Reply by Abhishek Ballaney●January 1, 20092009-01-01
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!
Reply by furqan ahmed●December 29, 20082008-12-29
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!
Reply by mash...@yahoo.com●December 26, 20082008-12-26
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!