Spread spectrum

Spread spectrum deliberately smears a signal across far more bandwidth than it needs, which sounds wasteful but is brilliantly useful. In direct-sequence spread spectrum (DSSS) you multiply each data bit by a fast pseudo-random code, fracturing it into many tiny 'chips' that fill a wide band at low power per hertz — the signal sinks below the noise floor, looking like faint static to anyone without the code. In frequency-hopping spread spectrum (FHSS) the carrier leaps among many frequencies dozens of times a second on a secret schedule, so an interferer or eavesdropper can never camp on it.

Spreading buys three superpowers. It resists jamming and narrowband interference, because a jammer that blasts one frequency only dents a sliver of the spread signal. It's hard to detect or intercept, prized by the military that invented it. And it lets many users share one band at once using different codes (CDMA, the basis of 3G and GPS). The receiver 'de-spreads' by correlating against the same code — a matched-filter operation that simultaneously lifts the wanted signal out of the noise and rejects everyone else's.