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xaelsouth / rtl-wmbus

Software defined receiver for wireless M-Bus with RTL-SDR
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RSSI value in dBm #41

Closed pbasista closed 1 year ago

pbasista commented 1 year ago

Hi,

I have noticed that the RSSI values reported by rtl-wmbus are always positive. Packets received via weak signals have RSSI of around 20 or less. Packets received via strong signals have RSSI as high as 130 or more.

Is there a reasonable way to convert these values to decibel-milliwatts (dBm)?

pswiatki commented 1 year ago

I guess there is no way that is reasonable. You see: AGC is enabled as far as I know and it changes as necessary during the reception.

xaelsouth commented 1 year ago

Hello pbasista,

you can calculate dBm instead of RSSI by using following formula even it doesn't make much sense, see below:

dBm = 10 log10[ (I^2 + Q^2) / (2 R * 0.001) ]

Where R is receiver's input impedance, which is typically 75 ohms for DVB-T receivers (some of them have got 50 ohms!), referenced to 0.001W.

This formula can be applied only if no AGC comes into play, as pswiatki has already mentioned. Digital receivers usually take AGC into account and put a corrected RSSI and/or dBm value into their registers.

Xael