DRX

DRX is the main reason a 5G phone's battery survives a day. Without it, the UE would monitor PDCCH every slot, keeping its receiver awake continuously. DRX instead lets the device sleep on a schedule and wake only at defined moments to check for downlink scheduling. RRC configures the whole cycle, and the scheduler has to respect it — it can only address the UE during its awake windows.

The mechanism is a set of timers. The On Duration is the window where the UE wakes and watches PDCCH. If it's scheduled, an Inactivity Timer keeps it awake a while longer in case more data follows. When the timers expire it sleeps until the next cycle. Networks often configure both a short and a long DRX cycle — short for responsiveness during bursty activity, long for deep sleep when traffic is sparse. The trade is fundamental: longer sleep saves more power but adds latency, since data arriving mid-sleep waits for the next wake-up. 5G-Advanced adds wake-up signals and relaxed monitoring to push the power saving further.