Idle Mode DRX in 5G NR

What is Idle Mode DRX?

Idle Mode DRX is a power-saving mechanism that allows UEs to periodically switch to a low-power state during idle periods. When the UE is not actively transmitting or receiving data, it enters a sleep mode, conserving energy. The UE periodically wakes up to check for any incoming data, minimizing unnecessary power consumption.

Discontinuous Reception (DRX) is a power-saving feature designed to extend the battery life of mobile devices. It achieves this by allowing the device to switch off its radio receiver when not expecting network communication, thus reducing power consumption significantly. DRX is applicable in various states of a device’s network connection, with Idle Mode DRX being particularly important for devices not actively engaged in data transmission or reception

How Idle Mode DRX Works

In Idle Mode, a device periodically wakes up to listen for Paging Occasions (PO), which are opportunities for the network to inform the device of incoming calls, messages, or notifications. The DRX cycle defines the frequency and duration of these wake-up periods. A typical DRX cycle involves a DRX On-duration, during which the device listens for the PO, and a DRX Off-duration, during which the device turns its radio off to save power.

DRX Cycles

  • DRX Cycle Duration: The DRX cycle defines the interval between consecutive wake-up periods. It consists of an On Duration (active period) and an Off Duration (sleep period).
  • On Duration: During the On Duration, the UE monitors the control channels for incoming data. If no data is scheduled, it transitions to the Off Duration.
  • Off Duration: In the Off Duration, the UE remains in a low-power state, reducing energy consumption.
Idle Mode DRX in 5G NR
idle_mode_drx

Paging and Scheduling

  • Paging: The network sends paging messages to wake up UEs during the Off Duration. These messages indicate that data is waiting for the UE.
  • Scheduling: When the UE wakes up during the On Duration, it checks the scheduling information to determine if any data transmissions are scheduled.

A UE gets the IDLE mode DRX information from the paging parameters in the SIB1 for NR

PagingCycle ::= ENUMERATED _{rf32, rf64, rf128, rf256}
rf32 indicates 32 subframes
rf64 indicates 64 subframes and so on
Challenges and Considerations
  1. Latency: Longer DRX cycles may introduce latency, affecting real-time applications.
  2. Balancing Act: Choosing appropriate DRX parameters (cycle duration, paging frequency) requires striking a balance between power savings and responsiveness.
The Impact of Idle Mode DRX
  • On Devices: Significantly longer battery life and improved user experience, especially for IoT devices that require long-term connectivity with minimal power usage.
  • On Networks: Reduced signaling overhead, improved resource allocation, and the ability to support a larger number of connected devices without compromising network performance.

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Further reading