FFT and orthogonality....

Hi all!
I am really new in DSP, finally i found this group. I hope members of DSP could help me honestly.
My question is for what we use FFT (DFT, IFFT, IDFT) and why is needed?
I read we use IFFT (or IDFT) in OFDM modulation scheme to transmit signals, what is the aim using it? (to change some properties ?)

My second quesyion is about orthogonality. I can explain with mathemathical rules that we have 2 functions sin(x) and cos(x)
if they are orthogonal each other cos(x)*sin(x)=0 means x=(pi/2)*k; k=1,2,3....
Hovewer, in OFDM technique, there are many signals which are obey in orthogonality rule, for example
1-subcarrier sin(x)/x;
2-subcarrier sin(x-2)/(x-2);
3-subcarrier sin(x+2)/(x+2);
....
....
Could somebody helps me? I really appreciate!
Thanks in advance! Start a New Thread

IFFT is used in OFDM to modulate the data on orthogonal subcarriers, as they are orthogonal so they dont interfere with each other even when they overlap in frequency domain, this makes OFDM spectrally efficient.

As far as orthogonality is concerned different harmonics of sinusoid are orthogonal, as they have integer number of cycles in one period, so positive and negative areas under the curve cancel. In OFDM modulation is based on this concept of orthogonality.

Furqan
--- On Fri, 12/26/08, m...@yahoo.com wrote:

From: m...@yahoo.com
Subject: [audiodsp] FFT and orthogonality....
To: a...
Date: Friday, December 26, 2008, 4:00 PM

Hi all!
I am really new in DSP, finally i found this group. I hope members of DSP could help me honestly.
My question is for what we use FFT (DFT, IFFT, IDFT) and why is needed?
I read we use IFFT (or IDFT) in OFDM modulation scheme to transmit signals, what is the aim using it? (to change some properties ?)

My second quesyion is about orthogonality. I can explain with mathemathical rules that we have 2 functions sin(x) and cos(x)
if they are orthogonal each other cos(x)*sin(x) =0 means x=(pi/2)*k; k=1,2,3....
Hovewer, in OFDM technique, there are many signals which are obey in orthogonality rule, for example
1-subcarrier sin(x)/x;
2-subcarrier sin(x-2)/(x- 2);
3-subcarrier sin(x+2)/(x+ 2);
....
....
Could somebody helps me? I really appreciate!
Thanks in advance!


Start a New Thread

Dear Mashhurs,

FFT is an efficient algorithm to compute DFT & IDFT.  A DFT decomposes a sequence of values into components of different frequencies.  Computing DFT as per its definition is often slow & not practical.  FFT is a way to compute it quickly.

Regarding orthogonality, if inner product of two functions is zero then they are orthogonal.

Regards,
abhishek

--- On Fri, 26/12/08, m...@yahoo.com wrote:

From: m...@yahoo.com
Subject: [audiodsp] FFT and orthogonality....
To: a...
Received: Friday, 26 December, 2008, 4:30 PM


Hi all!

I am really new in DSP, finally i found this group. I hope members of DSP could help me honestly.

My question is for what we use FFT (DFT, IFFT, IDFT) and why is needed?

I read we use IFFT (or IDFT) in OFDM modulation scheme to transmit signals, what is the aim using it? (to change some properties ?)

My second quesyion is about orthogonality. I can explain with mathemathical rules that we have 2 functions sin(x) and cos(x)

if they are orthogonal each other cos(x)*sin(x) =0 means x=(pi/2)*k; k=1,2,3....

Hovewer, in OFDM technique, there are many signals which are obey in orthogonality rule, for example

1-subcarrier sin(x)/x;

2-subcarrier sin(x-2)/(x- 2);

3-subcarrier sin(x+2)/(x+ 2);

....

....

Could somebody helps me? I really appreciate!

Thanks in advance! Start a New Thread