Public warning over 5G Broadcast carries emergency alerts (earthquake and tsunami warnings, and CMAS-style civil alerts) to any receiver in coverage, using the same free-to-air LTE-based broadcast carrier as linear TV and radio. Because it needs no SIM, no subscription and no return channel, it keeps working when the cellular network is congested or unavailable, which is exactly when warnings matter most.
The warning-message handling is added to the existing 5G Broadcast - TV and Radio Services transmit and receive chain rather than introduced as a separate stack: it is best understood as a feature set layered on that broader platform, developed and tracked together with it.
The Problem It Solves
A warning delivered only over a data connection fails exactly when it matters most: when the cellular network is congested with everyone trying to check on each other, or down entirely. Carrying the alert as a Cell Broadcast message on the same free-to-air broadcast carrier as TV and radio sidesteps that — it needs no SIM, no subscription and no return channel, so it reaches idle, roaming and otherwise unattached devices the same way it reaches every other receiver in coverage. Layering this on the existing 5G Broadcast chain, rather than building a separate alerting stack, also means no new receiver hardware is needed beyond what TV and radio delivery already requires.
Open Source, Built Together
Reference Tools for This Project
The real, running software for this technology — 4 repositories.
Also used by 5G Broadcast - TV and Radio Services
How It Works
An alert is passed to the transmitter, encoded as a Cell Broadcast Service (CBS) message carried in a System Information Block, broadcast over the FeMBMS carrier, and received and presented on a device -- reusing the 5G Broadcast receiver rather than requiring an alert-specific one.
Key specifications: ETSI TS 103 720 (5G Broadcast System for linear TV and radio services -- clause 5.15.3.3 defines PWS support, the delivery of warning messages via SIB broadcast on the E-UTRAN Uu downlink), 3GPP TS 23.041 (Technical realization of Cell Broadcast Service, CBS: message structure, message identifiers, serial numbers and data coding for ETWS and CMAS), 3GPP TS 36.331 (defines SystemInformationBlockType12 and the UE actions on receiving a warning-message SIB), 3GPP TR 36.976 (overall description of LTE-based 5G broadcast).
Public warning here follows the Public Warning System (PWS) approach: CBS is the delivery mechanism, and the alert types it can carry include the Earthquake and Tsunami Warning System (ETWS) and the Commercial Mobile Alert System (CMAS).
Transmit and receive roles
The Emergency Alerts feature set sits on the LTE-based 5G Broadcast transmit chain, in the roles the standard itself defines:
- RAN / physical-layer transmitter. The 5G Broadcast transmitter builds the E-UTRA system information carried on the broadcast carrier. For public warning, this is where the warning-message System Information Blocks are scheduled and encoded onto the radio frame, so that a connected 5G Broadcast device receives the SIB12 message that triggers a CMAS alert.
- SIB12 encoding. The transmitter constructs the SIB12 message body from a set of CBS fields (message identifier, serial number, data coding scheme, and the warning-message segment carrying the alert text) — the fields a standards-conformant transmitter needs to populate to broadcast a given alert (for example a tsunami or earthquake warning).
- Receiver / device. A 5G Broadcast capable device tuned to the carrier decodes the system information, detects the warning-message SIB, decodes the CBS payload, and presents the alert. 3GPP does not define an alert-specific receiver: the same procedure that decodes ordinary system information also decodes SIB10/11/12, so a warning is just another scheduled SIB.
The mapping to the standard: the transmitter plays the role of the E-UTRAN cell (the eNB in a live network); the SIB12 it emits is the same warning-message structure a live network would broadcast; and the device applies the standard UE procedure for acquiring and acting on warning-message system information.
The CBS message model on the transmit side
CBS carries a warning message as a short, self-contained data structure rather than as an IP flow, which is what lets it reach idle and non-attached devices. The main fields a developer will encounter when configuring or extending the transmitter are:
| CBS field | Role | Notes |
|---|---|---|
| Message Identifier | Identifies the warning type / source (for example a CMAS or ETWS category) | Assigned in 3GPP TS 23.041 clause 9.4.1.2.2; the reference transmitter's default configuration uses the fixed identifier 0x1102, which TS 23.041 assigns to the ETWS combined earthquake and tsunami warning. |
| Serial Number | Distinguishes and versions a message (geographical scope, message code, update number) | Changing the update number signals an updated message for the same event. |
| Data Coding Scheme | Selects the character set / language of the message body | Follows the CBS data coding scheme rules in TS 23.041. |
| Warning Message Segment | Carries the alert text, possibly split across segments | The transmitter marks the last segment and sets the segment number; the payload is the encoded warning text. |
For CMAS-style alerts the message is carried in SIB12. For ETWS the notification and body are carried in SIB10 and SIB11 respectively, each following the same scheduling and encoding model as SIB12.
The complete public warning chain
A live Public Warning System has more upstream steps than the broadcast hop this page focuses on: a Cell Broadcast Entity feeds a Cell Broadcast Centre, which runs the CBC-to-MME Write-Replace Warning procedure, and the MME distributes the warning to the relevant eNBs. This page's analysis covers the last hop of that chain — network to device over the broadcast carrier — which is where CBS/SIB12 delivery and the standard UE procedure for acting on it are defined.
5G-MAG's own reference tools overview slide deck introduces the alert path, the Roadmap tracks current implementation work, and the MBMS: Public Warning System board tracks the underlying developer issues.
Related: 5G Broadcast - TV and Radio Services (the underlying transmit and receive platform this extends) · Standards: 5G Broadcast - TV, Radio and Emergency Alerts (the full specification set, including ETSI TS 103 720, that this builds on)
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