Below the noise, not through interference

Noise is a level, and a ratio decides

Every receiver hears noise in its own band. Thermal noise is -174 dBm in each hertz, so -123 dBm across 125 kHz, and a receiver with an assumed noise figure of 6 dB has a floor of about -117 dBm. A conventional radio needs the signal above that.

A LoRa receiver knows the waveform it is looking for, a chirp, and adds the energy of the whole sweep. It needs a signal to noise ratio above a limit that is negative and depends on the spreading factor: -12.5 dB at SF9, down to -20 dB at SF12. A signal 7 dB under the floor, a ratio of -7 dB, decodes at SF9.

Limit and sensitivity

The limit is a ratio in dB. Sensitivity is a level in dBm: the noise floor plus the limit, about -129.5 dBm at SF9 and 125 kHz, nominal, for the assumed receiver. Another bandwidth or noise figure moves the sensitivity and leaves the limit alone. The bench draws them apart.

An interferer is not noise

Noise has no waveform. Another LoRa transmission on the same channel does. On the same spreading factor the wanted packet has to beat it by a few dB; about 6 dB is commonly quoted, and the bench uses it as an example. Spreading factors are only quasi-orthogonal, so a packet on another one is mostly rejected, up to a margin the bench puts at about 16 dB as an example. A strong enough one still gets through.

So there are two checks. One asks whether the signal clears the noise by the limit. The other asks whether another signal is close enough in level to take the packet. A link can pass the first with room to spare and lose to the second. On the bench, an interferer 2 dB under the wanted signal loses the packet with the ratio untouched. The next lessons cover the rules for the band and sharing it.

Common mistakes

• Reading "below the noise" as "immune to interference". Decoding under the noise floor says nothing about another transmission on the channel.
• Taking the demodulation limit for the sensitivity. The limit is a ratio. Sensitivity is the level it gives for one bandwidth and noise figure.
• Quoting a margin as a constant. The 6 dB and 16 dB are examples; real figures depend on the settings and the receiver.