question:what is the need of IFFT and FFT in OFDM?

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raghavenderece

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what is the need of IFFT and FFT in OFDM?
what is cyclic prefix?
what is out of band energy in OFDM transmitters ?how it will happen?
 

Salam(= hello in farsi)
by using IFFT and FFT in OFDM Tx/Rx, systems can do modulation in digital form. and it is important.
in OFDM idea each channel should modulate by 1 different frequency for example 32 frequency needed. in simple form u should implement your sestem by using 32 oscillators. it is expensive, hard to implement in high frequency, need more space and ...
but by using IFFT and FFT in digital form just 1 oscillator is needed. and IFFT convert time domain data to frequency domain data.

because in OFDM, carriers are orthogonal and near each other, if IF and RF ocsillators is not perfect, maybe some power of 1 channel go to the next channels.
This is out of band energy
 

cyclic prefix is the guard interval between symbole and others to avoid isi
 

we know that an OFDM(A) signal consists number of subchannels and frequency of each subchannel is unique from the rest.
and the basic principle of OFDM is that the basis functions are orthogonal to each other.
i.e. hence after mapper we use IFFT function to modulate and hence choose the functions that are orthogonal to each other.
 

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