ofdm circular convolution
assume that the sequnce is,
x(0),x(1),x(2),x(3)
and the channel impulse response is h(0), h(1);
if we take DFT of x and h, h has to be extended upto N (4)points,
h(0), h(1), 0,0;
now if we multiply DFTs of x(that is X) and h(that is H), then coreespondingly there should be circular con in time domain;
for example for the first symbol;
x(0)*h(o) + x(1)*0 + x(2)*0+x(3)*h(1);------(A)
obviously just sending x through the channel you cannot get this;we do not get the last term
so what we do is
add taplength-1 (here it is 2-1=1)cyclic extension to x sequence as,
x(3) x(0) x(1) x(2) x(3), noW if we do the linear convolution
for the "second term" you get
x(3)*h(1)+x(0)*h(o) + x(1)*0 + x(2)*0
which exactly the cicular convolution in (A); similary you can get the other term....
so what you have to do is; add channel_tap_length -1 cyclic extension to x sequence and just pass through the channel;
and remove first channel_tap_length -1 symbols and take the rest N symbols(here is is 4), and take FFT
that will be H
*X
Hope this will help