)
shortDRX-cycle: the drx cycle running only within drxshortcycletimer period at the expiration of DRX-inactivity timer.

Where can you find CDRX value?

Network informs UE of this timing using RRC Connection Reconfiguration or RRC Connection Setup msg.
drx-Config: setup (1)
                         onDurationTimer: psf7 (5)
                         drx-InactivityTimer: psf19o0 (20)
                         drx-RetransmissionTimer: psf20 (5)
                         longDRX-CycleStartOffset: sf1280 (13)
                             sf1280: 0
                             shortDRX-Cycle: sf7 (3)
                             drxShortCycleTimer: 9 shortDRX-Cycles

Is operator able to perform long DRX only?

Yes, Operator are able to configure long DRX only and long DRX along with short DRX.


  1. What happens when PDCCH is not decoded during CDRX?


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