how does the ifft work in OFDM

[abdullah gill]

Hi to all and everyone
after reading so much about ofdm all around i m still unable to find the solution of how IFFT makes the carrier signals orthogonal .whats the mathematics involve in it.actually i m looking for the mathematical proof for that....can someone help me out

 

[tabraiz]

IFFT takes the time domain chunk of Signals (Sub carriers) and Adds them to produce a carrier. This is similar to the FFT. Then the Frequency domain signal is produced. Which is further processed

 

[abdullah gill]

@tabraiz
now u r confusing me more that how can a sum of subcarriers can produce a single carrier. please elaborate it a bit thanks

 

[abdullah gill]

thanks mathuranthan i have read all these tutorials but my query reamains there please help me out

 

[abdullah gill]

thanks mathuranathan
i have tried it in matlab that after taking the ifft of any random numbers it will give u complex numbers and if u multiple and two adjacent complex nos u will get the resultant zero which proves that they are orhtogonal to each other

 

[abdullah gill]

hi everybody ,
after so much study and discussions with u people i have simulated OFDM in matlab simulink ..........but now the problem is i cant understand what the graph is showing to me it in negative dbs on y-axis.
can someone helo me out?
i ll be very grateful

 

[Communications_Engineer]

upload the graph so that we can read it for you

 

[uk.engr]

Can any one tell me: how OFDM can be used to transmit at high data rates over a wireless channel

 

[Communications_Engineer]

Sure.

The reason why we do not transmit at high data rates over a wireless channel without using OFDM is that most of our transmitted signals will be corrupted by Inter Symbol Interference in the time-domain and by Frequency selective fading in the frequency domain. This happens because the symbol time becomes smaller than the maximum excess delay of the channel.

Now, OFDM helps by making sure the symbol time remains larger than the maximum excess delay of the channel by dividing a wide band, high symbol rate carrier into multiple narrower band, smaller symbol rate carriers.

To be honest, it would more appropriately be called multicarrier transmission, it is just that OFDM is the most efficient form of MC trans.


I hope this helps.


Feel free to google any term you do not understand or contact by yahoo group telecom_research

Added after 2 minutes:

Tutorials by mathuranathan and at www.complex2real.com are good for beginners, if you are one with OFDM

 

[uk.engr]

Thank you communications_Engineer

 

[uk.engr]

I want to make OFDM code using 16-PSK modulation and having FFT Size=64. As we know the predefined function for M-PSK which is given as: modulate(modem.pskmod(modindex),data).
How can i utilize this function for OFDM? I mean, how should i put FFT size in this function?

 

[Communications_Engineer]

Can you make OFDM code using 2-PSK modulation and having FFT Size=64?

Added after 17 seconds:

If not, try that first!

 

[uk.engr]

Ok Thank you Communications_Engineer, now i am trying for 2-PSK FFT size=64.

 

[abdullah gill]

the graph is uploaded now if some one can help me in understanding it

 

[uk.engr]

I am trying to understand the Matlab program for BPSK OFDM with FFT size=64. At the time of taking ifft it takes fftshift. It also normalizes the power of transmit symbol to 1 i.e. xt = (nFFT/sqrt(nDSC))*ifft(fftshift(xF.’)).’ , i have following queries:

1. Why there is a need of normalization?

2. Before ifft, it have taken fftshift that swaps the data elements, why there is need of fftshift?

Please tell me. Thank you