Saturday, May 23, 2009
Heddy Lamarr, George Antheil..."frequency hopping"
Heddy Lamarr, George Antheil, World War II, and the development of "frequency hopping"...a methodology to scramble radio communications . Legend holds that they were the creators of the methodology but that may be somewhat doubtful. Wikipedia offers...
Perhaps the earliest mention of frequency hopping in the open literature is 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.
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 army officer, Leonard Danielewicz, came up with his own version of the idea in 1929. Several other patents were taken out in the 1930s, including one by Willem Broertjes (Germany 1929, U.S. Patent No. 1,869,659 (issued Aug. 2, 1932)).
During World War II, the US Army Signal Corp was inventing a communication system called SIGSALY, which incorporated spread spectrum in a single frequency context. However, SIGSALY was 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 patent number 2,292,387 for their "Secret Communications System." This early version of frequency hopping used a piano-roll to change between 88 frequencies, and was intended to make radio-guided torpedoes 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.
Nevertheless, I prefer the Lamarr/Antheil story.
Frequency hopping is one of two basic modulation techniques used in spread spectrum signal transmission. It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications. It also is known as frequency- hopping code division multiple access (FH-CDMA).
Spread spectrum modulation techniques have become more common in recent years. Spread spectrum enables a signal to be transmitted across a frequency band that is much wider than the minimum bandwidth required by the information signal. The transmitter "spreads" the energy, originally concentrated in narrowband, across a number of frequency band channels on a wider electromagnetic spectrum. Benefits include improved privacy, decreased narrowband interference, and increased signal capacity.
In an FH-CDMA system, a transmitter "hops" between available frequencies according to a specified algorithm, which can be either random or preplanned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. Frequency hopping requires a much wider bandwidth than is needed to transmit the same information using only one carrier frequency.
The spread spectrum approach that is an alternative to FH-CDMA is direct sequence code division multiple access (DS-CDMA), which chops the data into small pieces and spreads them across the frequency domain. FH-CDMA devices use less power and are generally cheaper, but the performance of DS-CDMA systems is usually better and more reliable. The biggest advantage of frequency hopping lies in the coexistence of several access points in the same area, something not possible with direct sequence.
Certain rules govern how frequency-hopping devices are used. In North America, the Industrial, Scientific, and Medial (ISM) waveband is divided into 75 hopping channels, with power transmission not to exceed 1 watt on each channel. These restrictions ensure that a single device does not consume too much bandwidth or linger too long on a single frequency.
The Federal Communications Commission (FCC) has amended rules to allow frequency hopping spread spectrum systems in the unregulated 2.4 GHz band. The rule change is designed to allow wider bandwidths, thus enabling Internet devices to operate at higher speeds and fostering development of wireless LANs and wireless cable modems.
Movie star Hedy Lamarr is generally credited as co-originator of the idea of spread spectrum transmission. She and her pianist were issued a patent for the technique during World War II. They discovered the technique using a player piano to control the frequency hops, and envisioned it as a way to provide secure communications during wartime. The pair never made any money off the invention and their patent eventually expired. Sylvania introduced a similar concept in the 1950s and coined the term "spread spectrum."
The complete patent
Thanks to Patrick Neas for the tip.