In the datasheet, the calculation for RSSI is like below, "The RSSI values reported by the LoRa modem differ from those expressed by the FSK/OOK modem. The following formula shows the method used to interpret the LoRa RSSI values: RSSI (dBm) = -157 + Rssi, (when using the High Frequency (HF) port)" May I ask, what does this -157 mean? Is it a noise power for rough estimates?
I searched the internet, and found no answer.
It's -157 for HF, and -164 for LF. Basically it's the lowest RSSI value possible. And it's an easy way to encode a negative value on an 8-bit register:
RegPktRssiValue(0x1A) encodes the packet RSSI in one byte. Add that value to -157 or -164 and you have your RSSI.