Frequency-hopping spread spectrum (FHSS) transmission is the repeated switching of frequencies during radio transmission to reduce interference and avoid interception. It is useful to counter eavesdropping, or to obstruct jamming of telecommunications. And it can minimize the effects of unintentional interference.
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In FHSS, the transmitter hops between available narrowband frequencies within a specified broad channel in a pseudo-random sequence known to both sender and receiver. A short burst of data is transmitted on the current narrowband channel, then transmitter and receiver tune to the next frequency in the sequence for the next burst of data. In most systems, the transmitter will hop to a new frequency more than twice per second. Because no channel is used for long, and the odds of any other transmitter being on the same channel at the same time are low, FHSS is often used as a method to allow multiple transmitter and receiver pairs to operate in the same space on the same broad channel at the same time.
Direct sequence spread spectrum (DSSS) is a related technique. It also spreads a signal across a wide channel, but it does so all at once instead of in discrete bursts separated by hops. It can achieve higher throughput, but DSSS is more susceptible to interference and less effective as a spectrum-sharing method.
Governments regulate broadcast spectrum and often dictate some aspects of FHSS behavior. For example, in North America, the industrial, scientific and medical (ISM) waveband is divided into 75 hopping channels, and those using them may not transmit with more than 1 watt of power on any one channel. These restrictions ensure a single device does not consume too much bandwidth or linger too long on a single frequency. In the 2000s, the U.S. Federal Communications Commission (FCC) allowed FHSS systems to operate in the unregulated 2.4 GHz band in order to support use of FHSS in 802.11b, 802.11g and 802.11n Wi-Fi deployments.
The idea behind FHSS was discovered and rediscovered several times in the 20th century. The concept was initially mentioned in print in 1908 -- by German Johannes Zenneck. But credit for FHSS really belongs to actress Hedy Lamarr, who worked with composer George Antheil to bring the technology into existence during World War II.