Idle Mode DRX in 5G NR

Idle Mode DRX in 5G NR

Discontinuous Reception (DRX) is a power-saving technology implemented in mobile communications to extend the battery life of User Equipment (UE). In 5G New Radio (NR), Idle Mode DRX allows devices that are not actively transferring data to enter a low-power "sleep" state and periodically wake up to check for incoming calls or data.

Without DRX, a mobile phone would have to constantly monitor the downlink control channels, quickly draining its battery within hours even when not in use.

How Idle Mode DRX Works

When a UE is in the RRC_IDLE or RRC_INACTIVE state, it is not connected to a dedicated radio channel. Instead, it sleeps and periodically wakes up at designated intervals called Paging Occasions (PO) to monitor the Physical Downlink Control Channel (PDCCH) for paging messages.

A single DRX Cycle consists of:

1. On Duration (Active Period): The UE receiver turns on, synchronizes with the cell timing, and decodes control channel schedules to see if it is being paged.
2. Off Duration (Sleep Period): If no paging message matches the UE's identity, the UE shuts down its RF transceiver and enters a sleep state to conserve power.

Paging Occasions and Scheduling

The exact frame and slot where a UE monitors paging is calculated using the UE's identity (5G-S-TMSI) and configuration parameters broadcast in System Information Block 1 (SIB1). This ensures that different UEs are distributed across different paging occasions, preventing control channel congestion.

If the network has incoming data for the UE (e.g., an incoming voice call or downlink packet):

1. The gNB transmits a paging DCI (scrambled with P-RNTI) during the UE's Paging Occasion.
2. The UE wakes up, detects the P-RNTI DCI, and decodes the corresponding paging message on the PDSCH.
3. The UE initiates a Random Access (RACH) procedure to transition to the RRC_CONNECTED state and receive the data.

SIB1 Parameters for Idle Mode DRX

The DRX configuration parameters are broadcast by the network in SIB1 under the pcch-Config (Paging Control Channel Configuration) structure:

PagingCycle ::= ENUMERATED {rf32, rf64, rf128, rf256}

• rf32: 32 Radio Frames (320 ms)
• rf64: 64 Radio Frames (640 ms)
• rf128: 128 Radio Frames (1280 ms)
• rf256: 256 Radio Frames (2560 ms)

Choosing a paging cycle involves a trade-off:

• Shorter Cycles (e.g., 320 ms): Reduced paging latency (faster call setup), but higher UE power consumption.
• Longer Cycles (e.g., 2.56 s): Maximum battery life, but increased latency for incoming traffic.

Impact of Idle Mode DRX

• For the Device: Significantly longer battery life. This is particularly crucial for Internet of Things (IoT) or RedCap devices that operate in the field for years without a battery replacement.
• For the Network: Lowers signaling overhead and optimizes resource usage by scheduling paging messages only during configured intervals.

References

• 3GPP TS 38.304: User Equipment (UE) procedures in Idle mode and RRC Inactive state.
• 3GPP TS 38.331: Radio Resource Control (RRC) protocol specification.

Further Reading

• 5G NR Timers and Constants
• RRC Reestablishment Request in 5G NR