Frequency-hopping spread spectrum
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Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
- Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.
- Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.
- Spread-spectrum transmissions can share a frequency band with many types of 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.
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Military use
If the sequence of channel changes is not known to potential adversaries, spread-spectrum signals are highly resistant to deliberate jamming. Military radios use cryptographic techniques to generate the channel sequence under the control of a secret Transmission Security Key that the sender and receiver share. By itself, frequency hopping provides only limited protection against eavesdropping, so military frequency hopping radios often employ separate encryption devices such as the KY-57. U.S. military radios that use frequency hopping include HAVE QUICK and SINCGARS.
Technical considerations
The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission only occurs on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Use of the Shannon limit shows that the signal to noise ratio (SNR) required for the carrier relative to the background decreases as a wider range of frequencies is 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.
In a real multipoint radio system, space allows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system data rates that are higher than the Shannon limit for a single channel. This property is also seen in MIMO and DSSS systems. Beam steering and directional antennas also facilitate increased system performance by providing isolation between remote radios.
Invented by a movie star
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 (http://www.ncafe.com/chris/pat2/index.html) 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.