RE IFFT IN ofdm MODULATION

[bvishwanath]

what is the use of ifft block in ofdm ?
asn: to provide orthogonal between carriers..
and to conver freq to time domain but how can we know ( before ifft block )it is in frequency domain?

why can't we use fft block in transmitter and ifft blok in rx?
diff bet mapper and modulator??

 

[ChakerQ]

Hi,

what is the use of ifft block in ofdm ?

- ifft is used in ofdm to process modulation no more. It is a mathematical utility (not get confused with conversion to frequency domain or inverse)

asn: to provide orthogonal between carriers..
and to conver freq to time domain but how can we know ( before ifft block )it is in frequency domain?

- The orthogonality between frequencies is guaranteed by separation of 1/T. T is the time to transmit one symbol.

why can't we use fft block in transmitter and ifft blok in rx?

-like i said, the use of ifft is only to facilitate mathematical process of modulation.

diff bet mapper and modulator??

- the mapper block in ofdm schema is used to convert symbol to one of constellation ASK, PSK or QAM.
for example with BASK the sequence [0 1 1 1 0 0] will be [-1 1 1 1 -1 -1 ]

- Modulation block add a waveform to symbols
[-1 1 1 1 -1 -1 ] will be [-1*cos(2*pi*f1*t) 1*cos(2*pi*f2*t) 1*cos(2*pi*f3*t) 1*cos(2*pi*f4*t) -1*cos(2*pi*f5*t) -1*cos(2*pi*f6*t) ]

regards,

Chaker

 

[bvishwanath]

can ifft provide orthogonlity?
which block in ofdm will provide separation of 1/T. T is the time to transmit one symbol. is ifft right?
how precoded transform(wht ,dht etc..) decreases PAPR?

---------- Post added at 12:35 ---------- Previous post was at 12:32 ----------

if mapper does not do modulation in ofdm then why it is called as modulation?
which block does modulation? is ifft or other??

 

[yonemo]

what is the use of ifft block in ofdm ?

I was confused on this issue myself until I ran into this excellent tutorial.

**broken link removed**

Basically the guy is saying that after you take the time-domain serial input, parallelize it, and modulate it (in this example BPSK), you can actually view the each separate stream as the coefficients of a frequency domain signal. And that is exactly what the IFFT wants as an input. Thus, the output can be viewed as a time-domain sum of a set of frequency signals, which is what is transmitted on the channel. But he also makes the excellent point that the IFFT and FFT are virtually identical except for some sign changes. They are commutable and you can just as easily use an FFT in the transmitter and an IFFT in the receiver. The convention in literature is IFFT and that's how it is today. At the end of the day, you have a set of discrete points, you point them through a transform, you get another set of discrete points.

Read the tutorial and it should be clear.

 

[Quadrant Quadrifoglio]

You could use FFT in xmtr and IFFT in recvr - nothing will go wrong. You can do IFFT then FFT or FFT then IFFT. Both will give the same result. See **broken link removed** and select Tutorial 22 and look at page 11.

However, you obviously can't do FFT then FFT nor IFFT then IFFT. You have to do one of them (pick one) then the other.

So, if you make a transmitter and I make a receiver, we better know which uses which transform - we can't both use the same one. They are different, but given any input, they both produce the same output. But given that output, if you want the input back, you have to use the 'other' one. So you must do either IFFT then FFT or FFT then IFFT.

My educated guess is that we mostly think in terms of the Fourier Transform as converting signals from the time domain into the frequency domain. The inverse of that would convert from the frequency domain into the time domain. Since we want to transform frequency domain signals in the form of multiple carriers ('subcarriers') into a time domain signal that can be transmitted, then it's logical to think of that process as the inverse Fourier Transform. But it could be done just as well using the ('forward') Fourier Transform. Anyway, to keep everyone (all those who will make transmitters and receivers) on the same page, let's decree that it will be IFFT in the transmitter and FFT in the receiver. Could have been the other way 'round!