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Vincent van BeverenVincent van Beveren 

Realistic figure for Received Signal Strength

We did extensive measurements with a number of motes at different locations and two gateways. 20k packets have been transmitted. We wish to make an estimate of the 'real' received signal strength as the reported RSSI has a lower limit at about -123dBm (reported by our Kerlink Wirnet gateways). Around this point the SNR can drop below 0dB. So, the actual signal would be below the -123dBm.

What would a realistic figure be for received signal strength? I would expect:

 RSS = RSSI if SNR > 0, otherwise it is RSSI + SNR

Is this correct?

Also:
For signals with an RSSI > 100dBm, the SNR seems to top in the range of 7-14dB.
What limits the SNR to this range, as I would expect to continue to increase with RSSI?
Best Answer chosen by Vincent van Beveren
SebastienSebastien (Semtech Corporation) 
Hi Vincent,

The calculation you are proposing is mostly right, and is the simplification we actually propose in our drivers and documentation. It may be made better when SNR is close to 0 (energies sum up), but this approxximation is good enough.

Concerning the SNR, by implementation choice is tails off when it becomes quite positive. You just have to consider that SNR is "suffucient" when over 5dB, and then just rely on RSSI.

Actually, in your calculation, a better description would be "packet power" instead of RSSI, which is effectively what you are trying to quantify.

Best,

All Answers

SebastienSebastien (Semtech Corporation) 
Hi Vincent,

The calculation you are proposing is mostly right, and is the simplification we actually propose in our drivers and documentation. It may be made better when SNR is close to 0 (energies sum up), but this approxximation is good enough.

Concerning the SNR, by implementation choice is tails off when it becomes quite positive. You just have to consider that SNR is "suffucient" when over 5dB, and then just rely on RSSI.

Actually, in your calculation, a better description would be "packet power" instead of RSSI, which is effectively what you are trying to quantify.

Best,
This was selected as the best answer
Lawrence GriffithsLawrence Griffiths

Follow up to this we are trying to set threshold alerts for RSSI

So for 125 kHz bandwidth SX1276/77/78/79  where would set an warning and action level.  For example I have one sensor on SF8 125k and is faling off a LoRaWAN net just over -112.2 RSSI


SF = 6 -118
SF = 7 -123
SF = 8 -126
SF = 9 -129
SF = 10 -132
SF = 11 -133
SF = 12 -136

Thanks

Lawrence

 

 

SebastienSebastien (Semtech Corporation) 
Hi Lawrence,

You have to be ab it careful with that. RSSI, from which you are extrapolating these numbers, measures ALL the energy in the channel, and that may (will) include interference, noise, etc... A better way is to trigger actions on the "link margin", which is the excess of SNR. Minimum SNR is -20dB for SF12, and cranks up 2.5dB or so per NF, to end up at -5dB for SF7.

Best,
Lawrence GriffithsLawrence Griffiths
Sebastien many thanks for this.  
Madhukar VarshneyMadhukar Varshney
For  SX1276/77/78/79  on SF8 125k are the values of RSSI ?

SF = 6 -118
SF = 7 -123
SF = 8 -126
SF = 9 -129
SF = 10 -132
SF = 11 -133
SF = 12 -136