Frequency-hopping spread spectrum

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Frequency-hopping spread spectrum (FHSS) is a 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:

  1. 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.
  2. Spread-spectrum signals are difficult to intercept. A frequency-hop spread-spectrum signal simply sounds like an increase in the background noise to a narrowband receiver.
  3. 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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[edit] 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 (TRANSEC) 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.

[edit] 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.

One of the challenges of frequency hopping systems is to synchronize the transmitter and receiver. One approach is to have a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can then find the transmitter by picking a random channel and listening for valid data on that channel. The transmitter's data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel and the segment can have a checksum for integrity and further identification. The transmitter and receiver can use fixed tables of channel sequences so that once synchronized they can maintain communication by following the table. On each channel segment, the transmitter can send its current location in the table.

In the US, FCC part 15 on unlicensed system in the 900MHz and 2.4GHz bands permits more power than non-hopping systems. The limit is increased from 1 milliWatt to 1 Watt or a thousand times increase. The FCC prescribes a minimum number of channels and a maximum dwell time for each channel.

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.

[edit] 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 # 1,869,695, 1932). During WWII, the US Army Signal Corp was inventing a communication system called SIGSALY, which incorporated spread spectrum, but as it was top secret, its existence did not become known until the 1980s. The most celebrated invention of frequency hopping was that of actress Hedy Lamarr and composer George Antheil, who in 1942 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 rediscovered in the 1950s during patent searches when private companies independently developed Code Division Multiple Access, a civilian form of spread-spectrum.

[edit] 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 can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order.

[edit] See also

[edit] External links

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