OFDM

OFDM is the waveform that made wideband mobile broadband practical. The trick is counterintuitive: instead of sending one fast data stream that gets mangled by multipath, you split the channel into hundreds of narrow subcarriers and send a slow stream on each. A slow stream means a long symbol, and a long symbol is far more resistant to the echoes (delay spread) that wreck high-speed single-carrier transmission in a real, cluttered radio environment.

"Orthogonal" is the clever part. The subcarriers are spaced so that each one's peak lands exactly where its neighbours hit zero, so they can overlap in frequency without interfering — you pack them tight and lose almost no spectrum to guard bands. Receivers process the whole thing efficiently with an FFT, which is what made it cheap enough for handsets. A short guard interval (the cyclic prefix) at the front of each symbol soaks up residual multipath. The catch every RF engineer learns is the high peak-to-average power ratio: combine many subcarriers and the peaks get large, which stresses the power amplifier — the specific problem that pushed LTE toward SC-FDMA on the uplink.