WKT UE will receive downlink data over PDCCH. So UE has to monitor PDCCH everytime to check if there is any downlink data available.
By doing so, UE will loose battery very quickly. So to save battery life UE will go to sleep and wakeup at specific time interval and this cycle continues. This is called as Discontinuous Reception (DRX).
So to maintain this, there should be synchronization between eNB and UE. Because if network sends the data when UE is sleep or when UE is awake, network does not send the data, then DRX is of no use.
So the time when UE should wakeup and sleep will be provided by the network in RRC ConnectionReconfiguration or RRC Connection Setup as below:
mac-MainConfig drx-Config: setup (1) setup onDurationTimer: psf6 (5) drx-InactivityTimer: psf1920 (20) drx-RetransmissionTimer: psf16 (5) longDRX-CycleStartOffset: sf1280 (13) sf1280: 0 shortDRX shortDRX-Cycle: sf10 (3) drxShortCycleTimer: 10 shortDRX-Cycles
Explanation of DRX parameters:
DRX Cycle: It is the duration of one “On Time” + “Off time”.
onDurationTimer: It is the duration of one “On Time”
drx-Inactivity timer: It will specify how long UE should remain ‘ON’ after the reception of a PDCCH.
drx-Retransmission timer: It will specify the maximum number of consecutive PDCCH subframes the UE should remain active to wait an incoming retransmission after the first available retransmission time
shortDRX-Cycle: DRX cycle which can be implemented within the ‘OFF’ period of a long DRX Cycle.
drxShortCycleTimer: The consecutive number of subframes the UE shall follow the short DRX cycle after the DRX Inactivity Timer has expired
UE will check the scheduling messages during ON period and if it finds scheduling message it will start “drx-Inactivity timer” and will check PDCCH.
DRX can be implemented in idle called as Idle Mode DRX. In idle mode, IDRX is same as paging cycle.
DRX can be implemented in connected mode called as Connected DRX (C DRX).
DRX Command MAC Control Element
Apart from the above parameters, eNB can also set UE’s DRX behavior by transmitting DRX Command as a MAC CE.
eNB, if it doesn’t have any data to be sent to UE, it will transmit DRX Command as a MAC CE to UE.
Then UE will enter into short DRX cycle if configured, else will go to long DRX cycle.
When UE is in RRC COnnected mode, it will continuously monitor PDCCH data.
If there is a DL Grant and downlink data, the DRX inactivity timer and RRC Inactivity timer are restarted.
If there is UL grant for UE along with DL Grant both DRX and RRC inactivity timers are restarted. 4 ms later UE sends data in uplink
If there are no further grants for uplink or downlink, then DRX Inactivity timer gets expired.
At this point, UE will enter short DRX cycle. If eNB has configured only long DRX cycle, then UE will enter long DRX cycle.
when short DRX cycle timer expires, UE will enter into long DRX cycle.
Then, if there is no activity in uplink or downlink for the duration of RRC Inactivity timer, once that timer expires, UE will enter into RRC_IDLE state. In this state, UE will use paging DRX cycle.