RFM95W RSSI in LoRa Mode

Question

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.

Answer 1

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.