5G Paging Procedure

Paging is the mechanism by which the network notifies a UE in idle or inactive mode about incoming data, voice, system updates, or alerts. While LTE already supports paging via Discontinuous Reception (DRX) and efficient signaling, 5G brings additional requirements: supporting massive IoT, URLLC, finer privacy guarantees, and more energy efficiency.
3GPP TS 38.304 defines how a UE behaves in RRC_IDLE and RRC_INACTIVE in 5G and describes the paging procedures, showing how they evolve relative to LTE (TS 36.304).

LTE Paging-A Quick Recap

To better appreciate the differences, let’s briefly restate how paging works in LTE:

• When the core network (via the MME) needs to reach a UE in idle mode (e.g. downlink data arrives), it sends a paging request toward all eNodeBs in the UE’s registered Tracking Area (TA).
• The eNodeBs broadcast the paging message on the Paging Control Channel (PCCH), including a UE identifier (often a TMSI).
• The UE, which is conserving power, does not continuously monitor the downlink. Instead, it “wakes up” periodically according to a DRX cycle.
• Using formulas in TS 36.304, the UE computes:
  • A Paging Frame (PF) — the radio frame in which it should listen,
  • A Paging Occasion (PO) — the subframe / slot within the PF where it monitors the PCCH.
• If the UE finds its identity in the paging message, it transitions to the connected state by initiating a Service Request / RRC Connection Request, etc.

This scheme offers good efficiency and power savings, but has limitations in context of 5G’s expanded goals (massive IoT, tighter latency, privacy, etc.).

5G Paging Architecture & States

Paging Control Entity

In 5G, the AMF replaces the role of the MME in triggering paging. When downlink data arrives or when the network must reestablish contact, the AMF sends NGAP Paging messages to relevant gNBs. The gNBs then deliver the paging over the radio interface via PCCH/PDCCH.

Idle vs Inactive

One of the major enhancements in 5G is the introduction of RRC_INACTIVE in addition to RRC_IDLE:

• RRC_IDLE: Similar to LTE’s idle state,the UE camps, may do cell reselection, and monitors paging.
• RRC_INACTIVE: The UE’s signaling context is maintained at the RAN, but its RRC connection is suspended. The UE does not fully release its context but is not actively connected.

Because of RRC_INACTIVE, paging can be scoped more narrowly via RNA (RAN Notification Area) rather than paging across the entire Tracking Area. This reduces signaling overhead and improves efficiency, especially in dense networks.

Thus, 5G paging is more adaptive based on whether the UE is idle or merely inactive.

DRX, PF, and PO in 5G

Just like LTE, paging in 5G leverages DRX cycles to save power:

• The UE periodically wakes up to check for paging, rather than continuously listening.
• Paging Frame (PF): the specific radio frame in which the UE expects paging. It is computed using the UE’s identity (e.g. derived from 5G-S-TMSI) and the DRX cycle.
• Paging Occasion (PO): within each PF, there are one or more POs (slots/subframes) where the UE monitors PCCH. The mapping of PF + PO ensures that different UEs spread out their wakeups, reducing collisions and congestion.

Parameters like DRX cycle length and PO configuration are signaled via SIB1  in 5G.

The formal formulas are defined in TS 38.304 and TS 38.213.

If a UE does not yet have a 5G-S-TMSI, it uses a default identity (e.g. UE_ID = 0) in the PF/PO formulas until assignment.

Paging Procedure: Idle vs Inactive

Case A: UE in RRC_IDLE

• Downlink data arrives at SMF/UPF, which conveys to AMF.
• AMF triggers paging, sends NGAP paging to serving gNB(s).
• UE wakes up at its PO (determined by PF/PO).
• UE decodes paging, recognizes its identifier.
• UE issues RRC Connection Request / Service Request to gNB.
• gNB forwards message to AMF; AMF activates the PDU session or context.
• gNB and AMF set up context.

UE transitions from RRC_IDLE → RRC_CONNECTED, and data flow begins

Case B: UE in RRC_INACTIVE (RNA paging)

• Similar paging trigger by AMF, within the RNA scope.
• UE wakes in its RNA paging occasion.
• UE sees identity in the paging message, sends RRC Resume Request.
• gNB forwards UE Context Resume Request to AMF.
• AMF reactivates or restores context with SMF/UPF, context setup over gNB.
• gNB sends RRC Resume Accept and the UE transitions to RRC_CONNECTED.
• Data flow continues.

Because the UE’s context is already known to the RAN in the INACTIVE state, the resume process is more lightweight compared to a full connection setup, improving latency and reducing signaling.

Paging Message Content

A 5G paging message carries several key fields:

• UE Identity: Typically, the 5G-S-TMSI or I-RNTI, avoiding exposure of IMSI and improving privacy.
• TAI or RNA list: Defines which Tracking Areas or RNAs the UE is supposed to monitor.
• Paging DRX: The cycle length the UE should adopt.
• Paging Priority: Enables prioritization (for example, for emergency alerts, URLLC, or public warning systems like ETWS/CMAS).
• System Information Change Indicator: Tells the UE if it must re-acquire updated system info or broadcast changes.

These fields are defined in standards such as TS 38.331 (RRC message structure) and 3GPP system architecture specs (e.g. TS 23.501).

Key Enhancements & Advantages in 5G Paging

Compared to LTE, 5G paging introduces several improvements:

• RNA Paging / RRC_INACTIVE-Paging is constrained to a local RAN Notification Area rather than the entire Tracking Area, thus reducing signaling.
• Improved Privacy-LTE paging could sometimes leak identifiers like IMSI or static TMSI.5G only uses temporary identifiers (5G-S-TMSI / I-RNTI), which are refreshed, reducing tracking risk.
• Beamformed Paging-In high-frequency or mmWave deployments, paging might be delivered over directional beams rather than omnidirectionally, improving coverage and reducing interference.
• Early Paging Indication (EPI)-Introduced in 3GPP Release 17, EPI lets the UE know in advance whether a paging is coming, helping it avoids unnecessary wakeups. This yields energy savings (e.g. 17–34% for some IoT use cases).
• Support for Public Warning Systems-Paging supports ETWS / CMAS signaling, ensuring critical alerts reach UEs even in idle/inactive states.
• Adaptive Latency & Signaling Efficiency-The combination of RNA-based scoping, more intelligent wake-up scheduling, beamforming, and optimized resume procedures leads to lower latency and fewer signaling messages, which is vital for 5G applications like URLLC and massive IoT.

Differences Between LTE and 5G Paging

Feature / Aspect	LTE Paging	5G Paging
Central paging entity	MME	AMF
Inactive / suspended state	No explicit inactive state	RRC_INACTIVE with RNA paging
Paging scope	Entire Tracking Area	For inactive UEs, paging limited to RNA
Identity used in paging	TMSI / sometimes IMSI exposure	Only temporary IDs (5G-S-TMSI, I-RNTI)
Wakeup mechanism	DRX cycles, PF / PO	DRX cycles, PF / PO, with enhancements (EPI)
Beamforming / directional paging	Not typical	Possible, especially in mmWave / high frequencies
Energy optimization	Good, but less flexible	More aggressive via EPI and selective wakeups
Latency & signaling load	Acceptable for LTE use cases	Lower latency, less signaling overhead in many cases

Although the fundamental intent of paging remains the same (wake up a sleeping UE to deliver data or signaling), 5G paging is far more sophisticated, efficient, and adaptive than LTE paging. It adds a new inactive mode (RRC_INACTIVE), narrower paging scopes (RNA), better privacy (temporary identifiers), beamforming-based delivery, and advanced energy-saving mechanisms like Early Paging Indication.

These enhancements help 5G better support diverse use cases, from smartphones needing low latency to IoT devices needing long battery life, while reducing signaling overhead and improving overall system scalability.

References

• 3GPP TS 38.304 – NR; User Equipment (UE) procedures in idle mode and in RRC Inactive state.
• 3GPP TS 38.331 – NR; Radio Resource Control (RRC) protocol specification.
• 3GPP TS 38.213 – NR; Physical layer procedures for control.
• 3GPP TS 23.501 – System Architecture for the 5G System (5GS); Stage 2.
• 3GPP TS 36.304 – E-UTRA; User Equipment (UE) procedures in idle mode (for LTE comparison).
• 3GPP TS 36.331 – E-UTRA; Radio Resource Control (RRC) protocol specification.
• Dahlman, E., Parkvall, S., & Skold, J. – 5G NR: The Next Generation Wireless Access Technology. Academic Press, 2020.
• ETSI White Paper – 5G System Overview. European Telecommunications Standards Institute.
• Nokia Bell Labs – RAN Evolution for 5G. Technical Report, 2021.