Everything about Spread Spectrum totally explained
Spread-spectrum techniques are methods by which
energy generated in a particular
bandwidth is deliberately spread in the
frequency domain, resulting in a
signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural
interference and
jamming, and to prevent detection.
History
Frequency Hopping
The concept of
frequency hopping was first alluded to in the 1903 and filed by
Nikola Tesla in July 1900. Tesla came up with the idea after demonstrating the world's first radio-controlled submersible boat in 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfered with in any way." His patents covered two fundamentally different techniques for achieving immunity to interference, both of which functioned by altering the carrier frequency or other exclusive characteristic. The first had a transmitter that worked simultaneously at two or more separate frequencies and a receiver in which each of the individual transmitted frequencies had to be tuned in, in order for the control circuitry to respond. The second technique used a variable-frequency transmitter controlled by an encoding wheel that altered the transmitted frequency in a predetermined manner. These patents describe the basic principles of frequency hopping and frequency-division multiplexing, and also the electronic AND-gate logic circuit.
Frequency hopping is also mentioned 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 several years earlier. Zenneck's book was a leading text of the time and it's likely that many later engineers were aware of it. A
Polish army officer, named 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,, 1932). During
World War II, the US Army Signal Corp was inventing a communication system called
SIGSALY for communication between Roosevelt and Churchill, which incorporated spread spectrum, but due to its top secret nature, SIGSALY's existence didn't 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". Lamarr had learned about the problem at defense meetings she'd attended with her former husband
Friedrich Mandl, who was an Austrian arms manufacturer. The Antheil-Lamarr version of frequency hopping used a
piano-roll to change among 88 frequencies, and was intended to make radio-guided
torpedoes harder for enemies to detect or to jam. The patent came to light during patent searches in the 1950s when
ITT Corporation and other private firms began to develop
Code Division Multiple Access (CDMA), a civilian form of spread spectrum, though the Lamarr patent had no direct impact on subsequent technology. It was in fact ongoing military research at
MIT Lincoln Laboratory,
Magnavox Government & Industrial Electronics Corporation, ITT and
Sylvania Electronic Systems that led to early spread-spectrum technology in the 1950s. Parallel research on radar systems and a technologically similar concept called "phase coding" also had an impact on spread-spectrum development.
Commercial use
The 1976 publication of
Spread Spectrum Systems by Robert Dixon, ISBN 0-471-21629-1, was a significant milestone in the commercialization of this technology. Previous publications were either classified military reports or academic papers on narrow subtopics. Dixon's book was the first comprehensive unclassified review of the technology and set the stage for increasing research into commercial applications.
Initial commercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System's very small aperture (
VSAT) satellite terminal system for newspaper newswire services, Del Norte Technology's radio navigation system for navigation of aircraft for crop dusting and similar applications, and
Qualcomm's
OmniTRACS
system for communications to trucks. In the Qualcomm and Equatorial systems, spread spectrum enabled small antennas that viewed more than one satellite to be used since the
processing gain of spread spectrum eliminated interference. The Del Norte system used the high bandwidth of spread spectrum to improve location accuracy.
In 1981, the Federal Communications Commission started exploring ways to permit more general civil uses of spread spectrum in a Notice of Inquiry docket. . This docket was proposed to FCC and then directed by Michael Marcus of the FCC staff. The proposals in the docket were generally opposed by spectrum users and radio equipment manufacturers, although they were supported by the then
Hewlett-Packard Corp. The laboratory group supporting the proposal would later become part of
Agilent.
The May 1985 decision in this docket permitted unlicensed use of spread spectrum in 3 bands at powers up to 1 Watt. FCC said at the time that it would welcome additional requests for spread spectrum in other bands.The resulting rules, now codified as permitted
Wi-Fi,
Bluetooth, and many other products including cordless telephones. These rules were then copied in many other countries. Qualcomm was incorporated within 2 months after the decision to commercialize CDMA.
Spread-spectrum telecommunications
This is a technique in which a (
telecommunication) signal is transmitted on a
bandwidth considerably larger than the
frequency content of the original information.
Spread-spectrum telecommunications is a signal structuring technique that employs direct sequence, frequency hopping or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Spread spectrum generally makes use of a sequential
noise-like signal structure to spread the normally
narrowband information signal over a relatively
wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes called
low probability of intercept (
LPI).
Frequency-hopping spread spectrum (FHSS),
direct-sequence spread spectrum (DSSS),
time-hopping spread spectrum (THSS),
chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. Each of these techniques employs
pseudorandom number sequences — created using
pseudorandom number generators — to determine
and control the spreading pattern of the signal across the alloted bandwidth.
Ultra-wideband (UWB) is another modulation technique that accomplishes the same purpose, based on transmitting short duration pulses. Wireless Ethernet standard
IEEE 802.11 uses either FHSS or DSSS in its radio interface.
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