Continuous reception/Continuous transmission means that “UE is always on Wake-up mode“. In this way it it simple to figure out that it will cause a lot of battery consumption. To save this kind of battery consumption, in LTE there’s a special mechanism called Discontinous Reception/Transmission DTx & DRx. It means that UE is in Sleeping Mode most of the time and only periodically ”Wake up” to receive or transmit the data.
Basically, LTE provides methods for the UE to micro-sleep even in the active state to reduce power consumption while providing high QoS and connectivity. DRX in the LTE sense means that the UE is not monitoring the PDCCH in the given sub frame and is allowed to go to power saving mode. As uplink is scheduled in downlink PDCCH, the DRX parameter impacts both uplink and downlink performance for a UE.
The DRX concept contains different user-specific parameters that are configured via higher layer signaling. Basically, upon knowledge of the activity requirements in uplink and downlink for a certain UE, the regular DRX period including a certain planned on-time can be set.
When you are talking about ”Reception” and ”Transmission”, you may often get confused with ”direction of data flow”. So let”s make it clear about ”who is receiving?” …and ”who is transmitting”? When we are talking about DRX and DTX: The answer is ”UE”:
Therefore, DRX means ”DISCONTINOUS RECEPTION BY UE” and DTX means ”DISCONTINOUS TRANSMISSION BY UE”
Power Save Protocols in LTE architecture
- Discontinuous Reception (DRX)
- Discontinuous Transmission (DTX)
Both reducing transceiver duty cycle while in active operation DRX also applies to the RRC_Idle state with a longer cycle time than active mode
DTX: Discontinuous Transmission
When an MS is engaged but no speech signals are transferred, the MS sends only comfort noises periodically to the peer end. The data volume of these comfort noises is smaller than the volume of normal speech data.
An MS does not keep transmitting speech signals during a call. Typically, only 40% of the duration of the call is occupied for speech transmission. During the non-speech transmission period, an MS reduces the transmitted data to suppress the interference to other channels and to help reserve system resources.
In addition, DTX relieves the workload of the TX module of the MS. The MS can enjoy a longer call duration and standby time. DTX affects only the transmission of TCH frames.
DTX can reduce the transmit power of an MS when the MS does not receive any speech signals. The quality of speech services in the entire radio network is thus improved.
DTX involves the following technical aspects:
- Silence Descriptor (SID) frames
- Voice Activity Detection (VAD)
- Global measurement and local measurement
When there is no speech signal transmission during a call, the MS sends SID frames to the BTS.
The VAD enables the MS to accurately detect speech signals
DRX: Discontinuous Reception
An MS in idle mode detects only the paging channels within a specific paging group.
When other paging groups send paging messages to an MS, the MS blocks the receive channel.
Each MS is mapped to a paging group, and each paging group is mapped to a paging sub-channel in the serving cell. When operating in idle mode, an MS detects the paging messages broadcast by the system only on the mapped paging sub-channel.
The MS blocks other paging sub-channels by powering off some hardware of the MS, and this also saves power.
Long and Short DRX
The RRC sets a cycle where the UE is operational for a certain period of time when all the scheduling and paging information is transmitted.
The eNodeB knows that the UE is completely turned off and is not able to receive anything. Except when in DRX, the UE radio must be active to monitor PDCCH (to identify DL data). During DRX, the UE radio can be turned off. The DRX/DTX functionality is an effective way to reduce the UE ‘s battery power usage, but at the same time introduces further constraints in the scheduler ‘s tasks. The immediate consequence of them is an average increase of packets delivery delays. The short DRX/DTX represents a further attempt to exploit the inactivity periods of UE to save even more power.
This further saving could be remarkable with certain types of services, but can also be very limited with others, like VoIP:
- for highly predictable traffic (e.g. VoIP), the On Duration can be set to 1 sub-frame and the DRX Cycle to 20 ms or 40 ms if packet staggering is used
- for traffic that is more dynamic and in bursts with tight delay requirements, it is possible to configure the user with a DRX Inactivity Timer where the packet scheduler can keep the UE awake by scheduling it within a certain time window.
- HARQ re-transmissions are planned outside of the predefined DRX cycle to allow for a tighter DRX optimization without having to plan for worst-case re-transmissions.
Additionally, a short DRX cycle can be triggered that allows for periodic activity within the regular DRX cycle if additional and time-distributed scheduling resources are needed to facilitate the best power saving and QoS trade-off for the given UE.
It was shown that with a DRX period set to 100 ms and the On Duration set to 1ms, the DRX Inactivity timer could be set quite loosely from 25 to 50 ms while still achieving a large UE power saving, large scheduling freedom for the network, and allow for multiple web objects to be received within the active window thus improving the web browsing performance.
With such settings the UE saw a 95% reduction in the radio-related power consumption while achieving 90% of the maximum achievable throughput.