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This is part 16 of the LoRa/LoRaWAN tutorial.
In this video series different topics will be explained which will help you to understand LoRa/LoRaWAN.
It is recommended to watch each video sequentially as I may refer to certain LoRa/LoRaWAN topics explained earlier.
In this video I will explain what an SNR limit is and how to calculate the receiver sensitivity.
For each spreading factor there is a SNR limit, if this limit is reached the receiver will not be able to demodulate the signal.
In the table below, the SNR limit can be found for each Spreading Factor:
SF=7, SNR limit = -7.5 dB
SF=8, SNR limit = -10 dB
SF=9, SNR limit = -12.5 dB
SF=10, SNR limit = -15 dB
SF=11, SNR limit = -17.5 dB
SF=12, SNR limit = -20 dB
Note: If the SF increases by 1, the SNRlimit changes by -2.5 dB
To calculate the receiver sensitivity:
S = -174 + 10xlog10(BW) + NF + SNRlimit [3]
Receiver sensitivity (S) in dBm
Bandwidth (BW) in Hz
Receiver Noise Figure (NF) in dB
NF is fixed for a given hardware implementation
For LoRa end node transceiver chips SX1272 and SX1276 use NF = 6 dB [1]
Signal-to-Noise limit (SNRlimit) in dB
For example: BW=125kHz, NF=6dB [1]
Lets calculate the receiver sensitivity for SF 7-12.
Use the SNRlimit from the previous table.
SF=7: SNRlimit=-7.5dB
S = -174 + 10xlog10(125000) + 6 -7.5 = -125 dBm
SF=8: SNRlimit=-10dB
S = -174 + 10xlog10(125000) + 6 -10 = -127 dBm
SF=9: SNRlimit=-12.5dB
S = -174 + 10xlog10(125000) + 6 -12.5 = -130 dBm
SF=10: SNRlimit=-15dB
S = -174 + 10xlog10(125000) + 6 -15 = -132 dBm
SF=11: SNRlimit=-17.5dB
S = -174 + 10xlog10(125000) + 6 -17.5 = -135 dBm
SF=12: SNRlimit=-20dB
S = -174 + 10xlog10(125000) + 6 -20 = -137 dBm
If the distance between end node and gateway increases the signal gets weaker and therefore an increase spreading factor is needed for a lower receiver sensitivity to be able to demodulate the received signal. The Spreading Factor ranges from 7 when close to a gateway to 12 when far away from a gateway.
Check out all my other LoRa/LoRaWAN tutorial videos:
• LoRa/LoRaWAN tutorials
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