Lab5e AS

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.

Up- and downchirps

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.

A LoRaWAN packet in gqrx

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.

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.