Code-division multiple access

Code Division Multiple Access (or CDMA) is a form of spread spectrum multiplexing that allows many transmissions to be sent over a single wideband channel. Qualcomm pioneered the introduction of CDMA for use with mobile phones.

CDMA works by mixing the orignal signal with a carrier, and then a pseudo-random code sequence. At the receiving end the same code is used to decode the original signal's carrier. Mixing in the code "spreads" the signal out in frequency so the bandwidth uses is slightly larger than what is needed for the original signal alone.

The advantage is that the coding is random, and thus two broadcasters can share the same channel at the same time. The interference between the two (or more) signals is reduced by the degree of orthogonality of the unique codes used in each transmission.

Another advantage is that the signal is spread out over a wider frequency, making the energy use in any one channel lower. CDMA permits a more uniform distribution of energy in the emitted bandwidth.

Source: from Federal Standard 1037C and from MIL-STD-188