Power-saving cycles, On Duration / Inactivity timers, and scheduling latency trade-offs
Without DRX, the UE monitors PDCCH every slot (2000 times/sec at 15 kHz). DRX lets the UE sleep for most of the cycle and only wake during On Duration. This trades latency for battery life: longer sleep = more power saved but more scheduling delay.
Low latency. Good for VoIP, gaming. Moderate power saving (60-70%).
High latency. Good for background traffic. Excellent power saving (90-95%).
Zero latency. Only for URLLC or active bursts. Maximum battery drain.
Each DRX cycle starts with On Duration. If PDCCH is decoded, the Inactivity Timer extends wake time. When all timers expire, UE enters sleep. After N short cycles, UE transitions to long DRX.
Animated timeline showing the DRX cycle phases. Send DL data to trigger inactivity timer extension.
| Parameter | Values | Purpose |
|---|---|---|
| drx-onDurationTimer | 1–200 ms | How long UE stays awake at cycle start |
| drx-InactivityTimer | 0–2560 ms | Extended awake time after PDCCH decode |
| drx-ShortCycleTimer | 1–16 cycles | Number of short cycles before long DRX |
| drx-SlotOffset | 0–99 slots | Delay from cycle start to On Duration |
The UE transitions between Sleep, On Duration, Inactivity, and HARQ Wait states based on timer expiry and PDCCH activity. Any Active Time condition keeps the UE awake.
Click events to trigger state transitions. Watch how the UE moves between sleep, wake, and extended states.
HARQ retransmissions can force the UE awake during sleep. After a NACK, the UE starts drx-HARQ-RTT-Timer (can sleep) followed by drx-RetransmissionTimer (must wake).
See how a DL NACK triggers HARQ RTT wait, then forces UE awake for retransmission — even during sleep period.
| Timer | Purpose | UE Can Sleep? |
|---|---|---|
| drx-HARQ-RTT-TimerDL | Wait for minimum RTT before expecting retx | YES — retransmission can't arrive yet |
| drx-RetransmissionTimerDL | Monitor PDCCH for DL retransmission | NO — must be awake |
| drx-HARQ-RTT-TimerUL | Wait for UL retx grant | YES |
| drx-RetransmissionTimerUL | Monitor for UL retransmission grant | NO |
| Service | On Duration | Inactivity | Short Cycle | Long Cycle |
|---|---|---|---|---|
| VoIP | 10 ms | 40 ms | 20 ms | — |
| Video streaming | 20 ms | 100 ms | 80 ms | 640 ms |
| Web browsing | 20 ms | 100 ms | 80 ms | 1280 ms |
| Gaming | 5 ms | 20 ms | 10 ms | — |
| Background/IoT | 5 ms | 0 ms | — | 10240 ms |
| URLLC | Continuous | 2560 ms | — | — |
| Aspect | LTE | 5G NR |
|---|---|---|
| Timer granularity | Subframe (1 ms) | Slot (varies with SCS) |
| Max long cycle | 2560 ms | 10240 ms |
| WUS | Rel-15 limited | Rel-16 DCI 2_6 (PS-RNTI) |
| BWP-aware | No | Yes |
| Cross-slot scheduling | N/A | Reduces wake time |
DCI 2_6 sent before On Duration tells the UE whether to wake or skip. This saves 20-40% additional power by avoiding unnecessary wake-ups when no data is pending.
Animated timeline: WUS arrives before On Duration. Toggle whether data is pending to see the skip vs wake decision.
AI predicts UE traffic. Dynamically tunes DRX cycle in real-time. 30-50% more power saving over 5G.
Sleep between OFDM symbols within a slot. Monitor only first 2-3 symbols. 10-30% savings per active slot.
Multi-RAT sleep coordination (5G + Wi-Fi). UE sleeps on 5G when Wi-Fi handles traffic.
Compare DRX capabilities across generations.
| Dimension | 4G (LTE) | 5G NR | 6G (Target) |
|---|---|---|---|
| Max cycle | 2560 ms | 10240 ms | 60+ sec |
| WUS | Limited | DCI 2_6 | AI-predicted |
| Sleep granularity | Subframe | Slot | Symbol (microsleep) |
| Adaptation | Static RRC | Static RRC | AI-dynamic |
| Multi-RAT | None | None | Cooperative sleep |