Frequency-hopping spread spectrum
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 used as a multiple access method
in the frequency-hopping [[code division multiple access]]
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. A spread-spectrum signal may simply appear as an increase in the background noise to a narrowband receiver. An eavesdropper may have difficulty intercepting a transmission in real time if the pseudorandom sequence is not 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 used more efficiently.
Spread-spectrum signals are highly resistant to deliberate radio 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 in advance.
By itself, frequency hopping provides only limited protection against eavesdropping and jamming. There is a simple algorithm that effectively discovers the sequence of frequencies To get around this weakness most modern military frequency hopping radios employ separate encryption devices such as the KY-57 United States
military radios that use frequency hopping include the JTIDS/MIDS family, HAVE QUICK
In the US, since the Federal Communications Commission
(FCC) amended rules to allow frequency hopping spread spectrum
systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various spread-spectrum modes.
that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios are marketed under the name eXtreme Radio Service
(eXRS). Despite the names similarity to the FRS allocation, the system is a proprietary design, rather than an official U.S. Federal Communications Commission (FCC) allocated service.
Motorola has deployed a business banded radio too that uses FHSS technology. This is the Motorola DTR series radios. They are the 410, 550 and 650 series radios. They are license free digital radios.
The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information
using only one Carrier wave
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 interference sources.
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 46 CFR Part 15
on unlicensed system in the 902–928 MHz and 2.4 GHz 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
systems. Beam steering
and directional antennas also facilitate increased system performance by providing isolation between remote radios.
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]
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 among 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 non-frequency-hopping 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 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 to communicate.
* Dynamic frequency hopping
* List of multiple discoveries Twentieth century
* Maximum length sequence
* Orthogonal frequency-division multiplexing
* Radio frequency sweep
*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.
* http://www.access.gpo.gov/nara/cfr/waisidx_05/47cfr15_05.html 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 Quantized radio modulation modes
Category Radio frequency propagation
Category Radio resource management
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