IFFT in OFDM

 Dear sir / Madam, I am doing M.E in Communication  system, i have gone
through the concept and fundamentals of OFDM first time , but i could not
understand the process of IFFT in OFDM modulation, why FFT is not taken at
the first stage? if any kind personality explains the concepts behind the
IIFT in OFDM, which will helpful for me to understand better.

How can i get e-version of any OFDM text book in the net- kindly suggest
any suitable website to download .

With regards,
Velan

velan wrote:
>  Dear sir / Madam, I am doing M.E in Communication  system, i have gone
> through the concept and fundamentals of OFDM first time , but i could not
> understand the process of IFFT in OFDM modulation, why FFT is not taken at
> the first stage? if any kind personality explains the concepts behind the
> IIFT in OFDM, which will helpful for me to understand better.
> 

Because each symbol controls the amplitude/phase of a sub-carrier in the
frequency-domain.  However, the transmitted waveform is a time-domain
waveform, therefore one needs an IFFT to convert from frequency to
time-domain.

-- 
Oli

On Dec 29, 1:43&#4294967295;am, "velan" <velan.resea...@gmail.com> wrote:
> &#4294967295;Dear sir / Madam, I am doing M.E in Communication &#4294967295;system, i have gone
> through the concept and fundamentals of OFDM first time , but i could not
> understand the process of IFFT in OFDM modulation, why FFT is not taken at
> the first stage? if any kind personality explains the concepts behind the
> IIFT in OFDM, which will helpful for me to understand better.
>
> How can i get e-version of any OFDM text book in the net- kindly suggest
> any suitable website to download .
>
> With regards,
> Velan

There's lots of ways to think of this, but here is one...

An OFDM symbol comprises the sum of a set of carriers (over the finite
symbol period) at different frequencies, each of which has a complex
amplitude given by one of a series of modulated (e.g. QPSK) data.

The frequencies of these carriers are carefully chosen so that they
can be easily separated at the receiver without any interference
between them (orthogonality).

In turns out this is what happens if you perform an IFFT on the set of
modulated data at the transmitter (to get the transmitted time domain
signal), and perform an FFT on the received signal at the receiver (in
order to separate the received signals in the frequency domain).

So basically in the transmitter you are assigning modulated data to
carrier frequencies in the frequency domain. You could literally
create each carrier frequency one at a time in the time domain,
multiply each by the corresponding complex amplitude (equal to the
modulated data), and then sum them together, then transmit; but simply
doing an IFFT on the set of modulated data does exactly the same
thing, usually with a lot less complexity.

Note data is just data - it is neither a time domain nor a frequency
domain phenomenon.
You can do what you want with it to communicate it in the most
efficient / accurate way possible.
The only requirement is that the physical signal you transmit must be
in the time domain!

If you're a student your university has a library - try it out!
They probably also have a subscription to IEEE Xplore too, although a
simple textbook would be the best place to start.

-T

On Dec 28, 9:43&#4294967295;pm, "velan" <velan.resea...@gmail.com> wrote:
> &#4294967295;Dear sir / Madam, I am doing M.E in Communication &#4294967295;system, i have gone
> through the concept and fundamentals of OFDM first time , but i could not
> understand the process of IFFT in OFDM modulation, why FFT is not taken at
> the first stage? if any kind personality explains the concepts behind the
> IIFT in OFDM, which will helpful for me to understand better.
>
> How can i get e-version of any OFDM text book in the net- kindly suggest
> any suitable website to download .
>
> With regards,
> Velan



hope u know the basics behind the ofdm well,
here small intro,
u know that the high speed data is trannsmitted through several low
data rate subcarriers. the carrier frequency will be in GHz range.
in a single carrier system we need single local oscillator in GHz
range. but in ofdm we r using several subcarriers. it is too complex
to design te circuits with lot of subcarriers. because all the
subcarriers are closely placed. the oscilator stability also the issue
here. so instead of generating all the subcarriers in the ghz range,
we
generate the carrriers in baseband level. then upconverting to ghz
range. in baseband it is easy to generate the subcarriers orthogonoly
using IFFT. by using single local oscillator u can generate several
orthogonal subcarriers in rf(high freq) range.  Usinfg IFFT we can
generate sinusoidal carriers ortogonoly. for example, let try this in
matlab,
input_ifft=zeros(1,64);
input_ifft(2)=1;
output_ifft=ifft(input_ifft);
plot(real(output_ifft)) ;
here u can see one complete cycle.then
plot(imag(output_ifft));
here index 2 is fundamental frequency let Ff.
if u use input_ifft(3) means the frequency is 2*Ff.i.e u can see 2
cycles it is orthogonal to previous one.
now u can upconvert, i.e multiply with high frequency carrrier
to understand ofdm first u should know how FFT works.
for FFT  basics gothrough
http://www.complextoreal.com/tutorial.htm
in that read tutorials 4,5 and 6.
for ofdm tutorial read 22.
hope u will get the ans