Code RFM69.cpp question for canSend() RSSI test

[Explorer]

Hi,
Should the test for RSSI be ">" not "<" in canSend() since these are negative numbers?
bool RFM69::canSend()
{
  if (_mode == RF69_MODE_RX && PAYLOADLEN == 0 && readRSSI() < CSMA_LIMIT) //if signal stronger than -100dBm is detected assume channel activity
  {
    setMode(RF69_MODE_STANDBY);
    return true;
  }
  return false;
}

[Felix]

Lower (more negative) is less signal strength (ie a farther transmitting node will have lower value than a closer transmitting node, receiving base being the same).
To give some numbers ... -100dBm is very weak, compared to -60dBm. A -30dBm signal is very strong.

[Explorer]

So if I get -30 from readRSSI(), isn't that greater than -100 for the condition to be true and canSend returns true?

[Felix]

Yes -30 > -100
How you interpret it is up to you.

[Explorer]

So...  should the code I referred to be change to > from <?

[Felix]

No

[Explorer]

Thanks - was thinking in reverse (that it needed a good RSSI)

[Felix]

well the farther a transmitter is the weaker signal and (RSSI lower as in more towards -100 which is the limit in the lib by default). The decrease is logarithmic not linear, quite important to mind that detail.

[Explorer]

Yes, I understand.  Wondering if a great way to save power & battery life  is to have code to adjust the transmitter power based on the RSSI received (-30db means that the gateway would not need to be full power, but at -80db keep all PA's on)?
Thanks

[Felix]

RX mode doesn't have power levels only the TX mode. So you'd have to tell your node to turn power lower. Doable, but I'd like metrics before I recommend that.