There is NO standard way or equation to convert DBFS to DBm. dBfs is not an absolute measure than can be converted universally to voltage. You must know the voltage equivalent of 'full scale'. Convert it to Volts, then turn it to DBm with standard conversion for RF signals.
Thanks for the reply. Could you tell me how to find the voltage equivalent of full scale and also to get the value in dBm once i know this voltage equivalent, is just adding +30 to the resultant power right?
The full scale Voltage is described eighter in documentation of your device or in the code itself (it depends). Have a look there to find it out. The level of 0 dBFS is assigned to the maximum possible digital level. For example, a signal that reaches 50% of the maximum level has a level of −6 dBFS, which is 6 dB below full scale. So, it's logarithmic unit, and adding +30 to it, won't convert to dBm.
BTW, I don't really understand the reason behind that conversion. DBFS is designed for Digital signals, and dBm is for Analog ones. Why do you want to convert from one to another?
I am actually transmitting a qpsk signal from one USRP to another. I measured the signal power before entering the receiver USRP using a spectrum analyzer. Now, at the receiver end, I collect IQ samples in the PC and I want to ccompare the power at the receiver section with the power of the signal before entering the receiver. So I am doing an average of squares of magnitudes of received samples. But this power is digital power right. I need to cinvert it into analog power to compare the two powers.
I hope I am clear.
Also if I am collecting the IQ sample values in the PC, do they directky represent the voltage level or is there any conversion to be done?
They do. But there are numerous amplifiers and attenuators in your instrument. You must look at datasheets to find out how the real amplitude is changed during receiving the signal 😄
So yes, better to compare signal intensity to IQ intensity to get comparison between them.