LoRa is the radio, not the network
Semtech describes LoRa as a spread spectrum modulation technique derived from chirp spread spectrum (CSS). It is purely a physical layer: it decides how bits become a radio signal and back again, and nothing more. Addressing, joining a network, security and when to transmit belong to protocols built on top, such as LoRaWAN, which LoRa vs LoRaWAN separates in detail.
The Things Network lists the licence-free sub-gigahertz bands LoRa uses, such as 868 MHz, 915 MHz and 433 MHz, and notes it can also run at 2.4 GHz for higher data rates at the cost of range. The LoRa glossary entry defines the term.
Chirps carry the data
A chirp is a tone whose frequency sweeps continuously across the channel. Semtech’s application note AN1200.86 explains that LoRa spreads the signal’s energy by generating such a chirp, and that the chirp’s sweep covers the whole channel bandwidth.
Data rides on where each chirp begins. In their analysis of the LoRa receiver, Ghanaatian and colleagues describe the symbol this way: the channel bandwidth is divided into 2 to the power SF frequency steps, where SF is the spreading factor. A symbol starts at a frequency chosen by its value, rises linearly, and when it reaches the top edge of the band it folds back to the bottom and continues. Each symbol therefore carries as many bits as its spreading factor. A frame begins with a preamble of plain upchirps so the receiver can lock on before the payload arrives.
Spreading the signal this way buys processing gain. Semtech explains that this gain is what lets a receiver recover data even when the signal-to-noise ratio is negative, meaning the signal is weaker than the background noise. The chirp shape has practical benefits too. Semtech notes that timing and frequency offsets between transmitter and receiver become equivalent, which simplifies the receiver, avoids the need for a highly accurate reference clock, and makes the signal tolerant of Doppler shifts, multipath and fading.
Spreading factor: trading speed for reach
The spreading factor is the main dial. Semtech lists six spreading factors, SF7 to SF12. Meshtastic’s documentation adds that second-generation chips (SX126x, LR11xx, SX128x) also support SF5 and SF6, while first-generation SX127x chips are limited to SF7 to SF12.
Each step up slows the chirp. The Things Network explains that every increase in spreading factor halves the chirp sweep rate and so halves the data rate, and Meshtastic puts the same fact the other way round: each step doubles the airtime needed to send a message. What you get in return is sensitivity. The Things Network’s table for a 125 kHz channel runs from -123 dBm at SF7 to -137 dBm at SF12, so a slower signal can be decoded at a much lower received power.
Spreading factors also share a channel well. Semtech describes them as orthogonal: signals with different spreading factors, sent on the same frequency at the same time, appear as noise to each other and can both be demodulated. Two packets with the same spreading factor can collide, although Semtech notes that the stronger one survives if it arrives 6 dB stronger.
Bandwidth and coding rate
Two further settings shape the link.
| Setting | Turning it up gives | Turning it up costs |
|---|---|---|
| Spreading factor | Better sensitivity, longer reach | Half the data rate per step, double the airtime |
| Bandwidth | More bits per second | Sensitivity: Meshtastic puts it at almost 3 dB of link budget per doubling |
| Coding rate (more redundancy) | Better resistance to noise | Lower data rate |
The Things Network gives example bit rates at SF7: about 5.5 kbit/s on a 125 kHz channel, 10.9 kbit/s on 250 kHz and 21.9 kbit/s on 500 kHz, so doubling the bandwidth doubles the rate. Semtech’s note makes the matching point that, for a fixed spreading factor, a narrower bandwidth increases sensitivity.
The coding rate is forward error correction. Semtech explains that at the 4/5 rate, every four bits of information are followed by a fifth parity bit, and that LoRaWAN fixes the rate at 4/5. Meshtastic exposes rates from 4/5 to 4/8, where 4/8 doubles the transmitted bits.
What the trade-offs mean in practice
Airtime is the budget every LoRa design spends. In Europe, The Things Network explains, ETSI EN 300 220-2 limits each sub-band to a duty cycle, such as 1% or 0.1% of the time. A message at SF12 occupies the air far longer than the same message at SF7, so a slow setting cuts how often a device may transmit as well as draining its battery. The Things Network notes that higher spreading factors keep the radio active longer and shorten battery life. This is why LoRaWAN’s adaptive data rate, as Semtech describes it, moves devices close to a gateway down to low spreading factors and keeps high ones for distant devices.
Vendors publish range figures, and they are best read as conditions rather than promises. Semtech’s application note quotes up to 5 km in urban areas and 15 km or more in rural areas with line of sight. Real links depend on terrain, antenna height, interference and the settings above.
The result is a radio built for small, infrequent messages over distance: sensor readings, status updates, short texts. It is not suited to voice, video or large files.