RSSI and SNR: How Strong Is the Signal?

Imagine a friend calling your name from across a crowded room.

How well you hear them depends on two things. The first is how loud their voice is by the time it reaches you. A friend at the next table sounds loud; a friend at the far end of the room sounds quiet. The second is how noisy the room is. A quiet voice is easy to pick out in a silent library, and a loud voice can get lost next to a band.

A CropWatch sensor and its gateway have the same two problems. The sensor is your friend, the gateway is you, and the radio waves are the voice:

• RSSI is how loud the voice is when it arrives.
• SNR is how much louder the voice is than the chatter around it.

RSSI stands for Received Signal Strength Indicator. It is measured in a unit called dBm, and for LoRaWAN it is almost always a negative number, such as -75 or -112.

The scale is not linear. Every 10 points down means the signal is 10 times weaker, so a reading of -100 is 1,000 times weaker than a reading of -70. That sounds dramatic, but LoRa radios are built to hear very faint signals, and the gateway can still understand a message at around -130.

As a rough guide for LoRaWAN:

• -30 to -90: strong. The sensor is close or has a clear path to the gateway.
• -90 to -110: good. Normal for a sensor some distance away.
• -110 to -120: weak, but readings usually still arrive.
• Below -120: close to the limit. Some readings may go missing.

SNR stands for Signal-to-Noise Ratio. It compares the sensor's signal with the background radio noise at the gateway: static from motors, power lines, inverters, other electronics, and the faint hiss that is always present. It is measured in dB.

• A positive SNR (for example +8) means the signal is louder than the noise.
• An SNR of 0 means the signal and the noise are equally loud.
• A negative SNR (for example -12) means the signal is quieter than the noise.

LoRa can pick a message out of noise that is a little louder than the message itself, a bit like recognising a friend's voice through the chatter because you know how it sounds. Depending on how slowly the sensor transmits, a gateway can sometimes decode a message at -7.5 dB, or even -20 dB.

Sometimes is not good enough for a sensor you rely on. At those levels there is no margin left: a pump switching on or a wet morning adds a little more noise, and readings start to go missing. CropWatch therefore uses a stricter rule:

• Above 0: good. The signal stands out from the noise.
• 0 to -4: acceptable. The signal is a little quieter than the noise, and readings still arrive reliably.
• Below -4: bad. Readings may still arrive for now, but the link is unreliable and needs attention.

Each number tells only half of the story. Two examples:

• Loud but noisy. A sensor 20 metres from the gateway shows an RSSI of -65, which is strong. But the gateway sits beside a large pump motor, and the SNR is -9. The friend is shouting, but there is a band playing.
• Quiet but clear. A sensor 3 km away in open fields shows an RSSI of -118, which is weak. The countryside has little electrical noise, so the SNR is +2. The friend is whispering, but the room is silent.

The second sensor has the better connection of the two, even though its signal is far weaker. The first one's SNR of -9 is well below CropWatch's limit of -4. When readings go missing, look at both numbers before deciding what to change.

CropWatch shows both numbers in two places.

On a gateway's page. Open Gateways and choose a gateway. The table lists every device that gateway hears, with its RSSI in dBm, its SNR in dB, a set of Signal bars, and when the gateway last heard it. This is the view to use when you want to see how one gateway is doing with all of its sensors.

On a device's page. Open the device and press the Signal button. The button itself shows signal bars for the device's best gateway. Pressing it opens Connected gateways, which lists every gateway that heard the device, with the RSSI, SNR and signal bars for each one. A device often reaches more than one gateway. Gateways you don't have access to appear as "Other gateway".

The signal bars look at both numbers and show the worse of the two, so a strong RSSI cannot hide a poor SNR.

If RSSI is low, the signal is too faint by the time it arrives. Help it travel:

• Mount the gateway or its antenna higher, ideally above roofs and trees.
• Clear the path between sensor and gateway. Walls, metal sheds, hills and wet foliage all absorb radio signals.
• Move the sensor or gateway closer together, or add a second gateway.

If SNR is low but RSSI is fine, the problem is noise near the gateway. Move the gateway or its antenna away from motors, pumps, inverters, LED drivers and power cables.

For more on placement, see what affects range and choosing a mounting location.