All About Connected Mode Discontinuous Reception (DRX)

Connected Mode DRX:

- DRX in connected mode is a power-saving method.

- DRX is a method by which the UE can switch off its receiver for a period of time.

- Applicable when UE is in RRC_CONNECTED state.

- When UE is in RRC Connected state UE may be configured with a UE specific DRX.

- if DRX is configured, the UE is allowed to monitor the PDCCH discontinuously in RRC Connected state.

- Controls the UE’s PDCCH monitoring activity for the UE’s C-RNTI, TPC-PUCCH-RNTI, TPC-PUSCH-RNTI and Semi-Persistent Scheduling C-RNTI (if configured).

- In the RRC_CONNECTED state DRX mode is enabled during the idle periods during the

packet arrival process,when there are no outstanding/new packets to be transmitted/received, eNB/UE may initiate the DRX mode.

- If the UE is configured with DRX, the UE may delay the measurement reporting for event triggered and periodical triggered measurements until the Active Time.

- cqi-Mask - Limits CQI/PMI/PTI/RI reports to the on-duration period of the DRX cycle.

DRX Cycle : 

-On Duration followed by a possible period of inactivity.

DRX Configuration: 

onDurationTimer : 

- The duration of 'ON time' within one DRX cycle.

- The number of consecutive PDCCH-subframe(s) UE monitors at the beginning of a DRX Cycle.

- If both Long DRX and Short DRX is configured for a particular UE, onDurationTimer i.e. on duration time during Short DRX cycle or Long DRX cycle should be same.

- When to onDurationTimer should be started depends on : 

-     If (Short DRX Cycle) && If ([(SFN * 10) + subframe number] modulo (shortDRX-Cycle) == (drxStartOffset) modulo (shortDRX-Cycle)) 

or

-     If (Long DRX Cycle) && If ([(SFN * 10) + subframe number] modulo (longDRX-Cycle) == drxStartOffset)

-     start onDurationTimer.

drx-InactivityTimer : 

- During Active time if UE receives PDCCH indicates a new transmission (DL or UL) drx-InactivityTimer started or restarted.

- Determines the number of consecutive PDCCH-subframe(s) UE monitors before going to sleep, after successfully decoding a PDCCH during active time. 

- Value in number of PDCCH sub-frames.

drx-RetransmissionTimer :

- PDCCH subframe(s) the UE should remain active as soon as a DL re-transmission is expected by the UE.

- Value in number of PDCCH sub-frames.

longDRX-Cycle :

- Once drxShortCycleTimer  expires Long DRX cycle starts.

- longDRX-Cycle and drxStartOffset . The value of longDRX-Cycle is in number of sub-frames.

- If shortDRX-Cycle is configured,the value of longDRX-Cycle shall be a multiple of the shortDRX-Cycle value.

- The value of drxStartOffset value is in number of sub-frames.

shortDRX-Cycle : 

- It is a optional one.

- If Short DRX is configured, if drx-InactivityTimer expires or a DRX Command MAC control element is received, drxShortCycleTimer  is started and short DRX cycle is used.

drxShortCycleTimer : 

- Specifies the number of time(s) the UE shall follow the Short DRX cycle.

- Value in multiples of shortDRX-Cycle. A value of 1 corresponds to shortDRX-Cycle, a

value of 2 corresponds to 2 * shortDRX-Cycle and so on.

Active Time:

• Time when UE continuously monitors PDCCH.
• Includes when onDurationTimer or drx-InactivityTimer or drx-RetransmissionTimer or mac-ContentionResolutionTimer is running.
• SR is sent on PUCCH and is pending.
• Uplink grant for a pending HARQ re-transmission can occur, and there is data in the corresponding HARQ buffer.
• A PDCCH indicating a new transmission addressed to the C-RNTI of the UE has not been received after successful reception of a RAR for the preamble not selected by the UE i.e. Dedicated RACH.

DRX Command MAC Control Element : 

- DRX Command MAC control element is identified by a MAC PDU subheader with LCID (11110).

- It has a fixed size of zero bits.