A radio technique that continuously alters its transmission pattern either by constantly changing carrier frequencies or by constantly changing the data pattern.
Direct Sequence Spread Spectrum (DSSS)
DSSS multiplies the data bits by a very fast pseudo-random bit pattern (PN sequence) that "spreads" the data into a large coded stream that takes the full bandwidth of the channel. DSSS is the basis for CDMA cellphones and 802.11 Wi-Fi wireless transmission. For a technical example of DSSS technology, see CDMA
Frequency Hopping Spread Spectrum (FHSS)
FHSS continuously changes the center frequency of a conventional carrier several times per second according to a pseudo-random set of channels, while chirp spread spectrum changes the carrier frequency. Because a fixed frequency is not used, illegal monitoring of spread spectrum signals is extremely difficult, if not downright impossible depending on the particular method. FHSS is the transmission technology in Bluetooth. See 802.11
Invented in Hollywood
In 1940, Austrian-born Hedy Lamarr, considered one of the sexiest actresses of her time, was also a co-inventor of frequency hopping. She, along with George Antheil, an American composer, invented the concept of switching frequencies in order to develop torpedo radio signals that could not be jammed. In 1942, Lamarr and Antheil were awarded a patent on the concept, but the patent later expired, and they never made any money from the idea. It would be many years later before the technology was actually deployed. What made a famous actress take an interest in radio? Lamarr was previously married to an Austrian munitions tycoon and had carefully listened to his business discussions. She conceived the idea of hopping from frequency to frequency just like a pianist can play the same notes, but in different octaves.
A Dazzling Inventor
Hardly the typical geek, Hedy Lamarr was a famous actress and brilliant inventor. (Image courtesy of T.C. Candler, www.tccandler.com)