Making and OFDM modulator with ifft

[julian403]

Transmitter

Serie to Parallel converter + Modulation (QPSK, QAM,APSK,etc ) + iFFT + adding signal+ antenna

I can't understand why there is an IFFT module instead of a FFT in the transmitter.

Let's see what I understand.

Let's figure up that the modulator is an 8PSK and the Serie to Parallel register store 3 bit from the serie path. Thats combination of bit are mapped in a 8 PSK constellation following this (I invented that mapped)

0 0 0 -> angle1
0 0 1 -> angle2
0 1 0 -> angle3
0 1 1 -> angle4
1 0 0 -> angle5
1 0 1 -> angle6
1 1 0 -> angle7
1 1 1 -> angle8

And of course, angle is a constellation phase.

So, we have an output signal in the modulation output with the form:

s(t) = A sin (2 Π f t + angleX)

where angleX can be angle1, angle2, ... depends on the 3-bit combination

But s(t), it's a signal in time domain. Why there is so the IFFT?

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What I think the IFFT can do is take more than one SP register and allocated it in:

y(t) = A1 sin (2 Π f + angleX1) + A2 sin (4 Π f + angleX2) +...+ An sin (n 2 Π f + angleXn)

That's correct? all signal has an orthogonal frequency. But I developed an OFDM modulator without an IFFT. of course, this is all generated by a DSP and there isn't an oscilator for every frecuency.

 

[Pulkit Varshney]

Hey,
If you look at any standard OFDM modulation literature, there is IFFT at Tx side and FFT at Rx side, that's how they are implemented. FFT and IFFT blocks can be interchanged as they are inverse of each other.

But s(t), it's a signal in time domain. Why there is so the IFFT?

it depends how you analyze the signal, another way to analyze this is
Freq domain signal -> IFFT -> time domain signal -> channel -> time domain signal -> FFT -> Freq domain signal

after all these are all numbers, it depends on us how we we utilize these representation for our purpose.

 

[Sani.hashmi]

we take IFFT on transmitter side because of the multi subcarriers .