How does IFFT modulate in OFDM transmitter?

[nawabumar]

Hi.

I am having problem regarding the usage of IFFT in OFDM transmitter.The transmitter should take in input symbols and modulate different subcarriers with one of these symbols.On the other hand IFFT input is these symbols(X(k)'s) but the output is x's

N-1 j2Πnk/N
x=∑ X(k)e
k=0

which from this formula are not the individually modulated sub-carriers.Cn anyone explain how IFFT is able to achieve this goal of modulation.I'm writing c code of IFFT for OFDM .How can I include the sub-carrier information in the ouput of IFFT.???

 

[bulx]

Re: Question:FFT in OFDM

What is meant by "one of these symbols"? IFFT is a batch process, you have to collect all your symbols you want to create one OFDM symbol, before you do the IFFT. You dont modulate carrier by carrier, but all carriers simultaneously.

Your output after IFFT is the sum of all modulated carriers.
-b

 

[hamdy]

Re: Question:FFT in OFDM

u fist convert the serial samples to parallel to form the OFDM symbol then pass them to IFFT, so each sample has its subcarrier

 

[nawabumar]

Re: Question:FFT in OFDM

yes.thats whtat i am saying .you convert serial data(say 8 symbols) into 8 parallel symbols and pass this frame of 8 symbols to 8 point IFFT block.At the output of this block we get x's (from the formula),which supposedly are the modulated sub-carriers .But these are just numbers i mean.where is the information about the carrier frequecies??Is it just gonna be a simple IFFT algorithm which takes in 8 complex numbers and outputs 8 complex numbers or something extra.
here is my IFFT code..

//*********************************
#include <stdio.h>
#include <conio.h>
#include <math.h>


void main(void){

int n=8;
int m=3;
int i,j,k,n1,n2,l;
double c,s,e,a,t1,t2;
double x[8]={1,1,1,1,1,1,0,0};
double y[8]={0,0,0,0,0,0,0,0};
j = 0; /* bit-reverse */
n2 = n/2;



for (i=1; i < n - 1; i++){
n1 = n2;
while ( j >= n1 ){
j = j - n1;
n1 = n1/2;
}
j = j + n1;
if (i < j){
t1 = x;
x = x[j];
x[j] = t1;
t1 = y;
y = y[j];
y[j] = t1;
}
}


n1 = 0; /* FFT */
n2 = 1;

for (i=0; i < m; i++){
n1 = n2;
n2 = n2 + n2;
e = -6.283185307179586/n2;
a = 0.0;

for (j=0; j < n1; j++){
c = cos(a);
s = -sin(a);
a = a + e;

for (k=j; k < n; k=k+n2){
t1 = c*x[k+n1] - s*y[k+n1];
t2 = s*x[k+n1] + c*y[k+n1];
x[k+n1] = x[k] - t1;
y[k+n1] = y[k] - t2;
x[k] = x[k] + t1;
y[k] = y[k] + t2;
}
}
}
for(l=0;l<=7;l++){

// mod= sqrt(pow(x[l],2)+pow(y[l],2));
printf("\n%.2f %.2fj",x[l]/n,y[l]/n);

}

}

//********************

Added after 56 minutes:

what is happening after multiplication with different carriers in each parallel branch.should ther be P IFFT blocks??is that a product sign after the multiplication with carriers??

 

[bulx]

Re: Question:FFT in OFDM

Yes, they are just numbers, but they are also samples of a the signal that would have been formed if you had indivually generated modulated sine waves and added up.

Your question is very general: Any DSP operation leaves with a set of numbers, the numbers have meaning, only of you know at what sampling rate the original sampling was done, else you cannot recreate. Think about sound samples stored on the harddisk, thay can be played as the recorded song only if you know what was the sampling rate with which it was recorded.

In this case, there is no sampling because you are creating the waveform from scratch, so to speak. If you were to feed your IFFT output data serially to a AD converter at a mutiple of the rate you want the carrier frequncies to be, you will generate the modulated wave.

You can actually create any frequency (and it multiples) you like, depending on the rate at which you feed to an ADC
-b

 

[nawabumar]

Re: Question:FFT in OFDM

well thanks bulx.so now if i have to insert guard interval and do cyclic prefix what do i have to do??copy some of the ouputs(x's) of the IFFT block(depending upon the guard time calculations)at the beginning and transmit this extended frame serially.is that how it should be done??

I'm not planning on going into analog domain .all the stuff is to be done inside the DSP.So is my transmitter complete after the above process??Means it's time to implement the receiver..
thanks again

Added after 1 minutes:

kindly copy the link of the above figure in a new window and then open it..

 

[bulx]

Re: Question:FFT in OFDM

"..copy some of the ouputs(x's) of the IFFT block(depending upon the guard time calculations)at the beginning and transmit this extended frame serially.is that how it should be done??"

exactly.
-b

 

[vuday455]

Re: Question:FFT in OFDM

will anybody tell me how BER vs FFT plot looks like?

i need it urgently for analysis the reseults..
thanx regards..
uday

 

[goosecool]

Re: Question:FFT in OFDM

well thanks bulx.so now if i have to insert guard interval and do cyclic prefix what do i have to do??copy some of the ouputs(x's) of the IFFT block(depending upon the guard time calculations)at the beginning and transmit this extended frame serially.is that how it should be done??

I'm not planning on going into analog domain .all the stuff is to be done inside the DSP.So is my transmitter complete after the above process??Means it's time to implement the receiver..
thanks again

Added after 1 minutes:

kindly copy the link of the above figure in a new window and then open it..

-- you should actually try to understand the concept of guard period here. cyclic prefix guard period is needed to simplify the channel estimation process while providing temporal separation between the ofdm symbols to mitigate multipath interference.

the concept is, if an inner product of 2 sequences is ifft'ed what you get is a circular convolution of the fft of those sequences.

FFT[x (*) y] = FFT[x] . FFT[y] where (*) is circular convolution

this way you can estimate the channel frequency response at the pilot carriers by a simple scalar multiplication. if you were to fold the sequence x on itself while convolving it becomes circular convolution. in ofdm, instead of folding the time domain symbol from at the beginning, its folded at the end which means a part of ofdm symbol from the end is appended to the beginning.

 

[firewood80]

Re: Question:FFT in OFDM

Hi there,

I'm having problem in understanding the function of FFT (as well as its counter IFFT) in OFDM. Can anybody explain to me? Someone ask me, "What does FFT&IFFT do in OFDM?"

 

[samuraign]

Question:FFT in OFDM

IFFT is converting frequency domain signals to time domain and FFT does the reverse.

 

[firewood80]

Re: Question:FFT in OFDM

Thanks, but can you eloborate more on that specifically?

 

[rag.perfect]

Re: Question:FFT in OFDM

Hi all!!!
please don't Confuse IFFT and FFT here as time<->frequency domain converter. any signal is bascially available in both the form simultaniously.
They are infact mathematical operter here to map the samples on subcarriers.
at TX, you can use FFT also but at Rx u have to use IFFT.
its vice versa is also true.