Frequency-hopping spread spectrum (FHSS is a method of transmitting radio signals by rapidly switching a carrier wave among many frequency channel (communications) using a pseudorandom sequence known to both transmitter and receiver (radio) It is utilized as a multiple access method in the frequency-hopping [[code division multiple access]] (FH-CDMA) scheme. A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission: # Spread-spectrum signals are highly resistant to narrowband Interference (communication) The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background. # Spread-spectrum signals are difficult to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if the pseudorandom sequence was known. # 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.

Basic algorithm

Typically, the initiation of an FHSS communication is as follows # The initiating party sends a request via a predefined frequency or control channel. # The receiving party sends a number, known as a random seed # The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single base station to serve multiple connections. # The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence. # The communication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time. In some uses, most often military, a predefined frequency-hopping sequence is negotiated, and after completing the first step the procedure is continued from number 5.

Military use

Spread-spectrum signals are highly resistant to deliberate jamming unless the adversary has knowledge of the spreading characteristics. Military radios use cryptography techniques to generate the channel sequence under the control of a secret TRANSEC (TRANSEC that the sender and receiver share. By itself, frequency hopping provides only limited protection against eavesdropping and jamming. To get around this weakness most modern military frequency hopping radios often employ separate encryption devices such as the KY-57 United States 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. Whilst providing no extra protection against wideband thermal noise the frequency-hopping approach does reduce the degradation caused by narrowband interferers. 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 transmitters 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, Title 47 CFR Part 15 on unlicensed system in the 900MHz and 2.4GHz bands permits more power than non-spread spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal Communications Commission (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–Hartley theorem for a single channel. Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. 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 inventors

Perhaps the earliest mention of frequency hopping in the open literature is in radio pioneer Jonathan Zenneck s book Wireless Telegraphy(German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken had already tried it. The German military made limited use of frequency hopping for communication between fixed command points in World War I to prevent eavesdropping by British forces, who did not have the technology to follow the sequence. Denis Winter, Haigs Command - A Reassessment A Poles engineer, Leonard Danilewicz came up with the idea in 1929.Danilewicz later recalled: "In 1929 we proposed to the Polish General Staff a device of my design for secret radio telegraphy which fortunately did not win acceptance, as it was a truly barbaric idea consisting in constant changes of transmitter frequency. The commission did, however, see fit to grant me 5,000 [[złoty]]chfor executing a model and as encouragement to further work." Cited in Władysław Kozaczuk Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War II 1984, p. 27. Several other patents were taken out in the 1930s, including one by Willem Broertjes ( issued Aug. 2, 1932). During World War II the US Army Signal Corps was inventing a communication system called SIGSALY which incorporated spread spectrum in a single frequency context. However, SIGSALY was a top-secret communications system, so 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 for their "Secret Communications System." This early version of frequency hopping used a Player piano to change between 88 frequencies, and was intended to make radio-guided torpedo s 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.

Variations of FHSS

Adaptive Frequency-hopping spread spectrum (AFH)(as used in Bluetooth improves resistance to Co-channel interference by avoiding using crowded frequencies in the hopping sequence. This sort of adaptive transmission is easier to implement with FHSS than with DSSS The key idea behind AFH is to use only the “good” frequencies, by avoiding the "bad" frequency channels -- perhaps those "bad" frequency channels are experiencing frequency selective fading or perhaps some third party is trying to communicate on those bands, or perhaps those bands are being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good/bad channels. However, if the radio frequency interference is itself dynamic, then the strategy of “bad channel removal”, applied in AFH might not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth Piconet , then they are mutually interfering and the strategy of AFH fails to avoid this interference. In this case, there is a need to use strategies for dynamic adaptation of the frequency hopping pattern.lt;/ref> Such a situation can often happen in the scenarios that use unlicensed spectrum In addition, dynamic radio frequency interference is expected to occur in the scenarios related to cognitive radio where the networks and the devices should exhibit frequency-agile operation. Chirp#Chirp modulation can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order.

See also

* Direct-sequence spread spectrum * Maximum length sequence * Orthogonal frequency-division multiplexing * Dynamic frequency hopping * Spread spectrum * List of multiple discoveries#Twentieth century

Notes

References

*Władysław Kozaczuk Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two edited and translated by Christopher Kasparek Frederick, MD, University Publications of America, 1984, ISBN 0-89093-547-5.

External links

* http://www.ferret.com.au/articles/7f/0c01f17f.asp More FHSS Information] * http://www.fcc.gov/oet/info/rules/part15/part15-2-16-06.pdf FCC Part 15 Rules that cover frequency hopping ] * http://kom.aau.dk/~petarp/papers/DAFH-AFR.pdf Frequency hopping in unlicensed spectrum] describes strategies for adaptive hopping in crowded spectrum, while considering the issues of radio etiquette and compliance with FCC Part 15 Rules Category:Applications of cryptography Category:Multiplexing Category:Radio frequency propagation Category:Quantized radio modulation modes Category:Radio resource management ar:قفز ترددي ca:Espectre eixamplat per salt de freqüència cs:FHSS da:Frekvensspring de:Frequency Hopping Spread Spectrum et:Sagedushüplemine es:Espectro ensanchado por salto de frecuencia fr:Frequency-hopping spread spectrum it:Frequency-hopping spread spectrum he:FHSS nl:Frequentieverspringing pl:FHSS pt:FHSS fi:FHSS uk:FHSS zh:跳頻