Data Rate and Spreading Factor

Sooner or later you’ll come across the terms data rate (DR), spreading factor (SF) and bandwidth (BW). These three terms are related but the link between these aren’t all that obvious.

LoRa uses what is known as a “chirp” protocol, something which was developed for sonar and radar applications during the 2nd world war so it is by no means a new technique.

The chirp protocol uses fixed amplitude frequency modulation. It can utilise the entire allocated spectrum to transmit signals by producing a signal that sweeps across the channel. There are two kinds of chirps – the “up-chirp” that moves upward in frequency and (not surprisingly) a “down-chirp” that moves down in frequency, ie when there’s a chirp it will move across the allocated frequency in either direction.

Bandwidth

LoRa uses three bandwidths: 125kHz, 250kHz and 500kHz. The chirp uses the entire bandwidth. If you look at a LoRa packet in an SDR application like Gqrx you’ll see a clearly defined square with chirps.

Spreading factor

The spreading factors are - in short - the duration of the chirp. LoRa operates with spread factors from 7 to 12. SF7 is the shortest time on air, SF12 will be the longest. Each step up in spreading factor doubles the time on air to transmit the same amount of data.

With the same bandwidth longer time on air obviously results in less data transmitted per unit of time.

Data Rate

LoRaWAN uses a different configuration of frequencies, spreading factors and bandwidths depending on where you are located in the world. For the EU868, EU433, CN780 and AS923 bands the data rates are as follows:

Data Rate Configuration bits/s  Max payload
DR0 SF12/125kHz 250 59
DR1 SF11/125kHz 440 59
DR2 SF10/125kHz 980 59
DR3 SF9/125kHz 1 760 123
DR4 SF8/125kHz 3 125 230
DR5 SF7/125kHz 5 470 230
DR6 SF7/250kHz 11 000 230
DR7 FSK: 50kpbs 50 000 230

Note that the AS923 band might not use DR0 and DR1, depending on the configuration.

In the US902-928 an AU915-928 bands the data rates are as follows:

Data Rate Configuration bits/s  Max payload
DR0 SF10/125kHz 980 19
DR1 SF9/125kHz 1 760 61
DR2 SF8/125kHz 3 125 133
DR3 SF7/125kHz 5 470 250
DR4 SF8/500kHz 12 500 250
DR8 SF12/500kHz 980 41
DR9 SF11/500kHz 1 760 117
DR10 SF10/500kHz 3 900 230
DR11 SF9/500kHz 7 000 230
DR12 SF8/500kHz 12 500 230
DR13 SF7/500kHz 21 900 230

For the CN470-510 and KR920-923 bands there’s only five defined data rates:

Data Rate Configuration bits/s  Max payload
DR0 SF12/125kHz 250 59
DR1 SF11/125kHz 440 59
DR2 SF10/125kHz 980 59
DR3 SF9/125kHz 1 760 123
DR4 SF8/125kHz 3 125 230
DR5 SF7/125kHz 5 470 230

As you can see ‘DR4’ can be two different things in two different regions. Fortunately you won’t have to worry about this since the library on the device and the backend service usually manages to pick the best data rate to use.

info Max payload size matters!

Even though you usually can ignore the selected Data Rate the max payload size means you should limit your payload to the maximum. If your payload exceeds the maximum length you should split it up into multiple pieces.