Y-Axis is the amplitude "abs(of the complex signal)".
X-Axis is not pure frequency domain or time domain,
because the figure is drown from follow:
I generate ZC sequence "its amplitude equals to 1 in
frequency domain" then I made IFFT and transmit the signal
using USRP. The environment is a cable. I received the
signal from another antenna of the same USRP. In the
receiver I made FFT and plot the correspond figure. Since,
I am the source file in the transmitter transmit the
signal many times"repeat activated". The last figure which
I sent it is the signal in the frequency domain which are
repeated with each transmission "I put them all together
to compare them."
I attached the same figure with more explanation "each black
block is the signal in the frequency domain", but block 1
.....n is the same signal transmitted in different times.