LTE: CDRX - Connected Mode DRX

http://know-lte.blogspot.kr/2016/07/cdrx-connected-mode-drx.html

Connected Mode DRX (CDRX)

In addition to Idle mode DRX, LTE had RRC mode DRX as well also known as Connected Mode DRX (CDRX), the purpose of both the DRX is same to conserve the battery power. During RRC connected state when there is no data transmission in either direction (UL/DL) UE goes into the DRX mode. It starts monitoring the PDCCH channel discontinuously in other words UE is in sleep and wake cycle.

Without the DRX the UE needs to monitor PDCCH in every subframe to check if there is downlink data available which drains the battery fast.

Battery Consumption without DRX

In the figure above, without DRX feature the UE is monitoring the PDCCH every subframe, so the UE will never be in the sleep state.

With DRX support, The UE monitors PDCCH at regular intervals during wake cycle. The DRX configurations are configured by the network in RRC Connection Setup request and RRC connection reconfiguration request. Here is the drx configuration:

rrcConnectionReconfiguration

....

    ul-SCH-Config

    ...

        DRX-Config ::= CHOICE {

            release     NULL,

            setup       SEQUENCE {

                onDurationTimer     ENUMERATED {

psf1, psf2, psf3, psf4, psf5, psf6, psf8, psf10, psf20, psf30, psf40, psf50, psf60, psf80, psf100, psf200},

                drx-InactivityTimer ENUMERATED {

psf1, psf2, psf3, psf4, psf5, psf6, psf8, psf10, psf20, psf30, psf40,psf50, psf60, psf80, psf100, psf200, psf300, psf500, psf750, psf1280, psf1920, psf2560, spare10, spare9, spare8, spare7, spare6,

                                    spare5, spare4, spare3, spare2, spare1},

                drx-RetransmissionTimer ENUMERATED {

psf1, psf2, psf4, psf6, psf8, psf16, psf24, psf33},

                longDRX-CycleStartOffset  CHOICE {

                                    sf10  INTEGER(0..9),

                                    sf20  INTEGER(0..19),

                                    sf32  INTEGER(0..31),

                                    sf40  INTEGER(0..39),

                                    sf64  INTEGER(0..63),

                                    sf80  INTEGER(0..79),

                                    sf128 INTEGER(0..127),

                                    sf160 INTEGER(0..159),

                                    sf256 INTEGER(0..255),

                                    sf320 INTEGER(0..319),

                                    sf512 INTEGER(0..511),

                                    sf640 INTEGER(0..639),

                                    sf1024      INTEGER(0..1023),

                                    sf1280      INTEGER(0..1279),

                                    sf2048      INTEGER(0..2047),

                                    sf2560      INTEGER(0..2559)

                  },

                  shortDRX    SEQUENCE {

                                    shortDRX-Cycle ENUMERATED     {

                                    sf2, sf5, sf8, sf10, sf16, sf20,

                                    sf32, sf40, sf64, sf80, sf128, sf160,

                                    sf256, sf320, sf512, sf640},

                  drxShortCycleTimer      INTEGER (1..16)

            }          

DRX Cycle

       IEs of rrcConnectionReconfiguration with respect to drx-config setup is discussed here: 

DRX Cycle:- is the periodic repetition of ON Duration (Monitoring PDCCH) and OFF Duration (DRX activity).
DRX Inactivity Timer: - specifies the time in terms of TTI duration after successful decoded PDCCH, to go again in OFF Duration.
On Duration Timer: Specifies the number of consecutive PDCCH subframe(s) need to be decoded after wakeup from the DRX Cycle.
DRX Retransmission Timer:  Specifies the consecutive number of PDCCH subframe(s) to monitor when retransmission is expected by the UE. 

DRX short cycle: It is the first DRX cycle that the UE enters after successful expiration of DRX inactivity timer. UE will be in the short DRX cycle till the expiration of DRX short cycle timer after that it will be in Long DRX cycle. 
DRX Short Cycle Timer: This parameter specifies the number of consecutive subframe(s) the UE shall follow the short DRX cycle after the DRX Inactivity Timer has expired.

Lets understand CDRX with example: 

From RRC connection reconfiguration the following DRX parameters are configured

DrxInactivityTimer : 6
OnDurationTimer : 2
shortDrxTimer : 6
LongDrxCycle : 12
drxStartOffset : 0
drxShortCycleTimer : 2 

CDRX- Connected Mode DRX

 Here UE is in connected mode and monitor PDCCH, After successful attempt of DL data, the DrxInactivityTimer started. As per example here it starts for 6 subframes. If there is any UL or DL data transmission during DrxInactivityTimer the timer restarts again.
If DrxInactivityTimer expired, UE now enters the DRX cycle. Power saving mode started from here. It starts the ShortDrxCycle here it is of 6 subframes and OnDuration is 2sf. 
DrxShortCycleTimer = ShortDrxCycle *   DrxShortCycleTimer. So it runs for 12sf. 
After expiration of DrxShortCycleTimer UE now enters the LongDrxCycle. 
This process continues till RRC inactivity timer expires. Once the RRC inactivity timer expires the UE moved into the IDLE mode DRX.