SNR, RSSI, and demodulation floor
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Jason Reiss.
Eric TsaiI have wall-powered LoRa edge devices that I’d like to display some indication of the strength of the wireless link between it and the gateway, specifically for upstream data. I see a couple of commands that return useful info, but I’d like to get a better understanding of what these values mean:
AT+PING: Returns RSSI and SNR
AT+NLC: Returns # of gateways in range, and also the dbm level above demodulation floor.
My understanding of these values:
RSSI: signal strength in dBm, ranges from -20 (strong) to -110 (weak). That makes sense to me.
SNR: how is SNR calculated at the gateway? I see values ranging from 10.5 (good) to (2.2 or -10.2) which are the points where the packets reception get hairy. Is SNR actually the (dbm signal / dbm noise floor)? Or is it (dBm signal – dBm noise floor)? The sign on SNR is confusing.
Typically, SNR is a better indication of health of the wireless link than RSSI. But since LoRa is able to resolve packets below the noise floor, is the AT+NLC value a better indicator? How is AT+NLC value calculated, this “dbm above demodulation floor”? Does this value already take into account the spreading factor setting of that end device and the noise level? That is, as you increase the spreading factor, you’re able to resolve signals that are farther and farther below the noise level.
Jason ReissThe SNR is reported by the LORA demodulator.
The value reported by AT+NLC should calculate the difference between the floor and the SNR of the received signal at the gateway. There appear to be a bug in the current release of network server on Conduit. It is only returning the SNR above 0.
SF12BW125 -20 dBm floor
Each SF step raises the floor 2.5 dBm
A BW step from 125 to 250 or 250 to 500 raises the floor 3 dBm.
In US915 the gateway RX floor then ranges from -15(DR0) to -4(DR4)
See page 3 of SF12 and SF10 margins.
https://www.semtech.com/images/datasheet/LoraDesignGuide_STD.pdf
See page 6 for SNR calculations.
http://www.semtech.com/images/datasheet/an1200.22.pdf