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5G Broadcast - Emergency Alerts

Cell Broadcast emergency alerts over a 5G Broadcast transmitter

This tutorial describes a basic setup to deliver emergency alerts compliant with the Cell Broadcast Service (CBS) over the rt-mbms-tx-for-qrd-and-crd transmitter from Reference Tools.

rt-mbms-tx-for-qrd-and-crd 5G Broadcast transmitter used throughout this tutorial, built from the emergency-alerts branch.
In short

By the end of this tutorial, a real 5G Broadcast capable handset will display an emergency alert broadcast from your own Software Defined Radio (SDR). You will need the hardware listed under Prerequisites, in particular an SDR and a compatible receiving device.

What you will build: an end-to-end emergency alert chain, where a 5G Broadcast transmitter driven by an SDR broadcasts a CBS alert that is received and shown on a device.

Prerequisites​

The following components are required to set up the end-to-end chain for emergency alerts:

QRD/CRD device SDR (e.g. BladeRF) + antenna Host: Ubuntu 22.04
  • A QRD or CRD device. These are the 5G Broadcast capable receiving devices used with the transmitter (confirm the exact meaning of the QRD and CRD abbreviations against the rt-mbms-tx-for-qrd-and-crd repository).
  • A Software Defined Radio (SDR) such as the BladeRF with an antenna connected to the TX1 port.
  • A Linux machine running Ubuntu 22.04.

Architecture​

The setup consists of a 5G Broadcast transmitter (rt-mbms-tx-for-qrd-and-crd running the emergency-alerts branch) connected to a Software Defined Radio (SDR). A QRD or CRD device receives the broadcast signal and decodes the Cell Broadcast Service (CBS) alert messages. The transmitter reads alert content from a configuration file and encodes it as CBS messages compliant with 3GPP TS 23.041.

Installation​

Step 1: Install the 5G Broadcast Transmitter​

Install the dependencies and SDR drivers for the transmitter as documented here.

Next, clone the transmitter repository using the emergency-alerts branch:

git clone --recurse-submodules -b emergency-alerts https://github.com/5G-MAG/rt-mbms-tx-for-qrd-and-crd.git rt-mbms-tx-for-qrd-and-crd-emergency-alerts

Now build the transmitter running the following commands:

cd rt-mbms-tx-for-qrd-and-crd-emergency-alerts
git submodule update
mkdir build && cd build
cmake -GNinja ..
ninja

Configuration​

Step 2: Configuration of the 5G Broadcast Transmitter​

Follow the configuration instructions documented here. Make sure to adjust the dl_freq and the dl_earfcn in the enb.conf based on the frequency that your CRD or QRD device is operating on. To derive the right dl_earfcn you can use this tool. Note also that SoapySDR might detect the wrong output (e.g. an audio device instead of your SDR.). In that case make sure to use device_name and device_args to select the right output device.

Example configuration files are located under the configuration-template directory. These will be called when running.

Copy the bytecode file to a folder /home/fivegmag.

cd ~/rt-mbms-tx-for-qrd-and-crd-emergency-alerts/Config-Template
cp bytecode /home/fivegmag/bytecode

Running​

Step 3: Running the 5G Broadcast Transmitter​

The transmitter is made up of three components you start in separate terminals:

  • MBMS-GW (Multimedia Broadcast Multicast Service Gateway): receives the multicast alert traffic and forwards it towards the radio.
  • EPC (Evolved Packet Core): the LTE core network functions the transmitter relies on.
  • eNodeB (the LTE base station): produces the broadcast radio signal via the SDR.

The MBMS-GW receives multicast packets on one tunnel interface, packages them to GTP-U packets and sends them to the eNodeB over another tunnel interface. The command below creates the sgi_mb interface used for that multicast traffic:

sudo route add -net 239.11.4.0 netmask 255.255.255.0 dev sgi_mb

Start the MBMS Gateway, EPC and eNodeB in different terminals, in that order (gateway first, then EPC, then eNodeB):

cd ~/rt-mbms-tx-for-qrd-and-crd-emergency-alerts/build
sudo ./srsepc/src/srsmbms ../Config-Template/mbms.conf
cd ~/rt-mbms-tx-for-qrd-and-crd-emergency-alerts/build
sudo ./srsepc/src/srsepc ../Config-Template/epc.conf
cd ~/rt-mbms-tx-for-qrd-and-crd-emergency-alerts/build
sudo ./srsenb/src/srsenb ../Config-Template/enb.conf

Note that some of these files point to directories which should be adapted for your own setup. For instance user_db.csv inside epc.conf points to db_file = /home/fivegmag/rt-mbms-tx-for-qrd-and-crd-emergency-alerts/Config-Template/user_db.csv

Step 4: Start the UE​

Now that the transmitter is running you can turn on your receiving device (the UE). You should receive an alert shortly after the device is turned on. The screenshot below shows an example alert displayed on the device:

Emergency alert message displayed on the receiving device

Figure: an emergency alert shown on the receiving device.

You now have a working end-to-end emergency alert chain. The transmitter is broadcasting a CBS alert over your SDR, and it is being received and displayed on the device.

Step 5: Changing the type of the alert​

With the current implementation, the SIB 12 payload is static and defined in Config-Template/sib.conf.mbsfn. To change the type of the alert you need to open Config-Template/sib.conf.mbsfn and change the message_identifier. A list of possible values is defined in 3GPP TS 23.041 Section 9.4.1.2.2. The example below sets message_identifier to 0x1102, which corresponds to the combined earthquake and tsunami warning row in the table that follows:

sib12 =
{
    message_identifier = 0x1102;
    serial_number = 0x0001;
    data_coding_scheme = 01;
    warning_msg_segment_type = "lastSegment";
    warning_msg_segment_num = 0;
    warning_msg_segment_r9 = "01C576597E2EBBC7F950A8D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D168341A8D46A3D1000A";
};

The table below lists the message identifiers and their meaning. ETWS is the Earthquake and Tsunami Warning System and CMAS is the Commercial Mobile Alert System; both are public warning message categories carried over CBS.

message_identifierDescription
0x1100ETWS CBS Message Identifier for earthquake warning message
0x1101ETWS CBS Message Identifier for tsunami warning message
0x1102ETWS CBS Message Identifier for earthquake and tsunami combined warning message
0x1104ETWS CBS Message Identifier for messages related to other emergency types
0x1112-1130CMAS CBS Message Identifier
0x1131-113BNon-ETWS CBS Message Identifier

Step 6: Triggering multiple alerts​

To trigger a new alert the serial_number needs to be changed. At this point, there is no interface to change the serial_number while the eNodeB is still running. You will need to change the serial_number in build/sib.conf.mbsfn manually (this is the build-time copy the running transmitter reads) and then restart the eNodeB.

Wrapping up​

You have set up an end-to-end emergency alert chain: building and configuring the 5G Broadcast transmitter, broadcasting a CBS alert carried in SIB12 over your SDR, and receiving and displaying it on a device. You also saw how to change the alert type through message_identifier and how to trigger repeat alerts by changing serial_number.

warning

Transmitting radio signals is subject to local regulation. Only transmit in a shielded environment or under an appropriate authorisation, and check the rules that apply to your frequency and location before running the transmitter.

Next steps​