# ifft and fft in matlab

Started by 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

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
>
>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
>
> >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.

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