Even while there is no traffic between the network and UE, UE has to keep listening to Network. At least it should be ready to decode PDCCH. It means UE has to be "ON" all the time even when there is no traffic. But being ON all the time would drain the battery.
So What is the ideal solution to save battery consumption and still does not lose chance of receiving the data that Network sent to UE ?
To save this kind of battery consumption, in LTE there's a special mechanism called Discontinuous Reception/Transmission DTX & DRX.
DRX -Part-1
UE Capability Information for DRX Supportability
CDRX is a very important feature in terms of energy saving on UE side and a lot of live network enables this feature, it is not the mandatory requirement on UE. Therefore, even though most of UE would support this feature, there would be some UE (especially UE released at very early stage of LTE) that may not support this feature. So UE needs to inform the network about CDRX supportability and Network can optionally enable or disable based on UE capability and Network operator's requirement.
How can we synchronize UE-wakeup timing with Network transmission timing for the UE. If these two timing does not match, there can be a worst case where UE is awake but Network does not transmit anything or Network transmit something for the UE but UE is in sleep mode. One solution would be that Network decide when to let UE sleep and when towake it up and inform the timing to the UE using a RRC message.
Network informs UE of this timing using RRC ConnectionReconfiguration or RRC Connection Setup as follows (Following is the example configuration from RRC ConnectionReconfiguration, but you can do the samething in RRC Connection Setup as well).
DRX -Part-2
So What is the ideal solution to save battery consumption and still does not lose chance of receiving the data that Network sent to UE ?
To save this kind of battery consumption, in LTE there's a special mechanism called Discontinuous Reception/Transmission DTX & DRX.
One of the solution for this is let UE get into sleeping mode for a certain period of time and wake up again checking if there is any data coming from the network and getting into sleeping mode again if there is no data and wake up again... repeating this cycles. This kind of periodic repetition of "sleep mode and wake up mode" is called DRX (Discontinuous Reception".
DRX may not be as simple as you may expected because there should be well designed synchronization between UE and Network. In worst case, Network tries to send some data while UE is in sleep mode and UE tries to wake up when there is no data to be received. To prevent this kind of worst case scenario, UE and Network has a well defined agreement about when UE has to be in sleep mode and when UE has to wake up.
DRX may not be as simple as you may expected because there should be well designed synchronization between UE and Network. In worst case, Network tries to send some data while UE is in sleep mode and UE tries to wake up when there is no data to be received. To prevent this kind of worst case scenario, UE and Network has a well defined agreement about when UE has to be in sleep mode and when UE has to wake up.
DRX -Part-1
UE Capability Information for DRX Supportability
CDRX is a very important feature in terms of energy saving on UE side and a lot of live network enables this feature, it is not the mandatory requirement on UE. Therefore, even though most of UE would support this feature, there would be some UE (especially UE released at very early stage of LTE) that may not support this feature. So UE needs to inform the network about CDRX supportability and Network can optionally enable or disable based on UE capability and Network operator's requirement.
ueCapabilityInformation-r8
Ue-CapabilityRAT-ContainerList: 1 item
Item 0
UE-CapabilityRAT-Container
rat-Type: eutra (0)
UeCapabilityRAT-Container: c9980050c08616082058b58fff15b1ffe2fe3ffc53c7ff8b...
UE-EUTRA-Capability
AccessStratumRelease: rel10 (2)
Ue-Category: 4
featureGroupIndicators: 7fcffeb2
0... .... = Indicator 1: ...
.1.. .... = Indicator 2: ...
..1. .... = Indicator 3: ...
...1.... = Indicator 4: Short DRX cycle - Supported
.... 1... = Indicator 5: Long DRX cycle;
DRX command MAC control element - Supported
.... .1... = Indicator 6: ....
RRC Connection Reconfiguration for DRX SettingHow can we synchronize UE-wakeup timing with Network transmission timing for the UE. If these two timing does not match, there can be a worst case where UE is awake but Network does not transmit anything or Network transmit something for the UE but UE is in sleep mode. One solution would be that Network decide when to let UE sleep and when towake it up and inform the timing to the UE using a RRC message.
Network informs UE of this timing using RRC ConnectionReconfiguration or RRC Connection Setup as follows (Following is the example configuration from RRC ConnectionReconfiguration, but you can do the samething in RRC Connection Setup as well).
RrcConnectionReconfiguration-r8
dedicatedInfoNASList: 1 item
Item 0
DedicatedInfoNASList: 27bd5ad9de05620ec101050403696d730902000000000000...
mac-MeasConfig: explicitValue (0)
explicitValue
ul-SCH-Config
...
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
dedicatedInfoNASList: 1 item
Item 0
DedicatedInfoNASList: 27bd5ad9de05620ec101050403696d730902000000000000...
mac-MeasConfig: explicitValue (0)
explicitValue
ul-SCH-Config
...
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
TimeAlignmentTimerDedicated: infinity (7)
DRX Cycle:
The duration of one 'ON time' + one 'OFF time'. (This value does not explicitely specified in RRC messages. This is calculated by the subframe time and longdrx-CycleStartOffset)
onDurationTimer:
The duration of 'ON time' within one DRX cycle
drx-Inactivity timer:
Specify how long UE should remain 'ON' after the reception of a PDCCH. When this timer is on UE remains in 'ON state' which may extend UE ON period into the period which is 'OFF' period otherwise. (See the figure for < case 2 > below)
drx-Retransmission timer:
Specifies 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.(See the figure for < case 4 > below)
drxShortCycleTimer:
The consecutive number of subframes the UE shall follow the short DRX cycle after the DRX Inactivity Timer has expired(See the figure for < case 4 > below)
The duration of one 'ON time' + one 'OFF time'. (This value does not explicitely specified in RRC messages. This is calculated by the subframe time and longdrx-CycleStartOffset)
onDurationTimer:
The duration of 'ON time' within one DRX cycle
drx-Inactivity timer:
Specify how long UE should remain 'ON' after the reception of a PDCCH. When this timer is on UE remains in 'ON state' which may extend UE ON period into the period which is 'OFF' period otherwise. (See the figure for < case 2 > below)
drx-Retransmission timer:
Specifies 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.(See the figure for < case 4 > below)
drxShortCycleTimer:
The consecutive number of subframes the UE shall follow the short DRX cycle after the DRX Inactivity Timer has expired(See the figure for < case 4 > below)
DRX -Part-2
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