Description
On-Demand SIB1 (OD-SIB1) is a 5G New Radio (NR) feature introduced in 3GPP Release 19, specified across the RAN protocol stack (TS 38.300, 38.331, etc.). System Information Block 1 (SIB1) is the most critical system information block, containing parameters necessary for a User Equipment (UE) to determine if it is allowed to access a cell (cell barring status, cell reservation) and the scheduling information for other SIBs. Traditionally, SIB1 is broadcast periodically by the gNB, consuming downlink radio resources continuously.
OD-SIB1 changes this paradigm. In this mode, the gNB does not broadcast SIB1 in its default state. Instead, it broadcasts a minimal System Information (SI), including the Master Information Block (MIB) and possibly SIB1 scheduling information, indicating that SIB1 is available on-demand. A UE requiring SIB1—for example, during initial cell selection after power-on or when entering a new tracking area—must explicitly request it. This request is made via a random-access channel (RACH) procedure. The UE sends a Physical Random Access Channel (PRACH) preamble, and in the subsequent Message 2 (Random Access Response, RAR), the gNB includes an uplink grant. The UE then uses this grant to transmit a dedicated RRC message (e.g., an RRCSystemInfoRequest) to solicit SIB1. Upon receiving this valid request, the gNB then broadcasts SIB1 for a configured duration, allowing the requesting UE and any other UEs in the area to acquire it.
The architecture involves coordination across the PHY, MAC, and RRC layers. The MIB, broadcast via the Physical Broadcast Channel (PBCH), carries a flag (e.g., si-BroadcastStatus) informing the UE of the on-demand status. The RRC layer in the UE handles the logic to trigger the request based on its need for SIB1. The gNB's MAC layer schedules the RAR with the UL grant, and its RRC layer triggers the subsequent SIB1 broadcast. This mechanism significantly reduces the average overhead of system information broadcasting, especially in small cells, indoor cells, or cells serving primarily connected-mode UEs where initial access events are infrequent. It is a key enabler for energy-efficient network operation, aligning with 5G's goals of sustainability and massive IoT support.
Purpose & Motivation
OD-SIB1 was created to address the inefficiency of perpetual system information broadcasting in 5G networks, particularly for deployment scenarios where continuous broadcast is wasteful. In traditional networks, SIB1 is broadcast every 80 ms regardless of whether any UE needs it. In low-traffic scenarios—such as a small cell at night, a neutral host network in a building after hours, or a network slice dedicated to sparse IoT sensors—this represents a constant drain on gNB energy and a perpetual consumption of downlink radio resources (energy and spectrum) for potentially little to no benefit.
The motivation stems from the 5G design pillars of enhanced mobile broadband, massive IoT, and ultra-reliable low-latency communications, all of which demand more efficient resource utilization. Release 19's focus on 'Network Energy Savings' provided the direct context. OD-SIB1 directly reduces the gNB's power consumption by allowing the transmitter to turn off the SIB1 broadcast channel when not needed. It also frees up physical resource blocks (PRBs) for user data traffic or other purposes. This solves the limitation of the 'always-on' broadcast model, which was a legacy from earlier generations where UE complexity and battery life were the primary constraints, not network energy efficiency. By making SIB1 acquisition a triggered, on-demand process, networks can achieve substantial operational cost savings and reduced environmental impact, especially as 5G deployments become denser.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (226 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the OD-SIB1 function was introduced, allowing SIB1 to be provided on-demand via dedicated RRC signalling instead of only through broadcast, as detailed in the "System Information Provisioning" and "System Information Handling" Change Requests. This included clarifications for scenarios like handover, such as corrections to SIB1 transmission during handover and the handling of missing fields in SIB1. Furthermore, specific configurations like CORESET#0, which schedules SIB1, were defined for cases when SIB1 is not broadcast.
- Slicing assistance information TS 38.300CR0024
- System Information Handling in TS38.300 TS 38.300CR0071
- CR on RACH configuration during HO TS 38.300CR0077
- Corrections to System Information TS 38.300CR0074
- Correction to the system information in Handover Request message TS 38.300CR0086
- System Information Provisioning TS 38.300CR0089
+ 49 more changes
In Release 16, the key new feature for OD-SIB1 was the introduction of an on-demand system information procedure for UEs in the RRC_CONNECTED state, allowing connected devices to request specific SIBs instead of relying solely on periodic broadcasts. This was accompanied by necessary corrections and refinements to the SIB1 processing and acquisition procedures to support the new on-demand mechanism. The release also specified that a mobile IAB-node could indicate via a SIB1 indicator that it is a mobile-IAB cell, which is part of the broader system information enhancements.
- Introduction of on-demand SI procedure in RRC_CONNECTED TS 38.300CR0194
- Introduction of 2-step RACH TS 38.300CR0197
- Introduction of on-demand SI procedure in RRC_CONNECTED TS 38.331CR1462
- Introduction of additional RACH configurations for TDD FR1 TS 38.331CR1486
- Implementing confirmation of code block group based transmission TS 38.331CR1717
- Introduction of on-demand SIB(s) procedure in RRC_CONNECTED TS 38.300CR0237
+ 46 more changes
In Release 17, the enhancements for On-Demand System Information Block 1 (OD-SIB1) included corrections to the on-demand SI request procedure itself. Furthermore, the release introduced clarifications for the reception of SIB1 by a remote UE, ensuring proper system information acquisition in relay scenarios. These updates refined the existing OD-SIB1 mechanism for improved reliability and broader operational conditions.
- Introduction of RACH triggers for T_ADV in NR E-CID [NRTADV] TS 38.300CR0407
- Introduction of Common RACH Partitioning Aspects TS 38.331CR2951
- Signalling of Neighbour cell CSI-RS configuration information over Xn [CSIRSXn] TS 38.423CR0653
- Support of QoE information transfer TS 38.473CR0826
- Support of RACH-based SDT TS 38.473CR0834
- Introduction of RACH partitioning TS 38.300CR0466
+ 35 more changes
In Release 18, the enhancements for OD-SIB1 primarily involved the generalization and refinement of RACH-less handover procedures. Specifically, the release introduced support for RACH-less handover during mobile IAB-DU migration and provided corrections and clarifications for RACH-less HO use cases and configurations. These updates built upon the existing framework for on-demand system information to enable more efficient, contention-free cell transitions.
- RACH-less support generalization [RACH-lessHO] TS 38.300CR0799
- GNSS LOS/NLOS posSIB broadcast assistance information [GNSS LOS/NLOS] TS 38.331CR4109
- Generalization of RACH-less handover [RACH-lessHO] TS 38.331CR4636
- Introduction of XR Broadcast Information IE for XnAP [2Rx_XR_Device] TS 38.423CR1228
- Introduction of Measurement Quality and Time Stamp Information to E-CID [ECIDQualTimeStamp] TS 38.473CR1339
- Transfer PDU Set Information during data forwarding for Xn handover TS 38.300CR0828
+ 52 more changes
In Release 19, the primary new development for the OD-SIB1 function was the introduction of its full support, enabling UEs to request this system information block on-demand. This release also included necessary corrections to the stage-2 specification, the Xn interface support for OD-SIB1 coordination, and the configuration provision status update procedure to ensure robust operation. Additionally, a correction was made to the OD-SIB1 procedure specifically for network energy saving scenarios.
- Introduction of control parameters for on-demand posSIB request [OdPosSIB_Req] TS 38.300CR1009
- Support Aerial UE Flight Information Reporting TS 38.300CR1031
- Introduction of control parameters for on-demand posSIB request [OdPosSIB_Req] TS 38.331CR5406
- Introduction of UE assistance information for cell DTX/DRX [UAI_cellDTRX] TS 38.331CR5474
- Support for On-demand SIB1 TS 38.401CR0480
- Support Aerial UE Flight Information Reporting TS 38.423CR1460
+ 14 more changes
Explore further
Broader topics and technologies where OD-SIB1 plays a role.
Defining Specifications
3GPP specifications that define or reference OD-SIB1, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| 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 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |