lora driver: SX1276 reports incorrect RSSI

[davefes]

Maintainer edit: Continuation from discussion at https://github.com/orgs/micropython/discussions/12989

For a Hope Radio RF96 (RFM96W) the RSSI readings seem to calculated incorrectly. I generally see about -110dBm where errors start creeping-in. Two other Micropython LoRa implementations report -120 or thereabout dBm for similar "errors*.

Is the formulae:

# units: dBm
rx_packet.rssi = self._reg_read(_REG_PKT_RSSI_VALUE) - (157 if self._pa_boost else 164)

valid for this chip? I do not understand why pa_boost is in this formulae.

[projectgus]

Hi @davefes,

Thanks for noticing this, it's a bug. It should be based on this section of the SX1276 datasheet:

Meaning the implementation should be something like:

# units: dBm
rx_packet.rssi = self._reg_read(_REG_PKT_RSSI_VALUE) - (157 if self._rf_freq_hz > 525_000_000 else 164)

Are you in a position to test this version and submit a PR to change it? If not, I'll get to it as soon as I have time.

[davefes]

Not until the weekend. Submit a PR ... it would be a first for me. As long as it works it's way into library I am OK with the "under-reporting".

[davefes]

I did check the RSSI at the receiver increased by turning off PA_BOOST at the receiver. Of course I didn't get an ACK at the sender. Another odd behaviour noted: the first RSSI after a re-boot is 5dB better then subsequent RSSI readings.

[projectgus]

Thanks @davefes! Will leave this issue open until there's a fix.

Another odd behaviour noted: the first RSSI after a re-boot is 5dB better then subsequent RSSI readings.

Interesting. I assume that's probably an artifact of the module, although I don't know what it would be down to.

[davefes]

Change VALUE to VAL
rx_packet.rssi = self._reg_read(_REG_PKT_RSSI_VAL) - (157 if self._rf_freq_hz > 525_000_000 else 164)
and RSSI values around -117dBm are when errors started appearing. Still see 4 or 5dB higher on the first RSSI reading. I am sure I haven't seen this happen on other implementations, but I will go back and check.

Edit:
just re-checked Wei's implementation and it reads -123 or -124dBm (at onset of errors) after a re-boot and that stays the same after that. The distance between RX and TX remained the same, so TX power and RX sensitivity appear to be identical with both S/Ws.

I believe you are both using the same RSSI register value. I did notice in the datasheet that there is another RSSI register:

3. As signal strength increases (RSSI>-100dBm), the linearity of PacketRssi is not guaranteed and results will diverge
   from the ideal 1dB/dB ideal curve. When very good RSSI precision is required over the whole dynamic range of the
   receiver, two options are proposed:

Rssi in RegRssiValue offers better linearity. Rssi can be sampled during the reception of the payload (between
ValidHeader and RxDone IRQ), and used to extract a more high-signal RSSI measurement
When SNR>=0, the standard formula can be adjusted to correct the slope: RSSI = -157+16/15 * PacketRssi (or RSSI = -
164+16/15 * PacketRssi)

Practically-speaking, I think the most useful aspect of RSSI is to give the user some rough idea of the excess link budget one has. If I see errors at -117dBm and my RSSI is -100dBm then I know I have ~17dB of margin.

[davefes]

Confirm that RSSI is the same on the first loop and subsequent loops with your chips ... then I'll just say it is problem with Hope Radio RF96 chips and I will close this issue.