Note: Frequency hopping, direct sequence PN spreading, time scrambling, chirp, and combinations of these techniques are forms of spread spectrum. Ultra Wideband (UWB) is another modulation technique that accomplishes much the same purpose, based on transmitting short duration pulses.
Frequency-hopped spread spectrum was invented by actress Hedy Lamarr and musician George Antheil. U.S patent 2,292,387 was awarded in 1942. They proposed using punched paper rolls (like those in player pianos familiar to George Antheil) to coordinate the frequency shifts of sender and receiver[1].
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 and increasing the immunity of spread spectrum receivers to noise and interference. Spread spectrum generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wide band of frequencies. The receiver correlates the signals to retrieve the original information signal.
Source: from Federal Standard 1037C and from the NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management and from MIL-STD-188 and from the National Information Systems Security Glossary
See also: open spectrum
Spread-spectrum clock generation
Spread-spectrum clock generation (SSCG) is used in the design of synchronous digital systems, especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signal that, because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency and its harmonics, and would therefore radiate energy with an infinite spectral density. Practical synchronous digital systems radiate electromagnetic energy in a number of narrow bands at the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagnetic interference (e.g. those of the FCC in the United States, JEITA in Japan and the IEC in Europe).
To avoid this problem, which is of great commercial importance to manufacturers, spread-spectrum clocking is used. This consists of modulating the frequency of the clock signal by either a regular function such as a triangular wave, or by a pseudo-random function. This method distributes the energy of the clock signal over a wider frequency range, and so reduces its peak spectral density. The technique therefore reshapes the system's electromagnetic emissions to make them comply with the electromagnetic compatibility (EMC) regulations. It is a popular technique because it can be used to gain regulatory approval with only a simple modification to the equipment.
It is important to note that this method does not reduce the peak electrical or magnetic field strength emitted by the system, nor the total energy, and therefore does not make the system any less likely to interfere with sensitive equipment such as TV and radio receivers. It works because the EMI receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to radiate all of its energy at one frequency, then this energy would fall into a single frequency band of the receiver, which would register a large peak at that frequency. Spread-spectrum clocking distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits.
See also :electromagnetic compatibility (EMC) and electromagnetic interference (EMI), optimum utilisation of spectrum.