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Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio 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:
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 occurs only 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. Multiple Inventions Perhaps the earliest mention of frequency hopping in the open literature is in radio pioneer Johannes Zenneck's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken had already tried it. A Polish army officer Leonard Danielewicz came up with the idea in 1929. Several other patents were taken out in the 1930s, including one by Willem Broertjes (Germany 1929, US patent Variations Adaptive Frequency-hopping spread spectrum (AFH) (as used in Bluetooth) improves resistance to radio frequency interference by avoiding using crowded frequencies in the hopping sequence. This sort of adaptive modulation is easier to implement with FHSS than with DSSS. Chirp modulation (chirp modulation) can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order. See also | ||||||||
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