Code Division Multiple Access (CDMA)

Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with cdmaOne (often referred to as simply "CDMA"), which is a mobile phone standard that uses CDMA as its underlying channel access method.

CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code). In communications technology, there are only three domains that can allow multiplexing to be implemented for more efficient use of the available channel bandwidth and these domains are known as time, frequency and space. CDMA divides the access in signal space. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

Interestingly, CDMA is based on a patent granted in 1942 to two people, one of which was world famous actress Hedy Lamarr. Lamarr, probably best known for doing one of the first nude scenes in a major motion picture, worked with a partner-composer George Antheil-on a system that would make radio controlled torpedoes more difficult to detect through an early version of frequency hopping. Their system was inspired by the mechanical rolls that make self playing pianos work.

CDMA has been used in many communications and navigation systems, including the Global Positioning System and the OmniTRACS satellite system for transportation logistics.