Spreading factor, data rate and airtime
Why SF12 reaches further than SF7, what that costs in airtime and when ADR fits.
Spreading factor (SF)
The spreading factor sets how long a symbol lasts. Each step up roughly doubles the airtime and makes the signal more robust: a gateway can then still pick it out deeper in the noise.
| Data rate (EU868) | Setting | Bit rate | Max. payload | Minimum SNR | Sensitivity (typ.) |
|---|---|---|---|---|---|
| DR0 | SF12, 125 kHz | 250 bit/s | 51 bytes | −20 dB | approx. −137 dBm |
| DR1 | SF11, 125 kHz | 440 bit/s | 51 bytes | −17.5 dB | approx. −135 dBm |
| DR2 | SF10, 125 kHz | 980 bit/s | 51 bytes | −15 dB | approx. −133 dBm |
| DR3 | SF9, 125 kHz | 1760 bit/s | 115 bytes | −12.5 dB | approx. −130 dBm |
| DR4 | SF8, 125 kHz | 3125 bit/s | 222 bytes | −10 dB | approx. −126 dBm |
| DR5 | SF7, 125 kHz | 5470 bit/s | 222 bytes | −7.5 dB | approx. −123 dBm |
| DR6 | SF7, 250 kHz | 11,000 bit/s | 222 bytes | −7.5 dB | approx. −120 dBm |
| DR7 | FSK | 50,000 bit/s | 222 bytes | – | – |
The sensitivity values are guide values from the data sheets of the Semtech chips. They depend on the gateway and its antenna.
Airtime
Longer airtime means more power, more duty cycle and the channel being busy longer for everyone else. Hence the rule: as fast as possible, as slow as necessary.
Adaptive data rate (ADR)
With ADR the network server watches the reception quality of a device over several packets and tells it to send faster or lower its power. ADR fits sensors mounted in a fixed place.
It does not suit devices that move, because conditions keep changing. Field testers therefore usually work with a fixed spreading factor: you choose which data rate you want to test.