When I send a Janus frame between two nodes, at the receiving side in RxJanusFrameNtf there is rssi value, How to calculate the received power or the SNR using rssi?
The rssi value is good for making signal strength comparisons between different nodes. The rssi value in typically given in dB, but with an arbitrary reference. You'd need a noise measurement in dB with the same reference to convert rssi to SNR -- something that is specific to the implementation of a modem or simulation model. For example, Subnero modems publish the noise value in the same units (but potentially a larger bandwidth) as phy.noise.