How to measure RSSI via software?


I want to measure RSSI in software.
Searching previous mailing list
Matt mentioned 3-fold measurement

-Analog RSSI
-Digital RSSI in FPGA (from output of ADCs)
-Digital RSSI in host (computed however you like, from the channelized
signal sent over the bus by the USRP)

I’m not sure which part I should look into.
I greatly appreciate detailed explanation.