RSSI meter wildly ineffective

[caezar]

The nRF 52840 RSSI "meter" is not usable.

I tested in an RF (2.4Ghz) quiet neighborhood. Using a reference receiver, I measured RSSI variance around +/-3 dB (95% confidence interval) from each of 5 transmitters; this is in line with expectation. Noise was below -99 at all times during the 1 hour test. I positioned a nRF 52840 20 cm away from the reference measurement station. The Nordic RSSI values did not exhibit a Gaussian noise curve at all, but instead reported three strong modes at -79, -65, and -58; each with incredibly small tails. Binning the results according to proximity to these modes revealed a variance of 0.2 dB (95%), which is more precise than noise will allow. From this evidence I gather three important facts: a more distant node can have a higher RSSI, the RSSI cannot be averaged to reduce "noise" as the values are nonlinear and modal, and therefore there is no way to estimate relative signal strength from the relative signal strength indicator. I used 5 transmitters during this test and observed similar behavior from each, both reasonable values from my reference receiver and unreasonable values from the Nordic chip. RSSI does not work as intended. Please document this behavior, consider disabling the API for this part, and further document which nRF parts actually work.

Please tell me what part I have to replace in my next board design to achieve usable RSSI?

[Andreas]

Hi,

 

Assuming propagation loss is similar between the devices, e.g. by measuring conducted, the RSSI filter is specified to give a +/-2dB accuracy on RSSI readings. Note though that you need to initiate the measurement correctly, most significantly the RSSI will have to settle. From the PS:

The RSSI is measured continuously and the value filtered using a single-pole IIR filter. After a signal level change, the RSSI will settle after approximately RSSI_SETTLE.

RSSI_SETTLE is 15µs typical, meaning if you initate the RSSI measurement less than 15µs after the radio has started in RX, or less than 15µs after starting reception of a packet, the reported RSSI will not be correct. As you have not described how you are doing this it's hard for me to say decisively, but this is a somewhat common cause for incorrect RSSI readings.

 

Best regards,
Andreas