IT admin trying to understand the radio side well enough to review a LoRaWAN rollout. I get that LoRa uses chirps, but I’m lost on chip rate vs symbol rate vs bit rate, and how any of that relates to the Lowest data rate (DR) and Highest data rate (DR) fields in the LoRaWAN device profile. The meter vendor just says “our meters use SF9”. Where does SF9 land in DR terms, and should I change the profile for them?
Way over my head, but following because we’re getting LoRa sensors next year.
Short version: higher SF is slower and reaches further. The rest is detail.
Keisha’s short version is right. The detail, as I explain it to new starters:
The chip rate equals the bandwidth, so at 125 kHz you get 125,000 chips per second. One symbol is 2^SF chips, so a symbol lasts 2^SF / BW: about 1 ms at SF7 and 33 ms at SF12. Each symbol carries SF bits. With the 4/5 coding rate that gives a bit rate of SF × BW / 2^SF × 4/5, about 5.5 kbit/s at SF7 and 290 bit/s at SF12. Each SF step roughly halves the bit rate and doubles the airtime.
In EU868, assuming that’s what your gateways run, the DR numbers are just names for SF and bandwidth pairs:
DR0 SF12 125k DR3 SF9 125k
DR1 SF11 125k DR4 SF8 125k
DR2 SF10 125k DR5 SF7 125k
DR6 SF7 250k DR7 FSK 50 kbit/sSo the vendor’s SF9 is DR3. You don’t have to set anything for that. ADR (on by default) moves each device within the profile’s Lowest/Highest DR range, DR0 to DR5 by default, based on the best SNR over its last 20 uplinks. A meter that hears the gateway well ends up faster than SF9, a far one stays slower. After a change you can see it in the device’s lora.dr attribute. The one case where you’d turn ADR off is anything that moves, like a tracker.
That table is what I was missing. Left the profile at the defaults. After a week most meters sit at DR5 and four basement ones at DR2, all visible in lora.dr. Thanks Ciaran.
Those four at DR2 use roughly 8x the airtime of the DR5 ones per message. Worth knowing if you ever add many more down there.
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