LTE: Connected Mode DRX

In LTE, without Discontinuous Reception (DRX), the UE has to be awake all the time in order to decode downlink data, as the data in the downlink may arrive at any time. This means that UE has to be monitoring PDCCH in every subframe in order to check if there is downlink data available. This consumes a lot of the user equipment’s power

DRX in LTE is introduced to improve UE battery lifetime. In DRX, UE discontinuously receives PDCCH. This post discusses LTE Connected mode DRX.

The eNodeB configures DRX with a set of DRX parameters. These DRX parameters are selected based on the application type such that power and resource savings are maximized.

When DRX is enabled, there may be an extended delay in receiving data as, the UE may be in DRX Sleep state at the time of data arrival at the eNodeB, and the eNodeB would have to wait until the UE becomes ON. So the DRX parameters have to be carefully selected such that the packet delay is minimized and power saving is maximized.

During DRX mode, the UE powers down most of its circuitry when there are no packets to be received. During this time UE listens to the downlink (DL) occasionally which is called DRX Active state whereas the time during which UE doesn’t listen PDCCH is called DRX Sleep state

DRX is also beneficial to the eNodeB. Without DRX, the UE would be transmitting periodic CSI or SRS very frequently (based on the configuration). With DRX, during OFF periods, the UE is not allowed to transmit Periodic CSI or SRS, so the eNodeB can assign these resources to the other UEs to maximize resource utilization.

DRX Configuration

The eNodeB configures the following RRC parameters for DRX. Entire DRX configuration is sent under drx-config structure under MAC-MainConfig. Each DRX parameter and its’ purpose is explained below

In RRC specification, almost all the DRX timer values are specified in terms of psfs. psf is a PDCCH subframe in which UE listens for PDCCH. In FDD every subframe is a DL/UL subframe so every subframe can be a psf whereas in TDD, only DL subframes are considered to be psfs.

• drx-Inactivity-Timer specifies the number of consecutive PDCCH-subframe(s) for which the UE should be Active after successfully decoding a PDCCH indicating a new transmission (UL or DL) . This timer is restarted upon receiving PDCCH for a new transmission (UL or DL). Upon the expiry of this timer the UE should go to DRX mode.
• shortDRX-Cycle is the first type of DRX cycle (if configured) that needs to be followed when UE enters DRX mode. This IE indicates the length of the short cycle in subframes which include ON time followed by a possible OFF (inactivity) time.
• drxShortCycleTimer expressed as multiples of shortDRX-Cycle. The timer value can vary from 1 to 16 (short DRX cycles). This timer indicates the number of initial DRX cycles to follow the short DRX cycle before entering the long DRX cycle
• longDRX-CycleStartOffset defines long DRX cycle length as well as the DRX offset. DRX offset is used to calculate the starting subframe number for DRX cycle.
• onDurationTimer specifies the number of consecutive PDCCH-subframe(s) at the beginning of each DRX Cycle (DRX ON). i.e., is the number of subframes over which the UE shall read PDCCH during every DRX cycle before entering the power saving mode (DRX OFF)
• HARQ RTT Timer specifies the minimum amount of subframe(s) duration from the time new transmission is received and before the UE can expect a retransmission of the same packet. This timer is fixed and not configured by RRC. For FDD the HARQ RTT Timer is set to 8 subframes. For TDD the HARQ RTT Timer is set to k + 4 subframes, where k is the interval between the downlink transmission and the transmission of associated HARQ feedback
• drx-RetransmissionTimer indicates the maximum number of subframes for which UE should be monitoring PDCCH when a retransmission from the eNodeB is expected by the UE.

Same onDurationTimer value is applied for both long and short DRX cycles

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

UE indicates the support of Short DRX cycle in featureGroupIndicators bit- 4

UE indicates the support of Long DRX cycle and DRX command MAC control element (together) in featureGroupIndicators bit – 5

DRX Command MAC Control Element

We have seen that a number of timers which controls the UE’s DRX state (ON/OFF). In addition to these timers the eNodeB’s MAC can also control UE’s DRX behavior by transmitting a command called DRX Command as a MAC Control Element.

When the eNodeB doesn’t have any (more) data to be sent to the UE, it can transmit DRX Command MAC CE to the UE. Upon reception of DRX Command MAC CE, the UE enters short DRX cycle if configured, otherwise, the UE enters long DRX cycle.

In reality, DRX Command MAC CE shortens UE’s ON period. For example, if DRX Command MAC CE is received when either onDurationTimer or drx-Inactivity-Timer running, the UE stops the timer and enters into DRX cycle (Short/Long)

The DRX Command MAC control element is identified by a MAC PDU subheader with LCID as 11110. It has a fixed size of zero bits.

DRX Active Time

Active Time is the time during which the UE is considered to be monitoring PDCCH. The Active Time includes the time while:

onDurationTimer is running;

drx-InactivityTimer is running;

drx-RetransmissionTimer is running:

As an example, let us say that the UE has received new data in subframe #n on PDSCH. The UE starts HARQ RTT Timer in the same subframe #n. Upon the expiry of HARQ RTT timer, if the data for the corresponding HARQ process was not successfully decoded (CRC error) then the UE starts drx-RetransmissionTimer for the corresponding HARQ process. The UE needs to be monitoring PDCCH while this timer is running as the retransmission can be expected by the UE during this time.

mac-ContentionResolutionTimer is running:

The UE shall start mac-ContentionResolutionTimer from the next subframe after transmitting Msg3. Since the UE is waiting for the contention resolution which is via PDCCH reception on C-RNTI (in connected mode), the UE needs to be monitoring PDCCH. So the time duration in which mac-ContentionResolutionTimer is running is also considered as Active Time

Scheduling Request has been sent on PUCCH and is pending:

The UE has to be in active state from the next subframe after transmitting SR over the air on PUCCH. After transmitting SR, the UE should be DRX active in order to receive grant from the eNodeB

An uplink grant for a pending HARQ retransmission can occur and there is data in the corresponding HARQ buffer:

Let us say that the DCI0 for initial transmission is received at subframe #n, the UE shall become active at subframes n+8, n+16, n+24...n+(maxHARQTx-1)*8 for a possible retransmission

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

In the non-contention based RA, after receiving RAR, the UE should be in active state until PDCCH indicating new transmission addressed to C-RNTI of the UE is received

DRX Operation

When there is no data activity for drx-InactivityTimer amount of time (i.e., upon expiry of drx-InactivityTimer) or DRX Command MAC CE is received,

-  if the Short DRX cycle is configured, then the UE should  start or restart drxShortCycleTimer and start using Short DRX Cycle. Else if Short DRX cycle is not confired, the UE should use the Long DRX cycle

If drxShortCycleTimer expires, i.e., maximum number of short cycles are already used, then the UE should enter the Long DRX cycle

If a DRX Command MAC control element is received, the UE should stop onDurationTimer and drx-InactivityTimer

During the active time, the UE shall monitor the PDCCH; if the PDCCH indicates a new transmission (DL or UL) in subframe #n, then the UE should start/restart drx-InactivityTimer in subframe #n+1

The UE should start onDurationTimer in a subframe which satisfies the following equation (based on whether short DRX cycle is configured or not):

If the Short DRX Cycle is used,

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

else if the Long DRX Cycle is used

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

When downlink assignment has been configured (DL SPS), and if the configured assignment recurs in subframe that does not fall in Active time, the UE need not decode PDSCH. It is the eNodeB’s responsibility to make sure that the configured assignment falls in onDurationTimer

During RAR-window, if a subframe falls in non-Active time, then the UE monitors PDCCH only for RA-RNTI

Regardless of whether the UE is monitoring PDCCH or not, the UE receives and transmits HARQ feedback when such is expected.

When more than one serving cell is configured (CA), the same active time applies to all activated serving cell(s). For FDD, The UE maintains a set of 8 HARQ-RTT/drx-RetransmissionTimers for each serving cell. The UE monitors PDCCH on all serving cells, even if the active time corresponds to only one serving cell

Periodic CSI on PUCCH and SRS in DRX mode

When not in Active time, the UE shall not transmit periodic SRS (type-0 triggered SRS)

A release-8 UE shall not transmit periodic CSI on PUCCH when not in Active time.

In release-9, CQI-mask IE is introduced which limits CQI/PMI/PTI/RI reports to the on duration period of the DRX cycle. If the IE CQI-mask is not setup by RRC, CQI/PMI/RI/PTI on PUCCH shall not be reported when not in Active time; else UE should send CQI/PMI/RI/PTI on PUCCH only if onDurationTimer is running

When more than one serving cell is configured (CA), one value of cqi-Mask applies for all serving cells (the associated functionality is common i.e. not performed independently for each cell)

Reference: 3GPP TS 36.321, 36.213, 36.331

Long DRX-cycle           Offset          onDurationTimer           Short DRX-cycle
10 20 32 40 64 80 128 160 256 320 512 640 1024 1280 2048 2560                                 1 2 3 4 5 6 8 10 20 30 40 50 60 80 100 200                      2 5 8 16 20 32 40 64 80 128 160 256 320 512 640

                                                                                              UL-DL Config
        Is Short DRX?         Is TDD?          0 1 2 3 4 5 6

Long DRX Occasions

Long DRX Occasions will be displayed here

Short DRX Occasions

Short DRX Occasions will be displayed here