Muted parts in the transmitted signal

Hi,

I am using a flowgraph to make a SNR estimation of a transmitted OFDM
signal. The method I am using to estimate SNR calculates the difference
between the power of the received signal and the noise floor.

In the low SNR case(-20 dB for example) I was getting, for a transmitter
with constant gain, a SNR that varies a lot over time. Sometimes, I
could
get oscillations of 5 dB or maybe more for more than 200000 samples.

I took some samples of the OFDM signal and store them in a file. What I
noticed is that there are some “mute” parts in the received signal that
appear from time to time as you can see in the picture
http://old.nabble.com/file/p33688580/oscillation.jpg

I wonder what is the explanation for this phenomenon

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