# orthogonality not acheived after IFFT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Started by March 8, 2007
```Guys i am working on ofdm simulation.after taking the ifft of my
crriers (having baseband data in the form of matrix) mycarriers are
not orthogonal. As the ifft is responsible for making carriers
orthogonal in order to acheive OFDM. I am using built in command of
ifft in matlab.
For eg x=ifft (carrier_matrix,128)
then when i take dot product of my vectors the result is a non-zero
number
or when i perform x*x' the result is not identity matrix.
It means that orthogonality is not acheived among subcarriers.

```
```On Thu, 08 Mar 2007 18:47:03 -0000, awais <awais.engr@gmail.com> wrote:

> Guys i am working on ofdm simulation.after taking the ifft of my
> crriers (having baseband data in the form of matrix) mycarriers are
> not orthogonal. As the ifft is responsible for making carriers
> orthogonal in order to acheive OFDM. I am using built in command of
> ifft in matlab.
> For eg x=3Difft (carrier_matrix,128)
> then when i take dot product of my vectors the result is a non-zero
> number
> or when i perform x*x' the result is not identity matrix.
> It means that orthogonality is not acheived among subcarriers.

What do you mean by "baseband data in the form of matrix"?  The only thi=
ng  =

I can assume this means is that each column of carrier_matrix contains t=
he  =

data for one OFDM symbol.

If this is the case, then x*x' will not be the identity matrix (unless t=
he  =

original data was especially chosen).

-- =

Oli
```
```origional data is random and is not especially chosen
by baseband data i mean  a stream of 1's and -1's in the form of
complex no
for eg 1 -> 1+ j0
-1 -> 1 - j0
this data is then transformed into matrix which has 4 columns.
i have to take ifft of this matrix in which my columns act as
sub_carriers
so the problem is that when i take ifft my resulting suncarriers are
not orthogonal
thats where the problem lies

Should i write my own function for taking DFT and dont use matlab
function

help me
thanks

```
```awais wrote:

> origional data is random and is not especially chosen
> by baseband data i mean  a stream of 1's and -1's in the form of
> complex no
> for eg 1 -> 1+ j0
>         -1 -> 1 - j0
> this data is then transformed into matrix which has 4 columns.
> i have to take ifft of this matrix in which my columns act as
> sub_carriers

If you arrange your input data like this, then the result of y = ifft(x)
will be a matrix; each column of y will be the IFFT of the corresponding
column in x.  Therefore:

* Each column of x and y is a separate OFDM symbol.

* The rows of x correspond to sub-carriers.

* Neither the rows nor the columns of y correspond to sub-carriers.

* y contains time-domain data.

* In general, the columns of y won't be orthogonal.

> so the problem is that when i take ifft my resulting suncarriers are
> not orthogonal
> thats where the problem lies
>

--
Oli
```
```how can i acheive orthogonality in frequency or time domain
i-e how can i make my subcarriers orthogonal in time domain

```
```On Mar 12, 5:19 pm, "awais" <awais.e...@gmail.com> wrote:
> how can i acheive orthogonality in frequency or time domain
> i-e how can i make my subcarriers orthogonal in time domain

They are orthogonal.  But neither the columns nor the rows of y (in
y=ifft(x)) correspond to sub-carriers, so neither the columns nor the
rows of y will be orthogonal.

--
Oli

```