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ifft and fft in matlab

Started by shamma May 7, 2009
hi 
i want to ask whether there is any difference between taking n-point ifft
and then fft for OFDM Vs taking ifft and then n-point fft in matlab.


the difference i get is the signal with n-point ifft and then fft has a
spectrum which do not have good resolution can anyone help me in this
regard.



Regards
On May 7, 2:59&#2013266080;am, "shamma" <shammashah...@yahoo.com> wrote:
> hi > i want to ask whether there is any difference between taking n-point ifft > and then fft for OFDM Vs taking ifft and then n-point fft in matlab. >
One is correct and the other is incorrect.
> the difference i get is the signal with n-point ifft and then fft has a > spectrum which do not have good resolution can anyone help me in this > regard. >
I cannot understand this last statement, can you say it in a different way, please? Julius
>On May 7, 2:59=A0am, "shamma" <shammashah...@yahoo.com> wrote: >> hi >> i want to ask whether there is any difference between taking n-point
ifft
>> and then fft for OFDM Vs taking ifft and then n-point fft in matlab. >> > >One is correct and the other is incorrect. > >> the difference i get is the signal with n-point ifft and then fft has
a
>> spectrum which do not have good resolution can anyone help me in this >> regard. >> > >I cannot understand this last statement, can you say it in a >different way, please? > >Julius >
Thanx for the reply ......Foe example actual data =[1 0 0 0] x = [0 0 1 0 0 0 0 0] %note the zeros at both ends of signal x=ifft(x) x=fft(x) plot(abs(x)) hold on; y = [1 0 0 0] y=ifft(y) y=fft(y,8) plot(abs(y),'r') why the result is not same In the first case i have interpolated 2 zeros on either side of the signal and in the second case i am using the matlab comand to interpolate zeros .Can you please tell me which one is wrong Regards
On May 7, 11:03&#2013266080;pm, "shamma" <shammashah...@yahoo.com> wrote:
> >On May 7, 2:59=A0am, "shamma" <shammashah...@yahoo.com> wrote: > >> hi > >> i want to ask whether there is any difference between taking n-point > ifft > >> and then fft for OFDM Vs taking ifft and then n-point fft in matlab. > > >One is correct and the other is incorrect. > > >> the difference i get is the signal with n-point ifft and then fft has > a > >> spectrum which do not have good resolution can anyone help me in this > >> regard. > > >I cannot understand this last statement, can you say it in a > >different way, please? > > >Julius > > Thanx for the reply ......Foe example > actual data =[1 0 0 0] > x = [0 0 1 0 0 0 0 0] %note the zeros at both ends of signal > x=ifft(x) > x=fft(x) > plot(abs(x)) > hold on; > > y = [1 0 0 0] > y=ifft(y) > y=fft(y,8) > plot(abs(y),'r') > > why the result is not same > > In the first case i have interpolated 2 zeros on either side of the signal > > and in the second case i am using the matlab comand to interpolate zeros > .Can you please tell me which one is wrong > > Regards
You need to review basic interpolation via the Fourier transform. That's the root cause of your misunderstanding. Julius
On May 7, 2:59&#2013266080;am, "shamma" <shammashah...@yahoo.com> wrote:
> hi > i want to ask whether there is any difference between taking n-point ifft > and then fft for OFDM Vs taking ifft and then n-point fft in matlab. > > the difference i get is the signal with n-point ifft and then fft has a > spectrum which do not have good resolution can anyone help me in this > regard.
Please do send separate copies of the same post to multiple newsgroups. Use one copy and a comma separated send list. See my replies in comp.soft-sys.matlab. http://groups.google.com/group/comp.soft-sys.matlab/ msg/b513535c2ef60b22?hl=en Hope this helps. Greg