Also known as: LoRa, chirp spread spectrum
LoRa (from “long range”) is a low-power, long-range modulation that encodes data as chirps — frequency sweeps that ramp linearly across the channel. Its chirp spread spectrum (CSS) waveform spreads each symbol over a wide bandwidth, giving LoRa exceptional sensitivity: it can be demodulated several dB below the noise floor, which is what makes it popular for low-rate IoT telemetry over kilometres in unlicensed ISM bands.1
Overview
A LoRa symbol is an up-chirp that sweeps the full channel bandwidth; the value of the symbol is set by where in the sweep the chirp starts and wraps around the band edge (a cyclic time shift). A receiver de-chirps by multiplying the incoming signal with a locally generated down-chirp, which collapses each chirp to a constant tone. An FFT then reads off the tone’s frequency bin, and that bin index is the symbol. Because de-chirping concentrates a wideband chirp into a single narrow bin, it delivers large processing gain — the mechanism behind LoRa’s sub-noise sensitivity, conceptually related to direct-sequence spread spectrum but using a chirp instead of a pseudorandom code.
Two parameters trade range against data rate. Bandwidth (typically 125, 250, or 500 kHz) sets the chirp’s sweep width. The spreading factor SF7–SF12 sets the chirp length: each step up doubles the symbol duration (2^SF chips per symbol), adding roughly 2.5 dB of link budget but halving the bit rate. High SF reaches farthest and slowest; low SF is faster and shorter-range. Payloads are protected by a header CRC and forward error correction (a configurable coding rate), with Gray-mapping and interleaving applied to the symbols.
Technical characteristics
| Property | Value |
|---|---|
| Modulation | Chirp spread spectrum (CSS), up/down-chirps |
| Bands | 433 / 868 / 915 MHz sub-GHz ISM (also 2.4 GHz) |
| Bandwidth | 125 / 250 / 500 kHz (sub-GHz) |
| Spreading factor | SF7–SF12 (2^SF chips/symbol) |
| Bit rate | ~0.25–50 kbps depending on SF and BW |
| Coding | Configurable FEC (4/5–4/8), CRC, interleaving |
| Sensitivity | down to about −137 dBm at SF12 |
History
LoRa’s chirp-modulation PHY was developed by the French startup Cycleo and acquired by Semtech in 2012, which keeps the physical layer proprietary and sells the radio chips. To standardise the network layer on top, industry members formed the LoRa Alliance in 2015, which publishes the open LoRaWAN specification — the MAC, addressing, and encryption that turn raw LoRa links into a low-power wide-area network. The PHY and the network layer are thus governed separately.12
Deployment
LoRa underlies a large share of long-range IoT: utility metering, agricultural and environmental sensors, asset tracking, and building automation, typically via LoRaWAN star-of-stars networks with battery devices reporting to gateways. Beyond LoRaWAN, the same PHY carries other protocols — most visibly Meshtastic, an open off-grid mesh-messaging project popular with hobbyists — and point-to-point telemetry links. It competes with other LPWAN technologies such as NB-IoT and Sigfox.
Decoding it with GopherTrunk
GopherTrunk does not decode LoRa. It targets trunked land-mobile voice and control systems (P25, DMR, NXDN, TETRA and the like), and LoRa’s chirp PHY and IoT MAC sit outside that scope; recovering it needs a de-chirp/FFT demodulator and, for LoRaWAN, the device session keys. LoRa is still a common and easily recognised sight on the waterfall in the 433/868/915 MHz bands — the diagonal chirp ramps are unmistakable — so it is worth knowing even though GopherTrunk leaves it to dedicated tools.
Sources
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LoRa — Wikipedia, for the chirp-spread-spectrum modulation, spreading factors and bandwidth trade-offs, ISM-band use, and the Semtech/LoRa Alliance split between PHY and LoRaWAN. ↩ ↩2
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LoRa and LoRaWAN technical papers and guides — Semtech, the chip vendor’s primary documentation for the CSS physical layer, de-chirp reception, link-budget/spreading-factor behaviour, and sensitivity figures. ↩