5G Paging Procedure and How It Differs from LTE Paging

Paging is the process by which the network contacts a UE in idle or inactive mode to notify it about incoming data, calls, or system information changes. In LTE, paging was already optimized with Discontinuous Reception (DRX) cycles and efficient signaling. However, 5G introduces new requirements such as supporting massive IoT devices, enabling URLLC, and improving privacy/security.

3GPP TS 38.304 specifies the details of UE behaviour in RRC_IDLE and RRC_INACTIVE states, highlighting how paging is handled differently from LTE (TS 36.304).

Paging in LTE – A Quick Recap

In LTE, paging is initiated by the MME when the network needs to reach a UE in idle mode. The MME sends a paging request to all eNodeBs within the Tracking Area where the UE is registered. The eNodeBs then broadcast paging messages using the PCCH, carrying identities such as the TMSI.

The UE does not continuously listen to the paging channel; instead, it wakes up periodically based on its Discontinuous Reception (DRX) cycle. Using formulas defined in 3GPP TS 36.304, the UE determines its Paging Frame (PF) and Paging Occasion (PO) where it should monitor for messages. If the UE finds its identity, it transitions to connected mode via the RACH and responds with a Service Request.

This LTE framework ensures efficient network signaling and power saving for UEs but still has limitations when addressing modern demands such as massive IoT connections and privacy threats.

5G Paging Architecture

In 5G, the AMF replaces the LTE MME as the paging control entity. The AMF determines when paging is needed, e.g., when downlink data arrives or when the network must re-establish contact with a UE.

The AMF sends NGAP Paging messages to the appropriate gNBs within the Tracking Area (TA) or RAN Notification Area (RNA). These gNBs then broadcast the paging message to UEs over the PCCH using the PDCCH as scheduling control.

Paging in Idle and Inactive States

Unlike LTE, 5G introduces two modes of UE inactivity:

• RRC_IDLE – like LTE idle; UE camps on a cell, performs cell reselection, and monitors paging for notifications.

• RRC_INACTIVE – a new 5G state where the UE maintains context in the RAN but suspends the connection. Paging in this state is scoped to an RNA, which is more granular than LTE’s Tracking Area, reducing signaling and improving efficiency.

This dual-state design allows 5G to optimize paging depending on whether the UE is completely idle or only temporarily inactive.

Discontinuous Reception (DRX) for Paging

Paging relies on DRX cycles where the UE does not continuously monitor the channel but wakes up at defined intervals to check for paging messages.

• The UE determines its Paging Frame (PF) and Paging Occasion (PO) using formulas like LTE, based on its UE ID and DRX cycle.

• Each UE is assigned one or more Paging Occasions to avoid collisions.

• SIB1 in 5G provides DRX cycle length, PO configuration, and other relevant parameters.

• This ensures efficient energy saving, especially for IoT UEs with long cycles and for smartphones where low latency is critical

Paging Frame (PF)

A Paging Frame is a specific radio frame where a UE in idle or inactive mode should look for paging messages.

• Since the UE doesn’t continuously monitor the downlink (to save battery), the network defines DRX cycles.

• A DRX cycle tells the UE how often it should wake up to check paging.

• Based on the UE’s identity (UE_ID) and the configured default paging cycle, the UE computes a PF number.

• This ensures that different UEs "spread out" across frames so that not all devices wake at the same instant.

In simple terms:

PF = “the exact frame within the DRX cycle when the UE must wake up and listen.”

The PF is calculated using this formula:

Where:

• SFN: System Frame Number.

• PF_offset: Offset for PF calculation.

• T: DRX cycle length (shortest of UE-specific DRX or default broadcast in SIB1).

• N: Number of total paging frames in one DRX cycle.

• UE_ID: Usually derived from 5G-S-TMSI mod 1024 (temporary UE identifier).

Paging Occasion (PO)

A Paging Occasion is a specific subframe (slot) within the Paging Frame where the UE monitors the PCCH.

• Within each PF, there may be multiple POs.

• Each PO corresponds to a specific set of UEs based on their UE_ID modulo calculation.

• The network uses formulas (defined in 3GPP TS 38.304) that take the UE_ID, PF, Ns (number of paging occasions per PF) into account to map UEs to their correct PO.

In simple terms:PO = “the exact slot inside the PF where my UE should look for its paging message.”

The PO index (i_s) is calculated as:

Where:

§  Ns = Number of paging occasions per PF (1, 2, 4, or 8).

If the UE has no 5G-S-TMSI (e.g., before initial registration), then it assumes:

• UE_ID = 0 in PF and PO formulas.

• 5G-S-TMSI is a 48-bit binary number (defined in TS 23.501). In formulas, it is treated as a normal integer (most significant bit = leftmost).

5G Paging Procedure – RRC_IDLE vs RRC_INACTIVE

• Downlink data arrives at the SMF/UPF for a UE that is not in RRC_CONNECTED.

• SMF/UPF notifies the AMF about the pending downlink data (N11/N4 interface).

• AMF constructs the Paging message.

• AMF sends the NGAP Paging message to the serving gNB(s).

• gNB broadcasts Paging via PCCH/PDCCH in the configured area.

Case A: UE in RRC_IDLE

• UE wakes at its Paging Occasion (PO), based on PF/PO calculation from DRX cycle.

• UE detects its identity in the paging message and sends RRC Connection Request / Service Request to the gNB.

• gNB forwards the Initial UE Message to the AMF.

• AMF activates the UE’s PDU session with the SMF/UPF.

• AMF sends a Context Setup Request to the gNB.

• gNB responds with RRC Connection Setup to the UE.

• UE completes the procedure with RRC Connection Setup Complete.

• UE transitions from RRC_IDLE → RRC_CONNECTED.

• Data delivery begins from SMF/UPF to UE.

Case B – UE in RRC_INACTIVE (RNA Paging)

• UE wakes at its RNA Paging Occasion within its configured RNA area.

• gNB broadcasts RNA Paging in the RNA list.

• UE detects its identity and sends RRC Resume Request to the gNB.

• gNB forwards UE Context Resume Request to the AMF.

• AMF restores the UE’s session context with the SMF/UPF.

• AMF sends a Context Setup Request to the gNB.

• gNB responds with RRC Resume Accept to the UE.

• UE transitions from RRC_INACTIVE → RRC_CONNECTED.

• Data delivery begins from SMF/UPF to UE.

5G Paging Message Content

A 5G paging message carries several key elements:

• UE Identity – usually 5G-S-TMSI or I-RNTI (avoids IMSI exposure).

• TAI or RNA list – defines the area where the UE is expected.

• Paging DRX – the cycle length the UE should use.

• Paging Priority – allows prioritization of emergency alerts, URLLC triggers, or public warning systems (ETWS/CMAS).

• System Information Change Indicator – signals when the UE must read updated broadcast information.

Key Enhancements in 5G Paging

• RRC_INACTIVE and RNA Paging: Unlike LTE which used only Tracking Areas, 5G introduces RNA for UEs in inactive state. This enables localized paging without involving the entire TA, reducing signaling overhead and improving efficiency.

• Improved Privacy: LTE paging could reveal IMSI or static TMSI values, leaving users vulnerable to tracking. 5G eliminates this risk by exclusively using temporary identifiers (5G-S-TMSI/I-RNTI) that are frequently updated.

• Beamforming-based Paging: With 5G’s reliance on mmWave and beamforming, paging messages are not always broadcast omnidirectionally. Instead, they can be transmitted via specific beams, enhancing coverage, especially for high-frequency bands.

• Early Paging Indication (EPI): Introduced in Release 17, EPI informs the UE whether to monitor an upcoming Paging Occasion. This prevents unnecessary wakeups and achieves 17–34% energy savings, particularly for IoT UEs.

• Public Warning Systems: 5G paging also integrates support for ETWS (Earthquake and Tsunami Warning System) and CMAS (Commercial Mobile Alert System), ensuring that critical alerts reach UEs promptly even in idle or inactive states.

Differences Between LTE and 5G Paging

In LTE, paging was triggered by the MME and delivered via the PCCH to all cells in a TA. UEs used TMSI-based identifiers and DRX cycles for power saving.

In 5G, the AMF takes over, and paging is more sophisticated:

• Two states (RRC_IDLE, RRC_INACTIVE) instead of just idle.

• RNA-based paging to reduce signaling.

• Stronger privacy with temporary identifiers.

• Smarter energy-saving via EPI.

• Beamformed delivery for mmWave scenarios.

Thus, while the fundamental purpose remains the same, 5G paging is more adaptive, secure, and energy-efficient than LTE paging.

Paging in LTE established the foundation for idle mode reachability with DRX cycles and TA-based broadcasts. 5G refines this further through architectural updates (AMF and RNA), state handling (RRC_INACTIVE), and advanced techniques like beamformed paging and Early Paging Indication.

The result is a paging system that is more efficient, private, and responsive to diverse 5G use cases ranging from massive IoT to URLLC services.

Reference:

• 3GPP TS 38.304 (Idle/Inactive Procedures) – Defines how Paging Frame (PF) and Paging Occasion (PO) are calculated for UEs in RRC_IDLE and RRC_INACTIVE.

• 3GPP TS 38.331 (RRC Protocol) – Provides the ASN.1 structure and fields of the Paging message (UE identity, TAI/RNA list, priority, etc.).

• 3GPP TS 38.213 (Physical Layer Control) – Specifies how PDCCH monitoring occasions map to Paging Occasions (POs).

• 3GPP TS 23.501 (5G System Architecture) – Explains core network–triggered paging and AMF/SMF/UPF interactions.