Overview

The FFT (Fast Fourier Transform) converts time-domain signals into frequency-domain signals, while the IFFT (Inverse Fast Fourier Transform) does the reverse, converting frequency-domain signals back into time-domain signals. It’s essential to note that FFT and IFFT are algorithms for efficient computation of the discrete Fourier transform (DFT) and its inverse, respectively.

The data to be transmitted, initially represented in the frequency domain, is converted into a time-domain signal by the IFFT, suitable for transmission over the channel. Each frequency component (or subcarrier) generated is orthogonal to the others. This orthogonality ensures that the subcarriers are mathematically independent and won’t interfere with each other, even when closely spaced.

In terms of visualization, the time domain of a signal displays the variance of its amplitude over time, as might be seen on an oscilloscope. In contrast, the frequency domain illustrates how a signal’s energy is distributed across different frequencies, similar to the output of a spectrum analyzer.

An analogy with music could be: the time domain corresponds to the loudness of a song over time, while the frequency domain represents the different pitches or frequencies present at a specific moment in the song, along with their respective amplitudes.

Purpose & Function

The IFFT is used to convert an outgoing frequency-domain signal for transmission onto all available subcarriers. These subcarriers then carry the signal across a medium, such as FSO (Free Space Optics). Upon receipt, the FFT is employed to revert the time-domain signal back to the frequency domain, enabling the extraction of data transmitted on each subcarrier. This data is then recompiled into the original data stream. The IFFT/FFT process ensures the orthogonality, or equal distance between subcarriers. As a result, these closely spaced frequencies remain non-interfering, crucial for preserving data integrity.

References

Mohsin, Marwa & Murdas, Ibrahim. (2022). Design an Outdoor Light Fidelity (Li-Fi) System Based on All-Optical OFDM Architecture. International Journal of Intelligent Engineering and Systems. 15. 2022. 10.22266/ijies2022.0630.16.