relation between FFT and IQ data

Question

I have been studying how the USRP works and, particularly, how to sense the energy from a signal. So far, I have understood that the USRP sense IQ data, and then, process it applying FFT. (I have been looking at usrp_spectrum_sense.py ) What are the units of the IQ samples? What are the units after the FFT is done? The only data that is needed to develop the FFT is the IQ samples?

Thanks in advance :D

Answer 1

Unfortunately, there are no units for the IQ samples, it is simply a magnitude (for the UHD driver, it is between -1 and 1). The specifications of many of the daughterboards of the USRP do not specify receiver sensitivity. You cannot use the IQ samples alone to figure out incoming power, you will need to you own calibration such as using a signal generator to generate a signal of known power into the USRP input and recording what you see using GNURadio. You would then need to wrap the data coming from your FFT with calculation derived from this calibration data you have collected.