Discontinous Reception (DRX)

To conserve energy it was proposed that UE keep it’s receiver circuitry closed for some time. i.e. it goes into sleep and wake up states. This mechanism is known as Discontinous Reception (DRX).

A DRX cycle represents 1 ON and 1 OFF period of DRX.

ON period is a period during which UE should monitor the PDCCH, whereas,

In OFF period UE enters in sleep state.

UE checks scheduling messages (by their C-RNTI on PDCCH) during ON period and if it finds a scheduling message, it starts DRX inactivity timer and monitor PDCCH in every consecutive subframe till the inactivity timer expires. A UE can send SR in OFF period but once it sends SR, it should continously monitor PDCCH till it gets DCI0.

Short DRX cycle is one that can be implemented within a long DRX cycle and is optional.

When a HARQ RTT timer expires and data for that HARQ process was still in the buffer and not decoded successfully then a DRX retransmission timer  is started.

DRXStartOffset indicates the subframe when DRX cycle starts. If short DRX cycle is configured value of long DRX cycle is it’s multiple.

DRX can be implemented in idle(I DRX) as well as connected mode(C DRX). In idle mode, DRX is same as paging cycle because UE only listens to P-RNTI.  The paging occasion and paging  frame are determined from DRX parameters provided in SI. In connected mode UE needs to listen to PDCCH for some data scheduling. The basic procedure for connected mode DRX is as follows-

• UE is in RRC Connected mode and is continuously monitoring PDCCH. At this point, there is DL Grant and downlink data. The DRX inactivity timer and the main RRC Inactivity timer are restarted
• There is UL grant for UE. With DL Grant both DRX and RRC inactivity timers are restarted. 4 ms later UE sends data in uplink
• The DRX Inactivity timer is expired since there were no further grants in uplink or downlink. Though UE was constantly monitoring PDCCH. UE now enters the short DRX cycle. The battery savings have just started
•  The DRX short cycle timer got expired therefore UE will end up its short DRX cycle and enter the long DRX cycle
• The main RRC inactivity timer got expired since there was no activity in uplink or downlink for the duration for RRC Inactivity timer. The UE will go to RRC IDLE state. In idle state UE will use paging DRX cycle

If both short and long DRX cycles are configured, UE starts with 16 short DRX cycles- 2 to 640 subframes and then enters a long DRX cycle- 10 to 2560 subframes if it does not receive any PDCCH scheduling. i.e. current specifications limits the DRX cycle to max 2.56 seconds.

UE wakeup timing and network transmission timing should be synchronized i.e. UE should not be in sleep when network sends data or vica versa. To ensure this, network decides the timings and sends to UE through RRC connection setup/reconfiguration message for DRX in connected mode. DRX in IDLE mode can be cell specific or UE specific. Default DRX cycle or cell specific DRX cycle is configured at eNB and broadcasted in SIB2 to ue’s. Dedicated or UE specific DRX value is indicated by UE to MME in attach request . Both eNB and UE uses smaller of the 2 values. The smaller of the 2 values is chosen because- Modification period is always a multiple of cell specific DRX cycle and therefore, choosing smaller of the 2 values ensures that paging is read once before modification period arrives. Only On-duration and inactivity timer are signalled to UE by eNB.