Paging Early Indication Radio Network Temporary Identifier

Description

The Paging Early Indication RNTI (PEI-RNTI) is a Radio Network Temporary Identifier in the 5G NR physical layer, specifically designed for the Paging Early Indication (PEI) feature. An RNTI is essentially a number used to scramble and identify control channel messages intended for a specific UE or group of UEs. The PEI-RNTI is a group identifier, meaning a single PEI-RNTI value is shared by a configured set of UEs monitoring the same Paging Early Indication-Occasion (PEI-O). The gNB uses this PEI-RNTI to scramble the Cyclic Redundancy Check (CRC) bits of the Downlink Control Information (DCI) format that conveys the PEI signal.

Operationally, when a gNB needs to alert a group of UEs that paging messages will follow, it generates a DCI on the PDCCH during the PEI-O. This DCI is scrambled with the PEI-RNTI known to the target UE group. Each UE in that group, upon waking at its PEI-O, attempts to blindly decode PDCCH candidates using its assigned PEI-RNTI. The process is highly efficient: the UE only needs to perform a CRC check with the PEI-RNTI. If the CRC check passes, the DCI is successfully decoded, and the payload (a simple indicator) informs the UE to monitor the subsequent Paging Occasion. If no DCI with the matching PEI-RNTI is detected, the UE interprets this as a 'negative' PEI and returns to sleep.

The PEI-RNTI is a critical component that enables the group-based addressing required for efficient PEI signaling. Without such a shared identifier, the network would need to signal to each UE individually, negating the power-saving benefits. The value of the PEI-RNTI is derived from parameters provided to the UE during configuration, such as via System Information Block (SIB) or dedicated RRC signaling. Its use is confined to the physical and MAC layers for the specific purpose of PEI detection. This mechanism exemplifies the layered approach to power saving in 5G, where a minimal physical layer signal, efficiently addressed via a group RNTI, gates the more energy-intensive higher-layer paging procedure.

Purpose & Motivation

The PEI-RNTI was created to solve the problem of efficiently and reliably addressing multiple UEs with a single, low-overhead wake-up signal within the Paging Early Indication framework. Prior to this feature, paging was addressed using the P-RNTI, which alerted all UEs in a paging occasion to decode a subsequent paging message, but did not provide an early, low-power filtering step. The need for a dedicated identifier arose from the design goal of the PEI: to be an ultra-low-power signal that a UE can detect with minimal processing.

Using a shared group RNTI is a proven, efficient method in cellular systems for multicast control signaling. The PEI-RNTI enables this for the PEI, allowing one compact DCI message to serve an entire group of UEs simultaneously. This design minimizes control channel overhead and UE processing complexity compared to individual signaling. It was motivated by the stringent power consumption targets for 5G, especially for IoT. The PEI-RNTI provides the necessary addressing mechanism that makes the two-step paging process (PEI check followed by potential PO decode) feasible and power-efficient, directly contributing to extended battery life for devices that are paged infrequently.

Key Features

• Group-based Radio Network Temporary Identifier for PEI signaling
• Used to scramble CRC of DCI carrying the Paging Early Indication
• Enables efficient blind decoding of PEI by a group of UEs
• Value derived from UE configuration parameters (e.g., from SIB or RRC)
• Fundamental to the physical layer operation of the PEI feature
• Reduces control channel overhead compared to individual UE signaling

Evolution Across Releases

Defining Specifications