【5G RRC】Introduction to 5G System Information SIB8

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I work for an internationally renowned terminal manufacturer and am responsible for the research and development of modem chips.
In the early days of 5G, he was responsible for the development of the terminal data service layer and the core network. Currently, he is leading the research on technical standards for 6G computing power networks.


The content of the blog mainly revolves around:
       5G/6G protocol explanation
       computing power network explanation (cloud computing, edge computing, end computing)
       advanced C language explanation
       Rust language explanation

1. Introduction to 5G system message SIB8

       The system information is downlink broadcast information periodically sent by the base station. SIB8 is used to provide Commercial Mobile Alert Service (CMAS, Commercial Mobile Alert Service) notifications. CMAS is used as a Public Warning System (PWS, Public Warning System) for sending multiple concurrent warning text messages. The FCC established CMAS, which allows carriers to send emergency alerts in the form of text messages to users. This emergency alert broadcast can be across the entire network or within a specific geographic area, down to the size of a single cell.

       Emergency alert messages are basically generated by an entity called a Cell Broadcast entity (CBE), which is managed by national authorities and deals with public safety or crime prevention or both in the countries where the network is deployed. The figure below shows the high-level broadcast flow for CMAS alerts.

2. Transmission details

• Signaling radio bearer: not applicable
• RLC mode: TM
• Logical channel: BCCH
• Shipping channel: DL-SCH
• Physical channel: PDSCH
• 加所RNTI：SI-RNTI
• Transmission direction: gNB–>UE

3. SIB8 message content

The following shows the content of the system information block used for CMAS alert in 5G NR:

• messageldentifier
  It is a 16-bit string that defines the type of CMAS message broadcast in SIB8. These types are specified in 3GPP TS 23.041, some of which include "Presidential Alert", "Extreme Alert with Severity of Extreme, Urgent or Immediate, and Observable Certainty" and "Severity of Critical and Urgent Anticipated serious alert".

• serialNumber
  which is a 16-bit string used to track any changes in CMAS notifications. The sequence number is updated every time a CMAS message with a specific message identifier is updated.

• warningMessageSegmentType
  It is an enumeration data used to indicate whether the message sent with the CMAS alert is the last segment of the message.

• warningMessageSegmentNumber
  indicates the number of the message segment, and its value ranges from 0 to 63. Use this number when reconstructing the complete message.

• warningMessageSegment
  This IE contains the actual CMAS message and allows to carry single or multiple OCTET information.

• dataCodingScheme
  which identifies the alphabet, encoding and language used for CMAS messages. Allowed values ​​are specified in 3GPP TS 23.038.

• warningAreaCoordinatesSegment
  It provides information about the valid area of ​​CMAS warning messages. The UE uses this information to determine whether to display a warning message.

The figure below shows an actual SIB8 content:

Four. Summary

• SIB8 is other system information (OSI);
• It is wrapped with the generic RRC message SystemInformation , which means that when you check the RRC message for SIB8 in the UE log, you won't see it is SystemInformationBlockType8 , instead you need to look for the SystemInformation RRC message and see if Contains SIB8;
• SIB8 scheduling information is provided by SIB1;
• SIB8 is mapped to BCCH logical channel, DL-SCH transmission channel and PDSCH physical channel;
• SIB8 can be received and decoded by UE in two states of RRC_IDLE and RRC_Connected;
• SIB8 is most commonly used in North America for emergency alerts;
• Emergency alerts can range from the highest level of presidential alert to the lowest level of normal alert;

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