MKR 1310 RSSI and SNR

Official Hardware MKR Family MKR WAN 1310
1

Hi everyone!

I have a question regarding how exactly the Lora.h library calculates the RSSI and the Signal to noise ratio of the lora packets that receives. This because in two tests, one at 43 km and one more than 100 km in Line - of - Sight, I was receiving at -120 dBm as lowest value, while with my SF = 12 the receiver sensitivity ( sx1276 ) should be less, -139 dBm using the calculators available online. This is a little confusing, because I was expecting to have at least one signal under -120 dBm while instead it seems that the receiver does not print messages under this value. In the same fashion, for the same bandwidth and during the same test I was receiving at -50 dBm and -100 dBm with 9 SNR.

Does anyone has an idea of what was going on? Is how the receiver performs the calculation? Is it reliable?

Thanks for your support!

3

First off, the LoRa.h library does not really calculate the RSSI and SNR, it just reads the SX127x registers and arrives at the RSSI and SNR according to the calculation given in the SX127x datasheet.

Second, there is no way you will receive LoRa signals in the real world at the sensitivity given in the datasheet because of the impact of real world RF noise.

There is a lot of RF noise about, circa maybe -100 to -105dBm @434Mhz, so with the SNR limit of SF12 being circa -20dBm, the weakest real World signals you will receive are around -120dBm to -125dBm.

Now if you measure the sensitivity of the SX127x inside a laboratory faraday shield then I guess you will receive weaker signals.

Myself I would not pay much attention to RSSI, its OK for strong signals but if you want to know how close to the limit of reception you are, read the SNR ............

4

Thank you for your answer! I get your point and I agree 100%. But there is my second question: In the same fashion with SF12 I never received signal with SN below -10 dB, either I received with that value, or I didn't receive at all.

P.S: I was communicating on 868 Mhz, do younhave an idea of the real world RF noise at that frequency? Where did you find that info? Thanks for your kind support.

5

Cannot say why your SF12 reception stops at -10dB, when doing decending power link tests I do see SF12 fail at circa -20dB.

The LoRa device can report the background or static RSSI it sees or you can use a portable RF tool.

I have not done much link testing at 868Mhz.

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