Frequency-hopping spread spectrum 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. It is used as a multiple access method in the frequency-hopping code division
multiple access scheme.
Spread-spectrum
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
Spread-spectrum signals are highly resistant to narrowband interference. 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. Military use
Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary
has knowledge of the spreading characteristics. 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 in advance.
By itself, frequency hopping provides only limited protection against eavesdropping and
jamming. Most modern military frequency hopping radios also employ separate encryption devices
such as the KY-57. U.S. military radios that use frequency hopping include the JTIDS/MIDS
family, HAVE QUICK and SINCGARS. Civilian use
In the US, since the Federal Communications Commission 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.
Some walkie-talkies 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. Despite the name's similarity to the FRS allocation, the system is a proprietary
design, rather than an official U.S. Federal Communications Commission 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
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
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 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 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 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. 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, 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.
A Polish engineer, Leonard Danilewicz, came up with the idea in 1929. Several other patents
were taken out in the 1930s, including one by Willem Broertjes.
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, though it came decades after others had come up with the concept and technologies
making use of it were in existence, was a patent awarded to actress Hedy Lamarr and
composer George Antheil, who in 1942 received U.S. Patent 2,292,387 for their "Secret Communications
System". This intended early version of frequency hopping was supposed to use a piano-roll to
change among 88 frequencies, and was intended to make radio-guided torpedoes harder for
enemies to detect or to jam, but there is no record of a working device ever being produced.
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
improves resistance to radio frequency 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, 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. 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 modulation can be seen as a form of frequency-hopping that simply scans through
the available frequencies in consecutive order to communicate.
See also Dynamic frequency hopping
List of multiple discoveries Maximum length sequence
Orthogonal frequency-division multiplexing Radio frequency sweep
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
FCC Part 15 Rules that cover frequency hopping 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