Tutorial LoRa raw

Tutorial: LoRa (raw)

What it is. LoRa is a chirp-spread-spectrum radio used for point-to-point links, sensors and mesh networks. This mode shows plain LoRa frames between radios as bytes, hex and text, with the signal quality. It is not LoRaWAN and not a mesh; for those use Tutorial-LoRaWAN, Tutorial-Meshtastic, Tutorial-MeshCore and Tutorial-MeshCom. It is useful for finding out what an unknown LoRa transmitter is doing.

Read this first. Receive only. Nothing is sent, decrypted or interpreted: the payload is just the bytes, and whatever is encrypted stays encrypted. Frames from other people's devices may contain personal data; do not publish them. Check the rules where you live. See Legal.

What you need

• A radio that reaches the frequency (the mode table covers 137 to 2500 MHz). The mode asks for 2 Msps and works from 1 Msps, so an RTL-SDR is enough for 125 and 250 kHz signals.
• An antenna for the band: a 433 MHz or 868/915 MHz antenna.
• Frequencies from the database: 868.1 MHz (LoRa raw EU868, the start frequency) and 915.0 MHz (915 MHz). The Preset list adds EU868 868.1, 868.3, 868.5, 867.1 and 869.525 MHz, US915 902.3, 902.5, 903.0, 915.0 and 923.3 MHz and 433 ISM 433.175, 433.920 and 434.000 MHz.

Before you start

LoRa signals are weak and below the noise in the spectrum; you will not see them in the waterfall as clearly as FSK. Keep gain moderate. If you know the settings of the transmitter (spreading factor, bandwidth), enter them; otherwise use Auto.

Step by step

1. In the mode list open IoT & sensors and choose IoT, sub-GHz. Several sub-GHz IoT entries share that label, so check that the panel shows the Frames, Settings and Decoders tabs.
2. Pick the radio and start the receiver.
3. Pick a frequency from Preset or type one.
4. Open Settings. For Spreading factor and Bandwidth you can tick Auto: then every spreading factor SF7 to SF12 at every bandwidth (125, 250, 500 kHz) that fits the capture is searched. Untick Auto to fix them.
5. Set the Header mode: explicit (the header carries the settings) or implicit (then set coding rate and CRC, and the length).
6. Choose the Sync word: 0x12 private, 0x34 LoRaWAN, 0x2B Meshtastic, any of these or custom. LDRO (low data rate optimisation) is auto, on or off.
7. Watch Frames.

What you should see

Each frame appears with time, spreading factor, bandwidth, SNR, level, carrier offset, length and the bytes in hex and text, plus a CRC flag. A status line shows the state.

Reading the screen

• Frames: the frame list. hide CRC errors hides frames that failed the CRC.
• Settings: the radio parameters above, a Test signal choice (nine frames SF7 to SF12 explicit header, or an implicit header example) for trying the mode without a transmitter.
• Decoders: the search table: one row per spreading factor, bandwidth and sync word with the preambles, frames and CRC errors counted.

Options and settings

Filter, Copy and Clear work on the frame list. Preamble length and coding rate can be set under Settings.

Troubleshooting

• Preambles but no frames. Wrong header mode, coding rate or sync word. Switch to any of these for the sync word and Auto for SF and bandwidth.
• Many CRC errors. Weak signal or a frequency offset; improve the antenna.
• High CPU with Auto. Fix spreading factor and bandwidth.
• SF5 or SF6 frames. The SF5 and SF6 layouts follow the SX126x frame layout and have not been checked against a real radio.
• A mesh network shows only noise. Meshtastic and others use their own modes with keys and presets.

Tips

Use the Test signal first to learn what a decoded frame looks like.

Legal and privacy notes

Receive only. Do not publish payloads. For default keys and what is decodable in mesh modes, see the mesh tutorials.

Under the hood

LoRa uses chirp spread spectrum with spreading factors SF5 to SF12 and bandwidths 62.5, 125, 250 and 500 kHz, with Hamming-style forward error correction at coding rates 4/5 to 4/8 and an optional CRC. The modulation is proprietary to Semtech; the decoder follows publicly documented analyses of the physical layer.

Related

Mode-LoRa-raw, Tutorial-LoRaWAN, Tutorial-Meshtastic, Tutorial-LoRa-APRS, Tutorial-ISM-sensors, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.