The Unreasonable Effectiveness of the Fourier Transform
Talk Track Two
Traditionally modulated radio interfaces suffer from a wide variety of problems. Some radio interfaces get very confused if the carrier is not centered where it’s supposed to be; some radio interfaces provide a high error rate on noisy channels; some radio interfaces suffer from the "digital cliff" in receivability; some radio interfaces use a wide bandwidth, and are very susceptible to uneven channel fading; some radio interfaces cannot tolerate multipath (signals bouncing on the way to a receiver); some wideband radio interfaces get completely muted by even a single tone of interference; and the list goes on. For each of these problems, an enormous amount of research went into mitigations for AM- and FM-style modulations, and each of these mitigations became computationally intensive and difficult to scale to higher data rates. With the advent of OFDM-derived modulations, however, almost all of these problems are very efficiently solved in one fell swoop! It seems like magic, but it is actually very understandable — at least, if you aren’t trying to read it out of a dry textbook.
We will begin with a brief refresher on frequency-domain signal processing, and the concept of linear time-invariant systems; for everyone’s sake, though, we’ll try to keep the math to a minimum. From there, we’ll give an overview of the core concept of OFDM — and how it’s implemented. Finally, for as many radio modulation challenges as we have time for, we’ll take an intuitive approach to try to visualize the impact the problem would have on an AM-modulated signal, and then we’ll see how OFDM neatly gives us a solution to each problem almost for free!