Frequency-hopping spread spectrumFrequency-hopping spread spectrum
) is a spread-spectrum
method of transmitting signals by rapidly switching a carrier
among many frequency channels
, using a sequence known to both transmitter
A spread-spectrum transmission offers two main advantages over a fixed-frequency transmission:
- Spread-spectrum signals are highly resistant to noise as well as deliberate jamming. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.
- Spread-spectrum signals are very hard to intercept. A spread-spectrum signal sounds like background noise on any receiver except a spread-spectrum receiver using the exact channel sequence used by the transmitter.
By virtue of these properties, spread-spectrum communication offers a third benefit: spread-spectrum transmissions can share a frequency band with conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.
The concept of frequency hopping was invented in 1942 during World War II by actress Hedy Lamarr and composer George Antheil, who received patent number 2,292,387 for their "Secret Communications System". This early version of frequency hopping used a piano-roll to change between 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam. The patent was little-known until recently because Lamarr applied for it under her married name of Hedy Keisler Markey. Neither Lamarr nor Antheil made any money from the patent.
- The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency.
- Use of the Shannon-Hartley law shows that the signal to noise ratio (SNR) required for the carrier relative to the background decreases the wider the range of frequencies used for transmission. It is even possible to have workable systems with negative SNRs (expressed in decibels), which correspond to wanted signals (on average) being lower than the noise level at any frequency.