IWR6843ISK: Why does ADC signal (I/Q) signal go down at the end of FFT bins ?

Part Number: IWR6843ISK
Other Parts Discussed in Thread: IWR6843

Hi 

I have a question about ADC signal 

The below signal is the ADC samples (I/Q) which are measured on each RX channel using IWR6843.

our configuration parameters are as follows.

profileCfg 0 60 10 11.2 76.4 0 0 51.42 1 128 2000 0 0 158
chirpCfg 0 0 0 0 0 0 0 1


In the figure, the horizontal axis is the sample number, and grid exists for every 10 samples.

The below figure shows that the size of ADC Amplitude from 90 to 128 samples is less than the size of ADC amplitude from 0 to 80 samples.

(the size of ADC amplitudes from 0 to 80 samples are bigger and the size of ADC amplitudes from 0 to 80 samples are smaller )

I want to know why the size of the ADC Amplitude is different between from 0 to 80 samples and from 90 to 128 samples