Orthogonal frequency-division multiplexing (OFDM)

OFDM is the idea behind Wi-Fi, 4G/5G, digital TV, and DSL. Instead of blasting one fast bit-stream down a single high-rate carrier — which a real-world channel with echoes smears into a useless blur — OFDM splits the data across hundreds or thousands of slow, narrow subcarriers running in parallel. Each subcarrier carries a modest QAM symbol at a leisurely pace, like turning one frantic checkout lane into 1000 calm ones. The 'orthogonal' part is the magic: the subcarriers are spaced so precisely that each one's peak sits exactly on its neighbours' nulls, so they overlap heavily yet never interfere.

Splitting into slow subcarriers tames the channel's worst enemy, multipath echo (inter-symbol interference), because each slow symbol lasts long enough that delayed copies arrive within the same symbol rather than smearing into the next. A short 'cyclic prefix' guard interval absorbs the remaining echo entirely. The whole transform is computed cheaply with an inverse FFT at the transmitter and an FFT at the receiver, which is why OFDM only became practical once DSP chips were fast enough — and then conquered nearly every modern wireless standard.