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Frequency-hopping spread spectrum

"FHSS" redirects here. For other uses, see FHSS (disambiguation).

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Passband modulation

Analog modulation

Digital modulation

Spread spectrum

See also

Multiplexing

Analog modulation

Circuit mode(constant bandwidth)

Statistical multiplexing(variable bandwidth)

Related topics

Channel access methodsMedia access control

Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching acarrier among many frequency channels, using a pseudorandom sequence known to both transmitter andreceiver. It is used as a multiple access method in the frequency-hopping code division multiple access (FH-CDMA) scheme.

Contents [hide]

Spread-spectrum[edit]Main article: Spread-spectrum

A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:

1. Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting aspread signal spreads out the interfering signal, causing it to recede into the background.

2. Spread-spectrum signals are difficult to intercept. A spread-spectrum signal may simply appear as anincrease in the background noise to a narrowband receiver. An eavesdropper may have difficultyintercepting a transmission in real time if the pseudorandom sequence is not known.

3. Spread-spectrum transmissions can share a frequency band with many types of conventionaltransmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be used more efficiently.

Military use[edit]

Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of thespreading characteristics. Military radios use cryptographic techniques to generate the channel sequence underthe control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.

By itself, frequency hopping provides only limited protection against eavesdropping and jamming. Most modernmilitary frequency hopping radios also employ separate encryption devices such as the KY-57. U.S. military radiosthat use frequency hopping include the JTIDS/MIDS family, HAVE QUICK and SINCGARS

Civilian use[edit]

In the US, since the Federal Communications Commission (FCC) amended rules to allow frequency hoppingspread spectrum systems in the unregulated 2.4 GHz band, many consumer devices in that band have employedvarious spread-spectrum modes.

Some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed forunlicensed use on the 900 MHz band. Several such radios are marketed under the name eXtreme Radio Service(eXRS). Despite the name's similarity to the FRS allocation, the system is a proprietary design, rather than anofficial U.S. Federal Communications Commission (FCC) allocated service.

Motorola has deployed a business-banded, license-free digital radio that uses FHSS technology: the DTR series,models 410, 550 and 650.

Technical considerations[edit]

The overall bandwidth required for frequency hopping is much wider than that required to transmit the sameinformation using only one carrier frequency. However, because transmission occurs only on a small portion ofthis bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providing no extraprotection against wideband thermal noise, the frequency-hopping approach does reduce the degradation causedby narrowband interference sources.

One of the challenges of frequency-hopping systems is to synchronize the transmitter and receiver. One approachis to have a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can thenfind the transmitter by picking a random channel and listening for valid data on that channel. The transmitter's datais 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 usefixed tables of channel sequences so that once synchronized they can maintain communication by following thetable. 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 902–928 MHz and 2.4 GHz bands permits more power thannon-spread-spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. Thelimit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal CommunicationsCommission (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 usingmultiple radios in a geographic area. This creates the possibility of system data rates that are higher than theShannon limit for a single channel. Spread spectrum systems do not violate the Shannon limit. Spread spectrumsystems rely on excess signal to noise ratios for sharing of spectrum. This property is also seen in MIMO andDSSS systems. Beam steering and directional antennas also facilitate increased system performance byproviding isolation between remote radios.

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Perhaps the earliest mention of frequency hopping in the open literature is in radio pioneer Jonathan Zenneck'sbook Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himselfstates that Telefunken had already tried it.

The German military made limited use of frequency hopping for communication between fixed command points inWorld War I to prevent eavesdropping by British forces, who did not have the technology to follow the sequence. [1

A Polish engineer, Leonard Danilewicz, came up with the idea in 1929.[2] Several other patents were taken out inthe 1930s, including one by Willem Broertjes (U.S. Patent 1,869,659, issued Aug. 2, 1932).

During World War II , the US Army Signal Corps was inventing a communication system called SIGSALY, whichincorporated spread spectrum in a single frequency context. However, SIGSALY was a top-secretcommunications system, so its existence did not become known until the 1980s.

The most celebrated invention of frequency hopping, though it came decades after others had come up with theconcept and technologies making use of it were in existence, was a patent awarded to actress Hedy Lamarr andcomposer George Antheil , who in 1942 received U.S. Patent 2,292,387 for their "Secret CommunicationsSystem". This intended early version of frequency hopping was supposed to use a piano-roll to change among 88frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam, but there isno record of a working device ever being produced. The patent was rediscovered in the 1950s during patentsearches when private companies independently developed Code Division Multiple Access, a non-frequency-hopping form of spread-spectrum.

Variations of FHSS[edit]

Adaptive Frequency-hopping spread spectrum (AFH) (as used in Bluetooth) improves resistance to radiofrequency interference by avoiding crowded frequencies in the hopping sequence. This sort of adaptivetransmission 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 partyis trying to communicate on those bands, or perhaps those bands are being actively jammed. Therefore, AFHshould 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”, appliedin AFH might not work well. For example, if there are several colocated frequency-hopping networks (as BluetoothPiconet), then they are mutually interfering and the strategy of AFH fails to avoid this interference.

The problem of dynamic interference, gradual reduction of available hopping channels and backward compatibilitywith legacy bluetooth devices was resolved in version 1.2 of the Bluetooth Standard (2003). Other Strategies fordynamic adaptation of the frequency hopping pattern have been reported in the literature.[3] Such a situation canoften 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 modulation can be seen as a form of frequency-hopping that simply scans through the available frequenciesin consecutive order to communicate.

See also[edit]

Notes[edit]

1. Jump up ^ Denis Winter, Haig's Command - A Reassessment

2. Jump up ^ Danilewicz later recalled: "In 1929 we proposed to the General Staff a device of my design forsecret radio telegraphy which fortunately did not win acceptance, as it was a truly barbaric idea consistingin constant changes of transmitter frequency. The commission did, however, see fit to grant me 5,000złotych for 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 WarII, 1984, p. 27.

3. Jump up ^ Petar Popovski; Hiroyuki Yomo and Ramjee Prasad (December 2006). "Strategies For AdaptiveFrequency Hopping In The Unlicensed Bands". IEEE Wireless Communications. Retrieved 2008-03-02.

References[edit]

External links[edit]

FCC Part 15 Rules that cover frequency hopping

Frequency hopping in unlicensed spectrum describes strategies for adaptive hopping in crowdedspectrum, while considering the issues of radio etiquette and compliance with FCC Part 15 Rules