Connected Mode DRX

We know that 2G and 3G terminal uses discontinuous reception in idle mode. In LTE the tradition has continued and we have similar DRX in idle mode but in addition to that we also have DRX in RRC mode.  In LTE, when there is no data to receive or transmit in RRC connected mode, UE would switch off its transceiver for a very short interval. It will start similar "wake up and sleep " cycle. During the wake up period, it will keep monitoring PDCCH channel for UL or DL grants whereas the sleep periods will improve the battery savings

Without Connected Mode DRX 

The main goal of Connected mode DRX is to minimize battery consumption by discontinuous monitoring of PDCCH channel. Without DRX, UE has to monitor PDCCH channel every time. This results in high battery consumption as shown below

With Connected Mode DRX

With DRX enabled in connected mode, UE only monitors PDCCH channel when it is awake during the sleep/wakeup cycles as seen below. During the sleep time, UE doesn't monitor PDCCH channels which results in energy savings. All the DL grants are delayed to nearest wake up period

Important DRX Parameters/Timers 

Some of the important parameters used in configuring the DRX for UE are shown below. The parameters are transferred to UE through RRC reconfiguration message

 

 

Basic Scenario

To better understand these parameters, see the below picture which shows each parameter

1.  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
2. 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
3. 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
4.  The DRX short cycle timer got expired therefore UE will end up its short DRX cycle and enter the long DRX cycle
5. 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

 

HARQ Retransmissions Scenario

In the above basic scenario it may seem complicated to include HARQ retransmission's scenarios, so here is another example below

1.  UE is in RRC Connected mode and is continuously monitoring PDCCH channel. At this point, there is DL Grant and downlink data. The DRX and RRC inactivity timer is restarted ( RRC Inactivity not shown here)
2. There is UL grant for UE. With DL Grant both DRX and RRC inactivity timers are restarted 4ms later UE sends data in uplink. And after additional 4ms later ACK is sent by eNB
3. There is DL grant for UE with DL data. For some reason UE is not able to decode the data. 4ms later UE will send NACK towards eNB. Harq RTT timer is started which has fixed duration of 8ms. Now UE is expecting retransmission in downlink
4. HARQ RTT got expired which will trigger the DRX retransmission timer as the retransmission is expected
5. There is DL grant with retransmission data. This time UE is able to decode it. 4ms later UE sends ACK in uplink. Note DL grant for retransmission data does not restart DRX inactivity timer
6. DRX retransmission timer expires and UE enters the short DRX cycle

Key points:

•  DRX cycles are synchronized at UE and eNB side i.e. eNB knows when UE is in DRX sleep or awake period so that it can schedule UE accordingly
• When UE is in DRX sleep state, it cannot read PDCCH channel therefore, the downlink grants must be delayed to nearest wake cycle as eNB is already aware of this UE DRX cycle. The introduces delay in dowlink transmission
• Uplink transmission is not affected as UE can send SR in uplink whenever it wants i.e. UE is in DRX sleep period and it has uplink data so it will just wake up and send SR to receive UL grants from eNB. 
• Other than the timers/parameters mentioned above, eNB MAC can also control UE DRX by transmitting MAC CE DRX commands