Description
Paging Early Indication-Occasion (PEI-O) is a Radio Access Network (RAN) concept introduced in 5G NR to enhance UE power efficiency. It refers to the predefined time resource (occasion) where a gNB can transmit a Paging Early Indication (PEI) signal. This signal is a brief, low-complexity physical layer indicator sent ahead of the actual Paging Occasion (PO). UEs operating in power-saving modes, specifically RRC_IDLE or RRC_INACTIVE states, wake up at their configured PEI-O to detect this signal. The PEI signal itself is a simple binary indicator, often conveyed via a Downlink Control Information (DCI) format scrambled by a PEI-RNTI, informing a group of UEs whether paging messages for them are present in the upcoming PO.
The operational workflow begins with the network configuring UEs with paging parameters, including their Paging Frame (PF), Paging Occasion (PO), and now, a corresponding PEI-O. This configuration is delivered via system information or dedicated RRC signaling. The PEI-O is temporally positioned before the associated PO, allowing a UE to first check the PEI. If the PEI indicates 'negative' (no paging for the monitored group), the UE can immediately return to a deep sleep state without powering up its full receiver chain to decode the PDCCH and PDSCH during the PO. This skip saves substantial battery power. If the PEI indicates 'positive', the UE remains awake to monitor its PO for a potential paging message.
Architecturally, PEI-O is a resource defined within the NR frame structure. Its periodicity and timing are derived from the UE's identity and paging configuration, ensuring alignment between the UE and gNB. The gNB's RRC and MAC layers determine which UEs need to be paged and then instruct the physical layer to transmit the appropriate PEI signal at the calculated PEI-Os. This mechanism is a key component of 5G's enhanced discontinuous reception (eDRX) and power saving features. It optimizes the trade-off between paging latency and UE battery life, which is especially critical for massive IoT deployments and always-on consumer devices, by drastically reducing the energy consumed during frequent paging monitoring cycles.
Purpose & Motivation
PEI-O was created to address the critical challenge of UE power consumption, particularly for devices that spend most of their time in low-power states (RRC_IDLE/INACTIVE) but must periodically wake up to check for paging messages. The traditional approach required UEs to fully decode the PDCCH at every Paging Occasion, which is a high-energy operation, even when no page is intended for them. This resulted in inefficient battery drain, a major limitation for IoT devices with decade-long battery life targets and for smartphone battery endurance.
The introduction of PEI-O in 3GPP Release 17 is part of the broader 'NR Power Saving' work item. It solves this by adding a low-power wake-up signal checkpoint. The motivation stems from the need to scale IoT and improve user experience. By allowing the UE to perform a very low-complexity detection (the PEI signal) before committing to full paging channel decoding, the energy cost of 'false wake-ups' is nearly eliminated. This innovation directly extends battery life for a wide range of devices without impacting the network's ability to reach them with low latency when necessary. It represents an evolution from LTE's paging mechanism, introducing an extra, efficient filtering step controlled by the network.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (59 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the concept of a Paging Occasion (PO) was formally defined as a set of PDCCH monitoring occasions where paging DCI can be sent. The specifications detailed the formulae for determining the Paging Frame (PF) and PO index based on parameters like UE_ID, DRX cycle (T), and the number of paging occasions per frame (N). It also clarified paging monitoring procedures for UEs in RRC_IDLE and RRC_INACTIVE states, including specific handling for CN-initiated paging and the role of L2 U2N Relay UEs.
- Clarification of Paging Monitoring Occasion TS 38.304CR0047
- CR on PDCCH monitoring occasions for paging TS 38.304CR0055
- Miscellaneous Corrections in Paging TS 38.304CR0075
- Correction to monitoring paging in Camped on Any Cell state TS 38.304CR0089
- Minor clarifications for paging TS 38.304CR0090
- 38.304 CR on derivation of N for paging TS 38.304CR0099
+ 18 more changes
In Release 16, the PEI-O (Paging Early Indication-Occasion) function was introduced as a new paging adaptation mechanism to reduce UE power consumption. This function allows the UE to monitor an early indication signal prior to the full Paging Occasion, enabling it to skip the subsequent, more power-intensive monitoring of the full paging DCI if the indication is not present. The details for this paging adaptation, including parameters like `pagingAdaptFirstPDCCH-MonitoringOccasionOfPO`, are specified within the paging procedures for RRC_IDLE and RRC_INACTIVE states.
- CR for Early Implementation in NR TS 38.331CR1961
- Introduction of voice fallback indication TS 38.331CR1312
- Redirection with MPS Indication [Redirect_MPS_I] TS 38.331CR2413
- Correction on the calculation of CG occasion TS 38.331CR2010
- Corrections on the UE capability of indication on supporting the extension of SRSresourceID TS 38.331CR2580
- Early implementation of eCall over IMS in NR TS 38.331CR2714
+ 2 more changes
In Release 17, the PEI-O function was enhanced to align the DRX for paging with RRC procedures for Small Data Transmission (SDT), ensuring coordinated paging monitoring during SDT. Furthermore, corrections were made to paging monitoring in extended DRX (eDRX) scenarios and to paging procedures for Multicast Broadcast Services (MBS). These updates refined the UE's behavior for monitoring paging occasions and short messages across different states and services.
- Explicit Indication of SI Scheduling window position [SI-SCHEDULING] TS 38.331CR2953
- Early Measurements for EPS fallback [IdleMeaEPSFB] TS 38.331CR2872
- Alignment of DRX for Paging with RRC for SDT TS 38.304CR0251
- Correction on UE behavior for NAS-based busy indication in RRC_INACTIVE TS 38.331CR3182
- Correction to explicit indication of SI Scheduling window position [SI-SCHEDULING] TS 38.331CR3486
- Corrections on satellite ephemeris indication TS 38.331CR3940
+ 3 more changes
In Release 18, enhancements for PEI-O (Paging Early Indication-Occasion) were introduced through clarifications and corrections for paging procedures during specific modes. This included clarifications on the order of the PagingRecordList and corrections for paging monitoring during Small Data Transmission (SDT) and extended Configured Grant (CG) periods. Additionally, corrections were made for group paging handling and for multicast reception in RRC_INACTIVE state upon paging.
- MUSIM paging cause forwarding [MUSIMpagingCause] TS 38.331CR4414
- Introduction of RRCRelease with resume indication for SDT [SDT_ReleaseEnh] TS 38.331CR4435
- [S081] Clarification for order of list in PagingRecordList TS 38.331CR4787
- B020 Correction of cell barred indication value for 2Rx XR [2Rx_XR_Device] TS 38.331CR4858
- Paging monitoring for extended CG period [CG-SDT-Enh] TS 38.331CR4788
- Correction for Paging monitoring during SDT [CG-SDT-Enh] TS 38.331CR4901
+ 10 more changes
In Release 19, the new PEI-O function introduced a band-specific paging capability, allowing the network to configure paging reception per frequency band. Additionally, the release introduced early CSI acquisition specifically for Layer 3 handover procedures, enhancing mobility performance. These additions provide more granular control over paging and handover processes.
Explore further
Broader topics and technologies where PEI-O plays a role.
Defining Specifications
3GPP specifications that define or reference PEI-O, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.304 vj00 | UE RRC_IDLE and RRC_INACTIVE Procedures | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |