Can Link Quality Indication be viewed as SNR?

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Hello Everyone!

I am using RF transceiver of atmel AT86RF231 and it gives me some reading of RSSI and LQI in its register.
I need to display signal to noise ratio of RF signal, but there is not direct register for that.
How can I get signal to noise ratio?
can LQI gives measure of SNR? or what is the equation for SNR from RSSI and LQI?

Regards,
shreyas.

Last Edited: Fri. Oct 16, 2015 - 01:44 PM
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There is no way to get the SNR exactly.

Indeed, LQI drops when the SNR goes bad, but basically (due to the spreading
gain), by the time LQI goes below 255, SNR is already 0 dB (or worse). In
other words, the spreading gain is then used to reconstruct those parts of
the signal that were damaged by noise.

The other way around, this means any RSSI above the RSSI base level can be
considered as that much SNR with respect to receiver *noise* (as opposed to
interferers). For the AT86RF231, the RSSI base level is -91 dBm, so if you
get an RSSI of -86 dBm, this means you've got +5 dB SNR.

There's a second reason for why the LQI can drop even if there's enough signal
strength: any kind of interference, be it other transmissions, or reflections
of the own signal due to multipath propagation (desctructive interference).
That's the reason why the LQI is much more important to know than the SNR.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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LQI (link quality indication) is a dimensionless number, per IEEE 802.15.4 it's 0 to 255, with units unknown.

Good vendors provide a transform from LQI to dBm (estimate). Usually, low cost radios estimate dBm by observing or being given the noise floor (absent ALL signals of all types), say, -100dBm or so. This is the noise floor of the radio. When a coherent signal is known to be present (demodulation is happening), the difference in measured signal strength vs. the noise floor, we have LQI. Then to dBm there needs to be a transform equation (linear) or a best fit curve as a lookup table.

The weakness in this is whether or not there is coherent or non-coherent power present in the receiver bandpass, and if that is not the desired signal - it is interference.

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> LQI (link quality indication) is a dimensionless number, per
> IEEE 802.15.4 it's 0 to 255, with units unknown.

However, the LQI as mandated by IEEE 802.15.4 is not necessarily the
same as the figure called "LQI" provided by the AT86RF231 hardware.

Since IEEE 802.15.4 doesn't say anything about how to establish their
LQI value, one can use the AT86RF231's hardware LQI directly. Keep
in mind that this value is normally (in the lack of interference) always
255 until the input power goes down to the noise range. The hardware
LQI value is solely a measure of the correlation in the demodulator.

Usually, it's best to combine the hardware LQI and the RSSI into a
measure that can tell a little more about the actual quality of the
link, so two paths that experience both a good demodulation but are
several 10 dBs apart in their signal strength can be distinguished
by the higher layers.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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An energy detect operation returns the average RF power in the channel. So an upper bound on S/N could be estimated by comparing the ED before and after a frame start.

Wifi can give a high background over several channels. This energy scan would indicate an active wifi channel 1 at 2412 MHz and maybe a distant and less-used wifi channel 8 (average power shown by #, peak by : )

 11: -83dB ##:::::::
 12: -59dB ############################:::::
 13: -47dB ##########################################:::
 14: -53dB ###########################::::::::::::
 15: -62dB ##################::::::::::::
 16: -86dB ::::::
 17: -83dB :::::::::
 18: -80dB ::::::::::::
 19: -80dB ::::::::::::
 20: -80dB ::::::::::::
 21: -80dB ::::::::::::
 22: -83dB :::::::::
 23: -83dB :::::::::
 24: -83dB :::::::::
 25: -86dB ::::::
 26: -92dB