Regarding plotting the graph for received data in USRP2 testbed node

Hi.
I have been trying to plot the PSD of data which was received on a
receiver node. I got a txt file which was a binary file having all the
received power data.
I had a matlab code to plot it and the graph had a peak in the center
frequency which was abt -48 dB . but then it went down till -220 dB as
we come away from the center freq. I have attached the image file in
here.
My professors said its not possible in real time to plot anything
below -120 dB itself. But I have been looking at the data it used and
they were very minimum numbers.
I am not sure why such low power data is coming. Can anyone help me
out with this?