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Vector frequency domain point and IFFT in OFDM

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richardhuang

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the inputting of the IFFT in OFDM should be the frequency domain point. which is the mapping of the constellation such as QAM or QPSK. but the vector in the consellation is just complex.why some book call this vector frequency domain point? such as 3-3j in conselation,it shoud be time domain signal,why is frequency
domain point?
 

IFFT In OFDM?

well it is just a traditional reason...nothing else..In OFDM consider FFT and IFFT as filtering..

The real channel effect is appeared in time domain(such as multipath channel delay spread)..So "assuming" transmit data are in frequency domain, perform the IFFT to get "time domain" signal to be transmitted to physical channel....that's it..
 

Re: IFFT In OFDM?

from the function of the OFDM, it is indeed that there has the IFFT expression.The output of the OFDM modulation is time domain signal.but we know in subcarriers modulation is on the frequency domain,how to understand this?is it just a expression in function but actually all the modulation in time domain?
 

Re: IFFT In OFDM?

That is not really difficult to understand. For myself, I confused when I started to learn OFDM. As the golden words of OFDM is 'To transmit N number of symbols in parallel or same time using N number of subcarriers'. This is why, we assume, each of the N symbols (could be QAM, BPSK or etc) in frequency domain. This is because each of the symbol will occupy a subcarrier during transmission. Therefore, we assume the QAM or BPSK symbol in frequency domain.
 

Re: IFFT In OFDM?

richardhuang said:
from the function of the OFDM, it is indeed that there has the IFFT expression.The output of the OFDM modulation is time domain signal.but we know in subcarriers modulation is on the frequency domain,how to understand this?is it just a expression in function but actually all the modulation in time domain?
Click to expand...
it is not true that subcarriers can be only in frequency domain. in OFDM when u see output of IFFT, it is in time domain, but u cannot see the subcarriers because signal is in time-domain and every point on graph is sum of the subcarriers at that particular point. as a whole when u draw it it seems like one time-domain signal, otherwise it is combination of subcarriers.
when u take FFT of this signal, u will find ur subcarriers, which are in frequency domain, so u can easily see and understand it.

Naveed
 

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