# OFDM demodulation with a single mixer and FFT

#### yef smith

Joined Aug 2, 2020
747
Hello,I the lecture shown bellow they mention a method where we only need one mixer and digital FFT to do demodulation .
I cold not find materials on the method.
How exactly FFT could help us memodulate OFDM signal?
Thanks.

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#### Papabravo

Joined Feb 24, 2006
21,225
The FFT algorithm can do either forward transform from the time domain to the frequency domain, or the inverse transform from the frequency domain to the time domain. Does that give you a clue about how it might be used.

https://en.wikipedia.org/wiki/Fast_Fourier_transform

A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in the frequency domain and vice versa.

#### yef smith

Joined Aug 2, 2020
747
Hello Papabravo,I know that FFT converts continues data into descrete frequency domain.but i cant see hoe it could be used for demodulation.

#### Papabravo

Joined Feb 24, 2006
21,225
Hello Papabravo,I know that FFT converts continues data into descrete frequency domain.but i cant see hoe it could be used for demodulation.
If the DFT (Discrete Fourier Transform) is used to convert sample data in the time domain, and transform it into the frequency domain, then it seems reasonable to surmise that if you perform the inverse transform on the frequency domain data you will be able to recover the original time domain data.

#### nsaspook

Joined Aug 27, 2009
13,254
If the DFT (Discrete Fourier Transform) is used to convert sample data in the time domain, and transform it into the frequency domain, then it seems reasonable to surmise that if you perform the inverse transform on the frequency domain data you will be able to recover the original time domain data.
Exactly.
https://www.researchgate.net/public...of_Orthogonal_Frequency_Division_Multiplexing
A. Discrete Fourier Transform

Throughout the development of OFDM technology, there have been a number of remarkable contributions. The first milestone came about in 1971 when Weinstein and Ebert [10] used a discrete Fourier transform (DFT) to perform baseband modulation and demodulation in the receiver. It should be noted that in 1970, the application of the DFT to an FDM system was first proposed by Darlington [11]. This renovation of the original analog multicarrier system to a digitally implemented OFDM eliminates banks of subcarrier oscillators and coherent demodulators and thus reduces the implementation complexity. This evolution makes the modern low-cost OFDM systems plausible today.

#### yef smith

Joined Aug 2, 2020
747
Hello nsaspook, I am having problem to imagine how it works ,is there some example of visualization i could use?
Thanks.

"This renovation of the original analog multicarrier system to a digitally implemented OFDM eliminates banks of subcarrier oscillators and coherent demodulators and thus reduces the implementation complexity"

#### nsaspook

Joined Aug 27, 2009
13,254
This is a old school analog example of HF digital data transmission, that when in diversity mode, operated similar (it still had guard bands due to analog filtering) to OFDM.

We would duplicate one or two digital channels on the multiple sets of DC loops to have several HF sub-carriers on each data link.

All of that analog complexity is replaced with digitally implemented OFDM.