LoRa / Meshtastic Signal Analysis

~120 minIntermediate

Understand Chirp Spread Spectrum modulation, visualize LoRa signals in a waterfall, and decode LoRa PHY frames using gr-lora in GNU Radio.

Prerequisites
Gather these before starting

Hardware

  • •RTL-SDR dongle or HackRF One
  • •Antenna for 868/915 MHz (1/4-wave whip)
  • •Computer capable of running GNU Radio
  • •Optional: Meshtastic node to generate known signals

Software / Knowledge

  • •GNU Radio 3.10+ (from FM Radio workshop)
  • •gr-osmosdr installed
  • •inspectrum for signal analysis
  • •Basic GNU Radio Companion experience
Step 1 of 714% complete
Step 115 min
Understanding LoRa CSS Modulation

LoRa (Long Range) is a proprietary spread-spectrum modulation technique developed by Semtech, using Chirp Spread Spectrum (CSS) to achieve remarkable range — up to 15 km line-of-sight — at extremely low power. It underlies the LoRaWAN IoT protocol and Meshtastic mesh networking.

What is CSS (Chirp Spread Spectrum)?

  • A chirp is a signal that continuously sweeps from low to high frequency (up-chirp) or high to low (down-chirp)
  • Data is encoded by the starting frequency offset of each chirp cycle
  • Each symbol = one complete chirp across the bandwidth
  • The chirp duration equals 2^SF / BW (where SF = Spreading Factor)
  • Chirps are highly resistant to multipath fading and narrowband interference
  • The receiver correlates against a reference chirp — works even at SNR = -20 dB

Key LoRa parameters

Spreading Factor (SF)7–12 (higher = longer range, lower data rate)
Bandwidth (BW)125, 250, 500 kHz
Coding Rate (CR)4/5, 4/6, 4/7, 4/8 (FEC overhead)
Symbol rateBW / 2^SF symbols/second
Bit rate (SF7/BW125)~5.5 kbps
Bit rate (SF12/BW125)~250 bps
Sensitivity (SF12)–137 dBm (exceptional!)

LoRa vs LoRaWAN

LoRa = the physical layer modulation (Semtech's CSS). LoRaWAN = the network protocol stack built on top of LoRa (MAC layer, gateways, network server). Meshtastic = a mesh networking protocol that uses LoRa PHY directly without LoRaWAN infrastructure. In this workshop we analyze the raw LoRa PHY layer signals visible to an SDR.

LoRa is a proprietary Semtech modulation scheme — it's patented but the physical layer has been thoroughly reverse-engineered by the open-source community, enabling SDR analysis.

Understanding LoRa CSS Modulation