Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Wireless mobile communications and wireless LANs can use a variety of schemes to transmit analog or digital information between base stations and users. One method is to transmit at a specific frequency, much like a radio station transmits at a frequency that you dial on your radio. For example, the AMPS mobile cellular telephone system operates in the 824-MHz to 894-MHz frequency range. This range is divided into a pool of 832 full-duplex channel pairs (one sends, one receives). Calls are made over the channels.

The only problem with this scheme is that anyone with an appropriate radio receiver can listen in on a target frequency. The other problem is that the frequency occupies a narrow band that is susceptible to interference, either accidental or malicious. Spread spectrum is a technique of spreading a signal out over a very wide bandwidth, often over 200 times the bandwidth of the original signal.

Spread spectrum technology was first used in World War II as a way to provide jamproof radio communication for guided torpedoes. A spread spectrum transmitter spreads the signals out over a wide frequency range using one of the following techniques:

• Direct sequence spread spectrum    In this scheme, the data to transmit is altered by a bit stream that is generated by the sender. The bit stream represents every bit in the original data with multiple bits in the generated stream, thus spreading the signal across a wider frequency band. If 100 bits are used to represent each bit of data, the signal is spread out to 100 times its original bandwidth. The source generates a pseudorandom bit stream to modulate the original data and the destination generates the same bit stream to demodulate what it receives. Spread spectrum broadcasts in bands where noise is prominent, but does not rise above the noise. Its radio signals are too weak to interfere with conventional radios and have fewer FCC (Federal Communications Commission) restrictions.


• Frequency hopping spread spectrum    In this technique, the original data signal is not spread out, but is instead transmitted over a wide range of frequencies that change at split-second intervals. Both the transmitter and the receiver jump frequencies in synchronization during the transmission so a jammer would have difficulty targeting the exact frequency on which the devices are communicating. The frequencies are derived from a table that both the sender and receiver follow.

CDMA (Code Division Multiple Access) is a digital cellular standard that uses wideband spread spectrum techniques for signal transmission. CDMA is employed in cellular phone systems and has become the top choice for new high-bandwidth 3G (third-generation) phones because of its spectral efficiency. CDMA is also used in wired systems such as shared cable access networks so that the shared spectrum is used more efficiently.

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